Parallel power supply system large disturbance stability analysis method considering coupling

Abstract

The invention belongs to the field of inverter transient synchronous stability analysis, and discloses a coupling-considered parallel power supply system large disturbance stability analysis method, and the topological structure of a parallel power supply system comprises a network-constructing inverter, a network-following inverter, a line impedance and a power grid. The method specifically comprises the following steps: simplifying a topological structure of the parallel power supply system, constructing an equivalent circuit mathematical model of the parallel power supply system, and constructing a voltage vector expression at a common bus for analyzing a coupling interaction relationship between a network-constructing inverter and a network-following inverter; based on the control block diagram of the network-constructing type inverter, constructing a network-constructing type control ring mathematical model in the large-disturbance transient process of the network-constructing type inverter; the method solves the problems that in the prior art, the coupling influence of a grid-forming inverter and a grid-following inverter parallel power supply system is not considered sufficiently, and the transient stability under large disturbance is difficult to analyze accurately.

Description

Technical Field

[0001] The present invention belongs to the field of transient synchronous stability analysis of inverters, and in particular relates to a large disturbance stability analysis method for a parallel power supply system considering coupling. Background Art

[0002] With the development of renewable energy and the penetration of distributed energy, the independent operation of a single converter often cannot meet the needs of large-scale grid connection. Therefore, the scenario of multiple converters in parallel has attracted more and more attention. In particular, the parallel power supply system of grid-forming inverters and grid-following inverters combines the voltage source external characteristics and weak grid adaptability of the former with the current source external characteristics and strong grid adaptability of the latter. This enables the parallel system to simultaneously meet the voltage, frequency regulation, and power quality requirements of both users and the power system, playing a complementary role. When multiple inverters operate in parallel, the output variables will affect each other, resulting in a relatively complex coupling interaction process, making the transient synchronous stability analysis of the parallel system under large disturbances more difficult.

[0003] Existing transient synchronous stability analysis of inverter parallel systems under large disturbances mainly focuses on application scenarios such as multiple grid-forming inverters in parallel, multiple grid-following inverters in parallel, or inverters in parallel with traditional synchronous machines. Scenarios of grid-forming converters and grid-following converters in parallel are rarely involved. In addition, there is still a general tendency to start from the perspective of a single inverter and ignore some coupling effects, resulting in an inaccurate description of the dynamic process. Therefore, the existing technology has insufficient consideration of the coupling effects of the power supply system of grid-forming inverters and grid-following inverters in parallel, making it difficult to accurately analyze transient stability problems under large disturbances. Summary of the Invention

[0004] In view of the shortcomings of the existing technology, the purpose of the present invention is to provide a large disturbance stability analysis method for a parallel power supply system taking coupling into account, which solves the problem in the existing technology that the coupling effect of the parallel power supply system between the grid-connecting inverter and the grid-following inverter is insufficiently considered, making it difficult to accurately analyze the transient stability under large disturbances.

[0005] The purpose of the present invention can be achieved through the following technical solutions:

[0006] A large disturbance stability analysis method for a coupled parallel power supply system is provided. The topology of the parallel power supply system includes a grid-forming inverter, a grid-following inverter, a line impedance, and a power grid. The method specifically includes the following steps:

[0007] The topology of the parallel power supply system is simplified, an equivalent circuit mathematical model of the parallel power supply system is constructed, and a voltage vector expression at the common bus is constructed to analyze the coupling interaction between the grid-forming inverter and the grid-following inverter.

[0008] Based on the control block diagram of the grid-type inverter, a mathematical model of the grid-type control loop of the grid-type inverter during large disturbance transient process is constructed;

[0009] Based on the control block diagram of the grid-following inverter, a mathematical model of the grid-following control loop of the grid-following inverter during large disturbance transient process is constructed;

[0010] Considering the coupling effect between grid-connected inverters and grid-following inverters, a coupling interaction model of the parallel power supply system is constructed for large disturbance stability analysis of the parallel power supply system.

[0011] The topological structure of the parallel power supply system is simplified, and an equivalent circuit mathematical model of the parallel power supply system is constructed, which specifically includes the following steps:

[0012] The grid-forming inverter adopts virtual synchronous control, and the grid-following inverter adopts phase-locked loop current control. The input terminals of the grid-forming inverter and the grid-following inverter are connected to the DC power generation unit. The input terminals of the grid-forming inverter and the grid-following inverter are respectively connected to the common bus in parallel through the line impedance. The common bus is connected to the grid through the line impedance.

[0013] When the parallel power supply system is subject to a large disturbance, the external characteristics of the grid-type inverter are equivalent to a controlled voltage source, and the external characteristics of the grid-type inverter are equivalent to a controlled current source;

[0014] The topological structure of the parallel power supply system is simplified into an equivalent circuit mathematical model consisting of a controlled voltage source and a controlled current source in parallel.

[0015] Constructing a voltage vector expression at the common bus for analyzing the coupling interaction between the grid-connected inverter and the grid-following inverter includes the following steps:

[0016] The voltage vector expression at the common bus is constructed by applying the superposition theorem to the mathematical model of the equivalent main circuit. The voltage vector expression at the common bus is as follows:

[0017]

[0018] Among them, U P is the phase voltage amplitude at the common bus;

[0019] U g is the grid voltage amplitude, which is used as a reference vector with a phase angle of 0 degrees;

[0020] U gfm is the output voltage amplitude of the grid-type inverter;

[0021] I gfl The current amplitude injected into the grid by the grid-following inverter;

[0022] Z gfmis the connection impedance between the grid-type inverter and the common bus,

[0023] Z gfm =R gfm +jX gfm , j is the imaginary unit, R gfm and X gfm are the resistance and inductance between the grid-connected inverter and the common bus, respectively;

[0024] Z grid is the connection impedance between the grid and the public bus, Z grid =R grid +jX grid , j is the imaginary unit, R grid and X grid are the resistance and inductive reactance between the grid and the public bus, respectively.

[0025] Based on the control block diagram of the grid-type inverter, a mathematical model of the grid-type control loop during the large disturbance transient process of the grid-type inverter is constructed, which specifically includes the following steps:

[0026] Analyze the control block diagram of the grid-type inverter with virtual synchronous control, which includes the main circuit, power control outer loop, and voltage and current control inner loop;

[0027] The control of the grid-type inverter with virtual synchronous control is decoupled from multiple time scales. The large disturbance stability analysis of the grid-type inverter focuses on the dynamic characteristics of the power control outer loop.

[0028] Construct the mathematical model of the grid-type control loop of the active-frequency control loop and the reactive-voltage control loop.

[0029] In the active power-frequency control loop, the synchronous generator rotor characteristics and primary frequency modulation characteristics are simulated by means of equivalent inertia and damping, and the droop characteristic is also provided. The control equation of the active power-frequency control loop is as follows:

[0030]

[0031] Where, J represents virtual inertia; D p Represents the active equivalent damping coefficient; P em Indicates the active power output of the grid-connected inverter, P setm Indicates the reference value of the active power output of the grid-connected inverter; ω gfm Indicates the output angular frequency of the grid-connected inverter; ω0 indicates the rated angular frequency of the grid; δ gfm Indicates the phase angle difference between the inverter output voltage and the grid voltage;

[0032] In the reactive power-voltage control loop, the control simulates the generator excitation system, supplements the inertia link and voltage regulation characteristics, and realizes the zero-difference control of the droop characteristics and reactive power. The control equation of the reactive power-voltage control loop is as follows:

[0033]

[0034] Where d / dt represents the differential with respect to time; k V Indicates the reactive inertia virtual excitation coefficient; D q Represents the reactive equivalent damping coefficient; Q em Indicates the reactive power output by the grid-connected inverter, Q setm Indicates the output reactive power reference value of the grid-connected inverter; U gfm Indicates the output voltage amplitude of the grid-type inverter, U gfm0 Indicates the output voltage reference amplitude of the grid-connected inverter.

[0035] Analyze the control block diagram of the grid-following inverter with phase-locked loop current control, which includes the main circuit, phase-locked loop and current control inner loop;

[0036] The control of the grid-following inverter with phase-locked loop current control is decoupled from multiple time scales. The large disturbance stability analysis of the grid-following inverter focuses on the dynamic characteristics of the phase-locked loop.

[0037] Based on the dynamic characteristics of the phase-locked loop, a mathematical model of the grid-following control loop is constructed. The control equation of the mathematical model of the grid-following control loop is as follows:

[0038]

[0039] Where d / dt represents the differential with respect to time; U gfl_q represents the q-axis component of the grid-following inverter output voltage; k p and k i are the proportional coefficient and integral coefficient in the phase-locked loop controller respectively; ω0 represents the rated angular frequency of the power grid; ω gfl is the grid-following inverter angular frequency given by the phase-locked loop; δ gfl Indicates the phase angle difference between the output voltage of the grid-following inverter and the grid voltage.

[0040] Considering the coupling effect between the grid-connected inverter and the grid-following inverter, a coupling interaction model of the parallel power supply system is constructed for large disturbance stability analysis of the parallel power supply system. The specific steps include:

[0041] Analyze the coupling effect of the interaction between grid-connected inverters and grid-following inverters;

[0042] Calculate and construct the grid-connected inverter output active power P em , grid-type inverter output reactive power Qem And the q-axis component U of the grid-following inverter output voltage gfl_q Expressions of

[0043] P em , Q em and U gfl_q Substituting the expression into the control equations of the active-frequency control loop, the reactive-voltage control loop, and the mathematical model of the grid-following control loop, we can obtain the coupled interaction model for large-disturbance stability analysis of the parallel power supply system considering coupling.

[0044] The large disturbance stability analysis of the coupled parallel power supply system is carried out through the coupled interaction model.

[0045] Define the following set of equivalent admittance amplitudes Y i , where i = 1, 2, 3, 4, 5;

[0046]

[0047] in, is the corresponding equivalent admittance phase angle, i=1,2,3,4,5;

[0048] Z gfl is the connection impedance between the grid-following inverter and the common bus, Z gfl =R gfl +jX gfl , R gfl and X gfl are the resistance and inductance between the grid-following inverter and the common bus respectively;

[0049] Based on the equivalent circuit mathematical model and Kirchhoff's law, the output current vector I of the grid-type inverter is obtained. gfm The following expression:

[0050]

[0051] Among them, θ I_gfl is the internal current phase angle of the grid-following inverter, θ I_gfl =arctan(I gfl_q / I gfl_d ), I gfl_q with I gfl_d are the vertical q-axis component and the horizontal d-axis component after orthogonal decomposition of the current injected by the grid-following inverter;

[0052] Furthermore, the complex power S output by the grid-connected inverter port is calculated. gfm , S gfm The expression is as follows:

[0053]

[0054] Where, I * gfm represents the conjugate of the output current of the grid-type inverter;

[0055] Then perform Euler transformation and get P em and Q em expression:

[0056]

[0057] Where, the output power term P em1 and Q em1 The corresponding active power and reactive power that the grid-connected inverter should output when it is connected to the grid. em2 and Q em2 is the active power and reactive power coupling interaction term, which represents the impact of the addition of grid-following inverters on the output power of grid-connected inverters in the parallel power supply system;

[0058] Based on the equivalent circuit mathematical model and Kirchhoff's law, the grid-connected segment voltage vector U of the grid-following inverter is obtained. gfl The expression:

[0059]

[0060] Then perform orthogonal coordinate transformation to obtain the voltage vector U gfl The q-axis component U gfl_q The expression:

[0061]

[0062] q-axis voltage term U gfl_q1 It is the voltage component that appears when a grid-following inverter is connected to the grid. It is generated by the interaction between its own output current and the network impedance and is affected by the grid operation status. gfl_q2 It is the voltage coupling interaction term, which represents the effect of adding grid-connected inverters on the q-axis voltage of grid-following inverters in the parallel power supply system.

[0063] Beneficial effects of the present invention:

[0064] The large-disturbance stability analysis method of a parallel system considering coupling proposed in the present invention clearly presents the coupling interaction relationship between the two inverters by constructing the system's equivalent circuit mathematical model and the voltage vector expression at the common bus; further, the control loop mathematical models of the grid-forming inverter and the grid-following inverter in the large-disturbance transient process are constructed respectively, and based on the coupling interaction model, the large-disturbance stability process mechanism of the system and the influence of the control loop parameters on the stability are deeply analyzed. The coupling interaction process and mutual influence in the transient process of the grid-forming inverter and the grid-following inverter in the parallel scenario are analyzed and studied, and a large-disturbance stability analysis method for the corresponding scenario is formed; compared with the prior art, the present invention not only considers the multi-time-scale characteristics of the inverter control loop, but also improves the analysis efficiency by reducing the order and simplifying the control equation; in addition, the present invention can more clearly present the interaction between the parallel inverters, provide strong support for optimizing the control strategy, and effectively improve the transient synchronization stability of the parallel power supply system under large disturbances. BRIEF DESCRIPTION OF THE DRAWINGS

[0065] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0066] Figure 1 It is a schematic diagram of the topological structure of the parallel power supply system of the present invention;

[0067] Figure 2 It is a circuit diagram of a parallel power supply system of the present invention;

[0068] Figure 3 This is a transient control block diagram of the grid-type inverter under large disturbance of the present invention;

[0069] Figure 4 This is a transient control block diagram of the grid-following inverter under large disturbances of the present invention;

[0070] Figure 5 It is a schematic diagram of the coupling interaction model of the present invention. DETAILED DESCRIPTION

[0071] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts shall fall within the scope of protection of the present invention.

[0072] like Figures 1 to 5As shown, a large disturbance stability analysis method for a parallel power supply system considering coupling is provided. The topology of the parallel power supply system includes a grid-forming inverter, a grid-following inverter, a line impedance, and a power grid. The method specifically includes the following steps:

[0073] The topology of the parallel power supply system is simplified, an equivalent circuit mathematical model of the parallel power supply system is constructed, and a voltage vector expression at the common bus is constructed to analyze the coupling interaction between the grid-forming inverter and the grid-following inverter.

[0074] Based on the control block diagram of the grid-type inverter, a mathematical model of the grid-type control loop of the grid-type inverter during large disturbance transient process is constructed;

[0075] Based on the control block diagram of the grid-following inverter, a mathematical model of the grid-following control loop of the grid-following inverter during large disturbance transient process is constructed;

[0076] Considering the coupling effect between grid-connected inverters and grid-following inverters, a coupling interaction model of the parallel power supply system is constructed for large disturbance stability analysis of the parallel power supply system.

[0077] The topological structure of the parallel power supply system is simplified, and an equivalent circuit mathematical model of the parallel power supply system is constructed, which specifically includes the following steps:

[0078] The grid-forming inverter adopts virtual synchronous control, and the grid-following inverter adopts phase-locked loop current control. The input terminals of the grid-forming inverter and the grid-following inverter are connected to the DC power generation unit. The input terminals of the grid-forming inverter and the grid-following inverter are respectively connected to the common bus in parallel through the line impedance. The common bus is connected to the grid through the line impedance.

[0079] When the parallel power supply system is subject to a large disturbance, the external characteristics of the grid-type inverter are equivalent to a controlled voltage source, and the external characteristics of the grid-type inverter are equivalent to a controlled current source;

[0080] The topological structure of the parallel power supply system is simplified into an equivalent circuit mathematical model consisting of a controlled voltage source and a controlled current source in parallel.

[0081] Constructing a voltage vector expression at the common bus for analyzing the coupling interaction between the grid-connected inverter and the grid-following inverter includes the following steps:

[0082] The voltage vector expression at the common bus is constructed by applying the superposition theorem to the mathematical model of the equivalent main circuit. The voltage vector expression at the common bus is as follows:

[0083]

[0084] Among them, U P is the phase voltage amplitude at the common bus;

[0085] U g is the grid voltage amplitude, which is used as a reference vector with a phase angle of 0 degrees;

[0086] U gfm is the output voltage amplitude of the grid-type inverter;

[0087] I gfl The current amplitude injected into the grid by the grid-following inverter;

[0088] δ gfl +θ I_gfl For I gfl The phase angle difference relative to the reference vector, where θ I_gfl is the internal current phase angle of the grid-following inverter, θ I_gfl =arctan(I gfl_q / I gfl_d ), I gfl_q with I gfl_d are the vertical q-axis component and the horizontal d-axis component after orthogonal decomposition of the injected current;

[0089] Z gfm is the connection impedance between the grid-type inverter and the common bus,

[0090] Z gfm =R gfm +jX gfm , j is the imaginary unit, R gfm and X gfm are the resistance and inductance between the grid-connected inverter and the common bus, respectively;

[0091] Z grid is the connection impedance between the grid and the public bus, Z grid =R grid +jX grid , j is the imaginary unit, R grid and X grid are the resistance and inductive reactance between the grid and the public bus, respectively.

[0092] Figure 1 In, V dc The DC voltage provided by the DC power generation unit; L f_gfm 、C f_gfm They are the filter inductor and filter capacitor of the grid-type inverter respectively;

[0093] L f_gfl 、C f_gfl They are the filter inductor and filter capacitor of the grid-following inverter respectively.

[0094] Based on the control block diagram of the grid-type inverter, a mathematical model of the grid-type control loop during the large disturbance transient process of the grid-type inverter is constructed, which specifically includes the following steps:

[0095] Analyze the control block diagram of the grid-type inverter with virtual synchronous control, which includes the main circuit, power control outer loop, and voltage and current control inner loop;

[0096] Since the voltage and current inner loops use the high-frequency modulation principle, conventional control strategies can enable them to accurately follow the reference signal generated by the power outer loop within a millisecond time scale and control the output characteristics of the grid-type inverter. When studying the transient synchronous stability problem under large disturbances, the analysis is usually carried out on a second time scale, mainly focusing on the power outer loop of the simulated synchronous generator. It can be assumed that the voltage and current inner loops have quickly reached a steady state, that is, the inverter output voltage is considered to be equal to its reference value.

[0097] Figure 3 In the main circuit part, V dc is the DC side voltage of the inverter, Z gfm is the line impedance. Lm,abc is the current flowing through the filter, u gfm,abc and i gfm,abc The above voltage and current are three-phase variables in the stationary abc coordinate system, which are converted into variables in the rotating dq coordinate system after sampling and coordinate transformation. Lm,dq 、u gfm,dq and i gfm,dq , used for three-phase inverter power calculation and voltage and current control;

[0098] Figure 3 In the control circuit part, P em and Q em They are respectively the active power and reactive power of the inverter output port obtained through the power calculation link, P setm and Q setm are their reference values. ω0 is the grid reference angular frequency, which is constant at 50Hz. gfm0 is the reference output voltage amplitude of the inverter. gfm is the inverter output angular frequency, θ gfm is the inverter phase signal, which serves as the inverter phase signal reference value output by the active power-frequency control loop; E gfm is a virtual potential signal, which serves as the reference value of the inverter voltage amplitude signal output by the reactive-voltage control loop and acts on the inverter operation process. Due to the time scale decoupling, the inverter output voltage is considered to be equal to its reference value, so U gfm =E gfm ; Among them, the power calculation process P em and Q emSuch as the formula:

[0099]

[0100] The control of the grid-type inverter with virtual synchronous control is decoupled from multiple time scales. The large disturbance stability analysis of the grid-type inverter focuses on the dynamic characteristics of the power control outer loop.

[0101] Construct the mathematical model of the grid-type control loop of the active-frequency control loop and the reactive-voltage control loop.

[0102] In the active power-frequency control loop, the synchronous generator rotor characteristics and primary frequency modulation characteristics are simulated by means of equivalent inertia and damping, and the droop characteristic is also provided. The control equation of the active power-frequency control loop is as follows:

[0103]

[0104] Where, J represents virtual inertia; D p Represents the active equivalent damping coefficient; P em Indicates the active power output of the grid-connected inverter, P setm Indicates the reference value of the active power output of the grid-connected inverter; ω gfm Indicates the output angular frequency of the grid-connected inverter; ω0 indicates the rated angular frequency of the grid; δ gfm Indicates the phase angle difference between the inverter output voltage and the grid voltage;

[0105] In the reactive power-voltage control loop, the control simulates the generator excitation system, supplements the inertia link and voltage regulation characteristics, and realizes the zero-difference control of the droop characteristics and reactive power. The control equation of the reactive power-voltage control loop is as follows:

[0106]

[0107] Where k V Indicates the reactive inertia virtual excitation coefficient; D q Represents the reactive equivalent damping coefficient; Q em Indicates the reactive power output by the grid-connected inverter, Q setm Indicates the output reactive power reference value of the grid-connected inverter; U gfm Indicates the output voltage amplitude of the grid-type inverter, U gfm0 Indicates the output voltage reference amplitude of the grid-connected inverter.

[0108] Analyze the control block diagram of the grid-following inverter with phase-locked loop current control, which includes the main circuit, phase-locked loop and current control inner loop;

[0109] In grid-following inverters, the inner current loop utilizes high-frequency modulation, enabling rapid response, typically accurately following a given current reference signal within milliseconds, thereby controlling the inverter's output characteristics. Therefore, when analyzing transient synchronization stability under large disturbances, the primary focus is on the phase-locked loop's timescale. It can be assumed that the inner current loop and power control link have quickly reached steady state, with the inverter output current equal to its reference value.

[0110] Figure 4 In the main circuit part, V dc is the DC side voltage of the inverter, Z gfl is the line impedance. Ll,abc is the current flowing through the filter, u gfl,abc and i gfl,abc The voltage and current at the coupling point of the inverter are all three-phase variables in the stationary abc coordinate system. After sampling and coordinate transformation, they are converted into variables i in the rotating dq coordinate system. Ll,dq 、u gfl,dq and i gfl,dq , used for three-phase inverter power calculation and voltage and current control.

[0111] Figure 4 In the control circuit part, ω0 is the grid reference angular frequency, which is constant at 50Hz. gfl The phase signal given by the phase-locked loop is used as the grid-connected reference for the grid-following inverter. Under normal circumstances, the grid-following inverter uses the power control link to give a current reference value to achieve maximum power tracking or unity power factor operation. el and Q el They are respectively the active power and reactive power of the inverter output port obtained through the power calculation link, P setl and Q setl When the grid voltage drops sharply due to a short circuit or other major disturbance, the grid-following inverter enters the low voltage ride-through stage, locks the power control link, and directly sets the active and reactive current reference values ​​that the inverter should output according to the grid guidelines to meet the grid connection requirements; i ref,d and i ref,q are the final d-axis and q-axis reference currents respectively; among them, P in the power calculation link el and Q el The calculation formula is as follows:

[0112]

[0113] That is, the control of the grid-following inverter controlled by the phase-locked loop current is decoupled from multiple time scales, and the large disturbance stability analysis of the grid-following inverter is targeted at the dynamic characteristics of the phase-locked loop;

[0114] Based on the dynamic characteristics of the phase-locked loop, a mathematical model of the grid-following control loop is constructed. The control equation of the mathematical model of the grid-following control loop is as follows:

[0115]

[0116] Where U gfl_q Represents the output voltage vector of the grid-following inverter Ugfl The q-axis component of k p and k i are the proportional coefficient and integral coefficient in the phase-locked loop controller respectively; ω0 represents the rated angular frequency of the power grid; ω gfl is the grid-following inverter angular frequency given by the phase-locked loop; δ gfl Indicates the phase angle difference between the output voltage of the grid-following inverter and the grid voltage.

[0117] Considering the coupling effect between the grid-connected inverter and the grid-following inverter, a coupling interaction model of the parallel power supply system is constructed for large disturbance stability analysis of the parallel power supply system. The specific steps include:

[0118] Analyze the coupling effect of the interaction between grid-connected inverters and grid-following inverters;

[0119] Calculate and construct the grid-connected inverter output active power P em , grid-type inverter output reactive power Q em And the q-axis component U of the grid-following inverter output voltage gfl_q Expressions of

[0120] P em , Q em and U gfl_q Substituting the expression into the control equations of the active-frequency control loop, the reactive-voltage control loop, and the mathematical model of the grid-following control loop, we can obtain the coupled interaction model for large-disturbance stability analysis of the parallel power supply system considering coupling.

[0121] The large disturbance stability analysis of the coupled parallel power supply system is carried out through the coupled interaction model.

[0122] Define the following set of equivalent admittance amplitudes Y i , where i = 1, 2, 3, 4, 5;

[0123]

[0124] in, is the corresponding equivalent admittance phase angle, i=1,2,3,4,5;

[0125] Z gfl is the connection impedance between the grid-following inverter and the common bus, Z gfl=R gfl +jX gfl , R gfl and X gfl are the resistance and inductance between the grid-following inverter and the common bus respectively;

[0126] First, consider the grid-connected inverter in the parallel system. Since its external characteristics are equivalent to a voltage source, the parallel connection with the grid-connected inverter branch changes its port output current, further changing the power sent to the grid by the grid-connected inverter. Under the action of the active-frequency control loop and the reactive-voltage control loop, its transient synchronization stability is affected.

[0127] Based on the equivalent circuit mathematical model and Kirchhoff's law, the output current vector I of the grid-type inverter is obtained. gfm The following expression:

[0128]

[0129] Among them, θ I_gfl is the internal current phase angle of the grid-following inverter, θ I_gfl =arctan(I gfl_q / I gfl_d ), I gfl_q with I gfl_d are the vertical q-axis component and the horizontal d-axis component after orthogonal decomposition of the current injected by the grid-following inverter;

[0130] Furthermore, the complex power S output by the grid-connected inverter port is calculated. gfm , S gfm The expression is as follows:

[0131]

[0132] Where, I * gfm represents the conjugate of the output current of the grid-type inverter;

[0133] Then perform Euler transformation and get P em and Q em expression:

[0134]

[0135]

[0136] Where, the output power term P em1 and Q em1 The corresponding active power and reactive power that the grid-connected inverter should output when it is connected to the grid. em2 and Q em2is the active power and reactive power coupling interaction term, which represents the impact of the addition of the grid-following inverter on the output power of the grid-connected inverter in the parallel power supply system;

[0137] For the grid-following inverter, its external characteristics are regarded as current sources. Therefore, the parallel connection of the grid-following inverter branches affects the output voltage of the grid-following inverter by changing the network parameters and voltage distribution. Under the dynamic control of the phase-locked loop, it affects its transient synchronization stability performance under large disturbances.

[0138] Based on the equivalent circuit mathematical model and Kirchhoff's law, the grid-connected segment voltage vector U of the grid-following inverter is obtained. gfl The expression:

[0139]

[0140] Then perform orthogonal coordinate transformation to obtain the voltage vector U gfl The q-axis component U gfl_q The expression:

[0141]

[0142] q-axis voltage term U gfl_q1 It is the voltage component that appears when a grid-following inverter is connected to the grid. It is generated by the interaction between its own output current and the network impedance and is affected by the grid operation status. gfl_q2 It is the voltage coupling interaction term, which represents the effect of adding grid-connected inverters on the q-axis voltage of grid-following inverters in the parallel power supply system.

[0143] Figure 5 The figure shows the coupled interaction model used in the present disclosure for large disturbance stability analysis of parallel power supply systems.

[0144] The coupled interaction model takes into account the influence of coupling terms. It clearly describes the impact of the grid-following inverter on the active power and reactive power output of the grid-forming inverter in the parallel system, as well as the impact of the grid-forming inverter on the output voltage of the grid-following inverter through coupling terms.

[0145] These influences will further affect the transient synchronization stability of the parallel power supply system under large disturbances through the active-frequency control loop, reactive-voltage control loop of the grid-type inverter, and the phase-locked loop of the grid-following inverter.

[0146] When the inverter control parameters change, the model can be used as a basis to explore the specific mechanism of the impact, and further combined with the equal area method, phase plane method, energy function method, etc. to carry out subsequent qualitative and quantitative analysis.

[0147] Throughout this specification, references to terms such as "one embodiment," "example," or "specific example" indicate that the specific features, structures, materials, or characteristics described in conjunction with that embodiment or example are included in at least one embodiment or example of the present invention. In this specification, schematic representations of these terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

[0148] The basic principles, main features, and advantages of the present invention are shown and described above. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions are merely illustrative of the principles of the present invention. Various changes and modifications may be made to the present invention without departing from the spirit and scope of the present invention, and such changes and modifications are intended to fall within the scope of the present invention.

Claims

1. A large disturbance stability analysis method for a parallel power supply system considering coupling, wherein the topology of the parallel power supply system includes a grid-forming inverter, a grid-following inverter, a line impedance, and a power grid, characterized in that: The specific steps include: The topology of the parallel power supply system is simplified, an equivalent circuit mathematical model of the parallel power supply system is constructed, and a voltage vector expression at the common bus is constructed to analyze the coupling interaction between the grid-forming inverter and the grid-following inverter. Based on the control block diagram of the grid-type inverter, a mathematical model of the grid-type control loop of the grid-type inverter during large disturbance transient process is constructed; Based on the control block diagram of the grid-following inverter, a mathematical model of the grid-following control loop of the grid-following inverter during large disturbance transient process is constructed; Considering the coupling effect between grid-connected inverters and grid-following inverters, a coupling interaction model of the parallel power supply system is constructed. Based on the coupled interaction model, the large disturbance stability process mechanism of the parallel power supply system and the influence of the control loop parameters on the large disturbance stability are analyzed.

2. The large disturbance stability analysis method of a parallel power supply system considering coupling according to claim 1 is characterized in that: The topological structure of the parallel power supply system is simplified, and an equivalent circuit mathematical model of the parallel power supply system is constructed, which specifically includes the following steps: The grid-forming inverter adopts virtual synchronous control, and the grid-following inverter adopts phase-locked loop current control. The input terminals of the grid-forming inverter and the grid-following inverter are connected to the DC power generation unit. The input terminals of the grid-forming inverter and the grid-following inverter are respectively connected to the common bus in parallel through the line impedance. The common bus is connected to the grid through the line impedance. When the parallel power supply system is subject to a large disturbance, the external characteristics of the grid-type inverter are equivalent to a controlled voltage source, and the external characteristics of the grid-type inverter are equivalent to a controlled current source; The topological structure of the parallel power supply system is simplified into an equivalent circuit mathematical model consisting of a controlled voltage source and a controlled current source in parallel.

3. The large disturbance stability analysis method of a parallel power supply system considering coupling according to claim 2 is characterized in that: Constructing a voltage vector expression at the common bus for analyzing the coupling interaction between the grid-connected inverter and the grid-following inverter includes the following steps: The voltage vector expression at the common bus is constructed by applying the superposition theorem to the mathematical model of the equivalent main circuit. The voltage vector expression at the common bus is as follows: Among them, U P is the phase voltage amplitude at the common bus; U g is the grid voltage amplitude, which is used as a reference vector with a phase angle of 0 degrees; U gfm is the output voltage amplitude of the grid-type inverter; I gfl The current amplitude injected into the grid by the grid-following inverter; Z gfm is the connection impedance between the grid-type inverter and the common bus, Z gfm =R gfm +jX gfm , j is the imaginary unit, R gfm and X gfm are the resistance and inductance between the grid-connected inverter and the common bus, respectively; Z grid is the connection impedance between the grid and the public bus, Z grid =R grid +jX grid , j is the imaginary unit, R grid and X grid are the resistance and inductive reactance between the grid and the public bus, respectively.

4. The large disturbance stability analysis method of a parallel power supply system considering coupling according to claim 3 is characterized in that: Based on the control block diagram of the grid-type inverter, a mathematical model of the grid-type control loop during the large disturbance transient process of the grid-type inverter is constructed, which specifically includes the following steps: Analyze the control block diagram of the grid-type inverter with virtual synchronous control, which includes the main circuit, power control outer loop, and voltage and current control inner loop; The control of the grid-type inverter with virtual synchronous control is decoupled from multiple time scales. The large disturbance stability analysis of the grid-type inverter focuses on the dynamic characteristics of the power control outer loop. Construct the mathematical model of the grid-type control loop of the active-frequency control loop and the reactive-voltage control loop.

5. The large disturbance stability analysis method of a parallel power supply system considering coupling according to claim 4 is characterized in that: In the active power-frequency control loop, the synchronous generator rotor characteristics and primary frequency modulation characteristics are simulated by means of equivalent inertia and damping, while also having droop characteristics. The control equation of the active power-frequency control loop is as follows: Where d / dt represents the differential with respect to time; J represents the virtual inertia; D p Represents the active equivalent damping coefficient; P em Indicates the active power output of the grid-connected inverter, P setm Indicates the reference value of the active power output of the grid-connected inverter; ω gfm Indicates the output angular frequency of the grid-connected inverter; ω0 indicates the rated angular frequency of the grid; δ gfm Indicates the phase angle difference between the grid-connected inverter output voltage and the grid voltage; In the reactive-voltage control loop, the control simulates the generator excitation system, supplements the inertia link and voltage regulation characteristics, and realizes the zero-difference control of droop characteristics and reactive power. The control equation of the reactive-voltage control loop is as follows: Where d / dt represents the differential with respect to time; k V Indicates the reactive inertia virtual excitation coefficient; D q Represents the reactive equivalent damping coefficient; Q em Indicates the reactive power output by the grid-connected inverter, Q setm Indicates the output reactive power reference value of the grid-connected inverter; U gfm Indicates the output voltage amplitude of the grid-type inverter, U gfm0 Indicates the output voltage reference amplitude of the grid-connected inverter.

6. The large disturbance stability analysis method of a parallel power supply system considering coupling according to claim 5 is characterized in that: Analyze the control block diagram of the grid-following inverter with phase-locked loop current control, which includes the main circuit, phase-locked loop and current control inner loop; The control of the grid-following inverter with phase-locked loop current control is decoupled from multiple time scales. The large disturbance stability analysis of the grid-following inverter focuses on the dynamic characteristics of the phase-locked loop. Based on the dynamic characteristics of the phase-locked loop, a mathematical model of the grid-following control loop is constructed. The control equation of the mathematical model of the grid-following control loop is as follows: Where d / dt represents the differential with respect to time; U gfl_q represents the q-axis component of the grid-following inverter output voltage; k p and k i are the proportional coefficient and integral coefficient in the phase-locked loop controller respectively; ω0 represents the rated angular frequency of the power grid; ω gfl is the grid-following inverter angular frequency given by the phase-locked loop; δ gfl Indicates the phase angle difference between the output voltage of the grid-following inverter and the grid voltage.

7. The large disturbance stability analysis method of a parallel power supply system considering coupling according to claim 6 is characterized in that: Considering the coupling effect between the grid-connected inverter and the grid-following inverter, a coupling interaction model of the parallel power supply system is constructed for large disturbance stability analysis of the parallel power supply system. The specific steps include: Analyze the coupling effect of the interaction between grid-connected inverters and grid-following inverters; Calculate and construct the grid-connected inverter output active power P em , grid-type inverter output reactive power Q em And the q-axis component U of the grid-following inverter output voltage gfl_q Expressions of P em , Q em and U gfl_q Substitute the expression into the control equations of the active-frequency control loop, the reactive-voltage control loop, and the mathematical model of the grid-following control loop to complete the construction of the coupled interaction model of the parallel power supply system. The large disturbance stability analysis of the coupled parallel power supply system is carried out through the coupled interaction model.

8. The large disturbance stability analysis method of a parallel power supply system considering coupling according to claim 7 is characterized in that: Define the following set of equivalent admittance amplitudes Y i , where i = 1, 2, 3, 4, 5; in, is the corresponding equivalent admittance phase angle, i=1,2,3,4,5; Z gfl is the connection impedance between the grid-following inverter and the common bus, Z gfl =R gfl +jX gfl , R gfl and X gfl are the resistance and inductance between the grid-following inverter and the common bus respectively; Based on the equivalent circuit mathematical model and Kirchhoff's law, the output current vector I of the grid-type inverter is obtained. gfm The following expression: Among them, θ I_gfl is the internal current phase angle of the grid-following inverter, θ I_gfl =arctan(I gfl_q / I gfl_d ), I gfl_q with I gfl_d are the vertical q-axis component and the horizontal d-axis component after orthogonal decomposition of the current injected by the grid-following inverter; Furthermore, the complex power S output by the grid-connected inverter port is calculated. gfm , S gfm The expression is as follows: Where, I * gfm represents the conjugate of the output current of the grid-type inverter; Then perform Euler transformation and get P em and Q em expression: Where, the output power term P em1 and Q em1 The corresponding active power and reactive power that the grid-connected inverter should output when it is connected to the grid. em2 and Q em2 is the active power and reactive power coupling interaction term, which represents the impact of the addition of grid-following inverters on the output power of grid-connected inverters in the parallel power supply system; Based on the equivalent circuit mathematical model and Kirchhoff's law, the grid-connected segment voltage vector U of the grid-following inverter is obtained. gfl The expression: Then perform orthogonal coordinate transformation to obtain the voltage vector U gfl The q-axis component U gfl_q The expression: q-axis voltage term U gfl_q1 It is the voltage component that appears when a grid-following inverter is connected to the grid. It is generated by the interaction between its own output current and the network impedance and is affected by the grid operation status. gfl_q2 It is the voltage coupling interaction term, which represents the effect of adding grid-connected inverters on the q-axis voltage of grid-following inverters in the parallel power supply system.

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