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

Abstract

The invention discloses a network construction type converter control method and system based on a virtual impedance feedforward controller, and the converter comprises a DC voltage source, a three-phase half-bridge converter, a low-pass output LC filter, a virtual synchronous generator control module, a virtual impedance control module, and a voltage and current double-closed-loop control module. The method specifically comprises the following steps: establishing a small signal model considering coupling on a network construction type converter based on virtual synchronous generator control; a small signal model for coupling is analyzed and considered, an active power loop, a reactive power loop and a coupling function all contain the same conjugate pole, synchronous frequency resonance can be caused, and coupling can intensify the synchronous frequency resonance; a feedforward controller containing virtual impedance is designed to increase system damping, and unstable factors introduced by the controller are inhibited by simplifying the controller; the damping and bandwidth of the system are comprehensively considered, and the setting condition of the virtual impedance is calculated. The device can restrain synchronous frequency resonance and has the advantages of being high in response speed and good in damping effect.

Description

Technical Field

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

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

[0003] At present, existing research has proposed a variety of solutions for the control of grid-connected converters, among which the most commonly used are power synchronization control (PSC) and virtual synchronous generator (VSG) control. Their active power control simulates the power droop characteristics of traditional synchronous generators (SG). In addition, virtual synchronous generator control adds virtual inertia and cooperates with energy storage to simulate the swing equation and inertial response of SG. However, since the power loop of VSG control is regarded as a second-order system when the dynamics of the grid inductance are ignored, system resonance may be caused when its damping coefficient is small. Therefore, VSG control also inherits the power synchronization and subsynchronous resonance problems of SG, and its stability is relatively poor when facing complex and changeable grid transient changes. Summary of the invention

[0004] The object of the present invention is to provide a control method and system for a grid-type converter 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 to achieve the purpose of the present invention is: a control method for a grid-connected converter 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, reactive power loop and coupling function all contain the same conjugate poles, which will cause synchronous frequency resonance, and coupling will aggravate synchronous frequency resonance.

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

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

[0010] A grid-type converter control system based on a virtual impedance feedforward controller, the system is used to implement the grid-type converter control method based on the virtual impedance feedforward controller, the system includes a model building module, a model analysis module, a feedforward controller module and a calculation module, wherein:

[0011] Model building module, which builds a small signal model considering coupling on the grid-connected converter based on virtual synchronous generator control;

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

[0013] Feedforward controller module, designing a feedforward controller with virtual impedance to increase system damping, and suppressing the unstable factors introduced by the controller by simplifying the controller;

[0014] The calculation module comprehensively considers the damping and bandwidth of the system and calculates the setting conditions of the virtual impedance.

[0015] A mobile terminal comprises a memory, a processor and a computer program stored in the memory and executable on the processor. When the processor executes the program, the control method of a grid-connected converter based on a virtual impedance feedforward controller is implemented.

[0016] A computer-readable storage medium stores a computer program, which, when executed by a processor, implements the steps in the control method of a grid-type converter based on a virtual impedance feedforward controller.

[0017] Compared with the prior art, the present invention has the following significant advantages: (1) by establishing a small signal model that takes into account coupled active and reactive power, the synchronous frequency resonance problem under weak damping can be suppressed compared to a power loop that does not consider coupling; (2) by considering the coupled small signal model, the control accuracy of the grid-type converter is improved; (3) by using a virtual impedance feedforward controller, the response speed, damping effect and stability of the grid-type converter control are improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a main circuit and decoupling control block diagram of a grid-type converter control method based on a virtual impedance feedforward controller of the present invention, wherein (a) is the main circuit of the grid-type converter and its control diagram, and (b) is the decoupling control diagram.

[0019] Figure 2Schematic diagram of the structure of the coupled small signal model in the embodiment of the present invention, 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 1 is a Bode diagram comparing the active and reactive power loops of the grid-type converter with and without coupling in an embodiment of the present invention, wherein (a) is the Bode diagram of the active loop and (b) is the Bode diagram of the reactive loop.

[0021] Figure 4 It is a Bode diagram of the active open-loop transfer function of the controller of the grid-connected converter with feedforward virtual impedance added in the embodiment of the present invention.

[0022] Figure 5 It is a Nyquist diagram of the active open-loop transfer function of the controller of the grid-connected converter with feedforward virtual impedance added in the embodiment of the present invention.

[0023] Figure 6 It is an active closed-loop Bode diagram with virtual resistance added in the embodiment of the present invention.

[0024] Figure 7 1 is a Bode diagram of the influence of virtual resistance and virtual inductance on the open-loop frequency response of the active loop in an embodiment of the present invention, wherein (a) is a Bode diagram of the open-loop frequency response of different virtual resistances, and (b) is a Bode diagram of the open-loop frequency response of different virtual inductances.

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

[0026] Fig. 9 It is a simulation waveform diagram of the synchronous frequency resonance after the active and reactive power parameters change in the embodiment of the present invention.

[0027] Fig.10 It is a waveform diagram of simulation results of active power, reactive power, grid voltage and grid current obtained by using a conventional control method in an embodiment of the present invention.

[0028] Fig.11 It is a waveform diagram of simulation results of output active power, reactive power, grid voltage and grid current obtained by using the control method of the present invention in an embodiment of the present invention.

[0029] Fig.12 It is a waveform diagram of the experimental results of active power, reactive power, grid voltage and grid current obtained by using a conventional control method in an embodiment of the present invention.

[0030] Fig.13 It is a waveform diagram of the experimental results of output active power, reactive power, grid voltage and grid current obtained by using the control method of the present invention in an embodiment of the present invention. DETAILED DESCRIPTION

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

[0032] like Figure 1 As shown, a control method for a grid-connected converter based on a virtual impedance feedforward controller comprises the following steps:

[0033] Step 1: Establish a small signal model considering coupling on a grid-connected converter based on virtual synchronous generator control;

[0034] Step 2: Analyze the small signal model considering coupling. The active power loop, reactive power loop and coupling function all contain the same conjugate poles, which will cause synchronous frequency resonance, and coupling will aggravate synchronous frequency resonance.

[0035] Step 3: Design a feedforward controller with virtual impedance to increase system damping, and suppress the unstable factors introduced by the controller by simplifying the controller;

[0036] Step 4: Consider the damping and bandwidth of the system and calculate the setting conditions of the virtual impedance.

[0037] As a specific example, in step 1, a small signal model considering coupling is established on a grid-connected converter based on virtual synchronous generator control, as follows:

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

[0039] Assume that the grid voltage remains constant and calculate the instantaneous active power P e and instantaneous reactive power Q e , give the initial electromotive force E0 an electromotive force disturbance ΔE to obtain the electromotive force E, give the initial power angle δ e0 A power angle disturbance Δδ e Get the power angle δ e ,Right now:

[0040] E=E0+ΔEδ e =δ e0 +Δδ e

[0041] Linearization is performed at the equilibrium point, and the VSG dynamic small signal model is obtained as follows:

[0042]

[0043] Among them, 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 The difference between the actual output value of active power and the rated power, ΔQ e It is the difference between the actual output value of reactive power and the rated power;

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

[0045]

[0046] Where R = R1 + R g , L=L1+L g ; L1 and R1 represent 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, respectively.

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

[0048]

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

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

[0051]

[0052] Among them, 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 The difference between the actual output value of active power and the rated power, ΔQ e It is the difference between the actual reactive power output value and the rated power.

[0053] As a specific example, in step 2, the coupled small signal model is analyzed. Since the active power loop, reactive power loop and coupling function all contain the same conjugate poles, synchronous frequency resonance will be caused, and coupling will aggravate synchronous frequency resonance, as follows:

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

[0055] Step 2.2: According to the G in the active loop and the reactive loop Pδ (s), G PE (s), G Qδ (s), G QE The poles of (s) have the same conjugate poles and cause synchronous frequency resonance. The conjugate poles are:

[0056]

[0057] Among them, R is the filter resistor R1 and the line resistor R g The sum of the filter inductance L1 and the line inductance L g sum;

[0058] Therefore, the coupling of reactive power in the active loop may cause the originally stable system to become unstable, and the coupling of active power in the reactive loop may also cause the originally stable system to become unstable.

[0059] As a specific example, in step 3, a feedforward controller with virtual impedance is designed to increase system damping, and the unstable factors introduced by the controller are suppressed by simplifying the controller, as follows:

[0060] Step 3.1: Design the feedforward controller G Eδ (s) and G δE (s) The transfer function is 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: Virtual impedance X in virtual impedance control link vc for:

[0064] X vc =R vc +ω n L vc

[0065] Among them, R vc is the virtual resistance, L vc is the virtual inductor;

[0066] Step 3.3: Since VSG is usually light-loaded and the system power angle is small, take δ e0 <20°,sinδ e0 ≈0, cosδ e0 ≈1, then the simplified feedforward branch is:

[0067]

[0068] Among them, G Eδ (s) and G δE (s) is the feedforward controller, U g is the grid voltage;

[0069] Step 3.4: Due to the introduction of the feedforward controller G Eδ (s) and G δE (s) contains differential terms, which will introduce high-frequency clutter, so in G Eδ (s) and G δE (s) A low-pass filter is introduced at the exit to attenuate the output high-frequency clutter signal. The introduced low-pass filter is:

[0070]

[0071] Among them, ω LPF is the cutoff frequency of the low-pass filter.

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

[0073]

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

[0075] As a specific example, in step 4, the setting conditions of the virtual impedance are calculated by comprehensively considering the damping and bandwidth of the system, 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 with virtual resistance added, set the virtual resistance, and at the same time ensure the bandwidth of the system after the introduction of the virtual resistance, sufficient damping during system oscillation, and the performance of the power loop;

[0078] Step 4.3: According to the Bode diagram with virtual inductance added, set the virtual inductance to reduce the resonance effect near the synchronization frequency.

[0079] The present invention also provides a grid-type converter control system based on a virtual impedance feedforward controller, which is used to implement the grid-type converter control method based on the virtual impedance feedforward controller. The system includes a model building module, a model analysis module, a feedforward controller module and a calculation module, wherein:

[0080] Model building module, which builds a small signal model considering coupling on the grid-connected converter based on virtual synchronous generator control;

[0081] Model analysis module, which analyzes the small signal model considering coupling. The active power loop, reactive power loop and coupling function all contain the same conjugate poles, which will cause synchronous frequency resonance, and coupling will aggravate synchronous frequency resonance.

[0082] Feedforward controller module, designing a feedforward controller with virtual impedance to increase system damping, and suppressing the unstable factors introduced by the controller by simplifying the controller;

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

[0084] A mobile terminal comprises a memory, a processor and a computer program stored in the memory and executable on the processor. When the processor executes the program, the control method of a grid-connected converter based on a virtual impedance feedforward controller is implemented.

[0085] A computer-readable storage medium stores a computer program, which, when executed by a processor, implements the steps in the control method of a grid-type converter based on a virtual impedance feedforward controller.

[0086] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0087] Example

[0088] This embodiment provides a control method for a grid-connected converter based on a virtual impedance feedforward controller. Figure 1As shown in the figure, it is the main circuit and decoupling control diagram of a grid-type converter control method. The voltage and current are obtained through the PCC point to calculate the power and transmit it to the virtual synchronous generator power control loop, output the virtual electromotive force, and after the virtual impedance damping, input it into 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-connected converter based on virtual synchronous generator control, as follows:

[0090] Step 1.1, according to Figure 1 For the main circuit shown in the figure, calculate the active power and reactive power of VSG at the PCC point:

[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 inductor and the corresponding parasitic resistance, L g and R g is the equivalent line inductance and resistance between the inverter and the grid, C f is the inverter output filter capacitor, e is the grid voltage at PCC point, i g is the grid voltage and current, P ref and P e are the active power command value and the active power actually output by 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-connected inverter output voltage and the grid voltage, u od ,u oq is the modulated wave voltage sent to the PWM link; the subscripts d and q are the corresponding dq axis components respectively, and the subscripts a, b, 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] Linearization is performed at the equilibrium point, and the VSG dynamic small signal model is obtained as follows:

[0097]

[0098] Among them, G Pδ (s), G PE (s), G Qδ (s), G QE (s) are the relationship between active power, reactive power, power angle and inverter electromotive force, respectively. The calculation formula is:

[0099]

[0100] Where R = R1 + R g ,L=L1+L g , subscript 0 is the steady-state value corresponding to the system stability;

[0101] Step 1.2: The power control of the voltage sag generator (VSG) is mainly divided into an active loop and a reactive loop. The active loop simulates the damping characteristics and inertia of the synchronous generator through the rotor motion equation. 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] In the formula, J p and D p are the virtual moment of inertia and damping coefficient of VSG respectively; ω n and ω e are the rated and actual output angular frequencies of VSG respectively;

[0104] The reactive loop adopts QE droop control by simulating the voltage regulation characteristics of the synchronous generator. The specific expression is as follows:

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

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

[0107] The active loop and reactive loop are linearized in the complex frequency domain at the steady-state operating point, and the relationship between the power angle and active power, voltage and reactive power, and frequency and active power of the VSG is as follows:

[0108]

[0109] Among them, ΔP e , ΔQ e , Δω e It is the difference between the actual output value of active power, reactive power and angular frequency and the rated power;

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

[0111]

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

[0113]

[0114] Then the open-loop transfer function of the active loop with coupling is:

[0115] G P_open (s)=G1(s)G P_δ (s)

[0116] Similarly, the reactive open-loop transfer function 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, synchronous frequency resonance will be caused, and coupling will aggravate synchronous frequency resonance, as follows:

[0119] The Bode diagram of the active power and reactive power open-loop transfer function is analyzed and compared with the uncoupled active power and reactive power open-loop transfer function 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 exceeding 0 dB and a phase lag of -180° are generated near 314 rad / s. This phase lag requires the control bandwidth of the VSG system to be limited to within 50 Hz. Therefore, the coupling of reactive power in the active loop may cause the originally stable system to become unstable, and the same is true for the reactive loop.

[0120] Step 3: Design a feedforward controller with virtual impedance to increase system damping and suppress the unstable factors introduced by the controller by simplifying the controller, as follows:

[0121] Step 3.1: Design the feedforward controller G Eδ (s) and G δE (s) The transfer function is as follows:

[0122] G Eδ (s)=-G PE (s) / G Pδ (s)

[0123] G δE (s)=-G Qδ (s) / G QE (s)

[0124]

[0125] Due to the introduction of the feedforward controller G δE (s)G QE The right half plane zero of the complex plane in (s) becomes G δE The right half plane poles in (s) will cause the feedforward controller to be unstable, so the controller needs to be redesigned;

[0126] Step 3.2: Virtual impedance X in virtual impedance control link vc for:

[0127] X vc =R vc +ω n L vc

[0128] Among them, R vc is the virtual resistance, L vc is the virtual inductor;

[0129] Step 3.3: In a power system with a higher voltage level, VSG usually operates under light load, so the system power angle is very small. 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 differential terms, which will introduce high-frequency clutter, so in G Eδ (s) and G δE (s) A low-pass filter is introduced at the exit to attenuate the output high-frequency clutter signal. The introduced low-pass filter is:

[0132]

[0133] Among them, ω LPF is the cutoff frequency of the low-pass filter.

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

[0135]

[0136] Step 4: Considering the damping and bandwidth of the system, calculate the setting conditions of the virtual impedance, as follows:

[0137] Step 4.1 Figure 6 The Bode diagram of the active power loop closed-loop transfer function is shown in Table 1. p =0.001, D p =1;

[0138] Table 1 System parameters

[0139]

[0140] When setting R vc When it increases from 0p.u to 0.5pu, it increases by 0.05pu each time; the system bandwidth decreases from 313rad / s to 143rad / s. When setting the virtual resistance, it cannot be set too large, otherwise the system bandwidth will be significantly reduced. Taking all factors into consideration, the virtual resistance is usually set within 0.2pu to ensure the system bandwidth.

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

[0142] Step 4.3: Figure 7 (b) in the figure shows the Bode diagram with virtual inductance added. The virtual inductance increases from 0pu to 0.5pu, with an increase of 0.1pu each time. It can be seen that although the added virtual inductance has a certain influence on the resonance peak near the synchronization frequency, the influence is very small, and it has almost no influence on the resonance peak at the synchronization frequency. Therefore, the virtual inductance is ignored and only the virtual resistance is set.

[0143] In order to verify the superiority of the control method provided by the present invention, the bode diagram containing coupling, virtual impedance and feedforward virtual impedance is drawn according to the parameters in Table 1. The Nyquist diagram is as follows: Figure 4 , Figure 5 The simulation analysis was carried out in MATLAB / simulink, and the system parameters are shown in Table 1. The simulation results are compared as follows Figure 8 , Fig. 9 , Fig.10 , Fig.11 As shown in the figure, the experimental results are compared with Fig.12 , Fig.13 shown.

[0144] First, according to the parameters in Table 1, the VSG small signal model is simulated to verify the oscillation phenomenon. ref For 10kW, Q ref After the system starts and the power becomes stable, 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, it can be seen that both the output active power and reactive power undergo synchronous frequency oscillation, with a frequency of about 49Hz.

[0145] Secondly, restart the system and wait for the power to stabilize. Set the system parameters to unstable parameters when t=2s: J p =0.001, D p = 1. When t = 3s, the system parameters switch to critical parameters: J p =0.013, D p =5. Fig. 9As shown in the figure, the system changes from synchronous frequency resonance to stability. Therefore, due to unreasonable setting of system parameters, synchronous frequency resonance is very likely to occur.

[0146] like Fig.10 As shown in the figure, when only virtual impedance is used to suppress synchronous frequency resonance, the virtual impedance is set to a virtual resistor R vc =0.068pu. When the system is stable at 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. Since 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 adopted, when t = 2s, the system parameters are switched to J p =0.001, D p =1. When t=3s, a virtual resistor R is introduced vc =0.068pu. Fig.11 As shown. Fig.11 It can be seen that the active power and reactive power oscillations are significantly suppressed due to feedforward control when the system oscillates at 2s-3s. The introduction of virtual impedance at t=3s also effectively suppresses the synchronous frequency resonance, and the system stable time is more stable than when only the virtual resistance of the same size is introduced. This shows that the virtual impedance feedforward strategy has a better damping effect on the synchronous frequency resonance in a weakly damped system.

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

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

[0150] like Fig.13 As shown, the introduction of virtual impedance at t=3s also effectively suppresses the synchronous frequency resonance, and the system stable time is more stable than when only the virtual resistance of the same size is introduced.

[0151] The above are only preferred embodiments of the present invention. It should be pointed out that, for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A control method for a grid-connected converter based on a virtual impedance feedforward controller, characterized in that: The following steps are involved: Step 1: Establish a small signal model considering coupling on a grid-connected converter based on virtual synchronous generator control; Step 2: Analyze the small signal model considering coupling. The active power loop, reactive power loop and coupling function all contain the same conjugate poles, which will cause synchronous frequency resonance, and coupling will aggravate synchronous frequency resonance. Step 3: Design a feedforward controller with virtual impedance to increase system damping, and suppress the unstable factors introduced by the controller by simplifying the controller; Step 4: Consider the damping and bandwidth of the system and calculate the setting conditions of the virtual impedance.

2. The control method of a grid-connected converter based on a virtual impedance feedforward controller according to claim 1, characterized in that: The grid-type converter comprises a DC voltage source, a three-phase half-bridge converter and a low-pass output LC filter, and also comprises a virtual synchronous generator control module, a virtual impedance control module and a voltage-current dual closed-loop control module.

3. The control method of a grid-connected converter based on a virtual impedance feedforward controller according to claim 1, characterized in that: In step 1, a small signal model considering coupling is established on the grid-connected converter based on virtual synchronous generator control, as follows: Step 1.1, calculate the power of the PCC point of the power grid: Assume that the grid voltage remains constant and calculate the instantaneous active power P e and instantaneous reactive power Q e , give the initial electromotive force E0 an electromotive force disturbance ΔE to obtain the electromotive force E, give the initial power angle δ e0 A power angle disturbance Δδ e Get the power angle δ e ,Right now: E=E0+ΔEδ e =d e0 +Dd e Linearization is performed at the equilibrium point, and the VSG dynamic small signal model is obtained as follows: Among them, G Pδ (s) is the relationship between active power and power angle, G PE (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 The difference between the actual output value of active power and the rated power, ΔQ e It is the difference between the actual output value of reactive power and the rated power; Step 1.2: Calculate the relationship between the power angle and active power G1(s), the relationship between voltage and reactive power G2(s), and the relationship between frequency and active power G3(s) of the VSG as follows: Where s is the complex variable in Laplace transform; J p , D p are the virtual inertia and damping coefficient in the frequency control of virtual synchronous generator, J q , D q are the virtual inertia and voltage droop coefficient in the virtual synchronous generator voltage control, Δω e is the actual angular frequency ω e and rated angular frequency ω n The difference between Step 1.3: The expression of the open-loop transfer function containing the coupled small signal model is obtained as follows: Among them, 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 The difference between the actual output value of active power and the rated power, ΔQ e It is the difference between the actual reactive power output value and the rated power.

4. The control method of a grid-connected converter based on a virtual impedance feedforward controller according to claim 3 is characterized in that: Relationship between active power and power angle G Pδ (s), the relationship between active power and inverter electromotive force G PE (s), the relationship between reactive power and power angle G Qδ (s), the relationship between reactive power and inverter electromotive force G QE (s), as follows: Where R = R1 + R g , L=L1+L g ; L1 and R1 represent 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, respectively.

5. The control method of a grid-connected converter based on a virtual impedance feedforward controller according to claim 4, characterized in that: The analysis described in step 2 considers the coupled small signal model. 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 aggravate the synchronous frequency resonance. Specifically: Step 2.1, draw the Bode diagram of the active and reactive open-loop transfer functions containing coupling, and calculate the negative damping brought by the coupling to the power loop; Step 2.2: According to the G in the active loop and the reactive loop Pδ (s), G PE (s), G Qδ (s), G QE The poles of (s) have the same conjugate poles and cause synchronous frequency resonance. The conjugate poles are: Therefore, the coupling of reactive power in the active loop may cause the originally stable system to become unstable, and the coupling of active power in the reactive loop may also cause the originally stable system to become unstable.

6. The control method of a grid-connected converter based on a virtual impedance feedforward controller according to claim 5, characterized in that: The design of the feedforward controller with virtual impedance described in step 3 increases the system damping and suppresses the unstable factors introduced by the controller by simplifying the controller, as follows: Step 3.1: Design the feedforward controller G Eδ (s) and G δE (s) The transfer function is as follows: G Eδ (s)=-G PE (s) / G Pδ (s) G δE (s)=-G Qδ (s) / G QE (s) Step 3.2: Virtual impedance X in virtual impedance control link vc for: X vc =R vc +oh n L vc Among them, R vc is the virtual resistance, L vc is the virtual inductor; Step 3.3: VSG is lightly loaded, so take δ e0 <20°,sinδ e0 ≈0, cosδ e0 ≈1, the simplified feedforward branch is: Among them, G Eδ (s) and G δE (s) is the feedforward controller, U g is the grid voltage; Step 3.4: Due to the introduction of the feedforward controller G Eδ (s) and G δE (s) contains differential terms, which will introduce high-frequency clutter, so in G Eδ (s) and G δE (s) A low-pass filter is introduced at the exit to attenuate the output high-frequency clutter signal. The introduced low-pass filter is: Among them, ω LPF is the cutoff frequency of the low-pass filter; Then the controller after adding virtual impedance and low-pass filter is: Among them, H LPF is a low-pass filter, and the subscript 0 is the corresponding steady-state value.

7. The control method of a grid-connected converter based on a virtual impedance feedforward controller according to claim 6, characterized in that: Considering the damping and bandwidth of the system as described in step 4, the setting conditions of the virtual impedance are calculated as follows: 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; Step 4.2, according to the active open-loop Bode diagram with virtual resistance added, set the virtual resistance, and at the same time ensure the bandwidth of the system after the introduction of the virtual resistance, sufficient damping during system oscillation, and the performance of the power loop; Step 4.3: According to the Bode diagram with virtual inductance added, set the virtual inductance to reduce the resonance effect of the synchronization frequency.

8. A grid-type converter control system based on a virtual impedance feedforward controller, characterized in that: The system is used to implement the grid-type converter control method based on the virtual impedance feedforward controller according to any one of claims 1 to 7, and the system includes a model building module, a model analysis module, a feedforward controller module and a calculation module, wherein: Model building module, which builds a small signal model considering coupling on the grid-connected converter based on virtual synchronous generator control; Model analysis module, which analyzes the small signal model considering coupling. The active power loop, reactive power loop and coupling function all contain the same conjugate poles, which will cause synchronous frequency resonance, and coupling will aggravate synchronous frequency resonance. Feedforward controller module, designing a feedforward controller with virtual impedance to increase system damping, and suppressing the unstable factors introduced by the controller by simplifying the controller; The calculation module comprehensively considers the damping and bandwidth of the system and calculates the setting conditions of the virtual impedance.

9. A mobile terminal comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that: When the processor executes the program, the grid-type converter control method based on the virtual impedance feedforward controller according to any one of claims 1 to 7 is implemented.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the program is executed by a processor, the steps in the control method of a grid-connected converter based on a virtual impedance feedforward controller as claimed in any one of claims 1 to 7 are implemented.

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