Modularized small-signal model of network construction type converter and grid-connected stability discrimination method

Abstract

The invention discloses a modular small-signal model of a network construction type converter and a grid-connected stability judgment method and system, and the method comprises the steps: determining a steady-state operation point of a grid-connected system of the network construction type converter based on a steady-state operation equation; establishing a small-signal transfer function model of the network construction type converter, establishing a small-signal transfer function model of the alternating-current power grid, and establishing a small-signal transfer function model of the network construction type converter grid-connected system; analyzing a small signal propagation path of the grid-connected system of the network-forming converter, and determining a key output variable; on the basis of the key output variables, reconstructing a small signal transfer function model of the grid-connected system of the grid-forming converter; carrying out equivalent transformation on the small-signal transfer function model of the grid-connected system of the network-forming converter, and deducing an analysis small-signal mathematical transfer function model of the grid-connected system; and on the basis of analyzing the small signal mathematical transfer function model, analyzing the small signal stability of the grid-connected system of the network construction type converter, and obtaining a small signal stability evaluation result of the grid-connected system of the network construction type converter.

Description

Technical Field

[0001] The present invention relates to the technical field of renewable energy power generation and grid connection, and more specifically, to a modular small signal model of a grid-connected converter and a grid connection stability determination method and system. Background Art

[0002] Constrained by the power density of wind and light resources, new energy units have small capacity and low voltage levels. Continuous access and multi-stage voltage boosting and transmission are the main ways to develop and utilize them on a large scale. With the increase in access capacity, the equivalent grid support strength of new energy grid-connected points gradually decreases, and weak power grids have become a common feature of my country's large-scale new energy grid-connected systems.

[0003] However, the mainstream wind and photovoltaic power generation equipment currently used all adopt a grid-following control scheme, which follows the synchronous voltage of the grid connection point through phase-locked control, and uses multi-loop control and power electronic converters to achieve power output. The interaction between the above control characteristics and the weak power grid can easily cause wide-band oscillations under steady-state conditions and overvoltage disconnection under transient faults.

[0004] Therefore, the existing grid-following control scheme is difficult to support the development of new energy into the main power source. A technology is needed to realize the judgment of grid-connected stability based on the modular small signal model of the grid-connecting converter. Summary of the invention

[0005] The technical solution of the present invention provides a modular small signal model of a grid-connected converter and a method and system for judging grid-connected stability, so as to solve the problem of how to judge grid-connected stability based on the modular small signal model of the grid-connected converter.

[0006] In order to solve the above problems, the present invention provides a modular small signal model of a grid-connected converter and a grid-connected stability determination method, the method comprising:

[0007] Based on the steady-state operation equation, determine the steady-state operation point of the grid-connected system of the grid-connected converter, and based on the steady-state operation point, control the grid-connected converter in a steady state;

[0008] Establishing a small signal transfer function model of a grid-forming converter and a small signal transfer function model of an AC power grid, and establishing a small signal transfer function model of a grid-forming converter grid-connected system based on the small signal transfer function model of the grid-forming converter and the small signal transfer function model of the AC power grid;

[0009] Analyze the small signal propagation path of the grid-connected system of the grid-connected converter and determine the key output variables;

[0010] Based on the key output variables, a small signal transfer function model of the grid-connected converter system is reconstructed;

[0011] Performing an equivalent transformation on the reconstructed small signal transfer function model of the grid-connected converter system, and deriving an analytical small signal mathematical transfer function model of the grid-connected system;

[0012] Based on the analytical small signal mathematical transfer function model, the small signal stability of the grid-connected system of the grid-connected converter is analyzed to obtain a small signal stability evaluation result of the grid-connected system of the grid-connected converter.

[0013] Preferably, the determining of the steady-state operation point of the grid-connected system of the grid-connected converter based on the steady-state operation equation, and the controlling of the grid-connected converter in a steady state based on the steady-state operation point, comprises:

[0014] The steady-state operation equations of various electrical quantities in the grid-connected system of the grid-connected converter in the dq coordinate system are:

[0015]

[0016]

[0017] Among them, U gd0 =U g0 cosδ0,U gq0 =U g0 sin(-δ0), δ0=θ PCC0 -θ g0 , δ0 is the steady-state power angle, U g0 and θ g0 are the amplitude and phase of the grid voltage, θ PCC0 is the steady-state value of the grid-connected point voltage phase, U PCCd0 is the steady-state value of the d-axis component of the grid-connected point voltage, U PCCq0 is the steady-state value of the q-axis component of the grid-connected point voltage, U gd0 is the steady-state value of the d-axis component of the grid voltage, U gq0 is the steady-state value of the q-axis component of the grid voltage, I gd0 is the steady-state value of the d-axis component of the current flowing into the AC grid, I gq0 is the steady-state value of the q-axis component of the current flowing into the AC grid, R g is the equivalent resistance of the AC grid, ω0 is the angular frequency of the ideal AC grid, L g is the equivalent power plant of the AC grid, U cd0 is the steady-state value of the d-axis component of the converter port voltage, U cq0 is the steady-state value of the q-axis component of the converter port voltage, R f is the AC filter resistor, I Ld0 is the steady-state value of the d-axis component of the filter inductor current, I Lq0 is the steady-state value of the q-axis component of the filter inductor current, L fis the AC filter inductor, C f is the AC filter capacitor, P0 and Q0 are the active power and reactive power output of the grid-connected converter in steady state respectively.

[0018] Preferably, the establishment of a small signal transfer function model of a grid-connected converter includes:

[0019] Establish the small signal transfer function model of the synchronization control submodule:

[0020]

[0021] Among them, Δ represents the small signal amount of the corresponding variable, s is the Laplace operator in the complex frequency domain, and θ GFM represents the output phase of the network control, K dc and T dc They are respectively the network-building controller G dc (s) proportionality coefficient and time constant, Δu dc is the DC bus voltage disturbance, Δu dcref is the change of DC bus voltage reference value;

[0022] Establish the small signal transfer function model of the reactive power-voltage control submodule:

[0023]

[0024] Among them, U ref represents the output voltage amplitude of the grid-connected converter, q ref and q e are the reactive power command and feedback value output by the grid-connected converter, k Qp and k Qi Respectively, reactive power-voltage controller G Q (s) proportional and integral coefficients;

[0025] Establish the small signal transfer function model of the virtual impedance control submodule:

[0026]

[0027] Among them, i dref represents the output of the virtual impedance control loop d-axis, i qref represents the output of the q-axis of the virtual impedance control loop, is the d-axis component of the grid-connected point voltage in the control coordinate system, is the q-axis component of the grid-connected point voltage in the control coordinate system, R v and L v are virtual resistance and virtual inductance respectively, G v (s) is the transfer function of virtual impedance control;

[0028] Establish the small signal transfer function model of the current vector control submodule:

[0029]

[0030] in, is the d-axis component of the converter modulation voltage in the control coordinate system, is the q-axis component of the converter modulation voltage in the control coordinate system, is the d-axis component of the converter output current in the control coordinate system, is the q-axis component of the converter output current in the control coordinate system, ω0 is the angular frequency of the ideal AC grid, L f is the filter inductance, k CCp and k CCi They are the current controller G CC (s) proportional and integral coefficients;

[0031] Establish the small signal transfer function model of the modulation link submodule:

[0032]

[0033] in, is the d-axis component of the converter output port voltage in the control coordinate system, is the q-axis component of the converter output port voltage in the control coordinate system, U md0 is the steady-state value of the d-axis component of the converter modulation voltage, U mq0 is the steady-state value of the q-axis component of the converter modulation voltage, U dc0 is the steady-state value of the DC bus voltage, u dc is the DC bus voltage feedback value.

[0034] Preferably, the establishing of a small signal transfer function model of an AC power grid comprises:

[0035] Establish a small signal transfer function model for the AC power grid:

[0036]

[0037] Among them, u cd is the d-axis component of the converter port voltage in the grid reference coordinate system, u cq is the q-axis component of the converter port voltage in the grid reference coordinate system, u pccd is the d-axis component of the grid connection point voltage in the grid reference coordinate system, u pccq is the q-axis component of the grid connection point voltage in the grid reference coordinate system, u gd is the d-axis component of the AC grid voltage in the grid reference coordinate system, u gqis the q-axis component of the AC grid voltage in the grid reference coordinate system, ω0 is the angular frequency of the ideal AC grid, R f and L f are the filter resistance and inductance respectively, s is the Laplace operator in the complex frequency domain, i Ld is the d-axis component of the output current in the grid reference coordinate system, i Lq is the q-axis component of the output current in the grid reference coordinate system.

[0038] Preferably, a small signal transfer function model of a grid-connected system of a grid-connected converter is established based on the small signal transfer function model of the grid-connected converter and the small signal transfer function model of the AC power grid, including:

[0039] Determine the conversion relationship between the dq components in the grid reference coordinate system based on the grid connection point voltage and the control coordinate system based on the output phase of the control link:

[0040]

[0041] in, is the d-axis component of the AC grid voltage in the control coordinate system, To control the q-axis component of the AC grid voltage in the coordinate system, consider Δθ GFM is a small disturbance, and the small signal linearization is performed on the conversion relationship to obtain the interface equation of the small signal variable in the power grid coordinate system and the control coordinate system, Δθ GFM It is the difference between the output phase of the control link and the actual phase of the grid-connected point voltage;

[0042]

[0043] Get the interface equations of different small signal variables in the grid coordinate system and the control coordinate system:

[0044]

[0045]

[0046]

[0047] Based on the implicit relationship of power balance, the small signal expressions of the active power output from the grid-connected converter port and the reactive power output from the grid-connected point are determined:

[0048]

[0049] Ignoring the power loss of the switching devices of the grid-type converter, the small signal transfer function model of the DC bus capacitor dynamics in the complex frequency domain is determined as follows:

[0050] C dc U dc0sΔu dc =Δp in -Δp o (14)

[0051] Among them, C dc and U dc0 are the steady-state values ​​of the DC bus capacitance and its voltage, Δp in is the active power disturbance injected into the DC bus capacitor on the new energy generator side, Δp o is the change in active power output by the grid-connected converter;

[0052] Assume that the DC bus voltage command value remains unchanged, that is, Δu dcref =0, the small signal transfer function model of the DC bus capacitor dynamics in the complex frequency domain is brought into the small signal transfer function model of the synchronous control submodule:

[0053]

[0054] Based on the interface equation, the small signal expressions of the active power output by the grid-connected converter port and the reactive power output by the grid-connected point, the small signal transfer function model of the grid-connected converter is combined to obtain the small signal transfer function model of the grid-connected converter grid-connected system.

[0055] Preferably, the analyzing the propagation path of the small signal transfer function model of the grid-connected system of the grid-connected converter to determine the key output variables includes:

[0056] The key output variable is the meshing control output phase.

[0057] Preferably, the reconstructing the small signal transfer function model of the grid-connected system of the grid-connected converter based on the key output variable comprises:

[0058] When the DC bus voltage command value and reactive power command value remain unchanged, Δu dcref =0, Δq ref =0, with the input power disturbance Δp in As the input signal, the output phase disturbance Δθ is controlled by the grid type GFM As the output signal, the small signal transfer function model of the grid-connected converter system is reconstructed:

[0059]

[0060]

[0061] Among them, K Pd (s) is the d-axis active power feedback gain, K Pq (s) is the q-axis active power feedback gain, K Qd(s) is the d-axis reactive power feedback gain, K Qq (s) is the q-axis reactive power feedback gain, H q0 is the coupling gain of phase disturbance to d-axis voltage, H d0 is the coupling gain of phase disturbance to q-axis voltage.

[0062] Preferably, performing equivalent transformation on the reconstructed small signal transfer function model of the grid-connected converter system to derive an analytical small signal mathematical transfer function model of the grid-connected converter system comprises:

[0063] The similar items in the small signal transfer function model of the grid-connected system of the grid-connected converter are merged, and the unit negative feedback subsystem composed of the current controller and the filter is equivalent to obtain the small signal transfer function model of the grid-connected system of the grid-connected converter:

[0064]

[0065]

[0066] Among them, G c1 (s) is the transfer function of the current control closed-loop subsystem, H U (s) is the voltage feedback equivalent gain, H θ1 (s) is the equivalent coupling gain of phase perturbation on d-axis control, H θ2 (s) is the equivalent coupling gain of phase perturbation on q-axis control, G CC (s) is the current controller transfer function, G v (s) is the virtual impedance control transfer function;

[0067] The grid-connected system of the grid-connected converter includes a positive feedback loop and a negative feedback loop:

[0068]

[0069]

[0070] Among them, G MC (s) is the equivalent transfer function of the synchronization link, H id1 (s) is the local d-axis current feedback equivalent gain, H iq2 (s) is the local q-axis current feedback equivalent gain, H iq1 (s) and H id2 (s) are the equivalent gains of dq axis current cross coupling, G dc (s) is the synchronous controller transfer function, G Q (s) is the reactive power control transfer function;

[0071] The negative feedback subsystem in the feedback path is equivalent, and the cross-coupling terms are decoupled, and the small signal transfer function model of the grid-connected system of the grid-connected converter is equivalent to a typical closed-loop feedback:

[0072]

[0073]

[0074] Where: G1(s) is the transfer function of the local equivalent subsystem of the d-axis current feedback, G2(s) is the transfer function of the local equivalent subsystem of the q-axis current feedback, H1(s) is the equivalent positive feedback gain of the entire closed-loop system, and H2(s) is the equivalent negative feedback gain of the entire closed-loop system;

[0075] Determine the open-loop transfer function G of the grid-connected system of the grid-connected converter op (s) and the closed-loop transfer function G c (s):

[0076] G op (s) = G MC (s)(H2(s)-H1(s))

[0077]

[0079] Preferably, the analyzing the small signal stability of the grid-connected system of the grid-connected converter and obtaining the small signal stability evaluation result of the grid-connected system of the grid-connected converter includes:

[0080] The expression of the open-loop transfer function in the frequency domain is G op The real and imaginary parts of (jω) are:

[0081]

[0082] Define σ(ω)=Re[G op (jω)], when the phase lag generated by the equivalent loop of the grid-connected system of the grid-connected converter reaches 180°, ∠G op (jω)=-180°,υ(ω)=0,the corresponding frequency is ω p ;

[0083] ω p Substituting into σ(ω), choose σ(ω p ) as an indicator for evaluating the small signal stability of the closed-loop system;

[0084] When σ(ω p )>0, the closed-loop system will be equivalent to a negative feedback loop, and the system will remain stable;

[0085] When -1<σ(ω p)<0, the closed-loop system will be equivalent to a positive feedback loop, but its amplitude does not meet the instability condition of the feedback system and can still remain stable;

[0086] When σ(ω p )≤-1, the closed-loop system will be equivalent to a positive feedback loop and meet the instability conditions of the feedback system, and there is a risk of instability.

[0087] Preferably, it also includes: based on the small signal stability evaluation result of the grid-connected converter grid-connected system, obtaining the instability mechanism of the grid-connected converter grid-connected system, and determining the dominant link causing the instability of the grid-connected converter grid-connected system.

[0088] Based on another aspect of the present invention, the present invention provides a modular small signal model of a grid-connected converter and a grid-connected stability determination system, the system comprising:

[0089] An initial unit, used to determine a steady-state operation point of the grid-connected system of the grid-connected converter based on a steady-state operation equation, and control the grid-connected converter in a steady state based on the steady-state operation point;

[0090] An establishing unit is used to establish a small signal transfer function model of a grid-forming converter and a small signal transfer function model of an AC power grid, and to establish a small signal transfer function model of a grid-forming converter grid-connected system based on the small signal transfer function model of the grid-forming converter and the small signal transfer function model of the AC power grid;

[0091] An analysis unit, used to analyze the small signal propagation path of the grid-connected system of the grid-connected converter and determine the key output variables;

[0092] A reconstruction unit, used for reconstructing a small signal transfer function model of the grid-connected system of the grid-connected converter based on the key output variable;

[0093] A transformation unit, used for performing an equivalent transformation on the reconstructed small signal transfer function model of the grid-connected system of the grid-connected converter, and deriving an analytical small signal mathematical transfer function model of the grid-connected system;

[0094] The result unit is used to analyze the small signal stability of the grid-connected system of the grid-connected converter based on the analytical small signal mathematical transfer function model, and obtain the small signal stability evaluation result of the grid-connected system of the grid-connected converter.

[0095] Preferably, the initial unit is used to determine the steady-state operation point of the grid-connected system of the grid-connected converter based on the steady-state operation equation, and control the grid-connected converter in a steady state based on the steady-state operation point, and is also used to:

[0096] The steady-state operation equations of various electrical quantities in the grid-connected system of the grid-connected converter in the dq coordinate system are:

[0097]

[0098]

[0099] Among them, U gd0 =U g0 cosδ0,U gq0 =U g0 sin(-δ0), δ0=θ PCC0 -θ g0 , δ0 is the steady-state power angle, U g0 and θ g0 are the amplitude and phase of the grid voltage, θ PCC0 is the steady-state value of the grid-connected point voltage phase, U PCCd0 is the steady-state value of the d-axis component of the grid-connected point voltage, U PCCq0 is the steady-state value of the q-axis component of the grid-connected point voltage, U gd0 is the steady-state value of the d-axis component of the grid voltage, U gq0 is the steady-state value of the q-axis component of the grid voltage, I gd0 is the steady-state value of the d-axis component of the current flowing into the AC grid, I gq0 is the steady-state value of the q-axis component of the current flowing into the AC grid, R g is the equivalent resistance of the AC grid, ω0 is the angular frequency of the ideal AC grid, L g is the equivalent power plant of the AC grid, U cd0 is the steady-state value of the d-axis component of the converter port voltage, U cq0 is the steady-state value of the q-axis component of the converter port voltage, R f is the AC filter resistor, I Ld0 is the steady-state value of the d-axis component of the filter inductor current, I Lq0 is the steady-state value of the q-axis component of the filter inductor current, L f is the AC filter inductor, C f is the AC filter capacitor, P0 and Q0 are the active power and reactive power output of the grid-connected converter in steady state respectively.

[0100] Preferably, the establishing unit is used to establish a small signal transfer function model of the grid-type converter, and is also used to:

[0101] Establish the small signal transfer function model of the synchronization control submodule:

[0102]

[0103] Among them, Δ represents the small signal amount of the corresponding variable, s is the Laplace operator in the complex frequency domain, and θ GFM represents the output phase of the network control, K dc and T dcThey are respectively the network-building controller G dc (s) proportionality coefficient and time constant, Δu dc is the DC bus voltage disturbance, Δu dcref is the change of DC bus voltage reference value;

[0104] Establish the small signal transfer function model of the reactive power-voltage control submodule:

[0105]

[0106] Among them, U ref represents the output voltage amplitude of the grid-connected converter, q ref and q e are the reactive power command and feedback value output by the grid-connected converter, k Qp and k Qi Respectively, reactive power-voltage controller G Q (s) proportional and integral coefficients;

[0107] Establish the small signal transfer function model of the virtual impedance control submodule:

[0108]

[0109] Among them, i dref represents the output of the virtual impedance control loop d-axis, i qref represents the output of the q-axis of the virtual impedance control loop, is the d-axis component of the grid-connected point voltage in the control coordinate system, is the q-axis component of the grid-connected point voltage in the control coordinate system, R v and L v are virtual resistance and virtual inductance respectively, G v (s) is the transfer function of virtual impedance control;

[0110] Establish the small signal transfer function model of the current vector control submodule:

[0111]

[0112] in, is the d-axis component of the converter modulation voltage in the control coordinate system, is the q-axis component of the converter modulation voltage in the control coordinate system, is the d-axis component of the converter output current in the control coordinate system, is the dq axis component of the converter output current in the control coordinate system, ω0 is the angular frequency of the ideal AC grid, L f is the filter inductance, k ccp and k CCi They are the current controller G CC(s) proportional and integral coefficients;

[0113] Establish the small signal transfer function model of the modulation link submodule:

[0114]

[0115] in, is the d-axis component of the converter output port voltage in the control coordinate system, is the q-axis component of the converter output port voltage in the control coordinate system, U md0 is the steady-state value of the d-axis component of the converter modulation voltage, U mq0 is the steady-state value of the q-axis component of the converter modulation voltage, U dc0 is the steady-state value of the DC bus voltage, u dc is the DC bus voltage feedback value.

[0116] Preferably, the establishing unit is used to establish a small signal transfer function model of the AC power grid, and is also used to:

[0117] Establish a small signal transfer function model for the AC power grid:

[0118]

[0119] Among them, u cd is the d-axis component of the converter port voltage in the grid reference coordinate system, u cq is the q-axis component of the converter port voltage in the grid reference coordinate system, u pccd is the d-axis component of the grid connection point voltage in the grid reference coordinate system, u pccq is the q-axis component of the grid connection point voltage in the grid reference coordinate system, u gd is the d-axis component of the AC grid voltage in the grid reference coordinate system, u gq is the q-axis component of the AC grid voltage in the grid reference coordinate system, ω0 is the angular frequency of the ideal AC grid, R f and L f are the filter resistance and inductance respectively, s is the Laplace operator in the complex frequency domain, i Ld is the d-axis component of the output current in the grid reference coordinate system, i Lq is the q-axis component of the output current in the grid reference coordinate system.

[0120] Preferably, the establishing unit is used to establish a small signal transfer function model of the grid-connected system of the grid-connected converter based on the small signal transfer function model of the grid-connected converter and the small signal transfer function model of the AC power grid, and is also used to:

[0121] Determine the conversion relationship between the dq components in the grid reference coordinate system based on the grid connection point voltage and the control coordinate system based on the output phase of the control link:

[0122]

[0123] in, is the d-axis component of the AC grid voltage in the control coordinate system, To control the q-axis component of the AC grid voltage in the coordinate system, consider Δθ GFM is a small disturbance, and the small signal linearization is performed on the conversion relationship to obtain the interface equation of the small signal variable in the power grid coordinate system and the control coordinate system, Δθ GFM It is the difference between the output phase of the control link and the actual phase of the grid-connected point voltage;

[0124]

[0125] Get the interface equations of different small signal variables in the grid coordinate system and the control coordinate system:

[0126]

[0127]

[0128]

[0129] Based on the implicit relationship of power balance, the small signal expressions of the active power output from the grid-connected converter port and the reactive power output from the grid-connected point are determined:

[0130]

[0131] Ignoring the power loss of the switching devices of the grid-type converter, the small signal transfer function model of the DC bus capacitor dynamics in the complex frequency domain is determined as follows:

[0132] C dc U dc0 sΔu dc =Δp in -Δp o (14)

[0133] Among them, C dc and U dc0 are the steady-state values ​​of the DC bus capacitance and its voltage, Δp in is the active power disturbance injected into the DC bus capacitor on the new energy generator side, Δp o is the change in active power output by the grid-connected converter;

[0134] Assume that the DC bus voltage command value remains unchanged, that is, Δu dcref =0, the small signal transfer function model of the DC bus capacitor dynamics in the complex frequency domain is brought into the small signal transfer function model of the synchronous control submodule:

[0135]

[0136] Based on the interface equation, the small signal expressions of the active power output by the grid-connected converter port and the reactive power output by the grid-connected point, the small signal transfer function model of the grid-connected converter is combined to obtain the small signal transfer function model of the grid-connected converter grid-connected system.

[0137] Preferably, the analysis unit is used to analyze the propagation path of the small signal transfer function model of the grid-connected system of the grid-connected converter, determine the key output variables, and also to:

[0138] The key output variable is the meshing control output phase.

[0139] Preferably, the reconstruction unit is used to reconstruct the small signal transfer function model of the grid-connected system of the grid-connected converter based on the key output variable, including:

[0140] When the DC bus voltage command value and reactive power command value remain unchanged, Δu dcref =0, Δq ref =0, with the input power disturbance Δp in As the input signal, the output phase disturbance Δθ is controlled by the grid type GFM As the output signal, the small signal transfer function model of the grid-connected converter system is reconstructed:

[0141]

[0142]

[0143] Among them, K Pd (s) is the d-axis active power feedback gain, K Pq (s) is the q-axis active power feedback gain, K Qd (s) is the d-axis reactive power feedback gain, K Qq (s) is the q-axis reactive power feedback gain, H q0 is the coupling gain of phase disturbance to d-axis voltage, H d0 is the coupling gain of phase disturbance to q-axis voltage.

[0144] Preferably, the transformation unit is used to perform equivalent transformation on the reconstructed small signal transfer function model of the grid-connected converter system, derive the analytical small signal mathematical transfer function model of the grid-connected converter system, and is also used to:

[0145] The similar items in the small signal transfer function model of the grid-connected system of the grid-connected converter are merged, and the unit negative feedback subsystem composed of the current controller and the filter is equivalent to obtain the small signal transfer function model of the grid-connected system of the grid-connected converter:

[0146]

[0147]

[0148] Among them, G c1 (s) is the transfer function of the current control closed-loop subsystem, H U (s) is the voltage feedback equivalent gain, H θ1 (s) is the equivalent coupling gain of phase perturbation on d-axis control, H θ2 (s) is the equivalent coupling gain of phase perturbation on q-axis control, G CC (s) is the current controller transfer function, G v (s) is the virtual impedance control transfer function:

[0149] The grid-connected system of the grid-connected converter includes a positive feedback loop and a negative feedback loop:

[0150]

[0151]

[0152] Among them, G MC (s) is the equivalent transfer function of the synchronization link, H id1 (s) is the local d-axis current feedback equivalent gain, H iq2 (s) is the local q-axis current feedback equivalent gain, H iq1 (s) and H id2 (s) are the equivalent gains of dq axis current cross coupling, G dc (s) is the synchronous controller transfer function, G Q (s) is the reactive power control transfer function;

[0153] The negative feedback subsystem in the feedback path is equivalent, and the cross-coupling terms are decoupled, and the small signal transfer function model of the grid-connected system of the grid-connected converter is equivalent to a typical closed-loop feedback:

[0154]

[0155]

[0156] Where: G1(s) is the transfer function of the local equivalent subsystem of the d-axis current feedback, G2(s) is the transfer function of the local equivalent subsystem of the q-axis current feedback, H1(s) is the equivalent positive feedback gain of the entire closed-loop system, and H2(s) is the equivalent negative feedback gain of the entire closed-loop system;

[0157] Determine the open-loop transfer function G of the grid-connected system of the grid-connected converter op (s) and closed-loop transfer function Gc(s):

[0158] G op (s) = G MC (s)(H2(s)-H1(s))

[0159]

[0160] Preferably, the result unit is used to analyze the small signal stability of the grid-connected system of the grid-connected converter, obtain the small signal stability evaluation result of the grid-connected system of the grid-connected converter, and is also used to:

[0161] The expression of the open-loop transfer function in the frequency domain is G op The real and imaginary parts of (jω) are:

[0162]

[0163] Define σ(ω)=Re[G op (jω)], when the phase lag generated by the equivalent loop of the grid-connected system of the grid-connected converter reaches 180°, ∠G op (jω)=-180°,υ(ω)=0,the corresponding frequency is ω p ;

[0164] ω p Substituting into σ(ω), choose σ(ω p ) as an indicator for evaluating the small signal stability of the closed-loop system;

[0165] When σ(ω p )>0, the closed-loop system will be equivalent to a negative feedback loop, and the system will remain stable;

[0166] When -1<σ(ω p )<0, the closed-loop system will be equivalent to a positive feedback loop, but its amplitude does not meet the instability condition of the feedback system and can still remain stable;

[0167] When σ(ω p )≤-1, the closed-loop system will be equivalent to a positive feedback loop and meet the instability conditions of the feedback system, and there is a risk of instability.

[0168] Preferably, it also includes: based on the small signal stability evaluation result of the grid-connected converter grid-connected system, obtaining the instability mechanism of the grid-connected converter grid-connected system, and determining the dominant link causing the instability of the grid-connected converter grid-connected system.

[0169] Based on another aspect of the present invention, the present invention provides a computer-readable storage medium, wherein the computer-readable storage medium stores a computer program, and the computer program is used to execute a modular small signal model of a grid-connected converter and a grid-connected stability judgment method.

[0170] Based on another aspect of the present invention, the present invention provides an electronic device, the electronic device comprising: a processor and a memory; wherein,

[0171] The memory is a memory for storing instructions executable by the processor;

[0172] The processor is used to read the executable instructions from the memory and execute the instructions to implement a modular small signal model of a grid-connected converter and a grid-connected stability determination method.

[0173] The technical solution of the present invention provides a modular small signal model of a grid-connected converter and a method and system for judging grid-connected stability, wherein the method includes: determining the steady-state operation point of the grid-connected converter grid-connected system based on the steady-state operation equation, and controlling the grid-connected converter in a steady state based on the steady-state operation point; establishing a small signal transfer function model of the grid-connected converter and a small signal transfer function model of the AC power grid, and establishing a small signal transfer function model of the grid-connected converter grid-connected system based on the small signal transfer function model of the grid-connected converter and the small signal transfer function model of the AC power grid; analyzing the small signal propagation path of the grid-connected converter grid-connected system to determine key output variables; reconstructing the small signal transfer function model of the grid-connected converter grid-connected system based on the key output variables; performing equivalent transformation on the reconstructed small signal transfer function model of the grid-connected converter grid-connected system to derive an analytical small signal mathematical transfer function model of the grid-connected system; analyzing the small signal stability of the grid-connected converter grid-connected system based on the analytical small signal mathematical transfer function model, and obtaining a small signal stability evaluation result of the grid-connected converter grid-connected system. The technical solution of the present invention proposes a modular small signal model of a grid-connected converter and a grid-connected stability determination method thereof. The present invention adopts a component connection method to establish a small signal block diagram model and an analytical transfer function model of a grid-connected converter, which effectively improves the modeling efficiency of complex systems and has good flexibility. The present invention proposes a grid-connected stability determination method, which can determine the overall small signal stability of the system and quantitatively analyze the impact of key factors on the small signal stability of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0174] A more complete understanding of exemplary embodiments of the present invention may be obtained by referring to the following drawings:

[0175] Figure 1 A schematic diagram of a grid-connected system structure and a control scheme of a grid-connected converter according to a preferred embodiment of the present invention;

[0176] Figure 2 A flowchart of a modular small signal model of a grid-connected converter and a grid-connected stability determination method according to a preferred embodiment of the present invention;

[0177] Figure 3 A small signal transfer function model diagram of a grid-type converter according to a preferred embodiment of the present invention;

[0178] Figure 4 A small signal transfer function model diagram of an AC power grid according to a preferred embodiment of the present invention;

[0179] Figure 5 is a schematic diagram of the relationship between two synchronously rotating reference coordinate systems according to a preferred embodiment of the present invention;

[0180] Figure 6 A connection relationship diagram between different submodules of a grid-connected system of a grid-connected converter according to a preferred embodiment of the present invention;

[0181] Figure 7 A schematic diagram of a small signal propagation path of a grid-connected system of a grid-connected converter according to a preferred embodiment of the present invention;

[0182] Figure 8 A block diagram of a small signal control of a grid-connected system of a grid-connected converter based on a synchronous leading loop according to a preferred embodiment of the present invention;

[0183] Fig. 9 It is an equivalent small signal control block diagram after merging the unit negative feedback subsystem composed of the current controller and the filter according to the preferred embodiment of the present invention;

[0184] Fig.10 According to a preferred embodiment of the present invention, Figure 8 The form diagram of the small and medium signal control block after equivalent transformation;

[0185] Fig.11 According to a preferred embodiment of the present invention, a small signal control block diagram of a grid-connected system of a grid-connected converter is equivalent to a simple closed-loop feedback structure diagram;

[0186] Fig.12 A flowchart of small signal stability analysis and instability leading link identification of a grid-connected system of a grid-connected converter according to a preferred embodiment of the present invention;

[0187] Fig.13 A real / imaginary characteristic diagram of system equivalent loop gain under different grid impedances according to a preferred embodiment of the present invention;

[0188] Fig.14 A simulation waveform diagram of a grid-connected system of a grid-connected converter under different grid impedances according to a preferred embodiment of the present invention; and

[0189] Fig.15 A modular small signal model of a grid-connected converter and a structure diagram of a grid-connected stability judgment system according to a preferred embodiment of the present invention. DETAILED DESCRIPTION

[0190] Now, exemplary embodiments of the present invention are described with reference to the accompanying drawings. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. These embodiments are provided to disclose the present invention in detail and completely and to fully convey the scope of the present invention to those skilled in the art. The terms used in the exemplary embodiments shown in the accompanying drawings are not intended to limit the present invention. In the accompanying drawings, the same units / elements are marked with the same reference numerals.

[0191] Unless otherwise specified, the terms (including technical terms) used herein have the commonly understood meanings to those skilled in the art. In addition, it is understood that the terms defined in commonly used dictionaries should be understood to have the same meanings as those in the context of the relevant fields, and should not be understood as idealized or overly formal meanings.

[0192] Figure 1 Schematic diagram of the structure and control scheme of a grid-connected system of a grid-connected converter according to a preferred embodiment of the present invention.

[0193] In recent years, grid-connected renewable energy power generation has become one of the important research and exploration directions at home and abroad. The basic idea is to change the synchronization mode of traditional renewable energy tracking the voltage phase of the grid connection point, and use its endogenous potential amplitude and frequency / phase as the control target, and through additional power droop, virtual inertia and other external loop controls, to achieve AC synchronous voltage support and power output. At present, scholars at home and abroad have proposed many grid-connected control schemes, such as droop control, virtual synchronous generator control, virtual oscillator control and matching control. The basic circuit structure of the grid-connected converter system is as follows: Figure 1 As shown, it mainly includes the front-stage new energy power generation unit (photovoltaic array, wind turbine, etc.), DC bus capacitor, three-phase bridge inverter, AC filter and AC power grid. dc is the equivalent output current of the previous power generation unit, C dc and u dc is the DC bus capacitance and its voltage, R f , Lf and C f is the AC filter resistance, inductance and capacitance, R g and L g is the equivalent resistance and inductance of the AC grid, u c,abc ,u PCC,abc and u g,abc is the voltage at the port of the grid-connected converter, the voltage at the common coupling point and the infinite grid voltage, i L,abc and i g,abc is the current flowing through the AC filter and the grid. Figure 1 The detailed control structure of the grid-connected converter is also given, which mainly includes grid-connected synchronization control, reactive voltage control, virtual impedance control, vector current control and PWM modulation. The subscript "ref" indicates the reference value of the corresponding variable, the subscripts "d" and "q" indicate the dq axis components of the corresponding variables, the superscript "c" indicates the variable in the reference coordinate system based on the grid-connected control output phase, and the variables without superscripts indicate the variables in the reference coordinate system based on the grid-connected point voltage, θ GFM is the output phase of the grid-type control, ω0 is the angular frequency of the ideal AC grid, S i It is the driving signal output by the modulation link.

[0194] The control structure of the grid-connected converter mainly includes the following four links: 1) grid-connected synchronous control; 2) reactive power-voltage control; 3) virtual impedance control; 4) current vector decoupling control. Among them, the grid-connected synchronous control is based on the DC bus voltage command u dcref And DC bus voltage feedback u dc Generate output AC voltage frequency reference value ω GFM , and the output AC voltage phase reference value is obtained by integration. The reactive power-voltage control link is based on the reactive power command Q ref , reactive power feedback q e The virtual impedance control link is implemented in the rotating dq coordinate system according to the voltage dq axis command and the grid point voltage feedback u PCC,dq Generate the reference value of the current inner loop control. The current vector decoupling control link is also implemented in the rotating dq coordinate system. dq,ref And output current feedback i L,dq Generate modulation voltage. Finally, the PWM link controls the output voltage of the grid-type converter according to the modulation voltage. Compared with the grid-type control method, the frequency and phase of the grid-type control are generated by the synchronous control loop, without the need for PLL to synchronize with the grid. It can independently generate and maintain the grid voltage and frequency, without the need for an external power supply to build a grid, and presents voltage source characteristics.

[0195] The basic working principle of the grid-type converter is introduced below. The core idea of ​​grid-type control is based on the similarity between the dynamic equation of the converter DC capacitor voltage and the synchronous generator rotor motion equation, as shown in equation (1). dc Analogous to the synchronous generator speed ω, therefore, the DC capacitor voltage can be used to map the angular frequency of the converter output AC voltage, that is, through u dc To adjust ω.

[0196]

[0197] In the formula, C dc and u dc is the DC bus capacitance and voltage, P in and P e is the converter input and output active power, J and ω are the moment of inertia and rotor speed of the synchronous generator, P T and P E It is the input and output active power of the synchronous generator.

[0198] Based on the above control concept, the grid-type converter can simulate the external characteristics of the synchronous generator, in which the converter DC bus capacitor is equivalent to the rotor of the synchronous generator and has a buffering effect on the unbalanced energy. When the power sent by the new energy machine-side converter to the DC capacitor increases, the DC bus voltage increases, the angular frequency of the grid-side converter output voltage increases, and the corresponding power angle δ increases, so that the output power of the grid-side converter increases, thereby maintaining the DC bus voltage constant; vice versa.

[0199] The grid-type control establishes a self-synchronization mechanism that uses the angular frequency matching relationship between the DC bus voltage and the AC voltage. It can achieve autonomous synchronization by simply measuring the DC capacitor voltage, and has a faster response speed. In addition, the DC capacitor integration link directly provides equivalent inertia for the system, without the need for additional simulation of inertia.

[0200] In summary, grid-based control can autonomously sense the frequency of the AC power grid through the DC bus voltage, and quickly adjust the active output of the new energy generator set according to the fluctuation of the power grid frequency, thereby realizing the active support function for the AC power grid.

[0201] The present invention proposes a modular small signal analysis model and a grid-connected stability evaluation index based on equivalent loop gain for the grid-connected new energy converter system, so as to evaluate the small signal stability of the grid-connected converter when it is connected to different grid conditions, reveal the system instability mechanism, and quantitatively analyze the influence mechanism of different influencing factors on the small signal stability of the grid-connected converter.

[0202] Figure 2A flowchart of a modular small signal model of a grid-connected converter and a grid-connected stability determination method according to a preferred embodiment of the present invention.

[0203] like Figure 2 As shown, the present invention provides a modular small signal model of a grid-connected converter and a grid-connected stability determination method, the method comprising:

[0204] Step 201: determining a steady-state operation point of a grid-connected system of a grid-connected converter based on a steady-state operation equation, and controlling the grid-connected converter in a steady state based on the steady-state operation point;

[0205] Preferably, based on the steady-state operation equation, determining the steady-state operation point of the grid-connected system of the grid-connected converter, and based on the steady-state operation point, controlling the grid-connected converter in a steady state, comprises:

[0206] The steady-state operation equations of various electrical quantities in the grid-connected system of the grid-connected converter in the dq coordinate system are:

[0207]

[0208]

[0209] Among them, U gd0 =U g0 cosδ0,U gq0 =U g0 sin(-δ0), δ0=θ PCC0 -θ g0 , δ0 is the steady-state power angle, U g0 and θ g0 are the amplitude and phase of the grid voltage, θ PCC0 is the steady-state value of the grid-connected point voltage phase, U PCCd0 is the steady-state value of the d-axis component of the grid-connected point voltage, U PCCq0 is the steady-state value of the q-axis component of the grid-connected point voltage, U gd0 is the steady-state value of the d-axis component of the grid voltage, U gq0 is the steady-state value of the q-axis component of the grid voltage, I gd0 is the steady-state value of the d-axis component of the current flowing into the AC grid, I gq0 is the steady-state value of the q-axis component of the current flowing into the AC grid, R g is the equivalent resistance of the AC grid, ω0 is the angular frequency of the ideal AC grid, L g is the equivalent power plant of the AC grid, U cd0 is the steady-state value of the d-axis component of the converter port voltage, U cq0 is the steady-state value of the q-axis component of the converter port voltage, R f is the AC filter resistor, I Ld0 is the steady-state value of the d-axis component of the filter inductor current, ILq0 is the steady-state value of the q-axis component of the filter inductor current, L f is the AC filter inductor, C f is the AC filter capacitor, P0 and Q0 are the active power and reactive power output of the grid-connected converter in steady state respectively.

[0210] The present invention determines the steady-state operating point of the grid-connected system of the grid-connected converter. When the system is in a stable operating state, the control dynamics of the grid-connected converter can be ignored. According to Kirchhoff's voltage law (KVL), the steady-state operating equations of each electrical quantity in the system in the dq coordinate system are as follows:

[0211]

[0212] Among them, U gd0 =U g0 cosδ0,U gq0 =U g0 sin(-δ0), δ0=θ PCC0 -θ g0 , δ0 is the steady-state power angle, U g0 and θ g0 are the amplitude and phase of the grid voltage, θ PCC0 is the steady-state value of the grid-connected point voltage phase. PCC,dq0 is the steady-state value of the dq-axis component of the grid-connected point voltage, U g,dq0 is the steady-state value of the dq-axis component of the grid voltage, U c,dq0 is the steady-state value of the dq-axis component of the converter port voltage, I g,dq0 is the steady-state value of the dq-axis component of the current flowing into the AC grid, I L,dq0 is the steady-state value of the dq-axis component of the filter inductor current, P0 and Q0 are the active and reactive powers output by the grid-connected converter in the steady state, respectively.

[0213] Under steady-state conditions, the grid-type converter of the present invention can be equivalent to a controlled voltage source, and its output voltage amplitude U PCC0 and active power P0 are known quantities and satisfy U PCCd0 =U PCC0 , U PCCq0 =0. At the same time, assuming that the grid voltage amplitude U g is a constant value, θ g The initial value is 0. Based on the above initial conditions, the steady-state power angle δ0 and output reactive power Q0 of the grid-connected converter can be obtained through power flow calculation. On this basis, the steady-state values ​​of the above electrical quantities are substituted into the steady-state operation equation (2-3) for solution to obtain the steady-state operation point of the grid-connected converter system.

[0214] Step 202: establishing a small signal transfer function model of a grid-connected converter and a small signal transfer function model of an AC power grid, and establishing a small signal transfer function model of a grid-connected converter grid-connected system based on the small signal transfer function model of the grid-connected converter and the small signal transfer function model of the AC power grid;

[0215] Preferably, a small signal transfer function model of a grid-connected converter is established, including:

[0216] Establish the small signal transfer function model of the synchronization control submodule:

[0217]

[0218] Among them, Δ represents the small signal amount of the corresponding variable, s is the Laplace operator in the complex frequency domain, and θ GFM represents the output phase of the network control, K dc and T dc They are respectively the network-building controller G dc (s) proportionality coefficient and time constant, Δu dc is the DC bus voltage disturbance, Δu dcref is the change of DC bus voltage reference value;

[0219] Establish the small signal transfer function model of the reactive power-voltage control submodule:

[0220]

[0221] Among them, U ref represents the output voltage amplitude of the grid-connected converter, q ref and q e are the reactive power command and feedback value output by the grid-connected converter, k Qp and k Qi Respectively, reactive power-voltage controller G Q (s) proportional and integral coefficients;

[0222] Establish the small signal transfer function model of the virtual impedance control submodule:

[0223]

[0224] Among them, i dref represents the output of the virtual impedance control loop d-axis, i qref represents the output of the q-axis of the virtual impedance control loop, is the d-axis component of the grid-connected point voltage in the control coordinate system, is the q-axis component of the grid-connected point voltage in the control coordinate system, R v and L v are virtual resistance and virtual inductance respectively, G v(s) is the transfer function of virtual impedance control;

[0225] Establish the small signal transfer function model of the current vector control submodule:

[0226]

[0227] in, is the d-axis component of the converter modulation voltage in the control coordinate system, is the q-axis component of the converter modulation voltage in the control coordinate system, is the d-axis component of the converter output current in the control coordinate system, is the q-axis component of the converter output current in the control coordinate system, ω0 is the angular frequency of the ideal AC grid, L f is the filter inductance, k CCp and k CCi They are the current controller G CC (s) proportional and integral coefficients;

[0228] Establish the small signal transfer function model of the modulation link submodule:

[0229]

[0230] in, is the d-axis component of the converter output port voltage in the control coordinate system, is the q-axis component of the converter output port voltage in the control coordinate system, U md0 is the steady-state value of the d-axis component of the converter modulation voltage, U mq0 is the steady-state value of the q-axis component of the converter modulation voltage, U dc0 is the steady-state value of the DC bus voltage, u dc is the DC bus voltage feedback value.

[0231] Preferably, establishing a small signal transfer function model of an AC power grid includes:

[0232] Establish a small signal transfer function model for the AC power grid:

[0233]

[0234] Among them, u cd is the d-axis component of the converter port voltage in the grid reference coordinate system, u cq is the q-axis component of the converter port voltage in the grid reference coordinate system, u pccd is the d-axis component of the grid connection point voltage in the grid reference coordinate system, u pccq is the q-axis component of the grid connection point voltage in the grid reference coordinate system, u gd is the d-axis component of the AC grid voltage in the grid reference coordinate system, ugq is the q-axis component of the AC grid voltage in the grid reference coordinate system, ω0 is the angular frequency of the ideal AC grid, R f and L f are the filter resistance and inductance respectively, s is the Laplace operator in the complex frequency domain, i Ld is the d-axis component of the output current in the grid reference coordinate system, i Lq is the q-axis component of the output current in the grid reference coordinate system.

[0235] Preferably, a small signal transfer function model of a grid-connected converter system is established based on a small signal transfer function model of a grid-connected converter and a small signal transfer function model of an AC power grid, including:

[0236] Determine the conversion relationship between the dq components in the grid reference coordinate system based on the grid connection point voltage and the control coordinate system based on the output phase of the control link:

[0237]

[0238] in, is the d-axis component of the AC grid voltage in the control coordinate system, To control the q-axis component of the AC grid voltage in the coordinate system, consider Δθ GFM is a small disturbance, and the small signal linearization is performed on the conversion relationship to obtain the interface equation of the small signal variable in the power grid coordinate system and the control coordinate system, Δθ GFM It is the difference between the output phase of the control link and the actual phase of the grid-connected point voltage;

[0239]

[0240] Get the interface equations of different small signal variables in the grid coordinate system and the control coordinate system:

[0241]

[0242]

[0243]

[0244] Based on the implicit relationship of power balance, the small signal expressions of the active power output from the grid-connected converter port and the reactive power output from the grid-connected point are determined:

[0245]

[0246] Ignoring the power loss of the switching devices of the grid-type converter, the small signal transfer function model of the DC bus capacitor dynamics in the complex frequency domain is determined as follows:

[0247] C dc Udc0 sΔu dc =Δp in -Δp o (14)

[0248] Among them, C dc and U dc0 are the steady-state values ​​of the DC bus capacitance and its voltage, Δp in is the active power disturbance injected into the DC bus capacitor on the new energy generator side, Δp o is the change in active power output by the grid-connected converter;

[0249] Assume that the DC bus voltage command value remains unchanged, that is, Δu dcref =0, the small signal transfer function model of the DC bus capacitor dynamics in the complex frequency domain is brought into the small signal transfer function model of the synchronous control submodule:

[0250]

[0251] Based on the interface equation, the small signal expressions of the active power output from the ports of the grid-connected converter and the reactive power output from the grid-connected point, the small signal transfer function model of the grid-connected converter is combined to obtain the small signal transfer function model of the grid-connected converter system.

[0252] The present invention establishes a small signal model of a grid-type converter. Figure 1 It can be seen from the control structure of the grid-type converter that it can be divided into five parts, and the transfer function model of each sub-module is constructed in the complex frequency domain.

[0253] 1) Small signal transfer function model of synchronous control submodule:

[0254]

[0255] Among them, Δ represents the small signal amount of the corresponding variable, s is the Laplace operator in the complex frequency domain, and θ GFM represents the output phase of the network control, which is used as the reference phase of the coordinate transformation in the vector decoupling control. dc and T dc They are respectively the network-building controller G dc (s) proportionality coefficient and time constant.

[0256] 2) Small signal transfer function model of reactive power-voltage control submodule:

[0257]

[0258] Among them, U ref represents the output voltage amplitude of the grid-type converter, which is used as the reference value of the virtual impedance control. ref and qe are the reactive power command and feedback value output by the grid-connected converter, k Qp and k Qi Respectively, reactive power-voltage controller G Q (s) proportional and integral coefficients.

[0259] 3) Small signal transfer function model of virtual impedance control submodule:

[0260]

[0261] Among them, i dq,ref Represents the output of the virtual impedance control loop, which serves as the reference value for the current inner loop control. is the dq axis component of the grid connection point voltage, R v and L v They are virtual resistance and virtual inductance respectively.

[0262] 4) Small signal transfer function model of current vector control submodule:

[0263]

[0264] in, and are the dq axis components of the converter modulation voltage and output current in the control reference coordinate system, ω0 is the angular frequency of the ideal AC grid, L f is the filter inductance, k CCp and k CCi They are the current controller G CC (s) proportional and integral coefficients.

[0265] 5) Small signal transfer function model of modulation link submodule:

[0266]

[0267] in, To control the dq-axis component of the converter output port voltage in the reference coordinate system, U m,dq0 is the steady-state value of the dq-axis component of the converter modulation voltage.

[0268] From the above, the small signal transfer function model of the grid-type converter is as follows: Figure 2 As shown. Among them, the input signals are the command value disturbance signal Δu dcref and Δq ref , feedback signal and Δu dc , Δq e , the output signals are Δθ GFM and

[0269] The present invention establishes a small signal model of the AC power grid. When analyzing the amplitude-frequency characteristics at a frequency lower than or equal to the cut-off frequency fc, since the capacitive reactance of the filter capacitor is much greater than the inductive reactance of the grid-side inductor, the filter capacitor branch can be considered to be an open circuit. In order to simplify the subsequent analysis process, the LC filter is simplified to an L-type filter. Figure 1 From the electrical topology in, we can see that the small signal transfer function model of the AC network submodule is:

[0270]

[0271] Among them, u c,dq 、u PCC,dq and u g,dq are the dq axis components of the converter port voltage, grid connection point voltage, and AC grid voltage in the grid reference coordinate system, respectively. L,dq is the dq axis component of the output current in the grid reference coordinate system, R f and L f They are filter resistance, inductance, R g and L g are the resistance and inductance of the equivalent grid impedance respectively. Figure 3 The small signal transfer function model of the AC network is given. Among them, the input signal is Δu c,dq , the output signals are Δu PCC,dq and Δi L,dq .

[0272] The present invention adopts the component connection method to establish the small signal model of the grid-connected system of the grid-connected converter. According to the physical connection and implicit relationship between the input and output signals of different submodules, each submodule is connected through an algebraic relationship to obtain the small signal transfer function model of the entire system.

[0273] 1) Interface equation based on coordinate transformation: When considering the dynamic control of the converter, there are two synchronous rotating reference coordinate systems. Here, the reference coordinate system based on the ideal grid-connected point voltage is called the grid coordinate system, and the reference coordinate system based on the output phase of the control link is called the control coordinate system, which is represented by the superscript 'c'. g As an example, we analyze the relationship between the two reference coordinate systems (here called the interface equation), such as Figure 5 shown.

[0274] According to the positional relationship between the rotation vectors, the relationship between the dq axis components in the two reference coordinate systems is as follows

[0275]

[0276] Consider Δθ GFMis a small disturbance. By linearizing the above equation, we can obtain the interface equations of the small signal variables in the two reference coordinate systems:

[0277]

[0278] Among them, Δθ GFM It is the difference between the output phase of the control link and the actual phase of the grid-connected point voltage. is the dq axis component in the control coordinate system, u g,dq is the dq axis component in the grid coordinate system, U g,dq0 is the steady-state value of the dq-axis component of the grid voltage. Similarly, the interface equations of different small signal variables in two reference coordinate systems can be obtained:

[0279]

[0280] 2) Implicit relationship based on power balance: According to the instantaneous power theory, the small signal expression of the active power output at the grid-connected converter port and the reactive power output at the grid-connected point is:

[0281]

[0282] Among them, q o and q e are the active and reactive power output by the grid-connected converter, I L,dq0 is the steady-state value of the dq-axis component of the output current, U PCC,dq0 is the steady-state value of the dq-axis component of the grid-connected point voltage.

[0283] Ignoring the power loss of the switching devices of the grid-type converter, the small signal transfer function model of the DC bus capacitor dynamics in the complex frequency domain is as follows:

[0284] C dc U dc0 sΔu dc =Δp in -Δp o (14)

[0285] Among them, C dc and U dc0 are the steady-state values ​​of the DC bus capacitance and its voltage, Δp in is the active power disturbance injected into the DC bus capacitor on the new energy generator side, Δp o is the change in active power output by the grid-connected converter. Assume that the DC bus voltage command value remains unchanged, that is, Δu dcref =0, substituting formula (14) into formula (4), we can get:

[0286]

[0287] 3) Construct the small signal transfer function model of the whole system: According to the interface equation and power balance relationship obtained above, as shown in Figure 6 As shown, by combining the small signal transfer function models of each sub-module, the small signal transfer function model of the grid-connected system of the grid-connected converter can be obtained.

[0288] Step 203: Analyze the small signal propagation path of the grid-connected system of the grid-connected converter to determine the key output variables;

[0289] Preferably, the propagation path of the small signal transfer function model of the grid-connected system of the grid-connected converter is analyzed to determine the key output variables, including:

[0290] The key output variable is the grid-type control output phase.

[0291] The present invention analyzes the propagation path of small signals in the grid-connected system of the grid-connected converter and determines the key output variables that can reflect the stability of the system. Here, the input power disturbance and reactive power command disturbance on the new energy machine side are taken as examples to analyze the propagation path of small signals inside the grid-connected system of the grid-connected converter in detail. Figure 7 As shown, this figure can intuitively show the interaction between different control links of the grid-type converter and the AC network.

[0292] Depend on Figure 7 It can be found that there are 6 closed loops in the grid-connected system of the grid-connected converter, among which loops I, II and III are introduced by the negative feedback control of current, voltage and reactive power respectively, and are used to adjust the above physical quantities to the corresponding command values, which are the inherent small signal propagation paths of the system; however, loops IV to VI are additionally introduced by the dynamic characteristics of the synchronous control link, which aggravates the interaction within the entire closed-loop system, resulting in more complex small signal propagation paths in the system. Therefore, when analyzing the small signal stability of the grid-connected system of the grid-connected converter, it is necessary to focus on the dynamic characteristics of the synchronous link, and take the grid-connected control output phase as the key variable reflecting the dynamic characteristics of the entire system, so as to more intuitively understand the stability mechanism of the grid-connected system of the grid-connected converter.

[0293] Step 204: reconstructing a small signal transfer function model of the grid-connected system of the grid-connected converter based on the key output variables;

[0294] Preferably, based on the key output variables, the small signal transfer function model of the grid-connected system of the grid-connected converter is reconstructed, including:

[0295] When the DC bus voltage command value and reactive power command value remain unchanged, Δu dcref =0, Δq ref =0, with the input power disturbance Δp inAs the input signal, the output phase disturbance Δθ is controlled by the grid type GFM As the output signal, the small signal transfer function model of the grid-connected system of the grid-connected converter is reconstructed:

[0296]

[0297]

[0298] Among them, K Pd (s) is the d-axis active power feedback gain, K Pq (s) is the q-axis active power feedback gain, K Qd (s) is the d-axis reactive power feedback gain, K Qq (s) is the q-axis reactive power feedback gain, H q0 is the coupling gain of phase disturbance to d-axis voltage, H d0 is the coupling gain of phase disturbance to q-axis voltage.

[0299] According to the key output variables determined in the above steps, the present invention reconstructs the established grid-connected converter grid-connected system small signal transfer function model. Assuming that the DC bus voltage command value and reactive power command value remain unchanged, that is, Δu dcref =0, Δq ref =0, at the same time, the input power disturbance Δp in As the input signal, the output phase disturbance Δθ of the grid-type control is GFM As the output signal, the small signal control block diagram of the grid-connected converter system based on the synchronous dominant loop can be obtained, such as Figure 8 shown.

[0300] Among them, K P,dq (s), K Q,dq (s) and H dq0 The expressions are:

[0301]

[0302] Step 205: performing equivalent transformation on the reconstructed small signal transfer function model of the grid-connected converter system, and deriving an analytical small signal mathematical transfer function model of the grid-connected system;

[0303] Preferably, an equivalent transformation is performed on the reconstructed small signal transfer function model of the grid-connected converter system to derive an analytical small signal mathematical transfer function model of the grid-connected converter system, including:

[0304] The similar items in the small signal transfer function model of the grid-connected system of the grid-connected converter are merged, and the unit negative feedback subsystem composed of the current controller and the filter is equivalent to obtain the small signal transfer function model of the grid-connected system of the grid-connected converter:

[0305]

[0306]

[0307] Among them, G c1 (s) is the transfer function of the current control closed-loop subsystem, H U (s) is the voltage feedback equivalent gain, H θ1 (s) is the equivalent coupling gain of phase perturbation on d-axis control, H θ2 (s) is the equivalent coupling gain of phase perturbation on q-axis control, G CC (s) is the current controller transfer function, G v (s) is the virtual impedance control transfer function;

[0308] The grid-connected system of the grid-connected converter includes a positive feedback loop and a negative feedback loop:

[0309]

[0310]

[0311] Among them, G MC (s) is the equivalent transfer function of the synchronization link, H id1 (s) is the local d-axis current feedback equivalent gain, H iq2 (s) is the local q-axis current feedback equivalent gain, H iq1 (s) and H id2 (s) are the equivalent gains of dq axis current cross coupling, G dc (s) is the synchronous controller transfer function, G Q (s) is the reactive power control transfer function;

[0312] The negative feedback subsystem in the feedback path is equivalent, and the cross-coupling terms are decoupled, and the small signal transfer function model of the grid-connected system of the grid-connected converter is equivalent to a typical closed-loop feedback:

[0313]

[0314]

[0315] Where: G1(s) is the transfer function of the local equivalent subsystem of the d-axis current feedback, G2(s) is the transfer function of the local equivalent subsystem of the q-axis current feedback, H1(s) is the equivalent positive feedback gain of the entire closed-loop system, and H2(s) is the equivalent negative feedback gain of the entire closed-loop system;

[0316] Determine the open-loop transfer function G of the grid-connected system of the grid-connected converter op (s) and closed-loop transfer function Gc(s):

[0317] G op (s) = G MC (s)(H2(s)-H1(s))

[0318]

[0319] The present invention performs equivalent transformation on the small signal control block diagram of the grid-connected system of the grid-connected converter constructed in the above steps, and derives the analytical mathematical model of the system. At the same time, the dynamic interaction in the entire closed-loop system is deeply analyzed from the perspective of feedback control theory, so as to better understand the small signal stability mechanism of the system. The specific equivalent transformation process is shown in Figure 9-11 .

[0320] According to the block diagram equivalent transformation rule, the present invention changes some connection points and lead-out points. Figure 8 By merging and simplifying the similar items in , and at the same time, making the unit negative feedback subsystem composed of the current controller and the filter equivalent, we can get a clear structure of the equivalent small signal control block diagram, as shown in Fig. 9 As shown. Among them, G c1 (s), H U (s) and H θ，12 The expression of (s) is:

[0321]

[0322] Depend on Fig. 9 It can be found that there are multiple interaction paths inside the grid-connected system of the grid-connected converter, and the signal connection relationship between the loops is relatively complex, making it difficult to directly analyze the impact of different feedback loops on system stability. Fig. 9 By carrying out equivalent transformation and adjusting the structure of the control block diagram, we can obtain Fig.10 The equivalent form shown in the figure reflects the existence of different feedback loops in the entire closed-loop system. At the same time, according to the signs of each feedback signal, it can be seen that there may be positive feedback loops and negative feedback loops in the closed-loop system at the same time. MC (s) and H idq,12 The expression of (s) is:

[0323]

[0324] The present invention will Fig.10 By equilibrating the negative feedback subsystem in the feedback path and decoupling the cross-coupling terms, the small signal control block diagram of the grid-connected converter system can be equivalent to a simple closed-loop feedback structure, such as Fig.11 As shown. This figure intuitively shows that there are both positive feedback and negative feedback in the final equivalent block diagram. The positive feedback loop in the figure is highlighted with a red line to facilitate the analysis of the impact of positive feedback on stability. Among them, the expressions of G1(s), G2(s), H1(s) and H2(s) are:

[0325]

[0326] The present invention is based on Fig.11 , we can get the open-loop transfer function G of the whole system op (s) and the closed-loop transfer function G c (s) are:

[0327]

[0328] By combining equations (16-22), we can get the analytical small signal transfer function model of the grid-connected converter system.

[0329] Step 206: Based on the analytical small signal mathematical transfer function model, the small signal stability of the grid-connected system of the grid-connected converter is analyzed to obtain the small signal stability evaluation result of the grid-connected system of the grid-connected converter.

[0330] Preferably, analyzing the small signal stability of the grid-connected system of the grid-connected converter and obtaining the small signal stability evaluation result of the grid-connected system of the grid-connected converter includes:

[0331] The expression of the open-loop transfer function in the frequency domain is G op The real and imaginary parts of (jω) are:

[0332]

[0333] Define σ(ω)=Re[G op (jω)], when the phase lag generated by the equivalent loop of the grid-connected system of the grid-connected converter reaches 180°, ∠G op (jω)=-180°,υ(ω)=0,the corresponding frequency is ω p ;

[0334] ω p Substituting into σ(ω), choose σ(ω p ) as an indicator for evaluating the small signal stability of the closed-loop system;

[0335] When σ(ω p )>0, the closed-loop system will be equivalent to a negative feedback loop, and the system will remain stable;

[0336] When -1<σ(ω p )<0, the closed-loop system will be equivalent to a positive feedback loop, but its amplitude does not meet the instability condition of the feedback system and can still remain stable;

[0337] When σ(ω p )≤-1, the closed-loop system will be equivalent to a positive feedback loop and meet the instability conditions of the feedback system, and there is a risk of instability.

[0338] The present invention analyzes the small signal stability of the grid-connected system of the grid-connected converter based on the real and imaginary characteristics of the system equivalent loop gain. For the entire closed-loop system, its equivalent loop gain is the expression of the system open-loop transfer function in the frequency domain G op (jω), which is obtained by replacing s with jω. The grid-connected stability determination method based on the equivalent loop gain characteristic will be elaborated in detail below.

[0339] According to the basic definition of complex function, G op The real and imaginary parts of (jω) are:

[0340]

[0341] Define σ(ω)=Re[G op (jω)],υ(ω)=Im[G op (jω)]. According to the feedback control principle, when the phase lag generated by the system equivalent loop reaches 180°, that is, ∠G op (jω)=-180°, then υ(ω)=0. Solving this equation, we can get the corresponding frequency as ω p , which is called the positive feedback frequency here. At this time, the amplitude of the system equivalent loop gain is determined only by its real part, so ω p Substituting into σ(ω), we can choose σ(ω p ) is used as an indicator to evaluate the small signal stability of the closed-loop system. At the same time, combined with the instability conditions of the feedback system, a stability judgment method suitable for closed-loop control systems can be proposed.

[0342] 1) When σ(ω p )>0, the closed-loop system will be equivalent to a negative feedback loop, and the system will remain stable;

[0343] 2) When -1<σ(ω p )<0, the closed-loop system will be equivalent to a positive feedback loop, but its amplitude does not meet the instability condition of the feedback system and can still remain stable;

[0344] 3) When σ(ωp )≤-1, the closed-loop system will be equivalent to a positive feedback loop and meet the instability conditions of the feedback system, and there is a risk of instability.

[0345] For the grid-connected converter used in practice, its electrical parameters and control parameters are substituted into the analytical small signal model obtained in the eighth step, and the grid-connected stability of the grid-connected converter can be evaluated using the stability judgment method proposed above.

[0346] Preferably, it also includes: based on the small signal stability evaluation result of the grid-connected converter system, obtaining the instability mechanism of the grid-connected converter system, and determining the dominant link causing the instability of the grid-connected converter system.

[0347] Based on the small signal stability evaluation results of the whole system, the present invention explains the instability mechanism of the grid-connected system of the grid-connected converter and quantitatively analyzes the influence of key factors on the small signal stability of the system. Figure 9-11 From the superposition theorem, it can be seen that the output response of the entire closed-loop system is equal to the sum of the output responses of the parallel interactive subsystems. Therefore, based on the idea of ​​reductionism, by analyzing the relationship between the equivalent loop gain characteristics of different interactive subsystems and the equivalent loop gain characteristics of the entire system step by step, the dominant link that causes the instability of the grid-connected system of the grid-connected converter can be determined. In addition, the stability judgment method proposed in the above steps can also quantitatively analyze the impact of key factors on the small signal stability of the system. In summary, the specific process of small signal stability analysis of the grid-connected system of the grid-connected converter can be refined, such as Fig.12 shown.

[0348] The key points of the present invention are a module small signal model of a grid-connected converter and a grid-connected stability determination method. The points to be protected include:

[0349] (1) Modular small signal block diagram model and analytical transfer function model of grid-connected converter;

[0350] (2) Grid-connected stability evaluation index based on the real and imaginary characteristics of equivalent loop gain;

[0351] (3) Identification method of the dominant link of instability in the grid-connected system of grid-connected converter.

[0352] The invention mentioned in the specification is a grid-connected new energy converter based on matching control, but the proposed small signal modeling method and grid-connected stability judgment method are also applicable to grid-connected converters using other control schemes.

[0353] Aiming at the stability problem of the new energy grid-connected system, the present invention proposes a modular small signal model of a grid-connected converter and a method for judging the grid-connected stability. The small signal block diagram model and analytical transfer function model of the grid-connected converter are established by the component connection method, which effectively improves the modeling efficiency of complex systems and has good flexibility. On this basis, a grid-connected stability evaluation index based on the real and imaginary characteristics of the equivalent loop gain is proposed. This index can not only judge the overall small signal stability of the system, but also reveal the instability mechanism of the system and quantitatively analyze the impact of key factors on the small signal stability of the system.

[0354] The following is an example of the implementation of the present invention:

[0355] The present invention takes a 2MW grid-connected converter as an example to introduce the implementation process and results of the present invention. The rated parameters and circuit element parameters of the grid-connected converter are shown in Table 1.

[0356] Table 1 Main parameters of grid-connected converter system

[0357] parameter value <![CDATA[Rated capacity S n / kVA]]> 2 <![CDATA[Rated AC line voltage U n / V]]> 690 <![CDATA[Rated DC bus voltage U dc / V]]> 1500 <![CDATA[AC grid frequency f g / Hz]]> 50 <![CDATA[DC capacitor C dc / mF]]> 30 <![CDATA[Filter inductor L f / mH]]> 0.2 <![CDATA[Filter capacitor C f / μF]]> 500 <![CDATA[Grid impedance R g / Ω, L g / mH]]> 0.1,0.2 <![CDATA[Matching control k dc ,T dc > 0.042,0.0001 <![CDATA[Reactive power control k qp ,k qi > 3,6 <![CDATA[Virtual impedance R v ,L v > 1,0.01 <![CDATA[Inner current loop k ip , k ii > 1,5 Active / reactive power / pu 1,0

[0358] Fig.13 The real / imaginary characteristic diagram of the system equivalent loop gain Gop(ω) under different AC grid impedances is given. It can be found that G op (ω) There are two positive feedback frequencies, namely ω p1 and ω p2 , where σ(ω) is p2 The value at is always greater than -1 and does not change with the grid impedance Z g Therefore, the system always remains stable at this frequency. p1 Nearby, the value of σ(ω) varies with the grid impedance Z g decreases with the decrease of Z g When σ(ω p1 ) is approximately -1. According to the proposed grid-connected stability judgment method, at this time, the grid-connected system of the grid-connected converter has the risk of instability.

[0359] Next, the above stability analysis results are verified by time domain simulation method. Fig.14 The simulation waveforms and analysis results of the grid-connected system of the grid-type converter under different grid impedances are given. It can be found that in the initial operation state, the grid equivalent impedance Z g =0.1pu, the system can start smoothly and run to a stable state, and has good output characteristics; when Z g When it decreases to 0.07pu, the system experiences attenuation oscillation, but can still recover to a stable operating state; when Z gWhen it decreases to 0.05pu, the system will experience constant amplitude oscillation. Fig.13 The theoretical analysis results are consistent with those of the proposed method, which effectively verifies the accuracy of the grid-connected stability judgment method proposed in this application.

[0360] Fig.15 A modular small signal model of a grid-connected converter and a structure diagram of a grid-connected stability judgment system according to a preferred embodiment of the present invention.

[0361] like Fig.15 As shown, the present invention provides a modular small signal model of a grid-connected converter and a grid-connected stability judgment system, the system comprising:

[0362] The initial unit 1501 is used to determine the steady-state operation point of the grid-connected system of the grid-connected converter based on the steady-state operation equation, and control the grid-connected converter in a steady state based on the steady-state operation point;

[0363] Establishing unit 1502, used to establish a small signal transfer function model of a grid-forming converter and a small signal transfer function model of an AC power grid, and to establish a small signal transfer function model of a grid-forming converter grid-connected system based on the small signal transfer function model of the grid-forming converter and the small signal transfer function model of the AC power grid;

[0364] An analysis unit 1503 is used to analyze the small signal propagation path of the grid-connected system of the grid-connected converter and determine the key output variables;

[0365] A reconstruction unit 1504, configured to reconstruct a small signal transfer function model of a grid-connected system of a grid-connected converter based on key output variables;

[0366] The transformation unit 1505 is used to perform an equivalent transformation on the small signal transfer function model of the reconstructed grid-connected converter system, and derive an analytical small signal mathematical transfer function model of the grid-connected system;

[0367] The result unit 1506 is used to analyze the small signal stability of the grid-connected system of the grid-connected converter based on the analytical small signal mathematical transfer function model, and obtain the small signal stability evaluation result of the grid-connected system of the grid-connected converter.

[0368] Preferably, the initial unit 1501 is used to determine the steady-state operation point of the grid-connected system of the grid-connected converter based on the steady-state operation equation, control the grid-connected converter in a steady state based on the steady-state operation point, and is also used to:

[0369] The steady-state operation equations of various electrical quantities in the grid-connected system of the grid-connected converter in the dq coordinate system are:

[0370]

[0371]

[0372] Among them, U gd0 =U g0 cosδ0,U gq0 =U g0 sin(-δ0), δ0=θ PCC0 -θ g0 , δ0 is the steady-state power angle, U g0 and θ g0 are the amplitude and phase of the grid voltage, θ PCC0 is the steady-state value of the grid-connected point voltage phase, U PCCd0 is the steady-state value of the d-axis component of the grid-connected point voltage, U PCCq0 is the steady-state value of the q-axis component of the grid-connected point voltage, U gd0 is the steady-state value of the d-axis component of the grid voltage, U gq0 is the steady-state value of the q-axis component of the grid voltage, I gd0 is the steady-state value of the d-axis component of the current flowing into the AC grid, I gq0 is the steady-state value of the q-axis component of the current flowing into the AC grid, R g is the equivalent resistance of the AC grid, ω0 is the angular frequency of the ideal AC grid, L g is the equivalent power plant of the AC grid, U cd0 is the steady-state value of the d-axis component of the converter port voltage, U cq0 is the steady-state value of the q-axis component of the converter port voltage, R f is the AC filter resistor, I Ld0 is the steady-state value of the d-axis component of the filter inductor current, I Lq0 is the steady-state value of the q-axis component of the filter inductor current, L f is the AC filter inductor, C f is the AC filter capacitor, P0 and Q0 are the active power and reactive power output of the grid-connected converter in steady state respectively.

[0373] Preferably, the establishing unit 1502 is used to establish a small signal transfer function model of the grid-connected converter, and is also used to:

[0374] Establish the small signal transfer function model of the synchronization control submodule:

[0375]

[0376] Among them, Δ represents the small signal amount of the corresponding variable, s is the Laplace operator in the complex frequency domain, and θ GFM represents the output phase of the network control, K dc and T dc They are respectively the network-building controller G dc (s) proportionality coefficient and time constant, Δu dcis the DC bus voltage disturbance, Δu dcref is the change of DC bus voltage reference value;

[0377] Establish the small signal transfer function model of the reactive power-voltage control submodule:

[0378]

[0379] Among them, U ref represents the output voltage amplitude of the grid-connected converter, q ref and q e are the reactive power command and feedback value output by the grid-connected converter, k Qp and k Qi Respectively, reactive power-voltage controller G Q (s) proportional and integral coefficients;

[0380] Establish the small signal transfer function model of the virtual impedance control submodule:

[0381]

[0382] Among them, i dref represents the output of the virtual impedance control loop d-axis, i qref represents the output of the q-axis of the virtual impedance control loop, is the d-axis component of the grid-connected point voltage in the control coordinate system, is the q-axis component of the grid-connected point voltage in the control coordinate system, R v and L v are virtual resistance and virtual inductance respectively, G v (s) is the transfer function of virtual impedance control;

[0383] Establish the small signal transfer function model of the current vector control submodule:

[0384]

[0385] in, is the d-axis component of the converter modulation voltage in the control coordinate system, is the q-axis component of the converter modulation voltage in the control coordinate system, is the d-axis component of the converter output current in the control coordinate system, is the q-axis component of the converter output current in the control coordinate system, ω0 is the angular frequency of the ideal AC grid, L f is the filter inductance, k CCp and k CCi They are the current controller G CC (s) proportional and integral coefficients;

[0386] Establish the small signal transfer function model of the modulation link submodule:

[0387]

[0388] in, is the d-axis component of the converter output port voltage in the control coordinate system, is the q-axis component of the converter output port voltage in the control coordinate system, U md0 is the steady-state value of the d-axis component of the converter modulation voltage, U mq0 is the steady-state value of the q-axis component of the converter modulation voltage, U dc0 is the steady-state value of the DC bus voltage, u dc is the DC bus voltage feedback value.

[0389] Preferably, the establishing unit 1502 is used to establish a small signal transfer function model of the AC power grid, and is also used to:

[0390] Establish a small signal transfer function model for the AC power grid:

[0391]

[0392] Among them, u cd is the d-axis component of the converter port voltage in the grid reference coordinate system, u cq is the q-axis component of the converter port voltage in the grid reference coordinate system, u pccd is the d-axis component of the grid connection point voltage in the grid reference coordinate system, u pccq is the q-axis component of the grid connection point voltage in the grid reference coordinate system, u gd is the d-axis component of the AC grid voltage in the grid reference coordinate system, u gq is the q-axis component of the AC grid voltage in the grid reference coordinate system, ω0 is the angular frequency of the ideal AC grid, R f and L f are the filter resistance and inductance respectively, s is the Laplace operator in the complex frequency domain, i Ld is the d-axis component of the output current in the grid reference coordinate system, i Lq is the q-axis component of the output current in the grid reference coordinate system.

[0393] Preferably, the establishing unit 1502 is used to establish a small signal transfer function model of the grid-connected system of the grid-connected converter based on the small signal transfer function model of the grid-connected converter and the small signal transfer function model of the AC power grid, including:

[0394] Determine the conversion relationship between the dq components in the grid reference coordinate system based on the grid connection point voltage and the control coordinate system based on the output phase of the control link:

[0395]

[0396] in, is the d-axis component of the AC grid voltage in the control coordinate system, To control the q-axis component of the AC grid voltage in the coordinate system, consider Δθ GFM is a small disturbance, and the small signal linearization is performed on the conversion relationship to obtain the interface equation of the small signal variable in the power grid coordinate system and the control coordinate system, Δθ GFM It is the difference between the output phase of the control link and the actual phase of the grid-connected point voltage;

[0397]

[0398] Get the interface equations of different small signal variables in the grid coordinate system and the control coordinate system:

[0399]

[0400]

[0401]

[0402] Based on the implicit relationship of power balance, the small signal expressions of the active power output from the grid-connected converter port and the reactive power output from the grid-connected point are determined:

[0403]

[0404] Ignoring the power loss of the switching devices of the grid-type converter, the small signal transfer function model of the DC bus capacitor dynamics in the complex frequency domain is determined as follows:

[0405] C dc U dc0 sΔu dc =Δp in -Δp o (14)

[0406] Among them, C dc and U dc0 are the steady-state values ​​of the DC bus capacitance and its voltage, Δp in is the active power disturbance injected into the DC bus capacitor on the new energy generator side, Δp o is the change in active power output by the grid-connected converter;

[0407] Assume that the DC bus voltage command value remains unchanged, that is, Δu dcref =0, the small signal transfer function model of the DC bus capacitor dynamics in the complex frequency domain is brought into the small signal transfer function model of the synchronous control submodule:

[0408]

[0409] Based on the interface equation, the small signal expressions of the active power output from the ports of the grid-connected converter and the reactive power output from the grid-connected point, the small signal transfer function model of the grid-connected converter is combined to obtain the small signal transfer function model of the grid-connected converter system.

[0410] Preferably, the analysis unit 1503 is used to analyze the propagation path of the small signal transfer function model of the grid-connected system of the grid-connected converter, determine the key output variables, and also to:

[0411] The key output variable is the grid-type control output phase.

[0412] Preferably, the reconstruction unit 1504 is used to reconstruct the small signal transfer function model of the grid-connected system of the grid-connected converter based on the key output variables, and is also used to:

[0413] When the DC bus voltage command value and reactive power command value remain unchanged, Δu dcref =0, Δq ref =0, with the input power disturbance Δp in As the input signal, the output phase disturbance Δθ is controlled by the grid type GFM As the output signal, the small signal transfer function model of the grid-connected system of the grid-connected converter is reconstructed:

[0414]

[0415]

[0416] Among them, K Pd (s) is the d-axis active power feedback gain, K Pq (s) is the q-axis active power feedback gain, K Qd (s) is the d-axis reactive power feedback gain, K Qq (s) is the q-axis reactive power feedback gain, H q0 is the coupling gain of phase disturbance to d-axis voltage, H d0 is the coupling gain of phase disturbance to q-axis voltage.

[0417] Preferably, the transformation unit 1505 is used to perform an equivalent transformation on the reconstructed small signal transfer function model of the grid-connected system of the grid-connected converter, derive an analytical small signal mathematical transfer function model of the grid-connected system of the grid-connected converter, and is also used to:

[0418] The similar items in the small signal transfer function model of the grid-connected system of the grid-connected converter are merged, and the unit negative feedback subsystem composed of the current controller and the filter is equivalent to obtain the small signal transfer function model of the grid-connected system of the grid-connected converter:

[0419]

[0420]

[0421] Among them, G c1 (s) is the transfer function of the current control closed-loop subsystem, H U (s) is the voltage feedback equivalent gain, H θ1 (s) is the equivalent coupling gain of phase perturbation on d-axis control, H θ2 (s) is the equivalent coupling gain of phase perturbation on q-axis control, G CC (s) is the current controller transfer function, G v (s) is the virtual impedance control transfer function;

[0422] The grid-connected system of the grid-connected converter includes a positive feedback loop and a negative feedback loop:

[0423]

[0424]

[0425] Among them, G MC (s) is the equivalent transfer function of the synchronization link, H id1 (s) is the local d-axis current feedback equivalent gain, H iq2 (s) is the local q-axis current feedback equivalent gain, H iq1 (s) and H id2 (s) are the equivalent gains of dq axis current cross coupling, G dc (s) is the synchronous controller transfer function, G Q (s) is the reactive power control transfer function;

[0426] The negative feedback subsystem in the feedback path is equivalent, and the cross-coupling terms are decoupled, and the small signal transfer function model of the grid-connected system of the grid-connected converter is equivalent to a typical closed-loop feedback:

[0427]

[0428]

[0429] Where: G1(s) is the transfer function of the local equivalent subsystem of the d-axis current feedback, G2(s) is the transfer function of the local equivalent subsystem of the q-axis current feedback, H1(s) is the equivalent positive feedback gain of the entire closed-loop system, and H2(s) is the equivalent negative feedback gain of the entire closed-loop system;

[0430] Determine the open-loop transfer function G of the grid-connected system of the grid-connected converter op (s) and the closed-loop transfer function G c (s):

[0431] G op (s) = G MC (s)(H2(s)-H1(s))

[0432]

[0433] Preferably, the result unit 1506 is used to analyze the small signal stability of the grid-connected system of the grid-connected converter, obtain the small signal stability evaluation result of the grid-connected system of the grid-connected converter, and is also used to:

[0434] The expression of the open-loop transfer function in the frequency domain is G op The real and imaginary parts of (jω) are:

[0435]

[0436] Define σ(ω)=Re[G op (jω)], when the phase lag generated by the equivalent loop of the grid-connected system of the grid-connected converter reaches 180°, ∠G op (jω)=-180°,v(ω)=0,get the corresponding frequency as ω p ;

[0437] ω p Substituting into σ(ω), choose σ(ω p ) as an indicator for evaluating the small signal stability of the closed-loop system;

[0438] When σ(ω p )>0, the closed-loop system will be equivalent to a negative feedback loop, and the system will remain stable;

[0439] When -1<σ(ω p )<0, the closed-loop system will be equivalent to a positive feedback loop, but its amplitude does not meet the instability condition of the feedback system and can still remain stable;

[0440] When σ(ω p )≤-1, the closed-loop system will be equivalent to a positive feedback loop and meet the instability conditions of the feedback system, and there is a risk of instability.

[0441] Preferably, the result unit 1506 is used to obtain the instability mechanism of the grid-connected converter system based on the small signal stability evaluation result of the grid-connected converter system, and determine the dominant link causing the instability of the grid-connected converter system.

[0442] The present invention provides a computer-readable storage medium, which stores a computer program. The computer program is used to execute a modular small signal model of a grid-connected converter and a grid-connected stability judgment method.

[0443] The present invention provides an electronic device, the electronic device comprising: a processor and a memory; wherein:

[0444] Memory, a memory for storing processor executable instructions;

[0445] The processor is used for reading executable instructions from a memory and executing the instructions to realize a modular small signal model of a grid-connected converter and a grid-connected stability determination method.

[0446] It will be appreciated by those skilled in the art that embodiments of the present invention may be provided as methods, systems, or computer program products. Therefore, the present invention may take the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware. Moreover, the present invention may take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program codes. The schemes in the embodiments of the present invention may be implemented in various computer languages, for example, object-oriented programming language Java and literal scripting language JavaScript, etc.

[0447] The present invention is described with reference to flowcharts and / or block diagrams of methods, devices (systems), and computer program products according to embodiments of the present invention. It should be understood that each process and / or block in the flowchart and / or block diagram, as well as the combination of processes and / or blocks in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 A process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0448] These computer program instructions may also be stored in a computer-readable memory capable of directing a computer or other programmable data processing device to operate in a specific manner, so that the instructions stored in the computer-readable memory produce an article of manufacture comprising an instruction device, which implements the process Figure 1 A process or multiple processes and / or boxes Figure 1 A function specified in one or more boxes.

[0449] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operating steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing instructions for implementing the process. Figure 1A process or multiple processes and / or boxes Figure 1 The steps for the functions specified in one or more boxes.

[0450] Although the preferred embodiments of the present invention have been described, those skilled in the art may make other changes and modifications to these embodiments once they have learned the basic creative concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of the present invention.

[0451] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include these modifications and variations.

[0452] The invention has been described above with reference to a few embodiments. However, it is readily apparent to a person skilled in the art that other embodiments than the ones disclosed above are equally within the scope of the invention, as defined by the appended patent claims.

[0453] Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a / / the [means, component, etc.]" are to be openly interpreted as at least one instance of a means, component, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not necessarily have to be performed in the exact order disclosed, unless explicitly stated otherwise.

Claims

1. A modular small signal model of a grid-connected converter and a grid-connected stability determination method, the method comprising: Based on the steady-state operation equation, determine the steady-state operation point of the grid-connected system of the grid-connected converter, and based on the steady-state operation point, control the grid-connected converter in a steady state; Establishing a small signal transfer function model of a grid-forming converter and a small signal transfer function model of an AC power grid, and establishing a small signal transfer function model of a grid-forming converter grid-connected system based on the small signal transfer function model of the grid-forming converter and the small signal transfer function model of the AC power grid; Analyze the small signal propagation path of the grid-connected system of the grid-connected converter and determine the key output variables; Based on the key output variables, a small signal transfer function model of the grid-connected converter system is reconstructed; Performing an equivalent transformation on the reconstructed small signal transfer function model of the grid-connected converter system, and deriving an analytical small signal mathematical transfer function model of the grid-connected system; Based on the analytical small signal mathematical transfer function model, the small signal stability of the grid-connected system of the grid-connected converter is analyzed to obtain a small signal stability evaluation result of the grid-connected system of the grid-connected converter.

2. The method according to claim 1, wherein the steady-state operation point of the grid-connected system of the grid-connected converter is determined based on the steady-state operation equation, and the grid-connected converter is controlled in a steady state based on the steady-state operation point, comprising: The steady-state operation equations of various electrical quantities in the grid-connected system of the grid-connected converter in the dq coordinate system are: Among them, U gd0 =U g0 cosδ0,U gq0 =U g0 sin(-δ0), δ0=θ PCC0 -θ g0 , δ0 is the steady-state power angle, U g0 and θ g0 are the amplitude and phase of the grid voltage, θ PCC0 is the steady-state value of the grid-connected point voltage phase, U PCCd0 is the steady-state value of the d-axis component of the grid-connected point voltage, U PCCq0 is the steady-state value of the q-axis component of the grid-connected point voltage, U gd0 is the steady-state value of the d-axis component of the grid voltage, U gq0 is the steady-state value of the q-axis component of the grid voltage, I gd0 is the steady-state value of the d-axis component of the current flowing into the AC grid, I gq0 is the steady-state value of the q-axis component of the current flowing into the AC grid, R g is the equivalent resistance of the AC grid, ω0 is the angular frequency of the ideal AC grid, L g is the equivalent power plant of the AC grid, U cd0 is the steady-state value of the d-axis component of the converter port voltage, U cq0 is the steady-state value of the q-axis component of the converter port voltage, R f is the AC filter resistance, I Ld0 is the steady-state value of the d-axis component of the filter inductor current, I Lq0 is the steady-state value of the q-axis component of the filter inductor current, L f is the AC filter inductor, C f is the AC filter capacitor, P0 and Q0 are the active power and reactive power output of the grid-connected converter in steady state respectively.

3. According to the method of claim 2, the step of establishing a small signal transfer function model of a grid-connected converter comprises: Establish the small signal transfer function model of the synchronization control submodule: Among them, Δ represents the small signal amount of the corresponding variable, s is the Laplace operator in the complex frequency domain, and θ GFM represents the output phase of the network control, K dc and T dc They are respectively the network-building controller G dc (s) proportionality coefficient and time constant, Δu dc is the DC bus voltage disturbance, Δu dcref is the change of DC bus voltage reference value; Establish the small signal transfer function model of the reactive power-voltage control submodule: Among them, U ref represents the output voltage amplitude of the grid-connected converter, q ref and q e are the reactive power command and feedback value output by the grid-connected converter, k Qp and k Qi Respectively reactive power-voltage controller G Q (s) proportional and integral coefficients; Establish the small signal transfer function model of the virtual impedance control submodule: Among them, i dref represents the output of the virtual impedance control loop d-axis, i qref represents the output of the q-axis of the virtual impedance control loop, is the d-axis component of the grid-connected point voltage in the control coordinate system, is the q-axis component of the grid-connected point voltage in the control coordinate system, R v and L v are virtual resistance and virtual inductance respectively, G v (s) is the transfer function of virtual impedance control; Establish the small signal transfer function model of the current vector control submodule: in, is the d-axis component of the converter modulation voltage in the control coordinate system, is the q-axis component of the converter modulation voltage in the control coordinate system, is the d-axis component of the converter output current in the control coordinate system, is the q-axis component of the converter output current in the control coordinate system, ω0 is the angular frequency of the ideal AC grid, L f is the filter inductance, k CCp and k CCi They are the current controller G CC (s) proportional and integral coefficients; Establish the small signal transfer function model of the modulation link submodule: in, is the d-axis component of the converter output port voltage in the control coordinate system, is the q-axis component of the converter output port voltage in the control coordinate system, U md0 is the steady-state value of the d-axis component of the converter modulation voltage, U mq0 is the steady-state value of the q-axis component of the converter modulation voltage, U dc0 is the steady-state value of the DC bus voltage, u dc is the DC bus voltage feedback value.

4. The method according to claim 3, wherein establishing a small signal transfer function model of an AC power grid comprises: Establish a small signal transfer function model for the AC power grid: Among them, u cd is the d-axis component of the converter port voltage in the grid reference coordinate system, u cq is the q-axis component of the converter port voltage in the grid reference coordinate system, u pccd is the d-axis component of the grid connection point voltage in the grid reference coordinate system, u pccq is the q-axis component of the grid connection point voltage in the grid reference coordinate system, u gd is the d-axis component of the AC grid voltage in the grid reference coordinate system, u gq is the q-axis component of the AC grid voltage in the grid reference coordinate system, ω0 is the angular frequency of the ideal AC grid, R f and L f are the filter resistance and inductance respectively, s is the Laplace operator in the complex frequency domain, i Ld is the d-axis component of the output current in the grid reference coordinate system, i Lq is the q-axis component of the output current in the grid reference coordinate system.

5. The method according to claim 4, establishing a small signal transfer function model of a grid-connected system of a grid-connected converter based on the small signal transfer function model of the grid-connected converter and the small signal transfer function model of the AC power grid, comprising: Determine the conversion relationship between the dq components in the grid reference coordinate system based on the grid connection point voltage and the control coordinate system based on the output phase of the control link: in, is the d-axis component of the AC grid voltage in the control coordinate system, To control the q-axis component of the AC grid voltage in the coordinate system, consider Δθ GFM is a small disturbance, and the small signal linearization is performed on the conversion relationship to obtain the interface equation of the small signal variable in the power grid coordinate system and the control coordinate system, Δθ GFM It is the difference between the output phase of the control link and the actual phase of the grid-connected point voltage; Get the interface equations of different small signal variables in the grid coordinate system and the control coordinate system: Based on the implicit relationship of power balance, the small signal expressions of the active power output from the grid-connected converter port and the reactive power output from the grid-connected point are determined: Ignoring the power loss of the switching devices of the grid-type converter, the small signal transfer function model of the DC bus capacitor dynamics in the complex frequency domain is determined as follows: C dc U dc0 sΔu dc =Δp in -Δp o Among them, C dc and U dc0 are the steady-state values ​​of the DC bus capacitance and its voltage, Δp in is the active power disturbance injected into the DC bus capacitor on the new energy generator side, Δp o is the change in active power output by the grid-connected converter; Assume that the DC bus voltage command value remains unchanged, that is, Δu dcref =0, the small signal transfer function model of the DC bus capacitor dynamics in the complex frequency domain is brought into the small signal transfer function model of the synchronous control submodule: Based on the interface equation, the small signal expressions of the active power output by the grid-connected converter port and the reactive power output by the grid-connected point, the small signal transfer function model of the grid-connected converter is combined to obtain the small signal transfer function model of the grid-connected converter grid-connected system.

6. The method according to claim 5, wherein the step of analyzing the propagation path of the small signal transfer function model of the grid-connected system of the grid-connected converter to determine the key output variables comprises: The key output variable is the meshing control output phase.

7. The method according to claim 6, wherein the small signal transfer function model of the grid-connected converter system is reconstructed based on the key output variable, comprising: When the DC bus voltage command value and reactive power command value remain unchanged, Δu dcref =0, Δq ref =0, with the input power disturbance Δp in As the input signal, the output phase disturbance Δθ is controlled by the grid type GFM As the output signal, the small signal transfer function model of the grid-connected converter system is reconstructed: Among them, K Pd (s) is the d-axis active power feedback gain, K Pq (s) is the q-axis active power feedback gain, K Qd (s) is the d-axis reactive power feedback gain, K Qq (s) is the q-axis reactive power feedback gain, H q0 is the coupling gain of phase disturbance to d-axis voltage, H d 0 is the coupling gain of phase disturbance to q-axis voltage.

8. The method according to claim 7, wherein the reconstructed small signal transfer function model of the grid-connected converter system is equivalently transformed to derive the analytical small signal mathematical transfer function model of the grid-connected converter system, comprising: The similar items in the small signal transfer function model of the grid-connected system of the grid-connected converter are merged, and the unit negative feedback subsystem composed of the current controller and the filter is equivalent to obtain the small signal transfer function model of the grid-connected system of the grid-connected converter: Among them, G c1 (s) is the transfer function of the current control closed-loop subsystem, H U (s) is the voltage feedback equivalent gain, H θ1 (s) is the equivalent coupling gain of phase perturbation on d-axis control, H θ2 (s) is the equivalent coupling gain of phase perturbation on q-axis control, G CC (s) is the current controller transfer function, G V (s) is the virtual impedance control transfer function; The grid-connected system of the grid-connected converter includes a positive feedback loop and a negative feedback loop: Among them, G MC (s) is the equivalent transfer function of the synchronization link, H id1 (s) is the local d-axis current feedback equivalent gain, H iq2 (s) is the local q-axis current feedback equivalent gain, H iq1 (s) and H id2 (s) are the equivalent gains of dq axis current cross coupling, G dc (s) is the synchronous controller transfer function, G Q (s) is the reactive power control transfer function; The negative feedback subsystem in the feedback path is equivalent, and the cross-coupling terms are decoupled, and the small signal transfer function model of the grid-connected system of the grid-connected converter is equivalent to a typical closed-loop feedback: Where: G1(s) is the transfer function of the local equivalent subsystem of the d-axis current feedback, G2(s) is the transfer function of the local equivalent subsystem of the q-axis current feedback, H1(s) is the equivalent positive feedback gain of the entire closed-loop system, and H2(s) is the equivalent negative feedback gain of the entire closed-loop system; Determine the open-loop transfer function G of the grid-connected system of the grid-connected converter op (s) and the closed-loop transfer function G c (s): G op (s)=G MC (s)(H2(s)-H1(s)) 。 9. The method according to claim 8, wherein analyzing the small signal stability of the grid-connected system of the grid-connected converter and obtaining the small signal stability evaluation result of the grid-connected system of the grid-connected converter comprises: The expression of the open-loop transfer function in the frequency domain is G op The real and imaginary parts of (jω) are: Define σ(ω)=Re[G op (jω)], when the phase lag generated by the equivalent loop of the grid-connected system of the grid-connected converter reaches 180°, ∠G op (jω)=-180°,υ(ω)=0,the corresponding frequency is ω p ; ω p Substituting into σ(ω), choose σ(ω p ) as an indicator for evaluating the small signal stability of the closed-loop system; When σ(ω p )>0, the closed-loop system will be equivalent to a negative feedback loop, and the system will remain stable; When -1<σ(ω p )<0, the closed-loop system will be equivalent to a positive feedback loop, but its amplitude does not meet the instability condition of the feedback system and can still remain stable; When σ(ω p )≤-1, the closed-loop system will be equivalent to a positive feedback loop and meet the instability conditions of the feedback system, and there is a risk of instability.

10. The method according to claim 9, further comprising: Based on the small signal stability evaluation results of the grid-connected converter system, the instability mechanism of the grid-connected converter system is obtained, and the dominant link leading to the instability of the grid-connected converter system is determined.

11. A modular small signal model of a grid-connected converter and a grid-connected stability determination system, the system comprising: An initial unit, used to determine a steady-state operation point of the grid-connected system of the grid-connected converter based on a steady-state operation equation, and control the grid-connected converter in a steady state based on the steady-state operation point; An establishing unit is used to establish a small signal transfer function model of a grid-forming converter and a small signal transfer function model of an AC power grid, and to establish a small signal transfer function model of a grid-forming converter grid-connected system based on the small signal transfer function model of the grid-forming converter and the small signal transfer function model of the AC power grid; An analysis unit, used to analyze the small signal propagation path of the grid-connected system of the grid-connected converter and determine the key output variables; A reconstruction unit, used for reconstructing a small signal transfer function model of the grid-connected system of the grid-connected converter based on the key output variable; A transformation unit, used for performing an equivalent transformation on the reconstructed small signal transfer function model of the grid-connected system of the grid-connected converter, and deriving an analytical small signal mathematical transfer function model of the grid-connected system; The result unit is used to analyze the small signal stability of the grid-connected system of the grid-connected converter based on the analytical small signal mathematical transfer function model, and obtain the small signal stability evaluation result of the grid-connected system of the grid-connected converter.

12. A computer-readable storage medium, characterized in that: The computer-readable storage medium stores a computer program, and the computer program is used to execute the method according to any one of claims 1 to 10.

13. An electronic device, characterized in that: The electronic device comprises: a processor and a memory; wherein, The memory is a memory for storing instructions executable by the processor; The processor is configured to read the executable instructions from the memory and execute the instructions to implement the method according to any one of claims 1 to 10.

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