Direct current side impedance modeling and parameter setting method for flexible soft switch of power distribution network

Abstract

The invention discloses a DC side impedance modeling method for a soft switch of a power distribution network, and the method comprises the steps: taking an inverter in an SOP as an example, building a small signal equation of an electrical part and a controller part, enabling the electrical part to be modeled in a system coordinate system, and enabling the controller part to be modeled in a control coordinate system; performing simultaneous solution on small signal equations of an electrical part and a controller part by utilizing a relation between a control coordinate system and a system coordinate system to obtain an inverter direct current side impedance model; similarly, a rectifier direct current side impedance model in the SOP is obtained, stability judgment is carried out by utilizing a Middlebrok criterion, and the influence of system parameters on the stability is analyzed; a test system is built in Matlab / Simulink, and a parallel current source is adopted to carry out small disturbance impedance test to verify the effectiveness of the built impedance model; and on the basis of a Matlab / Simulink test system, verifying the effectiveness of the influence of system parameters on the stability.

Description

Technical Field

[0001] The invention relates to a DC side impedance modeling and parameter setting method for a flexible soft switch of a distribution network. Background Art

[0002] In recent years, with the development of renewable energy, a high proportion of distributed energy and active loads have been connected to the distribution network, which has brought many challenges to the operation of the distribution network. To solve this problem, power electronic devices are widely used in the distribution network to achieve flexible regulation of electric energy and jointly support the realization of the distribution network operation goals. Among them, the flexible soft switch (soft open point, SOP) based on the back-to-back voltage source converter (B2B-VSC) can achieve the optimized operation and fault recovery of the distribution network. At present, there have been a lot of studies on the control and optimization operation of the distribution network SOP, which have realized the voltage reactive power support based on SOP, the reduction of operating losses, and the improvement of energy consumption. However, before the SOP is connected to the distribution network, its own stability must be guaranteed, so it is necessary to study the stability and parameter setting of the SOP.

[0003] There are two main methods for analyzing the stability of SOP: eigenvalue analysis based on state space and impedance modeling based on frequency domain theory. Eigenvalue analysis is a numerical calculation method that requires the establishment of a global small signal model of the system. However, when the system is complex, this method will have the problem of excessive calculation and it is difficult to reveal the instability mechanism of the system.

[0004] In 1976, R.D. Middlebrook first proposed the impedance model and divided the converter system into a source-load subsystem. At present, the impedance modeling of multi-VSC systems can be divided into the source-load division method of the equivalent source-load method from the AC side and the DC side. Some literature has established an impedance model from the AC side and found that the impedance model has dq coupling terms, which will produce frequency coupling effects and affect stability. In fact, the impedance model of the VSC system established from the AC side is naturally a coupled MIMO (multiple-in multiple-out; MIMO) model. Although there are some methods to solve this problem, most methods are highly complex. However, the impedance model established from the DC side is a natural SISO (Single-Input Single-Output; SISO) model, which is convenient for analyzing system stability and parameter setting, but there is currently no relevant literature that comprehensively establishes an impedance model from the DC side to analyze the stability of the SOP system. Summary of the invention

[0005] The present invention aims to solve the shortcomings of the existing modeling methods, such as high complexity, multiple couplings, and failure to consider the phase-locked loop control effect, and provides a DC side impedance modeling method for a distribution network soft switch, as well as parameter setting based on the established model.

[0006] In order to achieve the above object, the technical solution of the present invention comprises the following steps:

[0007] A DC side impedance modeling and parameter setting method for a flexible soft switch of a distribution network comprises the following steps:

[0008] S1: Using the inverter in SOP, establish the small signal equations of the electrical part and the controller part, where the electrical part is modeled in the system coordinate system and the controller part is modeled in the control coordinate system;

[0009] S2: Using the relationship between the control coordinate system and the system coordinate system, the small signal equations of the electrical part and the controller part are solved simultaneously to obtain the DC side impedance model of the inverter;

[0010] S3: Similarly, solve the rectifier DC side impedance model in SOP, use Middlebrook criterion to judge the stability, and analyze the influence of system parameters on stability;

[0011] S4: Build a test system in Matlab / Simulink and use a parallel current source to perform a small disturbance impedance test to verify the effectiveness of the impedance model.

[0012] S5: Based on Matlab / Simulink test system, verify the effectiveness of the influence of system parameters on stability.

[0013] In step S1, small signal equations of the electrical part and the controller part are established, including:

[0014] S1-1: Based on the Kirchhoff equations written in the circuit, the relationship between the power conversion on the AC and DC sides of the converter described by the average model of the converter, and the dynamic equations that establish the connection between the electrical quantity and the control signal, the linear expansion of the above equations can obtain the small signal dynamic equations of the electrical part of the system. The equations are converted to the frequency domain through Laplace transform. In addition, assuming that the voltage of the three-phase power supply is symmetrical and balanced, and there is basically no disturbance, the small signal equation of the electrical part is obtained:

[0015]

[0016] S1-2: According to the system control mode, the inverter adopts voltage and current dual-loop control, the outer loop is constant DC voltage control, and the converter control part and modulation part are linearized with small disturbances:

[0017]

[0018] Among them, G PI1 (s), G PI2(s) are the PI control transfer functions of the voltage outer loop and the current inner loop, respectively, and Z3 is the decoupling term;

[0019]

[0020] In step S2, the small signal equations of the electrical part and the controller part are solved simultaneously to obtain the impedance model, which specifically includes:

[0021] Due to the dynamic characteristics of the phase-locked loop (PLL), there is a certain deviation between the voltage phase of its output and the actual value of the PCC point; the system coordinate system (abbreviated as s system) uses the voltage phase of the PCC point as the rotation angle, while the control coordinate system (abbreviated as c system) uses the output of the PLL as the rotation angle; the impedance modeling of the system is completed in the system coordinate system. In steady-state operation, the angles of the two are consistent, but when the voltage at the PCC point fluctuates, the impact of its phase difference Δθ on the stability of the system must be considered;

[0022] The small signal model of general variables acting on PLL is:

[0023]

[0024] Among them, X d ,X q represents the steady-state value of the variable, G PLL (s) is the transfer function of PLL;

[0025]

[0026] in, for Steady-state value on the d-axis;

[0027] First, combine equation (1) and equation (2) to obtain

[0028]

[0029] Considering the effect of PLL, substituting equations (7), (5), and (8) into equation (6), we can obtain

[0030]

[0031] in:

[0032]

[0033] Y1(s)=[G1(s)+(I+G1(s)G i (s)+G u (s)-G m (s))Y(s)

[0034] Substituting equation (8) into equation (3), we get

[0035]

[0036] Combining equations (9) and (10), we can obtain:

[0037]

[0038] in:

[0039]

[0040] Q(s)=(Y1(s)P(s)+G2(s))

[0041] Substituting equations (11) and (12) into equation (4), the DC side impedance of the inverter station can be obtained:

[0042]

[0043] Among them: P(s)1 represents the first element of the matrix P(S), and the others are similar.

[0044] In step S3, the rectifier DC side impedance model and Middlebrook criterion are:

[0045] S3-1: Similar to the derivation process of the inverter side impedance, the topology of the two is the same, the only difference is the control method, the inverter uses constant DC voltage control, and the rectifier uses constant power control. Therefore, the linearized small perturbation equation of the control part will be different:

[0046]

[0047] According to the power conservation on both sides of the rectifier station:

[0048]

[0049] Substituting equation (7), equation (14), equation (15) and equation (8) into equation (6), we can obtain:

[0050]

[0051] in:

[0052]

[0053] Y3(s)=[G1(s)+(I+G1(s)G i (s)+G u (s)-G m (s))Y(s)

[0054] Similarly, by combining equations (4), (10) and (16), we can obtain the DC side impedance of the rectifier:

[0055]

[0056] S3-2: A typical cascade system consists of a source converter and a load converter. Although both the source converter and the load converter can work independently, the whole system may become unstable due to their interaction. If the Middlebrook criterion is satisfied, that is, Tm = Z o_S / Z in_L Satisfies the Nyquist criterion, where Z o_S is the output impedance of the source converter, Z in_L is the input impedance of the load converter, the stability of the cascade system can be guaranteed;

[0057] In the SOP system, the rectifier works with power control and is a source converter, while the inverter works with DC voltage control and is a load converter. The impedance of the DC capacitor should be connected in parallel with the output impedance of the rectifier for stability analysis. Therefore:

[0058] Z o_S =Z2

[0059] S3-3: Based on the established SOP impedance model, draw the Nyquist diagram of the impedance ratio TM to analyze the system stability, and then analyze the influence of the voltage outer loop parameters, power outer loop parameters, current outer loop parameters, system transmission power, and DC capacitance on the stability.

[0060] In step S4, a test system is built in Matlab / Simulink to verify the validity of the impedance model:

[0061] Small signal disturbances of different frequencies are injected into the SOP DC port by means of parallel current sources. The disturbance current and disturbance voltage of the corresponding port are measured, and the DC side impedance of the system is obtained by performing FFT analysis and ratio calculation.

[0062] In step S5, the influence of system parameters on stability is verified in the Matlab / Simulink test system:

[0063] Including the influence of voltage outer loop parameters, power outer loop parameters, current outer loop parameters, system transmission power, and DC capacitance on stability, verifying the consistency of the method for analyzing the influence of system parameters on stability described in step S3.

[0064] The present invention establishes small signal linearization equations from the electrical part and control part of the inverter and rectifier in the soft switch (SOP), and then derives the DC side impedance model of the inverter and rectifier through coordinate system transformation, and uses the Middlebrook criterion to perform stability analysis when different system parameters are used. The SOP simulation model is built by Matlab / Simulink software, and the impedance model test and the simulation of the influence on stability under different system parameters are performed, which proves the effectiveness of stability judgment and parameter setting using the proposed method.

[0065] The beneficial effects of the present invention are:

[0066] 1. Provide a DC side impedance modeling method for flexible soft switches in distribution networks to solve the problems of high complexity, multiple couplings, and failure to consider the control effect of phase-locked loops in existing modeling methods.

[0067] 2. Based on the established model, the AC / DC impedance ratio stability criterion can be used to conveniently control and adjust electrical parameters, thereby minimizing system instability.

[0068] 3. It is beneficial to quickly and accurately judge the stability and set the control parameters, provide a reference for SOP system debugging, reduce the amount of calculation and improve efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0069] Figure 1 It is the SOP topology structure of the present invention.

[0070] Figure 2 It is the SOP control method of the present invention.

[0071] Figure 3 It is a typical source-load cascade system of the present invention.

[0072] Figure 4 It is the Nyquist diagram of TM when different voltage outer loop ratio parameters of the present invention are used.

[0073] Figure 5 It is the Nyquist diagram of TM when different power outer loop ratio parameters of the present invention are used.

[0074] Figure 6 It is the Nyquist diagram of TM when different current inner loop ratio parameters of the present invention are used.

[0075] Figure 7 It is the Nyquist diagram of TM at different transmission powers of the present invention.

[0076] Figure 8 It is the Nyquist diagram of TM for different DC capacitances of the present invention.

[0077] Fig. 9This is the small disturbance signal testing method of the present invention.

[0078] Fig.10 This is the verification result of the DC port impedance model of the inverter station.

[0079] FIG11( a ) is a comparison diagram of DC voltage waveforms for different voltage outer loop ratio parameters, and FIG11( b ) is a comparison diagram of transmitted active power waveforms for different voltage outer loop ratio parameters.

[0080] FIG12( a ) is a comparison diagram of DC voltage waveforms for different power outer loop ratio parameters, and FIG12( b ) is a comparison diagram of transmitted active power waveforms for different power outer loop ratio parameters.

[0081] FIG13( a ) is a comparison diagram of DC voltage waveforms for different current inner loop ratio parameters, and FIG13( b ) is a comparison diagram of transmitted active power waveforms for different current inner loop ratio parameters.

[0082] FIG14( a ) is a comparison diagram of DC voltage waveforms at different transmission powers, and FIG14( b ) is a comparison diagram of transmission active power waveforms at different transmission powers.

[0083] FIG15( a ) is a comparison diagram of DC voltage waveforms for different DC capacitors, and FIG15( b ) is a comparison diagram of transmitted active power waveforms for different DC capacitors.

[0084] Fig.16 is a flow chart of the method of the present invention. DETAILED DESCRIPTION

[0085] The present invention will be further described below in conjunction with the accompanying drawings.

[0086] Reference Figure 1 to Figure 15(b) , a DC side impedance modeling method for soft switches in distribution networks, and parameter setting based on the established model.

[0087] The technical solution of the present invention comprises the following steps:

[0088] S1: Using the inverter in SOP, establish the small signal equations of the electrical part and the controller part, where the electrical part is modeled in the system coordinate system and the controller part is modeled in the control coordinate system;

[0089] S2: Using the relationship between the control coordinate system and the system coordinate system, the small signal equations of the electrical part and the controller part are solved simultaneously to obtain the DC side impedance model of the inverter;

[0090] S3: Solve the rectifier DC side impedance model in SOP, use Middlebrook criterion to judge stability, and analyze the influence of system parameters on stability;

[0091] S4: Build a test system in Matlab / Simulink and use a parallel current source to perform a small disturbance impedance test to verify the effectiveness of the impedance model.

[0092] S5: Based on Matlab / Simulink test system, verify the effectiveness of the influence of system parameters on stability.

[0093] In step S1, small signal equations for the electrical part and the controller part are established, specifically including:

[0094] S1-1: Figure 1 This is a flexible soft switch topology diagram using a back-to-back voltage source converter. The converter uses a two-level voltage source converter, and the system has a good symmetrical structure. In order to better derive the system impedance model, the inverter side is taken as an example to illustrate the meaning of system parameters, where u1 is the grid voltage, R1+jωL1 is the grid equivalent reactance, ω is the grid frequency; C1 is the filter capacitor, R2+jωL2 is the filter impedance; u2 is the common connection point (PCC) voltage; C dc1 is the DC capacitance of the inverter DC side, u dc1 is the DC voltage. In addition, there is a conversion between coordinate systems during the system impedance modeling process, where the superscript c represents the control coordinate system, the superscript s represents the system coordinate system, and the subscripts d and q represent the d and q components in the synchronously rotating coordinate system. is a small disturbance of the physical quantity; the subscript 0 represents the steady-state value of the physical quantity at the static operating point.

[0095] According to the Kirchhoff equation written in the circuit, the relational expression describing the power conversion of the AC and DC sides of the converter according to the average model of the converter, and the dynamic equation establishing the connection between the electrical quantity and the control signal, the above equations can be linearized to obtain the small signal dynamic equation of the electrical part of the system. The equation is transformed into the frequency domain through Laplace transform. In addition, considering the voltage symmetry and balance of the three-phase power supply, there is basically no disturbance, and the following small signal equation of the electrical part is obtained:

[0096]

[0097] S1-2: Figure 2 The control method used by the SOP system is shown in the figure, in which the inverter adopts a constant DC voltage control, which is controlled by u dc The reference value of i2 on the d-axis is obtained; the rectifier adopts constant power control, and the reference value of i2 on the d-axis is obtained from its power P. In addition, it is assumed that its power factor is 1, the reactive power Q is 0, and then the reference value of i2 on the q-axis is 0.

[0098] According to the system control block diagram, the small disturbance linearization is performed on the converter control part and the modulation part:

[0099]

[0100] Among them, G PI1 (s), G PI2 (s) are the PI control transfer functions of the voltage outer loop and the current inner loop, respectively, and Z3 is the decoupling term;

[0101]

[0102] In step S2, the small signal equations of the electrical part and the controller part are solved simultaneously to obtain the impedance model:

[0103] Due to the dynamic characteristics of the phase-locked loop (PLL), there is a certain deviation between the voltage phase of its output and the actual value of the PCC point; the system coordinate system (s) rotates with the voltage phase of the PCC point, while the control coordinate system (c) rotates with the output of the PLL; the impedance modeling of the system is completed in the system coordinate system. The two angles are consistent in steady-state operation, but when the voltage at the PCC point fluctuates, the impact of the phase difference Δθ on the system stability must be considered;

[0104] The small signal model of general variables acting on PLL is:

[0105]

[0106] Among them, X d ,X q represents the steady-state value of the variable, G PLL (s) is the transfer function of PLL;

[0107]

[0108] in, for Steady-state value on the d-axis;

[0109] First, combine equation (1) and equation (2) to obtain

[0110]

[0111] Considering the effect of PLL, substituting equations (7), (5), and (8) into equation (6), we can obtain

[0112]

[0113] in:

[0114]

[0115] Y1(s)=[G1(s)+(I+G1(s)G i (s)+G u (s)-Gm (s))Y(s)

[0116] Substituting equation (8) into equation (3), we get

[0117]

[0118] Combining equations (9) and (10), we can obtain:

[0119]

[0120] in:

[0121]

[0122] Q(s)=(Y1(s)P(s)+G2(s))

[0123] Substituting equations (11) and (12) into equation (4), the DC side impedance of the inverter station can be obtained:

[0124]

[0125] Among them: P(s)1 represents the first element of the matrix P(S), and the others are similar.

[0126] In step S3, the rectifier DC side impedance model and Middlebrook criterion are:

[0127] S3-1: Similar to the derivation process of the inverter side impedance, the topology of the two is the same, the only difference is the control method, the inverter uses constant DC voltage control, and the rectifier uses constant power control. Therefore, the linearized small perturbation equation of the control part will be different:

[0128]

[0129] According to the power conservation on both sides of the rectifier station:

[0130]

[0131] Substituting equation (7), equation (14), equation (15) and equation (8) into equation (6), we can obtain:

[0132]

[0133] in:

[0134]

[0135] Y3(s)=[G1(s)+(I+G1(s)G i (s)+G u (s)-G m(s))Y(s)

[0136] Similarly, by combining equations (4), (10) and (16), the DC side impedance of the rectifier can be obtained as:

[0137]

[0138] S3-2: Figure 3 A typical cascade system consisting of a source converter and a load converter is shown; although both the source converter and the load converter can work individually, the entire system may become unstable due to their interaction; if the Middlebrook criterion is satisfied, that is, Tm = Z o_S / Z in_L Satisfies the Nyquist criterion, where Z o_S is the output impedance of the source converter, Z in_L is the input impedance of the load converter, the stability of the cascade system can be guaranteed;

[0139] exist Figure 1 In the SOP system shown in the figure, the rectifier works with power control and is a source converter, while the inverter works with DC voltage control and is a load converter. The impedance of the DC capacitor should be connected in parallel with the output impedance of the rectifier for stability analysis. Therefore:

[0140]

[0141] S3-3: Based on the established SOP impedance model, draw the Nyquist diagram to analyze the system stability. The system parameters in Table 1 are for operating condition 1, and then analyze the influence of voltage outer loop parameters, power outer loop parameters, current outer loop parameters, system transmission power, and DC capacitance on stability.

[0142]

[0143]

[0144] Table 1 SOP simulation system parameters

[0145] S3-3-1: Based on working condition 1, the influence of the voltage outer loop proportional parameter Kp1 of the inverse conversion flow station on the stability is studied, and the Nyquist impedance ratio Tm is obtained for different Kp1. Figure 4 As can be seen from the figure, as Kp1 increases, the intersection of the Nyquist diagram of Tm and the real axis is getting farther and farther away from (-1j, 0), indicating that the stability margin of the system is getting larger and larger. Therefore, increasing the voltage outer loop proportional parameter Kp1 is beneficial to enhancing the stability of the system.

[0146] S3-3-2: Based on working condition 1, the influence of the power outer loop ratio parameter Kp3 of the rectifier converter station on the stability is studied, and the Nyquist impedance ratio Tm at different Kp3 is obtained. Figure 5 As can be seen from the figure, as Kp3 increases, the intersection of the Nyquist diagram of Tm and the real axis gets closer and closer to (-1j, 0), indicating that the stability margin of the system is getting smaller and smaller. Therefore, increasing the voltage outer loop proportional parameter Kp3 is not conducive to enhancing the stability of the system.

[0147] S3-3-3: Based on working condition 1, the influence of the current inner loop proportional parameter Kp2 of the rectifier converter station on the stability is studied, and the Nyquist impedance ratio Tm at different Kp2 is obtained. Figure 6 As can be seen from the figure, as Kp2 increases, the intersection of the Nyquist diagram of Tm and the real axis gets closer and closer to (-1j, 0), indicating that the stability margin of the system is getting smaller and smaller. Therefore, increasing the voltage outer loop proportional parameter Kp2 is not conducive to enhancing the stability of the system.

[0148] S3-3-4: Based on working condition 1, study the effect of transmission power P on stability and obtain the Nyquist impedance ratio Tm for different P. Figure 7 As can be seen from the figure, as P increases, the intersection of the Nyquist diagram of Tm and the real axis remains basically unchanged, indicating that the stability margin of the system remains basically unchanged. Therefore, changing the transmission power under the rated capacity of the system does not affect the stability of the system.

[0149] S3-3-5: Based on working condition 1, study the effect of DC capacitance Cdc on stability and obtain the Nyquist impedance ratio Tm at different Cdc. Figure 8 As can be seen from the figure, as Cdc increases, the intersection of the Nyquist diagram of Tm and the real axis moves further and further away from (-1j, 0), indicating that the stability margin of the system is getting larger and larger. Therefore, increasing the DC capacitance Cdc is beneficial to enhancing the stability of the system.

[0150] In step S4, a test system is built in Matlab / Simulink to verify the validity of the impedance model:

[0151] The SOP simulation model was built in Matlab / Simulink with the parameters in Table 1. Fig. 9 As shown in the figure, a 20A current source is connected in parallel to inject small signal disturbances of different frequencies into the SOP DC port, the disturbance current and disturbance voltage of the corresponding port are measured, and the DC side impedance of the system is obtained by performing FFT analysis and ratio calculation. Fig.10 The DC port impedance model of the inverter station controlled by constant DC voltage is compared with the swept frequency simulation verification results. It can be seen that in the frequency range of 1 to 1 000 Hz, the amplitude-frequency characteristics and phase-frequency characteristics of the impedance model obtained are basically consistent with the simulation results, which verifies the correctness of the constructed impedance model.

[0152] In step S5, the influence of system parameters on stability is verified in the Matlab / Simulink test system:

[0153] Including the influence of voltage outer loop parameters, power outer loop parameters, current outer loop parameters, system transmission power, and DC capacitance on stability, verifying the consistency of the method for analyzing the influence of system parameters on stability described in step S3.

[0154] S5-1: When the voltage outer loop proportional coefficient Kp1 is 1, 2, and 5, Figure 11(a) is a comparison diagram of the DC voltage waveform, and Figure 11(b) is a comparison diagram of the system transmission active power waveform. It can be seen that as Kp1 increases, the DC voltage regulation peak decreases, that is, the overshoot decreases, and the dynamic regulation is faster, reaching the stable value more quickly. When a disturbance is given to the system at 0.5s, that is, the DC voltage suddenly changes to 22KV, the DC voltage and active power can be stabilized again, and the larger Kp1 is, the faster the dynamic regulation is, indicating that increasing the voltage outer loop proportional coefficient is beneficial to improving the stability of the system. The simulation is consistent with the theoretical analysis in Section S3-3-1.

[0155] S5-2: When the power outer loop proportional coefficient Kp3 is 2, 5, and 10 respectively, Figure 12(a) is a comparison diagram of the DC voltage waveform, and Figure 12(b) is a comparison diagram of the system transmission active power waveform. It can be seen that as Kp3 increases, the DC voltage harmonics become more and more serious. When a disturbance is given to the system at 0.5s, the DC voltage and active power can stabilize again, but the larger Kp4 is, the lower the stable value of the transmitted active power is, indicating that increasing the power outer loop proportional coefficient is not conducive to improving the stability of the system. The simulation is consistent with the theoretical analysis in Section S3-3-2.

[0156] S5-3: When the current loop proportional coefficient Kp2 of the rectifier station is 0.5, 1, and 5 respectively, Figure 13(a) is a comparison diagram of the DC voltage waveform, and Figure 13(b) is a comparison diagram of the system transmission active power waveform. It can be seen that as Kp2 increases, the DC voltage harmonics become more and more serious. When a disturbance is given to the system at 0.5s, the DC voltage and active power can be stabilized again, but the larger the Kp4, the larger the harmonics of the transmitted DC voltage and active power, indicating that increasing the current inner loop proportional coefficient of the rectifier station is not conducive to improving the stability of the system. The simulation is consistent with the theoretical analysis in Section S3-3-3.

[0157] S5-4: When the system transmission active power is 12MW, 10MW, and 8MW respectively, Figure 14(a) is a comparison diagram of the DC voltage waveform, and Figure 14(b) is a comparison diagram of the system transmission active power waveform. It can be seen that as the transmission power decreases, the DC voltage overshoot decreases slightly and the harmonics slow down slightly. When a disturbance is given to the system at 0.5s, the DC voltage and active power can stabilize again, indicating that within the system transmission active power capacity, changing the transmission active power basically does not affect the stability of the system. The simulation is consistent with the theoretical analysis in Section S3-3-4.

[0158] S5-1: When the DC capacitor Cdc is 500uf, 800uf, and 1000uf respectively, Figure 15(a) is a comparison diagram of the DC voltage waveform, and Figure 15(b) is a comparison diagram of the system transmission active power waveform. It can be seen that as Cdc increases, the DC voltage regulation peak decreases, that is, the overshoot decreases, and the dynamic regulation is faster, reaching the stable value faster. When a disturbance is given to the system at 0.5s, the DC voltage and active power can stabilize again, and the larger the Cdc, the faster the dynamic regulation, indicating that increasing the voltage outer loop proportional coefficient is beneficial to improving the stability of the system. The simulation is consistent with the theoretical analysis in Section S3-3-5.

[0159] The contents described in the embodiments of this specification are merely an enumeration of the implementation forms of the inventive concept. The protection scope of the present invention should not be regarded as limited to the specific forms described in the embodiments. The protection scope of the present invention also extends to equivalent technical means that can be conceived by those skilled in the art based on the inventive concept.

Claims

1. A DC side impedance modeling and parameter setting method for a flexible soft switch in a distribution network, characterized in that: The following steps are involved: S1: Use the inverter in SOP to establish the small signal equations of the electrical part and the controller part, where the electrical part is modeled in the system coordinate system and the controller part is modeled in the control coordinate system; S2: Using the relationship between the control coordinate system and the system coordinate system, the small signal equations of the electrical part and the controller part are solved simultaneously to obtain the DC side impedance model of the inverter; S3: Solve the rectifier DC side impedance model in SOP, use Middlebrook criterion to judge the stability, and analyze the influence of system parameters on stability; S4: Build a test system in Matlab / Simulink and use a parallel current source to perform a small disturbance impedance test to verify the validity of the constructed impedance model. S5: Based on Matlab / Simulink test system, verify the effectiveness of the influence of system parameters on stability.

2. The DC side impedance modeling and parameter setting method of a distribution network flexible soft switch according to claim 1 is characterized in that: In step S1, small signal equations for the electrical part and the controller part are established, specifically including: S1-1: Based on the Kirchhoff equations written in the circuit, the relationship between the power conversion on the AC and DC sides of the converter described by the average model of the converter, and the dynamic equations that establish the connection between the electrical quantity and the control signal, the linear expansion of the above equations can obtain the small signal dynamic equations of the electrical part of the system. The equations are converted to the frequency domain through Laplace transform. In addition, assuming that the voltage of the three-phase power supply is symmetrical and balanced, and there is basically no disturbance, the small signal equation of the electrical part is obtained: S1-2: According to the system control mode, the inverter adopts voltage and current dual-loop control, the outer loop is constant DC voltage control, and the converter control part and modulation part are linearized with small disturbances: Among them, G PI1 (s), G PI2 (s) are the PI control transfer functions of the voltage outer loop and the current inner loop, respectively, and Z3 is the decoupling term; 3. The DC side impedance modeling and parameter setting method of a distribution network flexible soft switch according to claim 2 is characterized in that: In step S2, the small signal equations of the electrical part and the controller part are solved simultaneously to obtain the impedance model, which specifically includes: Due to the dynamic characteristics of the phase-locked loop (PLL), there is a certain deviation between the voltage phase of its output and the actual value of the PCC point; the system coordinate system s is the rotation angle of the voltage phase of the PCC point, while the control coordinate system c is the rotation angle of the output of the PLL; the impedance modeling of the system is completed in the system coordinate system. In steady-state operation, the two angles are consistent, but when the voltage at the PCC point fluctuates, the impact of its phase difference Δθ on the stability of the system must be considered; The small signal model of general variables acting on PLL is: Among them, X d ,X q represents the steady-state value of the variable, G PLL (s) is the transfer function of PLL; in, for Steady-state value on the d-axis; First, combine equation (1) and equation (2) to obtain Considering the effect of PLL, substituting equations (7), (5), and (8) into equation (6) yields in: Y1(s)=[G1(s)+(I+G1(s)G i (s)+G u (s)-G m (s))Y(s) Substituting equation (8) into equation (3), we get Combining equations (9) and (10), we can obtain: in: Q(s)=(Y1(s)P(s)+G2(s)) Substituting equations (11) and (12) into equation (4), the DC side impedance of the inverter station can be obtained: Among them: P(s)1 represents the first element of the matrix P(S), and the others are similar.

4. The DC side impedance modeling and parameter setting method of a distribution network flexible soft switch according to claim 3 is characterized in that: In step S3, the rectifier DC side impedance model and Middlebrook criterion are: S3-1: Similar to the derivation process of the inverter side impedance, the topological structures of the two are the same. The only difference is the control method. The inverter adopts constant DC voltage control, while the rectifier adopts constant power control; therefore, the linearized small disturbance equation of the control part will be different: According to the power conservation on both sides of the rectifier station: Substituting equation (7), equation (14), equation (15) and equation (8) into equation (6), we can obtain: in: Y3(s)=[G1(s)+(I+G1(s)G i (s)+G u (s)-G m (s))Y(s) Similarly, by combining equations (4), (10) and (16), we can obtain the DC side impedance of the rectifier: S3-2: A typical cascade system consists of a source converter and a load converter. Although both the source converter and the load converter can work independently, the whole system may become unstable due to their interaction. If the Middlebrook criterion is satisfied, that is, Tm = Z o_S / Z in_L Satisfies the Nyquist criterion, where Z o_S is the output impedance of the source converter, Z in_L is the input impedance of the load converter, the stability of the cascade system can be guaranteed; In the SOP system, the rectifier works with power control and is a source converter, while the inverter works with DC voltage control and is a load converter. The impedance of the DC capacitor should be connected in parallel with the output impedance of the rectifier for stability analysis. Therefore: S3-3: Based on the established SOP impedance model, draw the Nyquist diagram of the impedance ratio TM to analyze the system stability, and then analyze the influence of the voltage outer loop parameters, power outer loop parameters, current outer loop parameters, system transmission power, and DC capacitance on the stability.

5. The DC side impedance modeling and parameter setting method of a distribution network flexible soft switch according to claim 4 is characterized in that: In step S4, a test system is built in Matlab / Simulink to verify the impedance model: Small signal disturbances of different frequencies are injected into the SOP DC port by means of parallel current sources. The disturbance current and disturbance voltage of the corresponding port are measured, and the DC side impedance of the system is obtained by performing FFT analysis and ratio calculation.

6. The DC side impedance modeling and parameter setting method of a distribution network flexible soft switch according to claim 5 is characterized in that: In step S5, the influence of parameters on stability is verified in the Matlab / Simulink test system: Including the influence of voltage outer loop parameters, power outer loop parameters, current outer loop parameters, system transmission power, and DC capacitance on stability, verifying the consistency of the method for analyzing the influence of system parameters on stability described in step S3.

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