Self-stabilization control method for three-phase four-leg series flexible interconnection switch

Abstract

The application provides a direct-current self-voltage stabilization control method for a three-phase four-bridge-arm series flexible interconnection switch, is applied to a power distribution network connected with two three-phase alternating current sources, the two three-phase alternating current sources respectively constitute a first end network side and a second end network side of the power distribution network, and comprises the following steps: collecting a power grid multi-dimensional electrical state parameter set of the first end network side and the second end network side, wherein the power grid multi-dimensional electrical state parameter set comprises electrical state parameters and line structure parameters; determining series voltage vector control parameters based on the power grid multi-dimensional electrical state parameter set; and determining a pulse width modulation signal of an inverter fourth bridge arm based on the series voltage vector control parameters. The existing three-phase four-bridge-arm inverter follow-up control is fully utilized, the active power scheduling size between feeders can be controlled, the direct-current side can be self-voltage stabilized without other power electronic devices or direct-current sources, the withstand voltage requirement of the device is reduced, the number of devices is reduced, and the stability and economy of the system are improved.

Description

Technical Field

[0001] This application relates to the field of power control technology, and in particular to a self-stabilizing control method for a three-phase four-arm series-connected flexible interconnected switch. Background Technology

[0002] With the rapid development of power electronics and active distribution network technologies in recent years, soft open points (SOPs) for distribution networks have gained widespread attention due to their advantages such as more flexible power flow regulation capabilities, faster response speeds, smaller loop-closing impacts, and longer service life. SOPs can improve the overall power flow distribution, feeder voltage levels, and absorb distributed generation in low-voltage distribution networks, thereby enhancing grid reliability. Currently, SOPs can be broadly classified into two categories: parallel and series types. The main topology of parallel-type devices is a back-to-back AC-DC-AC converter, which achieves free power flow between adjacent feeders and fault isolation by adjusting port control strategies. However, when regulating power in a parallel structure, all power flows through the device body, leading to problems such as large capacity, a large number of components, and high component tolerance requirements.

[0003] In comparison, series-connected system-on-a-chip (SOP) offers advantages in terms of cost and size. While achieving the same power regulation capability as parallel-connected SOPs, series-connected equipment only needs to handle a small portion of the power to control a large power flow. Therefore, series-connected SOPs have become a focus of current research due to their advantages such as smaller required equipment capacity, lower device voltage ratings, lower cost, and lower losses.

[0004] Current series-type SOP control mainly focuses on precise power flow regulation. For example, patent application number 202510234084.3, entitled "Control Method of a Three-Phase Decoupled Series-Type Power Flow Regulation Equipment," analyzes a power flow control method for a series-type four-arm SOP with independent three-phase active and reactive power decoupling. Its topology is as follows: Figure 1 As shown, the proposed control method lacks self-regulation, and a stable DC source or additional voltage regulator port is still required on the DC side for voltage regulation. Therefore, existing series-type four-arm SOP control methods cannot simultaneously meet the dual requirements of DC-side voltage stability control and power flow regulation. Summary of the Invention

[0005] To overcome the aforementioned technical deficiencies, this application provides a self-regulating voltage control method for three-phase four-arm series-connected flexible interconnection switches. To achieve the above objective, this application implements it according to the following technical solution:

[0006] This application provides a DC self-regulating voltage control method for a three-phase four-arm series-connected flexible interconnection switch, applied to a distribution network connecting two three-phase AC sources, wherein the two three-phase AC sources respectively constitute the first and second end-network sides of the distribution network, including:

[0007] Collect a set of multi-dimensional electrical state parameters of the power grid from the first terminal network side and the second terminal network side. The set of multi-dimensional electrical state parameters of the power grid includes electrical state parameters and line structure parameters.

[0008] Based on the multi-dimensional electrical state parameter set of the power grid, the series voltage vector control parameters are determined;

[0009] Based on the series voltage vector control parameters, the pulse width modulation signal of the fourth bridge arm of the inverter is determined.

[0010] Optionally, the electrical state parameters include three-phase voltage amplitude and three-phase voltage phase; the line structure parameters include three-phase line resistance and three-phase line inductance; the series voltage vector control parameters include series reference output initial voltage amplitude and series reference output initial voltage phase; determining the series voltage vector control parameters based on the multi-dimensional electrical state parameter set of the power grid includes:

[0011] Based on the three-phase voltage amplitude, three-phase voltage phase, three-phase line resistance, and three-phase line inductance, determine the initial phase of the series reference output voltage;

[0012] The initial voltage amplitude of the series reference output is determined based on the initial voltage phase of the series reference output.

[0013] Optionally, determining the pulse width modulation signal of the fourth arm of the inverter based on the voltage vector control parameters includes:

[0014] Collect actual DC voltage values;

[0015] Obtain the target DC voltage value;

[0016] The phase change value is obtained by proportionally integrating the difference between the actual DC voltage value and the target DC voltage value.

[0017] The series output reference voltage phase is obtained by subtracting the initial phase of the series reference output voltage from the phase change value.

[0018] Based on the phase of the series output reference voltage and the amplitude of the initial series reference output voltage, the pulse width modulation signal of the fourth bridge arm of the inverter is determined.

[0019] Optionally, determining the pulse width modulation signal of the fourth bridge arm of the inverter based on the phase of the series output reference voltage and the initial amplitude of the series reference output voltage includes:

[0020] The phase of the series output reference voltage is sinusoidally calculated to obtain the sinusoidal value of the series reference output voltage phase; based on the sinusoidal value of the series reference output voltage phase and the initial amplitude of the series reference output voltage, the pulse width modulation signal of the fourth bridge arm of the inverter is determined.

[0021] Optionally, determining the pulse width modulation signal for the fourth arm of the inverter based on the sinusoidal value of the series reference output voltage phase and the amplitude of the initial series reference output voltage includes:

[0022] Multiply the sinusoidal value of the series reference output voltage phase by the amplitude of the series reference output initial voltage to obtain the three-phase reference output voltage vector;

[0023] Based on the three-phase reference output voltage vector, the pulse width modulation signal of the fourth bridge arm of the inverter is determined.

[0024] Optionally, determining the pulse width modulation signal of the fourth arm of the inverter based on the three-phase reference output voltage vector includes:

[0025] Collect the actual three-phase output voltage;

[0026] The difference between the actual three-phase output voltage and the three-phase reference output voltage vector is proportional to integral feedback to obtain the current feedback quantity;

[0027] Based on the current feedback, the pulse width modulation signal of the fourth bridge arm of the inverter is determined.

[0028] Optionally, determining the pulse width modulation signal of the fourth arm of the inverter based on the current feedback includes:

[0029] Collect the inverter filter capacitor current;

[0030] The difference between the current feedback quantity and the inverter filter capacitor current is proportionally fed back to obtain the modulated voltage signal.

[0031] Based on the modulated voltage signal, the pulse width modulation signal of the fourth bridge arm of the inverter is determined.

[0032] Optionally, determining the pulse width modulation signal of the fourth arm of the inverter based on the modulated voltage signal includes:

[0033] The waveform of the modulated voltage signal is compared with a triangular wave to obtain the pulse width modulation signal of the three-phase bridge arm of the inverter.

[0034] Based on the pulse width modulation signals of the three-phase bridge arms of the inverter, the pulse width modulation signal of the fourth bridge arm of the inverter is determined.

[0035] Optionally, determining the pulse width modulation signal of the fourth arm of the inverter based on the pulse width modulation signals of the three-phase arms of the inverter includes:

[0036] The pulse width modulation signal of the three-phase bridge arm of the inverter is decoupled and calculated through the fourth bridge arm to obtain the modulation voltage of the fourth bridge arm;

[0037] The modulation voltage of the fourth bridge arm is compared with a triangular wave to output the pulse width modulation signal of the fourth bridge arm of the inverter.

[0038] This application has the following beneficial effects:

[0039] The method proposed in this application makes full use of the follow-up control of the existing three-phase four-arm inverter, which can control the magnitude of active power dispatch between feeders, and can perform self-stabilized voltage control on the DC side without the need for other power electronic equipment or DC source, thereby reducing the voltage withstand requirements of the devices and reducing the number of devices, and improving the stability and economy of the system.

[0040] In addition to the purposes, features, and advantages described above, this application has other purposes, features, and advantages. The application will now be described in further detail with reference to the accompanying drawings. Attached Figure Description

[0041] The accompanying drawings, which form part of this application, are used to provide a further understanding of this application. The illustrative embodiments and descriptions of this application are used to explain this application and do not constitute an undue limitation of this application. In the drawings:

[0042] Figure 1 This is a schematic diagram of the topology used in the power flow control method of series-type four-arm SOP three-phase independent active and reactive power decoupling provided in the background technology of this application.

[0043] Figure 2 This is a schematic flowchart of a DC self-regulating control method for a three-phase four-arm series flexible interconnected switch provided in an embodiment of this application.

[0044] Figure 3 This is a schematic diagram of the topology used in the DC self-regulating control method for a three-phase four-arm series flexible interconnected switch provided in the embodiments of this application.

[0045] Figure 4 This is the circuit equivalent model of the power flow from the grid side at end 1 to the grid side at end 2 through the SF-SOP under grid-connected operating conditions in the embodiments of this application.

[0046] Figure 5 This is a vector diagram of the SF-SOP single-phase output circuit in the embodiments of this application;

[0047] Figure 6 This is a schematic diagram of the DC capacitor control principle in the embodiments of this application; Figure 6 (a) is a schematic diagram of capacitor discharge. Figure 6 (b) is a schematic diagram of DC-side balance. Figure 6 (c) is a schematic diagram of capacitor charging;

[0048] Figure 7 This is the experimental simulation stage in the embodiments of this application, showing the DC voltage and transformer area voltage waveforms of SF-SOP under simulated conditions; Figure 7 (a) is a waveform diagram comparing the phase a voltage on the network side of ports 1 and 2. Figure 7 (b) is the DC side voltage of the SF-SOP;

[0049] Figure 8 This is a simulation diagram of the voltage and current waveforms of the three-phase device of SF-SOP under simulated conditions during the experimental simulation stage in this application embodiment. Figure 8 (a) is a waveform diagram of the output voltage of the SF-SOP series connection. Figure 8 (b) is a waveform diagram of the output current of the SF-SOP series connection;

[0050] Figure 9 This is a simulation diagram of the three-phase power flow waveform of the SF-SOP under simulated conditions during the experimental simulation stage in this application embodiment. Figure 9 (a) is a waveform diagram of the active power output by the SF-SOP itself. Figure 9 (b) is a waveform diagram of the transmission power of the SF-SOP distribution area;

[0051] Figure 10 This is a topology diagram of an existing flexible interconnected switch connected to the power grid during the experimental simulation phase of this application embodiment. Figure 10 (a) is a topology diagram of a back-to-back power supply inverter system; Figure 10 (b) is a topology diagram of a three-phase H-bridge structure flexible interconnection equipment;

[0052] Figure 11 The above are simulation waveforms of the series output voltage and current of three flexible interconnected switches under the same simulation conditions during the experimental simulation stage in this application embodiment. Figure 11 (a) is a simulation waveform of the output voltage of back-to-back SOP series connection; Figure 11 (b) is a simulation waveform of the output current of back-to-back SOPs in series; Figure 11 (c) is a simulation waveform of the output voltage of a three-phase H-bridge in series; Figure 11 (d) is the simulation waveform of the output current of the three-phase H-bridge in series; Figure 11(e) is a simulation waveform of the SF-SOP series output voltage of this application; Figure 11 (f) is a simulation waveform of the SF-SOP series output current of this application;

[0053] Figure 12 The above are simulation waveforms of DC-side voltage and transformer area transmission power under the same simulation conditions for three types of flexible interconnected switches in the experimental simulation stage of this application embodiment. Figure 12 (a) is a simulation waveform of the DC side voltage of a back-to-back SOP; Figure 12 (b) is a simulation waveform diagram of the power transmission between back-to-back substations; Figure 12 (c) is a simulation waveform diagram of the DC side voltage of the three-phase H-bridge; Figure 12 (d) is a simulation waveform diagram of the transmission power of the three-phase H-bridge substation; Figure 12 (e) is a simulation waveform of the DC-side voltage of the SF-SOP in this application; Figure 12 (f) is a simulation waveform diagram of the transmission power of the SF-SOP distribution area in this application. Detailed Implementation

[0054] The embodiments of this application are described in detail below with reference to the accompanying drawings, but this application can be implemented in many different ways as defined and covered by the claims.

[0055] Therefore, in order to solve the above problems, such as Figure 2 As shown, this application proposes a DC self-regulating control method for a three-phase four-arm series-connected flexible interconnection switch, including:

[0056] Step S201: Collect the multi-dimensional electrical state parameter set of the power grid from the first terminal network side and the second terminal network side. The multi-dimensional electrical state parameter set of the power grid includes electrical state parameters and line structure parameters.

[0057] The method proposed in this application is applied to, for example, Figure 3 The topology shown is a distribution network connecting two three-phase AC sources. The two three-phase AC sources respectively constitute the first end network side 1 (hereinafter referred to as the 1-end network side) and the second end network side 2 (hereinafter referred to as the 2-end network side). DN1 represents the three-phase AC source area on the 1-end network side; DN2 represents the three-phase AC source area on the 2-end network side. R 1—x , R 2—x ( x =a, b, c represent the three-phase resistances a, b, and c) and L 1—x , L 2—x These represent the resistance and inductance of the three-phase lines a, b, and c between terminals 1 and 2 of the network; i 12This indicates the feeder current between terminals 1 and 2 on the grid side; u sx This indicates the three-phase series output voltage of the SF-SOP. U dc This is the actual value of the DC voltage.

[0058] During the calculation, it is first necessary to collect a multi-dimensional electrical state parameter set of the power grid from the first and second terminal grid sides. This multi-dimensional electrical state parameter set includes electrical state parameters and line structure parameters; the electrical state parameters include three-phase voltage amplitude and three-phase voltage phase. Phase of three-phase voltage (i=1,2 indicates ports 1 and 2); the line structure parameters include the three-phase line resistances R1-x and R2-x (x=a,b,c indicates three phases a, b, and c) and the three-phase line inductances L1-x and L2-x.

[0059] Step S202: Determine the series voltage vector control parameters based on the multi-dimensional electrical state parameter set of the power grid;

[0060] After obtaining the above parameters, corresponding calculations are required to determine the series voltage vector control parameters. The series voltage vector control parameters include the initial voltage amplitude of the series reference output and the initial voltage phase of the series reference output. The calculation process of steps S202 and S203 is explained in detail below:

[0061] First, based on the three-phase voltage amplitude, three-phase voltage phase, three-phase line resistance, and three-phase line inductance, the initial phase of the series reference output voltage is determined. The calculation process is as follows:

[0062] Taking phase a as an example, the circuit equivalent model of the flexible interconnection switch of SF-SOP under grid-connected operation is as follows: Figure 4 As shown, where The set of three-phase line impedances (a, b, c) between terminals 1 and 2 is represented by the line inductance and resistance calculated by formula (1). and These represent the amplitude and phase of the grid-side output voltage, respectively; power flow control equipment is equivalent to a controlled voltage source. , It is its actual output voltage amplitude. It is the phase of its actual output voltage.

[0063] like Figure 5 As shown, and Let these represent the single-phase voltage vectors of transformer substations 1 and 2, respectively. Establish a coordinate system for the real axis; This indicates the phase difference of the voltage in the transformer substation. This indicates the voltage drop caused by line impedance; This represents the single-phase output voltage vector of the SF-SOP. Represents the line current vector; Indicates the phase of the line current; This indicates the phase of the single-phase output voltage of the SF-SOP, where and The phase difference is 90°. If the output voltage amplitude at terminal 1 is greater than the output voltage amplitude at terminal 2, it will result in a larger line voltage. This leads to a significant line current. The flexible interconnection switch will provide a reverse potential to offset the voltage sag of DN2, or the phase of the grid-side output voltage at end 1 will lead the phase of the grid-side output voltage at end 2, and the power will flow from end 1 to end 2. Similarly, the condition for power to flow from end 2 to end 1 can be reversed.

[0064] like Figure 6 As shown, the DC capacitor regulates the magnitude of the DC voltage by charging and discharging. In order to stabilize the DC voltage, the output voltage of the SF-SOP needs to be orthogonal to the circuit.

[0065] Based on such Figure 5 The equivalent circuit model shown in this application can calculate the power transmitted by the power grid. Reactive power flows from the grid side with high output voltage amplitude to the grid side with low output voltage amplitude, and active power flows from the grid side with leading phase voltage phase to the grid side with lagging phase voltage phase. When connected to the grid, the active power output by the grid side flows from the grid with high frequency to the grid with low frequency. The calculation method of the power of each node of the distribution network under grid connection conditions is as follows.

[0066] First, calculations based on the equivalent circuit model yielded the following:

[0067] (1)

[0068] In the formula, The imaginary unit, The angular frequency of the power grid. and These represent the actual output voltage amplitude and phase of the SF-SOP, respectively. This represents the set of line impedances between terminals 1 and 2, including... and , Indicates the phase of the impedance of the distribution network line;

[0069] Then, by combining formula (1), the active power injected from the 1-end grid side to the 2-end grid side is obtained. Or active power injected from the 2nd grid side to the 1st grid side for:

[0070] (2)

[0071] Subsequently, when the phase difference between the series output voltage and the line current is 90°, its DC side voltage can remain stable. Combining formulas (1) and (2), the following formula (3) can be obtained:

[0072] (3)

[0073] Substituting the aforementioned collected data into formula (3), the initial phase of the series reference output voltage is calculated. .

[0074] Then the series reference output initial voltage phase Substituting the aforementioned collected data into equation (4), the initial voltage amplitude of the series reference output is calculated and determined. :

[0075] (4)

[0076] Step S203: Based on the series voltage vector control parameters, determine the pulse width modulation signal of the fourth bridge arm of the inverter.

[0077] The specific calculation process for determining the pulse width modulation (PWM) signal of the fourth bridge arm of the inverter is as follows:

[0078] At this point, the initial voltage control command of SF-SOP needs to be corrected. The following calculations are required, and the actual DC voltage value needs to be collected. and obtaining the target value of DC voltage Then, the difference between the actual DC voltage value and the target DC voltage value is proportionally integrated to output the phase change value. .

[0079] Then The input limiting circuit ensures that the SF-SOP series output voltage varies within a certain phase range, preventing overcorrection from causing the 90° phase difference between the series output voltage and the line current to fail, which would lead to DC-side voltage instability. Then, the initial phase of the series reference output voltage is used. and phase change value By subtracting the values, we obtain the phase of the series reference output voltage. Then, based on the phase of the series reference output voltage... and series reference output initial voltage amplitude This is used to determine the pulse width modulation signal for the fourth bridge arm of the inverter.

[0080] Connect the series reference output voltage phase Perform a sine calculation to obtain the sine value of the series reference output voltage phase. Then, based on the sine value of the series reference output voltage phase and the initial amplitude of the series reference output voltage... This is used to determine the pulse width modulation signal for the fourth bridge arm of the inverter.

[0081] The sinusoidal value of the phase of the series reference output voltage and the amplitude of the initial series reference output voltage. Multiplying them together yields the three-phase reference output voltage vector. ( x =a, b, c represent the three phases a, b, and c of the inverter. Then, based on the three-phase reference output voltage vector, the pulse width modulation signal of the fourth bridge arm of the inverter is determined.

[0082] After determining the three-phase reference output voltage vector, it is necessary to track the three-phase four-arm reference voltage command. The process is as follows:

[0083] Collect actual three-phase output voltage Then, the actual output voltage of the three phases and three-phase reference output voltage vector The difference between the subtraction and input is used for proportional-integral feedback control to obtain the current feedback quantity. Then based on the current feedback quantity This is used to determine the pulse width modulation signal for the fourth bridge arm of the inverter.

[0084] At this time, the inverter filter capacitor current is collected. Then the inverter filter capacitor current and current feedback quantity The difference is used for proportional feedback control to obtain the modulated voltage signal. Then, based on the modulated voltage signal, the pulse width modulation signal of the fourth bridge arm of the inverter is determined.

[0085] modulated voltage signal The waveform and the triangular wave are compared to obtain the pulse width modulation signal of the three-phase bridge arm of the inverter. Then, based on the pulse width modulation signal of the three-phase bridge arm of the inverter, the pulse width modulation signal of the fourth bridge arm of the inverter is determined.

[0086] The pulse width modulation signal of the three-phase bridge arm of the inverter is decoupled and calculated through the fourth bridge arm to obtain the modulation voltage of the fourth bridge arm. The modulation voltage of the fourth bridge arm is compared with the triangular wave, and the pulse width modulation signal of the fourth bridge arm of the inverter is output.

[0087] In this application, the PWM generation method employs a comparison between a modulated wave and a triangular wave. Regarding the PI adjustment value in this application, The larger the value, the smaller the equivalent inductance inside each phase of the flexible closed-loop equipment; however, from the perspective of feedback control, Too large a value can easily cause oscillations, therefore Choose wisely.

[0088] Experimental simulation

[0089] This application uses a novel three-phase four-arm series flexible interconnection switch model built in the MATLAB / Simulink simulation platform to verify the effectiveness and correctness of the proposed control method. The simulation parameters are shown in Table 1.

[0090] Table 1 Simulation Parameters

[0091] ;

[0092] To simulate the actual operating state of the novel three-phase four-arm series-connected flexible interconnection switch and verify the effectiveness of the control method in this application, this application fully considers the dynamic fluctuation of active power transmitted from the grid side. For example... Figure 7 The voltage amplitude of port 1 is 311V with an initial phase of 0°, and the voltage amplitude of port 2 is 280V with an initial phase of 15°.

[0093] In addition, such as Figure 8 As shown, the current waveform of the line and the output voltage waveform of the SF-SOP are shown. The SF-SOP can change the output voltage according to the different voltage control methods and control requirements of the device, so as to better control the active power and DC side voltage stability of the line.

[0094] Under simulated conditions of uneven three-phase load, 70kW of active power is transmitted from phase a to port 2 at port 1, 50kW of active power is transmitted from phase c, and 60kW of active power is transmitted from phase b to port 1 at port 2. The measured line active power and the SF-SOP's own output power are as follows: Figure 9 As shown.

[0095] To further verify the stability and superiority of this application for a three-phase four-arm series-connected flexible interconnection switch, three schemes are compared below. ① Scheme 1: Based on a four-arm back-to-back power electronic device, such as... Figure 10 (a) is shown; ② Scheme 2: Based on a three-phase H-bridge structure device, such as Figure 10 (b) shows; ③ This application provides a three-phase four-arm series flexible interconnection switch with self-stabilizing capability, such as Figure 2 As shown;

[0096] For ease of comparison and discussion, all the above schemes are implemented under the following conditions: the normal operating voltage amplitude on the AC side is 311V, the phase difference between the three-phase voltages is 120° sequentially, the DC side voltage is stable at 800V, the initial phase of the grid voltage at port 2 leads port 1 by 15°, and the voltage amplitudes are the same. Each phase transmits 70 / -60 / 50 kW of active power. A comparison of this application with existing schemes is shown in Table 2.

[0097] Table 2

[0098] ;

[0099] As shown in Table 2, the parallel SOP uses two three-phase isolation transformers in the distribution area, while the series SOP uses three single-phase isolation transformers. The output voltage of the parallel SOP is determined by the bus voltage of the distribution area, while the output voltage of the series SOP is calculated from the compensation voltage. Since the compensation voltage is often lower than the bus voltage, the output voltage of the parallel SOP is higher than that of the series SOP. The passive components of the four-bridge parallel back-to-back SOP include 6 filter inductors, 6 filter capacitors, and 1 DC capacitor, and the power electronic components include 16 IGBTs. The passive components of the three-phase H-bridge SOP include 3 filter inductors, 3 filter capacitors, and 3 DC capacitors, and the power electronic components include 12 IGBTs. The passive components of the SF-SOP include 4 filter inductors, 3 filter capacitors, and 1 DC capacitor. The SF-SOP has fewer components, a smaller system size, smaller capacity, and lower cost. Therefore, the SF-SOP can significantly reduce the requirements and quantity of components compared to existing SOPs, reducing the capacity and cost of the device itself. Furthermore, this application achieves DC-side self-regulation without the need for an external DC power source, and the voltage regulation effect is excellent. Therefore, compared with the prior art, this application can perform self-regulation control without other power electronic devices or DC power sources, reducing the requirements and number of devices, and further reducing the size and investment cost of series-type equipment.

[0100] To further verify the advantages of the proposed equipment, this application conducts simulation verifications of a four-arm parallel back-to-back SOP, a three-phase H-bridge SOP, and an SF-SOP. Simulation parameters are set according to the comparison conditions. The output voltage, line current, DC side voltage, and active power transmission of the back-to-back SOP, three-phase H-bridge SOP, and SF-SOP are as follows: Figures 11-12 As shown.

[0101] Simulation experiments show that when controlling the same power, the line current of the three types of SOPs is consistent. However, the output voltage of the series-connected SOP is much lower than that of the parallel-connected SOP, and its DC-side stability is slower than that of the SF-SOP. Therefore, the capacity of the series-connected SOP is smaller than that of the parallel-connected SOP, while the SF-SOP has stronger DC-side stability. The SF-SOP and the three-phase H-bridge SOP have the same control capability, but the three-phase H-bridge SOP has more components, resulting in a larger device size and lower utilization rate. Furthermore, single-phase DC-side control exhibits secondary ripple. Therefore, the SF-SOP offers better DC-side control and is more advantageous in terms of size and cost.

[0102] In summary, the method proposed in this application makes full use of the following control of the existing three-phase four-arm inverter, which can control the magnitude of active power dispatch between feeders, and can perform self-stabilizing control on the DC side without the need for other power electronic equipment or DC source, thereby reducing the voltage withstand requirements of the devices and reducing the number of devices, and improving the stability and economy of the system.

[0103] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A DC self-regulating voltage control method for a three-phase four-arm series-connected flexible interconnected switch, applied to a distribution network connecting two three-phase AC sources, wherein the two three-phase AC sources respectively constitute the first and second terminal sides of the distribution network, characterized in that... include: Collect a set of multi-dimensional electrical state parameters of the power grid from the first terminal grid side and the second terminal grid side. The set of multi-dimensional electrical state parameters of the power grid includes electrical state parameters and line structure parameters. The electrical state parameters include three-phase voltage amplitude and three-phase voltage phase. The line structure parameters include three-phase line resistance and three-phase line inductance. Based on the multi-dimensional electrical state parameter set of the power grid, series voltage vector control parameters are determined, including the amplitude of the initial voltage of the series reference output and the phase of the initial voltage of the series reference output. Based on the series voltage vector control parameters, the pulse width modulation signal of the fourth bridge arm of the inverter is determined; The step of determining the series voltage vector control parameters based on the multi-dimensional electrical state parameter set of the power grid includes: Based on the three-phase voltage amplitude, three-phase voltage phase, three-phase line resistance, and three-phase line inductance, determine the initial phase of the series reference output voltage; The initial voltage amplitude of the series reference output is determined based on the initial phase of the series reference output. The step of determining the pulse width modulation signal of the fourth arm of the inverter based on the voltage vector control parameters includes: Collect actual DC voltage values; Obtain the target DC voltage value; The phase change value is obtained by proportionally integrating the difference between the actual DC voltage value and the target DC voltage value. The series output reference voltage phase is obtained by subtracting the initial phase of the series reference output voltage from the phase change value. Based on the phase of the series output reference voltage and the amplitude of the initial series reference output voltage, the pulse width modulation signal of the fourth bridge arm of the inverter is determined.

2. The method according to claim 1, characterized in that, The step of determining the pulse width modulation signal of the fourth bridge arm of the inverter based on the phase of the series output reference voltage and the initial amplitude of the series reference output voltage includes: The phase of the series output reference voltage is calculated sinusoidally to obtain the sinusoidal value of the series reference output voltage phase. Based on the sinusoidal value of the series reference output voltage phase and the amplitude of the series reference output initial voltage, the pulse width modulation signal of the fourth bridge arm of the inverter is determined.

3. The method according to claim 2, characterized in that, The step of determining the pulse width modulation signal of the fourth bridge arm of the inverter based on the sinusoidal value of the series reference output voltage phase and the initial amplitude of the series reference output voltage includes: Multiply the sinusoidal value of the series reference output voltage phase by the amplitude of the series reference output initial voltage to obtain the three-phase reference output voltage vector; Based on the three-phase reference output voltage vector, the pulse width modulation signal of the fourth bridge arm of the inverter is determined.

4. The method according to claim 3, characterized in that, The step of determining the pulse width modulation signal of the fourth arm of the inverter based on the three-phase reference output voltage vector includes: Collect the actual three-phase output voltage; The difference between the actual three-phase output voltage and the three-phase reference output voltage vector is proportional to integral feedback to obtain the current feedback quantity; Based on the current feedback, the pulse width modulation signal of the fourth bridge arm of the inverter is determined.

5. The method according to claim 4, characterized in that, The step of determining the pulse width modulation signal of the fourth arm of the inverter based on the current feedback includes: Collect the inverter filter capacitor current; The difference between the current feedback quantity and the inverter filter capacitor current is proportionally fed back to obtain the modulated voltage signal. Based on the modulated voltage signal, the pulse width modulation signal of the fourth bridge arm of the inverter is determined.

6. The method according to claim 5, characterized in that, Determining the pulse width modulation signal of the fourth arm of the inverter based on the modulated voltage signal includes: The waveform of the modulated voltage signal is compared with a triangular wave to obtain the pulse width modulation signal of the three-phase bridge arm of the inverter. Based on the pulse width modulation signals of the three-phase bridge arms of the inverter, the pulse width modulation signal of the fourth bridge arm of the inverter is determined.

7. The method according to claim 6, characterized in that, The determination of the pulse width modulation signal of the fourth arm of the inverter based on the pulse width modulation signal of the three-phase bridge arm of the inverter includes: The pulse width modulation signal of the three-phase bridge arm of the inverter is decoupled and calculated through the fourth bridge arm to obtain the modulation voltage of the fourth bridge arm; The modulation voltage of the fourth bridge arm is compared with a triangular wave to output the pulse width modulation signal of the fourth bridge arm of the inverter.

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