Network configuration type converter control method and system based on virtual impedance feedforward controller

Abstract

The application discloses a network-constructed converter control method and system based on a virtual impedance feedforward controller, the converter comprising a direct-current voltage source, a three-phase half-bridge converter and a low-pass output LC filter, and further comprising a virtual synchronous generator control module, a virtual impedance control module and a voltage and current double-closed-loop control module, specifically: a small signal model considering coupling is established on the network-constructed converter based on virtual synchronous generator control; the small signal model considering coupling is analyzed, and it is found that the same conjugate pole is contained in the active power ring, the reactive power ring and the coupling function, which will cause synchronous frequency resonance, and the coupling will aggravate the synchronous frequency resonance; a feedforward controller containing a virtual impedance is designed to increase the system damping, and the controller is simplified to suppress the unstable factors introduced by the controller; the setting condition of the virtual impedance is calculated by comprehensively considering the damping and bandwidth of the system. The application can suppress the synchronous frequency resonance, and has the advantages of fast response speed and good damping effect.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of power electronics, in particular to a grid-connected converter control method and system based on a virtual impedance feedforward controller. BACKGROUND

[0002] In recent years, with the increasing penetration of distributed new energy such as wind power and photovoltaic power in the power system, large-scale new energy units are connected to the grid through power electronic devices. These connection methods have the advantages of flexible control and rapid response, but they also cause problems such as insufficient inertia and damping of the power grid, and weakened voltage and frequency support capability.

[0003] Currently, existing researches have proposed various schemes for the control of grid-connected converters, among which the most commonly used are power synchronization control (PSC) and virtual synchronous generator (VSG) control. The active power control of both of them simulates the power droop characteristic of traditional synchronous generators (SGs). In addition, the VSG control adds virtual inertia, which, together with energy storage, simulates the swing equation and inertial response of SGs. However, since the power loop of VSG control is regarded as a second-order system when the dynamic of grid inductance is ignored, it may cause system resonance when the damping coefficient is small. Therefore, VSG control also inherits the power synchronization and subsynchronous resonance problems of SGs, and its stability is relatively poor when facing complex and variable power grid transient changes. SUMMARY

[0004] The present application aims to provide a grid-connected converter control method and system based on a virtual impedance feedforward controller, which has high control accuracy, can suppress synchronous frequency resonance problems under weak damping, has fast response speed, good damping effect, and high stability.

[0005] The technical solution for achieving the present application is as follows: a grid-connected converter control method based on a virtual impedance feedforward controller, comprising the following steps:

[0006] Step 1: Establish a small-signal model considering coupling on a grid-connected converter based on virtual synchronous generator control.

[0007] Step 2: Analyze the small-signal model considering coupling. The active power loop, the reactive power loop, and the coupling function all contain the same conjugate poles, which will cause synchronous frequency resonance, and the coupling will exacerbate the synchronous frequency resonance.

[0008] Step 3: Design a feedforward controller containing virtual impedance to increase the system damping, and simplify the controller to suppress the unstable factors introduced by the controller.

[0009] Step 4: Consider the damping and bandwidth of the system to calculate the setting condition of the virtual impedance.

[0010] A networked converter control system based on a virtual impedance feedforward controller, which is used to implement the networked converter control method based on the virtual impedance feedforward controller, and comprises a model establishing module, a model analyzing module, a feedforward controller module and a calculation module, wherein:

[0011] The model establishing module establishes a small signal model considering coupling on the networked converter based on virtual synchronous generator control.

[0012] The model analyzing module analyzes the small signal model considering coupling, and the active power ring, the reactive power ring and the coupling function all contain the same conjugate pole, which will cause synchronous frequency resonance, and the coupling will aggravate the synchronous frequency resonance.

[0013] The feedforward controller module designs a feedforward controller containing virtual impedance to increase the system damping, and suppresses the unstable factors introduced by the controller through simplifying the controller.

[0014] The calculation module comprehensively considers the damping and bandwidth of the system to calculate the setting condition of the virtual impedance.

[0015] A mobile terminal comprises a memory, a processor and a computer program stored on the memory and executable on the processor, and the processor implements the networked converter control method based on the virtual impedance feedforward controller when executing the program.

[0016] A computer readable storage medium, which stores a computer program, and the program is executed by a processor to implement the steps in the networked converter control method based on the virtual impedance feedforward controller.

[0017] Compared with the prior art, the present application has the following advantages: (1) by establishing a small signal model considering coupling active and reactive power, compared with the power ring without considering coupling, the synchronous frequency resonance problem under weak damping can be suppressed; (2) by considering the small signal model with coupling, the accuracy of the networked converter control is improved; (3) by the virtual impedance feedforward controller, the response speed, damping effect and stability of the networked converter control are improved. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 is the main circuit and decoupling control block diagram of the networked converter control method based on the virtual impedance feedforward controller, wherein (a) is the main circuit and control diagram of the networked converter, and (b) is the decoupling control diagram.

[0019] Figure 2Figure 1 is a structure diagram of a small signal model with coupling in an embodiment of the present application, wherein (a) is a small signal model of active and reactive power loops with coupling, (b) is an active loop with coupling, and (c) is a reactive loop with coupling.

[0020] Figure 3 Figure 2 is a bode diagram of active and reactive power loops of a grid forming converter with and without coupling in an embodiment of the present application, wherein (a) is a bode diagram of an active loop, and (b) is a bode diagram of a reactive loop.

[0021] Figure 4 Figure 3 is a bode diagram of an active open loop transfer function of a controller of a grid forming converter with feedforward virtual impedance in an embodiment of the present application.

[0022] Figure 5 Figure 4 is a Nyquist diagram of an active open loop transfer function of a controller of a grid forming converter with feedforward virtual impedance in an embodiment of the present application.

[0023] Figure 6 Figure 5 is a bode diagram of an active closed loop with virtual resistance in an embodiment of the present application.

[0024] Figure 7 Figure 6 is a bode diagram of an influence of virtual resistance and virtual inductance on an open loop frequency response of an active loop in an embodiment of the present application, wherein (a) is a bode diagram of an open loop frequency response of different virtual resistances, and (b) is a bode diagram of an open loop frequency response of different virtual inductances.

[0025] Figure 8 Figure 7 is a simulation waveform diagram of active and reactive power synchronous frequency resonance in an embodiment of the present application.

[0026] Figure 9 Figure 8 is a simulation waveform diagram of active and reactive power synchronous frequency resonance after active and reactive power parameter changes in an embodiment of the present application.

[0027] Figure 10 Figure 9 is a simulation result waveform diagram of active, reactive, grid voltage and grid current using a conventional control method in an embodiment of the present application.

[0028] Figure 11 Figure 10 is a simulation result waveform diagram of output active, reactive, grid voltage and grid current using a control method of the present application in an embodiment of the present application.

[0029] Figure 12 Figure 11 is an experimental result waveform diagram of active, reactive, grid voltage and grid current using a conventional control method in an embodiment of the present application.

[0030] Figure 13 Figure 12 is an experimental result waveform diagram of output active, reactive, grid voltage and grid current using a control method of the present application in an embodiment of the present application. DETAILED DESCRIPTION

[0031] As shown in Figure 1 , the network-constructed converter of the application is composed of a DC voltage source, a three-phase half-bridge converter, a low-pass output LC filter, and a virtual synchronous generator control, a virtual impedance control, and a voltage and current double closed-loop control;

[0032] As shown in Figure 1 , a network-constructed converter control method based on a virtual impedance feedforward controller includes the following steps:

[0033] Step 1, on the network-constructed converter based on virtual synchronous generator control, a small-signal model considering coupling is established;

[0034] Step 2, analyzing the small-signal model considering coupling, it is found that the active power ring, the reactive power ring, and the coupling function all contain the same conjugate poles, which will cause synchronous frequency resonance, and the coupling will exacerbate the synchronous frequency resonance;

[0035] Step 3, a feedforward controller containing virtual impedance is designed to increase the system damping, and the controller introduced unstable factors are suppressed by simplifying the controller;

[0036] Step 4, considering the damping and bandwidth of the system, the setting condition of the virtual impedance is calculated.

[0037] As a specific example, in Step 1, on the network-constructed converter based on virtual synchronous generator control, a small-signal model considering coupling is established, which is as follows:

[0038] Step 1.1, calculate the grid PCC point power:

[0039] Set the grid voltage to be constant, calculate the instantaneous active power P e and the instantaneous reactive power Q e , give the initial electromotive force E0 an electromotive force disturbance ΔE to get the electromotive force E, give the initial power angle δ e0 a power angle disturbance Δδ e to get the power angle δ e , that is:

[0040] E = E0 + ΔE δ e = δ e0 + Δδ e

[0041] Linearize at the equilibrium point to get the VSG dynamic small-signal model:

[0042]

[0043] Where, G Pδ (s) is the relationship between active power and power angle, GPE (s) is the relationship between active power and inverter electromotive force, G Qδ (s) is the relationship between reactive power and power angle, G QE (s) is the relationship between reactive power and inverter electromotive force; ΔP e is the difference between actual output value of active power and rated power, ΔQ e is the difference between actual output value of reactive power and rated power;

[0044] Relationship between active power and power angle G Pδ (s), relationship between active power and inverter electromotive force G PE (s), relationship between reactive power and power angle G Qδ (s), relationship between reactive power and inverter electromotive force G QE (s), which is as follows:

[0045]

[0046] wherein R = R1 + R g , L = L1 + L g ; L1 and R1 represent filter inductance and corresponding parasitic resistance respectively, L g and R g are equivalent line inductance and resistance between inverter and power grid respectively.

[0047] In step 1.2, the relationship G1(s) between power angle and active power of VSG, the relationship G2(s) between voltage and reactive power, and the relationship G3(s) between frequency and active power are calculated as follows:

[0048]

[0049] wherein s is a complex variable in Laplace transform; J p , D p are virtual inertia and damping coefficient in virtual synchronous generator frequency control respectively, J q , D q are virtual inertia and voltage droop coefficient in virtual synchronous generator voltage control respectively, Δω e is the difference between actual angular frequency ω e and rated angular frequency ω n ;

[0050] In step 1.3, the expression of open-loop transfer function containing coupled small-signal model is obtained as follows:

[0051]

[0052] wherein G P_open(s) is the open-loop transfer function of the active power loop in VSG control, G Q_open (s) is the open-loop transfer function of the reactive power loop in VSG control, ΔP e is the difference between the actual output value and the rated value of the active power, ΔQ e is the difference between the actual output value and the rated value of the reactive power.

[0053] As a specific example, in step 2, the analysis considers the coupled small-signal model. Since the same conjugate poles are contained in the active power loop, the reactive power loop, and the coupling function, synchronous frequency resonance is caused, and the coupling exacerbates the synchronous frequency resonance, as follows:

[0054] Step 2.1, draw the bode diagram of the open-loop transfer function of the active power loop and the reactive power loop containing coupling, and calculate the negative damping brought by the coupling to the power loop;

[0055] Step 2.2, according to the poles of G Pδ (s), G PE (s), G Qδ (s), G QE (s), the same conjugate poles are caused to cause synchronous frequency resonance, and the conjugate poles are:

[0056]

[0057] wherein R is the sum of the filter resistance R1 and the line resistance R g , and L is the sum of the filter inductance L1 and the line inductance L g ;

[0058] Therefore, the coupling of the reactive power in the active power loop can make the originally stable system unstable, and the coupling of the active power in the reactive power loop can also make the originally stable system unstable.

[0059] As a specific example, in step 3, a feedforward controller containing a virtual impedance is designed to increase the damping of the system, and the controller is simplified to suppress the unstable factors introduced by the controller, as follows:

[0060] Step 3.1, design the feedforward controller G Eδ (s) and G δE (s) transfer functions as follows:

[0061] G Eδ (s) = -G PE (s) / G Pδ (s)

[0062] G δE (s) = -G Qδ (s) / G QE (s)

[0063] Step 3.2, the virtual impedance X in the virtual impedance control link vc is:

[0064] X vc = R vc + ω n L vc

[0065] wherein R vc is a virtual resistance, L vc is a virtual inductance;

[0066] Step 3.3, since the VSG is usually light load operation, the system power angle is very small, taking δ e0 <20°, sinδ e0 ≈0, cosδ e0 ≈1, then the simplified feedforward branch is:

[0067]

[0068] wherein G Eδ (s) and G δE (s) are feedforward controllers, U g is a grid voltage;

[0069] Step 3.4, since the introduced feedforward controllers G Eδ (s) and G δE (s) contain differential terms, which will introduce high-frequency noise, therefore low-pass filters are introduced at the outlet of G Eδ (s) and G δE (s) to attenuate the high-frequency noise signal output, the introduced low-pass filter is:

[0070]

[0071] wherein ω LPF is the cut-off frequency of the low-pass filter.

[0072] Then the controller after adding the virtual impedance and the low-pass filter is:

[0073]

[0074] wherein H LPF is a low-pass filter, and subscript 0 is a corresponding steady-state value.

[0075] As a specific example, in step 4, the setting condition of the virtual impedance is calculated by comprehensively considering the damping and bandwidth of the system, which is as follows:

[0076] Step 4.1, according to the bode diagram of the active power loop closed-loop transfer function, set the virtual resistance to ensure the bandwidth of the system;

[0077] Step 4.2, according to the active open-loop bode diagram of the added virtual resistance, setting the virtual resistance, while ensuring that the bandwidth of the system after introducing the virtual resistance, the damping of the system oscillation and the performance of the power ring are sufficient;

[0078] Step 4.3, according to the bode diagram of the added virtual inductance, setting the virtual inductance to reduce the resonance effect near the synchronous frequency.

[0079] The application also provides a networked converter control system based on a virtual impedance feedforward controller, which is used to implement the networked converter control method based on the virtual impedance feedforward controller.

[0080] The model establishing module establishes a small-signal model considering coupling on the networked converter based on the virtual synchronous generator control;

[0081] The model analyzing module analyzes the small-signal model considering coupling, and the active power ring, the reactive power ring and the coupling function all contain the same conjugate poles, which will cause the synchronous frequency resonance, and the coupling will aggravate the synchronous frequency resonance;

[0082] The feedforward controller module designs a feedforward controller containing virtual impedance to increase the system damping, and suppresses the unstable factors introduced by the controller through simplifying the controller;

[0083] The calculation module comprehensively considers the damping and bandwidth of the system, and calculates the setting condition of the virtual impedance.

[0084] A mobile terminal comprises a memory, a processor and a computer program stored on the memory and executable on the processor, and the processor implements the networked converter control method based on the virtual impedance feedforward controller when executing the program.

[0085] A computer readable storage medium has a computer program stored thereon, and the program is executed by a processor to implement the steps in the networked converter control method based on the virtual impedance feedforward controller.

[0086] The application will be further described in detail below in combination with the drawings and specific embodiments.

[0087] Embodiment

[0088] The embodiment provides a networked converter control method based on a virtual impedance feedforward controller. Figure 1The main circuit and decoupling control diagram of a grid-forming converter control method are shown. The voltage and current at the PCC point are obtained to calculate the power transmission to the virtual synchronous generator power control loop, output the virtual electromotive force, pass through the virtual impedance damping, input to the voltage and current double closed loop to generate PWM to control the inverter. The specific steps are as follows:

[0089] Step 1, a small signal model considering coupling is established on the grid-forming converter based on virtual synchronous generator control, as follows:

[0090] Step 1.1, according to Figure 1 The main circuit is shown, and the active power and reactive power of VSG at the PCC point are calculated:

[0091] E abc -U g_abc =(R+sL)i g_abc

[0092] E abc -e pcc_abc =(R1+sL1)i g_abc

[0093] In the main circuit diagram, V dc is the DC side voltage, E is the virtual internal potential of VSG, L1 and R1 represent the filter inductance and the corresponding parasitic resistance, L g and R g are the equivalent line inductance and resistance between the inverter and the grid, C f is the inverter output filter capacitance, e is the PCC point grid voltage, i g is the grid voltage and current, P ref and P e are the active power command value and the actual output active power of the inverter, Q ref and Q e are the reactive power command value and the actual output reactive power, i L is the current flowing through the filter, δ e is the phase angle difference between the grid voltage and the grid voltage, u od , u oq are the modulation wave voltages sent to the PWM link; the subscripts d and q are the corresponding dq axis components, and the subscripts a, b and c are the values of the corresponding three-phase coordinate system.

[0094] The grid voltage is set to remain constant, and a small disturbance is given to the electromotive force and power angle, that is:

[0095] E=E0+ΔEδ e =δ e0 +Δδ

[0096] The linearization at the equilibrium point can obtain the small-signal model of VSG as follows:

[0097]

[0098] where G Pδ (s), G PE (s), G Qδ (s) and G QE (s) are the relationships between active power, reactive power and power angle, inverter electromotive force, respectively, and the calculation formula is as follows:

[0099]

[0100] where R = R1+ R g , L = L1+ L g , and subscript 0 is the steady-state value corresponding to the stable state of the system;

[0101] Step 1.2, the voltage sag generator VSG power control is mainly divided into active ring and reactive ring, wherein the active ring simulates the damping characteristics and inertia of the synchronous generator through the rotor motion equation, and the specific expression is as follows:

[0102] P ref -P e -D p (ω e -ω n ) = J p ω n s(ω e -ω n )

[0103] where J p and D p are the virtual moment of inertia and damping coefficient of VSG, respectively, and ω n and ω e are the rated and actual output angular frequency of VSG, respectively;

[0104] The reactive ring adopts Q-E droop control, which simulates the voltage regulation characteristics of the synchronous generator, and the specific expression is as follows:

[0105] Q ref -Q e -D q (E-U n ) = J q sE

[0106] where J q and D q are the excitation coefficient and voltage droop coefficient, and U n is the rated voltage effective value of VSG;

[0107] Linearizing the active and reactive power loops in the complex frequency domain at their steady-state operating points yields the following relationships between the VSG's power angle and active power, voltage and reactive power, and frequency and active power:

[0108]

[0109] Where, ΔP e ΔQ e , Δω e This is the difference between the actual output values ​​of active power, reactive power, and angular frequency, and the rated power.

[0110] Step 1.3, G in the active and reactive power loops Pδ (s), G PE (s), G Qδ (s), G QE (s) contain the same conjugate poles, that is:

[0111]

[0112] according to Figure 2 The active and reactive multi-input multi-output coupled small-signal models in (a) are simplified as follows: Figure 2 In the diagram, (b) and (c) represent the coupled single-input single-output models for active and reactive power, respectively, where the feedback active power is P introduced by the power angle. δ And voltage introduction includes reactive coupling term P e It consists of two parts:

[0113]

[0114] The open-loop transfer function of the coupled active power loop is:

[0115] G P_open (s)=G1(s)G P_δ (s)

[0116] Similarly, the open-loop transfer function of reactive power with coupling is:

[0117]

[0118] Step 2: Analyze the small-signal model considering coupling. Since the active and reactive power loops and the coupling function all contain the same conjugate poles, this will cause synchronous frequency resonance, and coupling will exacerbate synchronous frequency resonance, as detailed below:

[0119] The analysis compares the Bode plots of the open-loop transfer functions of active and reactive power with those of uncoupled active and reactive power to reveal the influence of coupling on synchronous frequency resonance. The parameters are shown in Table 1. Figure 3As shown in (a) and (b) in the figure, a resonance peak over 0 dB and a phase lag of -180° are generated near 314 rad / s, which makes the control bandwidth of the VSG system have to be limited within 50 Hz, and the coupling of active and reactive in the active loop may make the originally stable system unstable, and the same goes for the reactive loop.

[0120] Step 3, design a feedforward controller containing virtual impedance to increase the damping of the system and suppress the instability factors introduced by the controller by simplifying the controller, as follows:

[0121] Step 3.1, design a feedforward controller G Eδ (s) and G δE (s) transfer function as follows:

[0122] G Eδ (s) = -G PE (s) / G Pδ (s)

[0123] G δE (s) = -G Qδ (s) / G QE (s)

[0124]

[0125] Since the introduced feedforward controller G δE (s) will change the right half plane zero point of G QE (s) into a right half plane pole in G δE (s), which will cause the feedforward controller to be unstable, so the controller needs to be redesigned;

[0126] Step 3.2, the virtual impedance X vc in the virtual impedance control link is:

[0127] X vc = R vc + ω n L vc

[0128] Where R vc is the virtual resistance, and L vc is the virtual inductance;

[0129] Step 3.3, in power systems with higher voltage levels, VSGs are usually operated at light load, so the power angle of the system is very small, in order to simplify the design, take δ e0 < 20°, sinδ e0 ≈ 0, cosδ e0 ≈ 1, then the simplified feedforward branch is:

[0130]

[0131] Step 3.4, due to the introduction of the feedforward controller G Eδ (s) and G δE (s) contains a differential term, which will introduce high frequency noise, so a low pass filter is introduced at the output of G Eδ (s) and G δE (s) to attenuate the high frequency noise signal output, the introduced low pass filter is:

[0132]

[0133] where ω LPF is the cut-off frequency of the low pass filter.

[0134] The controller after adding the virtual impedance and low pass filter is:

[0135]

[0136] Step 4, considering the damping and bandwidth of the system, the setting condition of the virtual impedance is calculated, which is as follows:

[0137] Step 4.1, Figure 6 The active power ring closed-loop Bode diagram is shown in (a), and the parameters are shown in Table 1, where J p = 0.001, D p = 1.

[0138] Table 1 System parameters

[0139]

[0140] When R vc is set from 0 p.u to 0.5 p.u, it is increased by 0.05 p.u each time; the system bandwidth is reduced from 313 rad / s to 143 rad / s, and the virtual resistance cannot be set too large, otherwise the system bandwidth will be significantly reduced. Considering comprehensively, the virtual resistance is usually taken within 0.2 p.u to ensure the bandwidth of the system.

[0141] Step 4.2, Figure 7 The active open-loop Bode diagram after adding the virtual resistance is shown in (a), and the parameters are also shown in Table 1, where J p = 0.001, D p = 1, R vc is set from 0 p.u to 0.2 p.u (increased by 0.04 p.u each time), as shown in Figure 7As shown in (a), a virtual resistor that is too small has poor damping effect, while a virtual resistor that is too large will cause low-frequency phase lag in the system, affecting the power loop performance. Therefore, in order to simultaneously ensure the system bandwidth, sufficient damping during system oscillation, and power loop performance after introducing a virtual resistor, according to... Figure 7 As shown in (a), the virtual resistor is set between 0.04 and 0.08 pu, and is finally set to 0.068 pu.

[0142] Step 4.3, as follows Figure 7 Figure (b) shows the Bode plot with a virtual inductor added. The virtual inductance increases from 0 pu to 0.5 pu, increasing by 0.1 pu each time. It can be seen that although the added virtual inductance has some influence on the resonant peak near the synchronization frequency, the influence is very small, and it has almost no influence on the resonant peak at the synchronization frequency. Therefore, the virtual inductance is ignored, and only a virtual resistance is set.

[0143] To verify the superiority of the control method provided by this invention, Bode plots with coupling, virtual impedance, and feedforward virtual impedance were drawn according to the parameters in Table 1. The Nyquist plots are shown below. Figure 4 , Figure 5 As shown in Table 1, simulation analysis was performed using MATLAB / Simulink, and the system parameters are shown in Table 1. The simulation results are compared below. Figure 8 , Figure 9 , Figure 10 , Figure 11 As shown, the experimental results are for example... Figure 12 , Figure 13 As shown.

[0144] First, based on the parameters in Table 1, the VSG small-signal model is simulated to verify the oscillation phenomenon. P is set... ref For 10kW, Q ref The value is 0Var. After the system startup power stabilizes, at t=2s, the system parameters switch to unstable parameters: J p =0.001, D p =1. The system output is as follows: Figure 8 As shown in the figure, both the output active power and reactive power oscillate synchronously at a frequency of approximately 49Hz.

[0145] Secondly, after restarting the system and waiting for the power to stabilize, set the system parameters to switch to unstable parameters at t=2s: J p =0.001, D p =1. When t=3s, the system parameter switches to the critical parameter: J p =0.013, D p =5. For example Figure 9As shown, the system is converted from synchronous frequency resonance phenomenon to stability. Therefore, due to unreasonable setting of system parameters, synchronous frequency resonance is easily caused.

[0146] As shown in the figure, Figure 10 when only virtual impedance is used to suppress synchronous frequency resonance, the virtual impedance is set as virtual resistance R vc = 0.068 p.u. When the system is stable, when t = 2s, the system parameters are switched to J p = 0.001, D p = 1. When t = 3s, the virtual impedance control strategy is introduced, and because the virtual resistance can effectively suppress the synchronous frequency resonance under weak damping, the system gradually stabilizes.

[0147] When the virtual impedance feedforward strategy is used, when t = 2s, the system parameters are switched to J p = 0.001, D p = 1. When t = 3s, the virtual resistance R vc = 0.068 p.u. is introduced. As shown in the figure, Figure 11 it can be seen from Figure 11 that the active power and reactive power oscillation are obviously suppressed when the system oscillates at 2s-3s by the feedforward control. When t = 3s, the virtual impedance is introduced, which also effectively suppresses the synchronous frequency resonance, and the system is more stable than when only the virtual resistance of the same size is introduced. It is proved that the virtual impedance feedforward strategy has a better damping effect on the synchronous frequency resonance in the weak damping system.

[0148] In order to further verify the effectiveness of the method for suppressing synchronous frequency resonance, an RT-LAB hardware-in-the-loop platform is built in the laboratory for further verification.

[0149] As shown in the figure, Figure 12 when t = 2s, the system parameters are switched to unstable parameters: Jp = 0.001, Dp = 1. When t = 3s, the virtual impedance Rvc = 0.068 p.u. is introduced, and the system gradually stabilizes due to the damping effect of the virtual resistance.

[0150] As shown in the figure, Figure 13 t = 3s, the virtual impedance is introduced, which also effectively suppresses the synchronous frequency resonance, and the system is more stable than when only the virtual resistance of the same size is introduced.

[0151] The above is only a preferred embodiment of the present application, and it should be noted that for ordinary skilled in the art, without departing from the principles of the present application, a number of improvements and refinements can be made, and these improvements and refinements should be considered as the protection scope of the present application.

Claims

1. A grid-forming converter control method based on a virtual impedance feedforward controller, characterized in that, The method comprises the following steps: Step 1, establishing a small-signal model considering coupling on a grid-forming converter based on virtual synchronous generator control; Step 2, analyzing the small-signal model considering coupling, wherein the active power loop, the reactive power loop and the coupling function all contain the same conjugate poles, which will cause synchronous frequency resonance, and the coupling will exacerbate the synchronous frequency resonance; Step 3, designing a feedforward controller containing a virtual impedance to increase system damping, and suppressing the unstable factors introduced by the controller through simplifying the controller; Step 4, comprehensively considering the damping and bandwidth of the system to calculate the setting condition of the virtual impedance; The feedforward controller containing a virtual impedance to increase system damping and suppress the unstable factors introduced by the controller through simplifying the controller is designed in step 3, and the specific process is as follows: Step 3.1, Designing the feedforward controller G Eδ (s) and G δE The transfer function of (s) is as follows: ； where s is a complex variable in Laplace transform; for the active power versus the power angle, for the active power versus the inverter electromotive force, for the reactive power versus the power angle, for the reactive power versus the inverter electromotive force; Step 3.2 Virtual impedance X in the virtual impedance control loop vc is: ； where R vc is a virtual resistance, L vc is a virtual inductance; ω n denotes the nominal angular frequency; Step 3.3, VSG light load, so take the initial power angle δ e0 <20°, sin δ e0 ≈0, cos δ e0 ≈1, simplify the feedforward branch as: ； where G Eδ (s) and G δE (s) are feed forward controllers, U g is the grid voltage; represents the initial electromotive force; R = R1+R g , L = L1+L g ; L g and R g are the equivalent line inductance and resistance between the inverter and the grid, respectively; L1and R1represent the filter inductance and the corresponding parasitic resistance, respectively. Step 3.4, due to the introduction of the feedforward controller G Eδ (s) and G δE (s) contains a differential term, which introduces high frequency noise, so at the output of G Eδ (s) and G δE (s) a low pass filter is introduced to attenuate the high frequency noise signal at the output of G ； where ω LPF is the cutoff frequency of the low-pass filter; The controller after adding the virtual impedance and the low-pass filter is: ； where H LPF is a low-pass filter with subscript 0 corresponding to the steady-state value.

2. The grid-forming converter control method based on a virtual impedance feedforward controller of claim 1, wherein, The grid-forming converter comprises a direct-current voltage source, a three-phase half-bridge converter and a low-pass output LC filter, and further comprises a virtual synchronous generator control module, a virtual impedance control module and a voltage-current double closed-loop control module.

3. The grid-forming converter control method based on a virtual impedance feedforward controller of claim 1, wherein, The small-signal model considering coupling on the grid-forming converter based on virtual synchronous generator control is established in step 1, and the specific process is as follows: Step 1.1, calculating the power at the grid PCC point: The grid voltage is set constant, the instantaneous active power P is calculated e and the instantaneous reactive power Q e , the initial electromotive force E is given An electromotive force disturbance The electromotive force E is obtained, the initial power angle δ is given A power angle disturbance The power angle δ is obtained i.e.: ； Linearization is performed at the equilibrium point, and the VSG dynamic small-signal model is obtained as follows: ； wherein P is the active power, E is the inverter electromotive force, Q is the reactive power, E is the inverter electromotive force; ΔP e is the difference between the actual output value of the active power and the rated power, ΔQ e is the difference between the actual output value of the reactive power and the rated power. Step 1.

2. Calculate the relationship between the power angle and the active power of the VSG The relationship between the voltage and the reactive power The relationship between the frequency and the active power As follows: ； where s is a complex variable in Laplace transform; J p , D p are virtual inertia and damping coefficients in virtual synchronous generator frequency control, respectively q , D q are virtual inertia and voltage droop coefficients in virtual synchronous generator voltage control, respectively e is the difference between actual angular frequency ω e and rated angular frequency ω n ​ Step 1.3, the expression of the open-loop transfer function of the small-signal model containing coupling is obtained as follows: ； ； where G P_open (s) is the open loop transfer function of the active power loop in VSG control, G Q_open (s) is the open loop transfer function of the reactive power loop in VSG control, ΔP e is the difference between the actual output value of active power and the rated power, ΔQ e is the difference between the actual output value of reactive power and the rated power.

4. The grid-forming converter control method based on a virtual impedance feedforward controller of claim 3, wherein, Relationship of active power to power angle Relationship of active power to inverter electromotive force Relationship of reactive power to power angle Relationship of reactive power to inverter electromotive force In particular, as follows: ； where R = R1+ R g , L = L1+ L g ; L1and R1represent the filter inductance and the corresponding parasitic resistance, respectively, L g and R g are the equivalent line inductance and resistance between the inverter and the grid.

5. The grid-forming converter control method based on a virtual impedance feedforward controller of claim 4, wherein, The small-signal model considering coupling is analyzed in step 2, wherein the active power loop, the reactive power loop and the coupling function all contain the same conjugate poles, which will cause synchronous frequency resonance, and the coupling will exacerbate the synchronous frequency resonance, and the specific process is as follows: Step 2.1, drawing the bode diagram of the active and reactive open-loop transfer functions containing coupling, and calculating the negative damping brought by the coupling to the power loop; Step 2.2, according to the poles of the active and reactive rings , , , with the same conjugate poles that cause the synchronous frequency resonance, the conjugate poles are: ； Therefore, the coupling of the reactive power in the active loop may make the originally stable system unstable, and the coupling of the active power in the reactive loop may also make the originally stable system unstable.

6. The grid-forming converter control method based on a virtual impedance feedforward controller of claim 5, wherein, The setting condition of the virtual impedance is calculated in step 4 by comprehensively considering the damping and bandwidth of the system, and the specific process is as follows: Step 4.1, setting the virtual resistance according to the bode diagram of the active power loop closed-loop transfer function to ensure the bandwidth of the system; Step 4.2, setting the virtual resistance according to the active open-loop bode diagram after adding the virtual resistance, while ensuring the bandwidth of the system, the damping of the system when oscillating and the performance of the power loop after adding the virtual resistance; Step 4.3, setting the virtual inductance according to the bode diagram after adding the virtual inductance to reduce the influence of synchronous frequency resonance.

7. A grid-forming converter control system based on a virtual impedance feedforward controller, characterized in that, The system is used to implement the grid-forming converter control method based on the virtual impedance feedforward controller in any one of claims 1-6, and the system comprises a model establishing module, a model analyzing module, a feedforward controller module and a calculating module, wherein: The model establishing module establishes a small-signal model considering coupling on a grid-forming converter based on virtual synchronous generator control; The model analyzing module analyzes the small-signal model considering coupling, wherein the active power loop, the reactive power loop and the coupling function all contain the same conjugate poles, which will cause synchronous frequency resonance, and the coupling will exacerbate the synchronous frequency resonance; The feedforward controller module is designed to increase system damping by adding a virtual impedance and to suppress the instability caused by the controller by simplifying the controller; The calculation module calculates the setting condition of the virtual impedance by comprehensively considering the damping and bandwidth of the system.

8. A mobile terminal comprising a memory, a processor, and a computer program stored on the memory and executable on the processor, characterized in that, The processor implements the network configuration type converter control method based on the virtual impedance feedforward controller according to any one of claims 1-6 when executing the program.

9. A computer-readable storage medium having stored thereon a computer program, characterized in that, The processor implements the steps in the network configuration type converter control method based on the virtual impedance feedforward controller according to any one of claims 1-6 when executing the program.

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