Three-phase imbalance control method and device for transformer area flexible interconnection system and medium

Abstract

The invention belongs to the technical field of power grid control, and relates to a transformer area flexible interconnection system three-phase imbalance control method and device and a medium. Taking the transformer area converter controlled by the PQ control algorithm as a target transformer area converter; collecting a first control parameter output by an active power PI controller of the target transformer area converter; based on the upper and lower limits of the voltage target value of the DC bus, performing amplitude limiting setting on the first control parameter of the target transformer area converter to obtain the voltage target value; subtracting the voltage target value from the actual voltage value of the direct-current bus, inputting the difference value into the PI controller, and outputting a second control parameter of the target transformer area converter; taking the second control parameter as a direct-axis current target value, subtracting the direct-axis current target value from a direct-axis current actual value, and taking the difference value as the input of a direct-axis current PI controller; and controlling the target transformer area converter based on the voltage modulation signal output by the PQ control framework. The scheme can effectively improve the power supply reliability of the power distribution area and stabilize the DC bus voltage.

Description

Technical Field

[0001] This invention relates to the field of power grid control technology, and in particular to a method, apparatus and computer-readable storage medium for three-phase imbalance control of a flexible interconnection system of distribution transformers. Background Technology

[0002] With economic development, electric vehicle charging stations and a large number of distributed power sources such as wind and solar power are being connected to distribution substations. Their randomness and volatility are making the "passivity" of the physical structure of these substations increasingly prominent. The high-density connection of these nonlinear loads is exacerbating the problem of three-phase unbalanced current in the distribution system. This unbalanced current circulates in the distribution lines, significantly increasing line losses and accelerating the aging of insulation in power transformers, cables, and other equipment, thus shortening their lifespan. Flexible Interconnection Devices (FIDs) based on power electronic devices can achieve power sharing between distribution substations, promoting the absorption of distributed power sources and meeting the demand for high-quality power supply. They can also provide voltage support after a substation fault, improving the controllability, reliability, and security of the interconnected substations.

[0003] Existing FID technology is mainly used for power mutual assistance between distribution stations, that is, through PQ control and U dc The Q-control mode in the shared control system can effectively improve the active power carrying capacity of the distribution area. However, under the existing PQ control mode, when one or more distribution areas in the flexible interconnection system experience a sudden increase or decrease in load or a three-phase imbalance, the power reference value output by the system is delayed. Under the PQ control framework, the control side's failure to receive and transmit the power reference value in a timely manner will cause an instantaneous power deficit. This deficit instantaneous power is borne by the DC bus, resulting in a surge or drop in the DC bus voltage, which in turn causes DC bus voltage instability and affects the operational stability and safety of the flexible interconnection system. In addition, when the flexible interconnection system switches operating modes, if the distribution area converters connected to the distribution area adopt the basic PQ control mode, the PI controller bandwidth mismatch during the mode switching process will increase the overshoot and control time, making the DC bus prone to instability, which will also affect the operation of the flexible interconnection system.

[0004] In summary, the existing three-phase imbalance control method for flexible interconnection systems in distribution substations has the problem that when the load of the distribution substation suddenly increases or decreases, the three-phase imbalance occurs, or the operating mode is switched, the PQ control mode will cause the DC bus voltage to be unstable, which in turn affects the safe and stable operation of the flexible interconnection system in the distribution substation. Summary of the Invention

[0005] Therefore, the technical problem to be solved by the present invention is to overcome the problem that the three-phase unbalance control method of the existing distribution area flexible interconnection system has the problem that when the load of the distribution area suddenly increases or decreases, the three-phase is unbalanced, or the operating mode is switched, the PQ control mode will cause the DC bus voltage to be unstable, which in turn affects the safe and stable operation of the distribution area flexible interconnection system.

[0006] To solve the above-mentioned technical problems, the present invention provides a three-phase imbalance control method for a flexible interconnection system of transformer substations, comprising: The transformer converters in the flexible interconnection system controlled by the PQ control algorithm are taken as target transformer converters; the first control parameters output by the active power PI controller in the PQ control framework of each target transformer converter are collected. Based on the upper and lower limits of the DC bus voltage target value, the first control parameter of each target transformer area is limited and tuned to obtain the voltage target value of each target transformer area. The voltage target value of each target transformer area converter is subtracted from the actual voltage value of the DC bus, and the difference is input into the PI controller to output the second control parameter of each target transformer area converter. The second control parameter of each target transformer area converter is used as the target value of the direct axis current of each target transformer area converter. The difference between the target value and the actual value of the direct axis current of each target transformer area converter is calculated, and the difference is used as the input of the direct axis current PI controller in the PQ control framework of each target transformer area converter. Based on the voltage modulation signal output by the PQ control framework of each target transformer area converter, the active power and reactive power of each target transformer area converter are controlled.

[0007] Preferably, based on the upper and lower limits of the target voltage value of the DC bus, the first control parameter of each target transformer area is limited and tuned to obtain the target voltage value of each target transformer area, including: The first control parameter of each target transformer area is compared with the upper and lower limits of the target voltage value of the DC bus. If the first control parameter is greater than or equal to the upper limit of the DC bus voltage target value, then the upper limit of the DC bus voltage target value shall be used as the voltage target value of the target transformer area. If the first control parameter is less than the upper limit of the DC bus voltage target value and greater than the lower limit of the DC bus voltage target value, then the first control parameter is taken as the voltage target value of the target transformer area. If the first control parameter is less than or equal to the lower limit of the DC bus voltage target value, then the lower limit of the DC bus voltage target value shall be used as the voltage target value of the target transformer area.

[0008] Preferably, the upper and lower limits of the target voltage value of the DC bus fluctuate within a preset fluctuation range; wherein, the preset fluctuation range is a preset proportion of the target voltage value of the DC bus.

[0009] Preferably, it further includes: The difference between the target reactive power value and the actual reactive power value of each target transformer area converter is used as the target quadrature axis current value of each target transformer area converter. The target value of the quadrature axis current of each target transformer area converter is subtracted from the actual value of the quadrature axis current of each target transformer area converter, and the difference is used as the input of the quadrature axis current PI controller in the PQ control framework of each target transformer area converter.

[0010] Preferably, the PQ control framework output voltage modulation signal of each target transformer area includes: Based on the actual direct-axis current values ​​of the converters in each target distribution area, construct the quadrature-axis decoupling terms for each converter in each target distribution area; based on the actual quadrature-axis current values ​​of the converters in each target distribution area, construct the direct-axis decoupling terms for each converter in each target distribution area. Based on the third control parameter output by the direct-axis current PI controller in the PQ control framework of each target transformer area converter, the direct-axis decoupling term, and the actual value of the direct-axis voltage of each target transformer area converter, the direct-axis voltage modulation signal of each target transformer area converter is calculated. Based on the fourth control parameter output by the quadrature-axis current PI controller in the PQ control framework of each target transformer area, the quadrature-axis decoupling term, and the actual value of the quadrature-axis voltage of each target transformer area, the quadrature-axis voltage modulation signal of each target transformer area is calculated.

[0011] Preferably, the method further includes obtaining the phase angle of each target transformer area converter using a dual second-order generalized integrator phase-locked loop, specifically including: Based on the A-phase voltage, B-phase voltage and C-phase voltage of the target transformer area converter, calculate the positive sequence voltage component and negative sequence voltage component of the target transformer area converter. The positive-sequence and negative-sequence voltage components of the target transformer are subjected to Clark transformation to output the α-axis DC component and β-axis DC component of the target transformer. The α-axis DC component and the preset angular frequency of the target transformer are input together into the first second-order generalized integrator, which outputs the first sine wave signal and the first cosine wave signal; the β-axis DC component and the preset angular frequency of the target transformer are input together into the second second-order generalized integrator, which outputs the second sine wave signal and the second cosine wave signal. Based on the vector product of the first and second sine wave signals, the positive sequence DC component of the α-axis is obtained; based on the vector product of the first and second cosine wave signals, the positive sequence DC component of the β-axis is obtained; based on the positive sequence DC components of the α-axis and β-axis, the positive sequence DC component of the target voltage of the target transformer area is obtained. The target voltage positive sequence component is subjected to Parker transformation to obtain the d-axis DC component and the q-axis DC component. The q-axis DC component is input into the PI regulator to output the instantaneous angular frequency. The vector product of the instantaneous angular frequency and the preset angular frequency is integrated to obtain the phase angle of the target transformer converter.

[0012] Preferably, controlling the active and reactive power of the converters in each target distribution area includes: Based on the direct-axis voltage modulation signal of each target transformer area converter, the direct-axis current reference value of each target transformer area converter is obtained; based on the quadrature-axis voltage modulation signal of each target transformer area converter, the quadrature-axis current reference value of each target transformer area converter is obtained. Based on the phase angle of each target transformer area converter, the direct-axis current reference value and quadrature-axis current reference value of each target transformer area converter are transformed by coordinate transformation to obtain the three-phase current reference value of each target transformer area converter; Based on the reference values ​​and actual values ​​of the three-phase current of each target transformer area converter, the error value of the three-phase current of each target transformer area converter is calculated. The actual values ​​of direct-axis current and quadrature-axis current of each target transformer area converter are filtered by a moving average filter. The filtered actual values ​​of direct-axis current and quadrature-axis current of each target transformer area converter are then input into a PI controller, which outputs the correction values ​​of direct-axis current and quadrature-axis current of each target transformer area converter. Based on the phase angle of each target transformer, the direct-axis current correction and quadrature-axis current correction of each target transformer are transformed by coordinate transformation to obtain the three-phase current compensation value of each target transformer. The three-phase current compensation value of each target transformer area converter is superimposed with its three-phase current error value to obtain the three-phase modulation error of each target transformer area converter; the three-phase modulation error of each target transformer area converter is subjected to triangular carrier modulation to generate the PWM drive signal of each target transformer area converter, thereby controlling the active power and reactive power of each target transformer area converter.

[0013] Preferably, after obtaining the three-phase current compensation values ​​for each target transformer area converter, the method further includes: Based on the averaging coefficient of the actual three-phase current of each target transformer area converter, the current sharing reference value of each target transformer area converter is obtained. The three-phase current compensation value of each target transformer area converter is superimposed with its current sharing reference value to obtain the new three-phase current compensation value of each target transformer area converter.

[0014] The present invention also provides a three-phase imbalance control device for a flexible interconnection system of transformer substations, comprising: The signal acquisition module is used to take the transformer converters controlled by the PQ control algorithm in the transformer flexible interconnection system as target transformer converters; and to acquire the first control parameters output by the active power PI controller in the PQ control framework of each target transformer converter. The limiting setting module is used to limit and set the first control parameter of each target transformer area based on the upper and lower limits of the DC bus voltage target value, so as to obtain the voltage target value of each target transformer area. The voltage regulation module is used to calculate the difference between the target voltage value of each target transformer area converter and the actual voltage value of the DC bus, input the difference into the PI controller, and output the second control parameter of each target transformer area converter. The current regulation module is used to take the second control parameter of each target transformer area converter as the target value of the direct axis current of each target transformer area converter, and calculate the difference with the actual value of the direct axis current of each target transformer area converter. The difference is then used as the input of the direct axis current PI controller in the PQ control framework of each target transformer area converter. The power control module is used to control the active and reactive power of each target transformer area converter based on the voltage modulation signal output by the PQ control framework of each target transformer area converter.

[0015] The present invention also provides a computer-readable storage medium storing a computer program, which, when executed by a processor, implements the steps of the above-described three-phase imbalance control method for a flexible interconnection system of transformer substations.

[0016] The three-phase imbalance control method for the flexible interconnection system of transformer substations provided in this application has the following beneficial effects: For the target transformer in PQ control mode, the first control parameter output by its active power PI controller is collected and limited based on the upper and lower limits of the DC bus voltage target value. This limits the active power regulation that may cause the DC voltage to exceed the limit, avoiding DC bus voltage instability caused by the delay in the system's output active power target value due to sudden increases or decreases in the distribution transformer load, three-phase imbalance, or operation mode switching. The difference between the limited voltage target value and the actual DC bus voltage value is calculated and output as a second control parameter, which is then used as the direct-axis current target value, thereby utilizing the DC voltage... The deviation directly drives the direct-axis current target correction, ensuring that the active power output in PQ control mode not only tracks the PQ command but also maintains the voltage stability of the DC bus. Simultaneously, the limiting setting based on the upper and lower limits of the DC bus voltage target value ensures that the active power regulation during operating mode switching does not exceed the DC side's carrying capacity, preventing voltage instability caused by energy surges during operating mode switching. This ensures that the flexible interconnection system always has a transformer in the distribution area maintaining the voltage of the connected branch distribution network bus, effectively improving the power supply reliability of the distribution area and stabilizing the DC bus voltage, thus enabling the flexible interconnection system to operate safely and stably. Attached Figure Description

[0017] To make the content of this invention easier to understand, the invention will be further described in detail below with reference to specific embodiments and accompanying drawings, wherein: Figure 1 The topology diagram of the flexible interconnection system between the two distribution radio stations provided in this application; Figure 2 A schematic diagram of the connection structure between the transformer substation and the distribution substation provided in this application; Figure 3 U provided for this application dc Q-control algorithm block diagram; Figure 4 The block diagram of the PQ control algorithm provided in this application; Figure 5 Flowchart of the three-phase imbalance control method for the flexible interconnection system of the transformer substation provided in this application; Figure 6 The block diagram of the PUI control algorithm provided in this application; Figure 7 A schematic diagram of the SOGI-based orthogonal signal generator structure provided in this application; Figure 8 This is a schematic diagram of the DSOGI-PLL algorithm provided in this application; Figure 9 A schematic diagram of the three-phase imbalance control principle of the flexible interconnection system for transformer substations provided in this application; Figure 10 The output current and its sequence components of the first area converter in the flexible interconnection system of two distribution areas provided in the embodiments of this application; Figure 11 The output current and its sequence components of the second area converter in the flexible interconnection system of two distribution areas provided in the embodiments of this application; Figure 12 A schematic diagram of the first AC feeder current in a flexible interconnection system of two distribution substations provided in this application embodiment; Figure 13 A schematic diagram of the second AC feeder current in a flexible interconnection system of two distribution substations provided in this application embodiment; Figure 14 A schematic diagram of the AC feeder current in the flexible interconnection system of two distribution substations provided in this embodiment of the application between 0s and 0.1s; wherein, Figure 14 (a) in the diagram is a schematic diagram of the first AC feeder current between 0s and 0.1s. Figure 14 (b) in the diagram is a schematic diagram of the second AC feeder current between 0s and 0.1s; Figure 15 A schematic diagram of the AC feeder current between 0.7s and 0.8s in the flexible interconnection system of two distribution substations provided in this embodiment of the application; wherein, Figure 15 (a) in the diagram is a schematic diagram of the first AC feeder current between 0.7s and 0.8s. Figure 15 (b) in the figure is a schematic diagram of the second AC feeder current between 0.7s and 0.8s. Detailed Implementation

[0018] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, so that those skilled in the art can better understand and implement the present invention. However, the embodiments described are not intended to limit the present invention.

[0019] A flexible interconnection device (FID) system based on power electronic devices consists of two or more voltage source converters (VSCs). Each VSC operates independently and can meet different control requirements of the flexible interconnection system. Furthermore, the DC link in the FID structure provides decoupling, enabling fault isolation between interconnected distribution substations, ensuring the normal operation of non-faulty substations, providing voltage support for faulty substations, and limiting fault current flow.

[0020] The following explanation uses a two-end back-to-back FID as an example to illustrate the structure, operation, and control principle of FID: like Figure 1The diagram shown is a topology of a flexible interconnection system for two distribution substations provided in this application. The FID (Flexible Interconnection System) consists of distribution substation 1 and distribution substation 2. If distribution substation 1 and distribution substation 2 are operating normally, the FID can balance the load and reduce network losses through power flow control. The VSC (Variable Switching Center) in one of the distribution substations operates in U... dc In Q control mode, the DC bus voltage needs to be stabilized and its reactive power output needs to be controlled. In another distribution substation, the VSC operates in PQ control mode and its active and reactive power output needs to be controlled.

[0021] like Figure 2 The diagram shown is a schematic diagram of the connection structure between the transformer substation and the distribution substation provided in this application. This indicates the voltage of the DC bus. , This indicates a split capacitor.

[0022] According to Kirchhoff's voltage law, VSC i The AC side voltage equation (i=1,2) is expressed as: , in, 、 、 This represents the three-phase voltage of AC feeder i; 、 、 VSC i The three-phase voltage; 、 、 This represents the three-phase current of VSCi; Indicates the equivalent resistance; This represents the equivalent inductance.

[0023] Performing a Parker transformation on the above AC side voltage equation yields VSC. i AC side voltage equation in dq coordinate system: , , in, The matrix used in the Park transformation; This represents the angle between the d-axis and the a-axis; , Represents the direct-axis (d-axis) voltage and quadrature-axis (q-axis) voltage of AC feeder i; 、 Represents the direct-axis voltage and quadrature-axis voltage of VSCi; , This represents the direct-axis and quadrature-axis currents of VSCi; This represents the fundamental angular frequency.

[0024] Furthermore, , , Furthermore, the active power of VSCi and reactive power It can be represented as: , , If the direction of the AC feeder i voltage vector is defined as the d-axis direction, then ,and ,in, Let VSCi represent the phase voltage amplitude of AC feeder i, then the active power of VSCi is... and reactive power It can be represented as: , , Therefore, the active power of VSCi reactive power The relationship between the AC side current and the AC side current is as follows: , .

[0025] Furthermore, such as Figure 3 The image shown is of the U provided in this application. dc Q-control algorithm block diagram, running at U dc In Q control mode, the VSC acts as the master station to maintain the stability of the DC bus voltage, ensuring the smooth operation of the flexible interconnected distribution substation area.

[0026] Specifically, U dc The Q control mode employs dual closed-loop control of voltage and current. After comparing the target value and actual value of the DC bus voltage, the target value of the d-axis component of the VSC output current, i, is obtained through the PI (Proportional Integral) controller. dref Then compare it with the actual current d-axis component i d Subtraction yields the d-axis component error value i of the VSC output current. d *; The upper-level system calculates the required reactive power command value Q for each transformer area based on actual operating conditions. ref Then through The command value i of the q-axis component of the current obtained from the calculation formula qref Then, compare it with the actual value of the q-axis component of the VSC output current i. q Subtraction yields the q-axis component error value i of the VSC output current. q * Next, the error values ​​i of the d-axis and q-axis components of the VSC output current. d * and i q *Voltage modulation signal V in dq coordinate system is output via PI controller d and V q , to obtain running on U dc Control commands in Q control mode.

[0027] like Figure 4 The diagram shown is a block diagram of the PQ control algorithm provided in this application. Specifically, the target active power value P output by the VSC is... ref After subtracting from the actual value P, the target value i of the d-axis component of the VSC output current is obtained through the PI controller. dref Then compare it with the actual current d-axis component i d Subtraction yields the d-axis component error value i of the VSC output current. d *; The upper-level system calculates the required reactive power command value Q for each transformer area based on actual operating conditions. ref Then through The command value i of the q-axis component of the current obtained from the calculation formula qref Then, compare it with the actual value of the q-axis component of the VSC output current i. q Subtraction yields the q-axis component error value i of the VSC output current. q * Next, the error values ​​i of the d-axis and q-axis components of the VSC output current. d * and i q *Voltage modulation signal V in dq coordinate system is output via PI controller d and V q This allows for the control of the active and reactive power output of the slave station's VSC.

[0028] Under the existing PQ control mode, when one or more distribution substations in the flexible interconnection system experience a sudden increase or decrease in load, or a three-phase imbalance, the system adjusts the power reference value P. ref There is a delay; under the PQ control framework, the control side for P... ref Delayed command transmission and reception will cause instantaneous power shortage, and the DC bus will bear the instantaneous power shortage. Therefore, the DC bus voltage will spike or drop, resulting in DC bus voltage instability, which will affect the operational stability and safety of the flexible interconnection system in the distribution area.

[0029] Addressing the problems of existing PQ control modes, this application proposes a PUI control mode to improve upon the PQ control mode in flexible interconnection systems for distribution transformer areas. Based on this control mode, a three-phase imbalance control method for such systems is also provided. Figure 5 The diagram shown is a flowchart of the three-phase imbalance control method for a flexible interconnection system in a transformer substation. Figure 6 The diagram shown is a block diagram of the PUI control algorithm.

[0030] It should be noted that this application only improves the control method of the transformer substation in PQ control mode. The system U dc The transformer substation converter in Q control mode still uses the traditional U... dc Q control.

[0031] The specific methods for controlling the three-phase imbalance of the flexible interconnection system in this transformer area include: S10: Take the transformer in the flexible interconnection system controlled by the PQ control algorithm as the target transformer; collect the first control parameter output by the active power PI controller in the PQ control framework of each target transformer.

[0032] S20: Based on the upper and lower limits of the target voltage value of the DC bus, the first control parameter of each target transformer area is limited and tuned to obtain the target voltage value of each target transformer area.

[0033] S30: Subtract the target voltage value of each target transformer area converter from the actual voltage value of the DC bus, input the difference into the PI controller, and output the second control parameter of each target transformer area converter.

[0034] S40: The second control parameter of each target transformer area converter is used as the target value of the direct-axis current of each target transformer area converter. The difference between the target value and the actual value of the direct-axis current of each target transformer area converter is calculated, and the difference is used as the input of the direct-axis current PI controller in the PQ control framework of each target transformer area converter.

[0035] S50: Based on the voltage modulation signal output by the PQ control framework of each target transformer area converter, control the active power and reactive power of each target transformer area converter.

[0036] The PUI control mode designed in this application consists of a power loop and a voltage loop. The inner loop control remains a current loop. The power loop in the outer loop control is used to optimize the load power of the interconnected distribution transformer areas. In the flexible interconnection operation mode, when each distribution transformer area is running stably, the operating parameters within the area are all within a controllable range and do not exceed the limits. The system will perform power calculation based on the operating status and operating parameters of the flexible interconnected distribution transformer areas, and the calculated active power reference value will be obtained. The signal is sent to the VSC to enable power flow control within the interconnected distribution substations; the voltage loop is used to stabilize voltage fluctuations on the DC bus, and can be considered as a U-loop with a limiting effect. dc Q control.

[0037] When the DC bus voltage experiences a drop or surge, the PUI control mode provided in this application limits the DC bus voltage. When the DC bus voltage exceeds the limit, it can be limited to the upper / lower limit through the voltage loop. At this time, the power loop is temporarily inactive, and the PUI control side is equivalent to operating in U... dc In the Q control mode, the VSC always maintains the voltage of the DC distribution bus connected to it. When the voltage of the DC bus returns to the normal range, the power loop resumes its function. Therefore, it solves the problem of DC bus voltage instability caused by large changes in the load power of the distribution substation. It can ensure stable operation of the distribution network within the rated voltage range without any mode switching, thus improving the safety and stability of the power system.

[0038] Specifically, step S20 includes: S200: Compare the first control parameters of each target transformer area converter with the upper and lower limits of the target voltage value of the DC bus.

[0039] S201: If the first control parameter is greater than or equal to the upper limit of the target voltage value of the DC bus, then the upper limit of the target voltage value of the DC bus shall be used as the target voltage value of the converter in the target area.

[0040] S202: If the first control parameter is less than the upper limit of the DC bus voltage target value and greater than the lower limit of the DC bus voltage target value, then the first control parameter is used as the voltage target value of the target transformer area.

[0041] S203: If the first control parameter is less than or equal to the lower limit of the DC bus voltage target value, then the lower limit of the DC bus voltage target value shall be used as the voltage target value of the target transformer area converter.

[0042] Furthermore, the upper / lower limit of the target voltage value of the DC bus ( / The voltage fluctuation range of the DC bus is closely related to the VSC regulation capability. The larger the allowable fluctuation range of the DC bus voltage, the larger the range of adjustable power of the system. Therefore, the upper and lower limits of the target DC bus voltage value fluctuate within a preset fluctuation range; where the preset fluctuation range is a preset proportion of the target DC bus voltage value. For example, the preset fluctuation range can be set as follows: 3%Udcref, where Udcref represents the target voltage value of the DC bus.

[0043] Furthermore, such as Figure 6 As shown, the method also includes: The difference between the target reactive power value and the actual reactive power value of each target transformer area converter is used as the target quadrature axis current value of each target transformer area converter.

[0044] The target value of the quadrature axis current of each target transformer area converter is subtracted from the actual value of the quadrature axis current of each target transformer area converter, and the difference is used as the input of the quadrature axis current PI controller in the PQ control framework of each target transformer area converter.

[0045] In this application, the reactive power control loop no longer uses the traditional reactive power loop to limit the reactive power target value. Instead, it directly uses the difference between the reactive power target value and the actual reactive power value as the quadrature axis current target value.

[0046] In addition, when the flexible interconnected system switches operating modes, if the VSC connected to the distribution substation operates in the basic PQ control mode, the performance of its PI controller is mainly reflected in the DC bus voltage. Due to the bandwidth mismatch of the PI controller during mode switching, the overshoot increases, the control time increases, and the DC bus is prone to instability. However, the PUI control mode can limit it to the upper / lower limit through the voltage loop, ensuring stable operation of the distribution network within the rated voltage range without any mode switching, thus alleviating the control pressure on the PI controller and optimizing the control system. It can be seen that the control strategy provided in this application can effectively ensure the stability of the DC bus voltage during large power fluctuations and operating mode switching, improve the operational reliability of the flexible interconnected distribution substation, and reduce the control requirements of the PI controller.

[0047] Furthermore, such as Figure 6 As shown, the PQ control framework output voltage modulation signal of each target transformer area converter in step S50 includes: S500: Based on the actual direct-axis current values ​​of the converters in each target distribution area, construct the quadrature-axis decoupling terms for each converter in each target distribution area; based on the actual quadrature-axis current values ​​of the converters in each target distribution area, construct the direct-axis decoupling terms for each converter in each target distribution area.

[0048] S501: Based on the third control parameter output by the direct-axis current PI controller in the PQ control framework of each target transformer area converter, the direct-axis decoupling term, and the actual value of the direct-axis voltage of each target transformer area converter, the direct-axis voltage modulation signal of each target transformer area converter is calculated.

[0049] S502: Based on the fourth control parameter output by the quadrature-axis current PI controller in the PQ control framework of each target transformer area converter, the quadrature-axis decoupling term, and the actual value of the quadrature-axis voltage of each target transformer area converter, the quadrature-axis voltage modulation signal of each target transformer area converter is calculated.

[0050] Furthermore, in some embodiments of this application, a dual second-order generalized integrator phase-locked loop (DSOGI-PLL) is used to obtain the phase angle of the converter to be controlled. The DSOGI-PLL consists of two parts: DSOGI and SRF-PLL (Synchronous Reference Frame-PLL). The DSOGI module extracts the positive-sequence fundamental frequency component of the grid voltage during grid disturbances and grid imbalances, and the SRF-PLL module accurately tracks this positive-sequence fundamental frequency component, thereby detecting its phase and frequency information. The PLL output phase angle obtained in this way can ensure the accuracy of the phase angle information required when the system uses the symmetrical component method to extract the current compensation command value, making the obtained unbalanced current compensation command value more accurate, thus improving the management of three-phase current imbalance.

[0051] like Figure 7 The diagram shows the structure of the SOGI-based Quadrant Signal Generator (SOGI-QSG) provided in this application, where d and q are the two orthogonal output signals, v is the error signal between the input f and the output d, and the transfer function of the SOGI circuit is: , The error transfer function of the SOGI circuit is: , , in, Indicates the preset resonant frequency; This indicates the damping ratio.

[0052] As can be seen from the above transfer function, For bandpass filters, As a low-pass filter, it can filter out high-order harmonics and select a specific frequency. Therefore, SOGI-QSG can extract a set of pure orthogonal signals.

[0053] Furthermore, such as Figure 8 As shown, obtaining the phase angle of each target transformer area converter using a dual second-order generalized integrator phase-locked loop (DSOGI-PLL) specifically includes: Step 1: Based on the A-phase voltage, B-phase voltage, and C-phase voltage of the target transformer area converter, calculate the positive-sequence voltage component and the negative-sequence voltage component of the target transformer area converter.

[0054] Specifically, the voltage of the transformer in the distribution area can be expressed as: ,in, Indicates the voltage of the transformer substation. Represents the positive sequence component of voltage. Represents the negative sequence component of voltage. Represents the zero-sequence component of voltage; Therefore, the positive-sequence voltage component, negative-sequence voltage component, and zero-sequence voltage component of the target transformer area can be calculated using the symmetrical component method. Specifically, the formula for calculating the positive-sequence voltage component of the target transformer area is as follows: , in, This represents the positive sequence voltage component of the target transformer area. This indicates the A-phase voltage of the target transformer area; This represents the B-phase voltage of the converter in the target distribution area; This represents the C-phase voltage of the converter in the target distribution area; ; The formula for calculating the negative sequence voltage component of the target transformer area is: , in, This represents the negative sequence voltage component of the target transformer area. The formula for calculating the zero-sequence voltage component of the target transformer area is: , in, This represents the zero-sequence voltage component of the target transformer area.

[0055] Step 2: Perform Clarke transform on the positive-sequence and negative-sequence voltage components of the target transformer area converter to output the α-axis DC component and β-axis DC component of the target transformer area converter.

[0056] Step 3: Input the α-axis DC component and the preset angular frequency of the target transformer into the first second-order generalized integrator to output the first sine wave signal and the first cosine wave signal; input the β-axis DC component and the preset angular frequency of the target transformer into the second second-order generalized integrator to output the second sine wave signal and the second cosine wave signal.

[0057] Step 4: Based on the vector product of the first sine wave signal and the second sine wave signal, obtain the positive sequence DC component of the α-axis; based on the vector product of the first cosine wave signal and the second cosine wave signal, obtain the positive sequence DC component of the β-axis; based on the positive sequence DC components of the α-axis and the β-axis, obtain the positive sequence component of the target voltage of the target transformer area converter.

[0058] Step 5: Perform Parker transformation on the positive sequence component of the target voltage to obtain the d-axis DC component and the q-axis DC component. Input the q-axis DC component into the PI regulator and output the instantaneous angular frequency. Integrate the vector product of the instantaneous angular frequency and the preset angular frequency to obtain the phase angle of the target transformer area converter.

[0059] Specifically, performing a Clarke transform on the positive-sequence and negative-sequence voltage components yields: , , , Furthermore, , , Through calculation, we can obtain: , , in, , indicating lag Phase.

[0060] Furthermore, such as Figure 8 The diagram shown illustrates the principle of the DSOGI-PLL algorithm provided in this application. SRF-PLL is a three-phase phase-locked loop based on dq synchronous rotating coordinate transformation. To reduce output disturbances and accelerate the system's steady-state response, a synchronous coordinate system phase-locked loop (SRF-PLL) is chosen to lock the grid voltage. Its working process is as follows: The obtained... and Performing the Park transformation yields and The phase angle required for the Parker transform is the phase angle of the phase-locked loop output. Then... The signal is fed into a PI controller, and the instantaneous angular frequency can be obtained from the output of the PI controller. Integrating the angular frequency gives the grid phase angle information.

[0061] Furthermore, such as Figure 9 The diagram shown is a schematic diagram of the three-phase imbalance control principle of the flexible interconnection system for distribution transformers provided in this application. Based on the voltage modulation signals and phase angles of the converters in each target distribution transformer area obtained from the above embodiments, step S50 controls the active power and reactive power of each target distribution transformer area converter, including: S503: Based on the direct-axis voltage modulation signal of each target transformer area converter, obtain the direct-axis current reference value of each target transformer area converter; based on the quadrature-axis voltage modulation signal of each target transformer area converter, obtain the quadrature-axis current reference value of each target transformer area converter.

[0062] S504: Based on the phase angle of each target transformer area converter, the direct-axis current reference value and quadrature-axis current reference value of each target transformer area converter are transformed by coordinate transformation to obtain the three-phase current reference value of each target transformer area converter.

[0063] S505: Based on the reference values ​​and actual values ​​of the three-phase current of each target transformer area converter, the error value of the three-phase current of each target transformer area converter is calculated.

[0064] S506: The actual values ​​of direct-axis current and quadrature-axis current of each target transformer area converter are filtered by a moving average filter. The filtered actual values ​​of direct-axis current and quadrature-axis current of each target transformer area converter are then input into the PI controller, and the direct-axis current correction and quadrature-axis current correction of each target transformer area converter are output.

[0065] S507: Based on the phase angle of each target transformer area converter, the direct-axis current correction and quadrature-axis current correction of each target transformer area converter are transformed by coordinate transformation to obtain the three-phase current compensation value of each target transformer area converter.

[0066] Optionally, such as Figure 9 As shown, step S507 is followed by: S507-A: Based on the averaging coefficient of the actual three-phase current values ​​of the converters in each target distribution area, the reference value of current sharing for each target distribution area converter is obtained.

[0067] S507-B: The three-phase current compensation value of each target transformer area converter is superimposed with its current sharing reference value to obtain the new three-phase current compensation value of each target transformer area converter.

[0068] S508: The three-phase current compensation value of each target transformer area converter is superimposed with its three-phase current error value to obtain the three-phase modulation error of each target transformer area converter; the three-phase modulation error of each target transformer area converter is subjected to triangular carrier modulation to generate the PWM drive signal of each target transformer area converter, thereby controlling the active power and reactive power of each target transformer area converter.

[0069] Specifically, with Figure 1 Taking the flexible interconnection system of the distribution substations shown as an example, when distribution substation 1 and distribution substation 2 are operating normally, it is assumed that VSC1 is working in PUI control mode and VSC2 is working in U... dc Q control mode utilizes the symmetrical component method to control the three-phase current i of load i. lia i lib and i lic Decomposed into positive, negative, and zero-order components: , in, , , These represent the magnitudes of the positive-sequence component, negative-sequence component, and zero-sequence component of load i, respectively. , , These represent the phase angles of the positive-sequence component, negative-sequence component, and zero-sequence component of load i, respectively. This represents the fundamental angular frequency.

[0070] Applying the Clarke transform to the above equation yields the decoupling equation for the current in a two-phase stationary coordinate system: , , Furthermore, the current in a two-phase stationary coordinate system Quantity and Quantity It can be decomposed into angular velocities of... The positive sequence components and angular velocities are Negative order components: , , , .

[0071] Furthermore, select the operating system in U dc The VSC in Q control mode enables its PUI controller's U... dc The limiter is forced to set the output to U. dcrmax That is U dcref At this time, power regulation is ineffective, and the converter station is essentially operating in constant voltage mode. Therefore, the VSC connected to this distribution substation operates in U... dc Q control mode stabilizes the system's DC bus voltage, ensuring system operational reliability. Secondly, the operating temperature at U... dc After the VSC in Q control mode, the VSCs connected to the remaining distribution radio areas i It then operates in PUI control mode for power mutual assistance, and the control flow is the active power command value P vi The difference between the actual active power value and the output is fed into a proportional-integral (PI) controller, where the output is limited to obtain the DC bus voltage reference value U. dc Finally, the DC bus voltage reference value U... dc * and actual value U dc The difference is calculated using a PI controller to determine VSC. i d-axis current reference value i vid *; Active power command Q is transmitted according to FID. vi * Request VSC i q-axis current reference value i viq *; To prevent VSC iOverload requires adjustment of the reactive power command Q. vi *Amplitude limiting is applied, with a limit value of Q. VSCi for: , in, express Rated capacity.

[0072] Further, calculate VSC i q-axis current reference value i viq *; Then, extract the three-phase voltage u of load i. lia u lib and u lic The required angle θ for coordinate transformation is obtained using DSOGI-PLL. Based on the angle θ, the reference values ​​i of the three-phase positive sequence current of the two VSCs in the abc three-phase coordinate system are obtained. pia *、i pib * and i pic *

[0073] To provide the negative sequence current of load i, based on angle -θ, the measured three-phase current i of load i is... lia i lib and i lic Transformed into the dq coordinate system for current i lid and i liq The DC component is extracted using a moving average filter (MAF): , in, For window width, Input signal The average value.

[0074] Then, the three-phase negative sequence current Δ of load i is obtained through a PI controller and Parker inverse transformation. inia *、Δ inib * and Δ inic *, and respectively with VSC i Add the reference values ​​of the three-phase positive sequence currents to obtain VSC. i Three-phase output current reference value i via *、i vib * and i vic *, and VSC i Actual value of three-phase output current i via i vib and i vic作 The difference is then used to obtain the gate signal of the Insulated Gate Bipolar Transistor (IGBT) through hysteresis comparison, and the corresponding VSC is then used.i It controls the output of the required compensation current.

[0075] If the distribution area i is a three-phase four-wire system, then the load current i may have a zero-sequence component. Therefore, VSC i The zero-sequence component Δ also needs to be provided. i0i *, and combine it with the above VSC i The three-phase output current reference values ​​are superimposed to obtain the three-phase current reference value i. via *、i vib * and i vic Finally, compare it with VSC respectively. i Measured three-phase output current i via i vib and i vic The difference is used to determine the control signal of the Insulated Gate Bipolar Transistor (IGBT) to achieve three-phase current imbalance compensation in distribution substation area i.

[0076] In the FID structure, the decoupling effect of capacitors allows each VSC to operate independently, thereby achieving independent compensation for the three-phase current imbalance of the connected distribution substations. By controlling VSC1 and VSC2 separately, the three-phase unbalanced current of distribution substation 1 and substation 2 can be effectively suppressed. Similarly, for multi-terminal FID flexible interconnection structures, the decoupling control capability for the unbalanced current of each substation can still be maintained. However, in scenarios where the operating mode of the flexible interconnection system frequently switches, if the VSC still uses traditional PQ control, the DC bus voltage stability heavily depends on the performance of the PI controller. When the PI controller bandwidth is not properly matched, the system is prone to problems such as large voltage overshoot and long settling time, leading to bus voltage fluctuations or even instability, which in turn affects the overall control accuracy and operational reliability. To address this, the PUI control strategy designed in this application dynamically limits the DC bus voltage within a safe range through a voltage loop, maintaining stable operation of the distribution network voltage without relying on frequent mode switching operations. This strategy not only ensures voltage stability but also provides a solid foundation for three-phase current imbalance control, enhancing the robustness and synergy of the system. It not only effectively improves the DC-side voltage control performance but also enhances the ability to suppress three-phase current imbalance in each transformer area, forming a control method that synergistically balances voltage stability and current balance.

[0077] The technical solution of this application will be described in detail below with reference to specific embodiments. However, it should be understood that the following embodiments are only for explaining and illustrating the technical solution, but do not limit the scope of this application.

[0078] This embodiment simulates the operating conditions of two distribution radio stations in a certain area, such as... Figure 1The diagram shows the topology of the flexible interconnection system between the two distribution substations. The PUI control algorithm is used to control the converter in distribution substation 1, and the U... dc The Q control algorithm controls the converter in distribution substation 2; among them, VSC (Voltage Source Converter) is a voltage source converter, and the operating conditions of the two distribution substations are shown in Table 1: Table 1

[0079] When FID performs power mutual assistance for the distribution substations, it monitors in real time whether there is three-phase unbalanced current in the AC feeder of the distribution substation; according to the operating conditions of the two distribution substations in Table 1, the power target value P of VSC2 is obtained by dynamic power flow calculation of the distribution substations. v2 If the power is 30kW, then the target power value P of VSC1 is... v1 * is -30kW. At 0.1s, the system detected a three-phase unbalanced current in distribution substation 2, and at 0.3s, the system detected a three-phase unbalanced current in distribution substation 1.

[0080] The system detected three-phase unbalanced current in the distribution substation at 0.1s and 0.3s respectively. At 0.1s, VSC2 connected to substation 2 and VSC1 connected to substation 1 started to implement the three-phase unbalanced current mitigation control strategy. (1) First, extract the three-phase voltage u of load i. lia u lib and u lic The positive sequence components are used to obtain the angle θ required for coordinate transformation using a phase-locked loop (PLL).

[0081] (2) The obtained VSC three-phase positive sequence current reference value i pia *、i pib * and i pic *With the three-phase negative sequence current i of the load nia *、i nib * and i nic *Add them together to obtain the target value of the VSC output current i. via *、i vib * and i vic *, then, i via *、i vib * and i vic * and the actual value of VSC output current i via i vib and i vic The difference is calculated, and the gate signal of the VSC switch is obtained by using the hysteresis comparison method. The VSC is then controlled to output the required compensation current.

[0082] Control VSC1 to transmit active power by injecting negative sequence and zero sequence compensation currents at 0.3 s and 0.4 s respectively, and control its transmitted active power value to increase from 0 kW to 30 kW at 0.5 s, with the direction being to flow into VSC1; control VSC2 to stabilize the DC bus voltage by injecting negative sequence and zero sequence compensation currents at 0.1 s and 0.2 s respectively.

[0083] like Figure 10 The figure shows the output current and its sequence components of the first transformer in the flexible interconnection system of two distribution substations provided in this application embodiment, namely, the output current i of VSC1. v1 and its positive order component i p1 Negative-order component i n1 and zero-order component i 01 ; like Figure 11 The figure shows the output current and its sequence components of the second transformer in the flexible interconnection system of two distribution transformer areas provided in this application embodiment, namely, the output current i of VSC2. v2 and its positive order component i p2 Negative-order component i n2 and zero-order component i 02 ; like Figure 12 The current i of the first AC feeder (AC feeder 1) in the flexible interconnection system of two distribution substations provided in this application embodiment s1 Schematic diagram Figure 13 The figure shows the current i of the second AC feeder (AC feeder 2) in the flexible interconnection system of two distribution substations provided in this application embodiment. s2 Schematic diagram; like Figure 14 The diagram shown illustrates the AC feeder current of a flexible interconnection system for two distribution substations provided in this embodiment of the application between 0s and 0.1s; wherein, Figure 14 (a) in the diagram is a schematic diagram of the first AC feeder current between 0s and 0.1s. Figure 14 (b) in the diagram is a schematic diagram of the second AC feeder current between 0s and 0.1s; like Figure 15 The diagram shows the AC feeder current between 0.7s and 0.8s in the flexible interconnection system of two distribution substations provided in this embodiment of the application; wherein, Figure 15 (a) in the diagram is a schematic diagram of the first AC feeder current between 0.7s and 0.8s. Figure 15 (b) in the figure is a schematic diagram of the second AC feeder current between 0.7s and 0.8s.

[0084] from Figures 12-15It can be seen that the three-phase imbalance of the load causes the current amplitude and phase of the two AC feeders to be unbalanced. By independently controlling the two VSCs to compensate for the negative sequence and zero sequence currents of their corresponding loads, the three-phase balance of the currents of the two AC feeders is achieved. Combined with power flow control, the amplitudes of the three-phase balanced currents are equal, that is, both are 198 A, thus achieving the balance of active power in the two distribution substations.

[0085] Based on the three-phase imbalance control method for a flexible interconnection system in a transformer substation provided in the above embodiments, this application also provides a three-phase imbalance control device for a flexible interconnection system in a transformer substation, which specifically includes: The signal acquisition module is used to take the transformer converters controlled by the PQ control algorithm in the transformer flexible interconnection system as target transformer converters; and to acquire the first control parameters output by the active power PI controller in the PQ control framework of each target transformer converter. The limiting setting module is used to limit and set the first control parameter of each target transformer area based on the upper and lower limits of the DC bus voltage target value, so as to obtain the voltage target value of each target transformer area. The voltage regulation module is used to calculate the difference between the target voltage value of each target transformer area converter and the actual voltage value of the DC bus, input the difference into the PI controller, and output the second control parameter of each target transformer area converter. The current regulation module is used to take the second control parameter of each target transformer area converter as the target value of the direct axis current of each target transformer area converter, and calculate the difference with the actual value of the direct axis current of each target transformer area converter. The difference is then used as the input of the direct axis current PI controller in the PQ control framework of each target transformer area converter. The power control module is used to control the active and reactive power of each target transformer area converter based on the voltage modulation signal output by the PQ control framework of each target transformer area converter.

[0086] This application also provides a computer-readable storage medium storing a computer program that, when executed by a processor, implements the steps of the above-described three-phase imbalance control method for a flexible interconnection system in a transformer substation.

[0087] Those skilled in the art will understand that embodiments of this application can be provided as methods, systems, or computer program products. Therefore, this application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, this application can take the form of a computer program product embodied on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0088] This application is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of this application. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart... Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0089] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0090] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0091] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A three-phase imbalance control method for a flexible interconnection system of transformer substations, characterized in that, include: The transformer converters in the flexible interconnection system controlled by the PQ control algorithm are used as the target transformer converters. Collect the first control parameter output by the active power PI controller in the PQ control framework of each target transformer area; Based on the upper and lower limits of the DC bus voltage target value, the first control parameter of each target transformer area is limited and tuned to obtain the voltage target value of each target transformer area. The voltage target value of each target transformer area converter is subtracted from the actual voltage value of the DC bus, and the difference is input into the PI controller to output the second control parameter of each target transformer area converter. The second control parameter of each target transformer area converter is used as the target value of the direct axis current of each target transformer area converter. The difference between the target value and the actual value of the direct axis current of each target transformer area converter is calculated, and the difference is used as the input of the direct axis current PI controller in the PQ control framework of each target transformer area converter. Based on the voltage modulation signal output by the PQ control framework of each target transformer area converter, the active power and reactive power of each target transformer area converter are controlled.

2. The three-phase imbalance control method for a flexible interconnection system in a transformer substation according to claim 1, characterized in that, Based on the upper and lower limits of the target voltage value of the DC bus, the first control parameter of each target transformer area is limited and tuned to obtain the target voltage value of each target transformer area, including: The first control parameter of each target transformer area is compared with the upper and lower limits of the target voltage value of the DC bus. If the first control parameter is greater than or equal to the upper limit of the DC bus voltage target value, then the upper limit of the DC bus voltage target value shall be used as the voltage target value of the target transformer area. If the first control parameter is less than the upper limit of the DC bus voltage target value and greater than the lower limit of the DC bus voltage target value, then the first control parameter is taken as the voltage target value of the target transformer area. If the first control parameter is less than or equal to the lower limit of the DC bus voltage target value, then the lower limit of the DC bus voltage target value shall be used as the voltage target value of the target transformer area.

3. The three-phase imbalance control method for a flexible interconnection system in a transformer substation according to claim 1, characterized in that, The upper and lower limits of the target voltage value of the DC bus fluctuate within a preset fluctuation range; wherein, the preset fluctuation range is a preset proportion of the target voltage value of the DC bus.

4. The three-phase imbalance control method for a flexible interconnection system in a transformer substation according to claim 1, characterized in that, Also includes: The difference between the target reactive power value and the actual reactive power value of each target transformer area converter is used as the target quadrature axis current value of each target transformer area converter. The target value of the quadrature axis current of each target transformer area converter is subtracted from the actual value of the quadrature axis current of each target transformer area converter, and the difference is used as the input of the quadrature axis current PI controller in the PQ control framework of each target transformer area converter.

5. The three-phase imbalance control method for a flexible interconnection system in a transformer substation according to claim 1, characterized in that, The PQ control framework outputs voltage modulation signals for each target transformer area, including: Based on the actual direct-axis current values ​​of the converters in each target distribution area, construct the quadrature-axis decoupling terms for each converter in each target distribution area; based on the actual quadrature-axis current values ​​of the converters in each target distribution area, construct the direct-axis decoupling terms for each converter in each target distribution area. Based on the third control parameter output by the direct-axis current PI controller in the PQ control framework of each target transformer area converter, the direct-axis decoupling term, and the actual value of the direct-axis voltage of each target transformer area converter, the direct-axis voltage modulation signal of each target transformer area converter is calculated. Based on the fourth control parameter output by the quadrature-axis current PI controller in the PQ control framework of each target transformer area, the quadrature-axis decoupling term, and the actual value of the quadrature-axis voltage of each target transformer area, the quadrature-axis voltage modulation signal of each target transformer area is calculated.

6. The three-phase imbalance control method for a flexible interconnection system in a transformer substation according to claim 5, characterized in that, It also includes using a dual second-order generalized integrator phase-locked loop to obtain the phase angle of each target transformer area converter, specifically including: Based on the A-phase voltage, B-phase voltage and C-phase voltage of the target transformer area converter, calculate the positive sequence voltage component and negative sequence voltage component of the target transformer area converter. The positive-sequence and negative-sequence voltage components of the target transformer are subjected to Clark transformation to output the α-axis DC component and β-axis DC component of the target transformer. The α-axis DC component and the preset angular frequency of the target transformer are input together into the first second-order generalized integrator, which outputs the first sine wave signal and the first cosine wave signal; the β-axis DC component and the preset angular frequency of the target transformer are input together into the second second-order generalized integrator, which outputs the second sine wave signal and the second cosine wave signal. Based on the vector product of the first and second sine wave signals, the positive sequence DC component of the α-axis is obtained; based on the vector product of the first and second cosine wave signals, the positive sequence DC component of the β-axis is obtained; based on the positive sequence DC components of the α-axis and β-axis, the positive sequence DC component of the target voltage of the target transformer area is obtained. The target voltage positive sequence component is subjected to Parker transformation to obtain the d-axis DC component and the q-axis DC component. The q-axis DC component is input into the PI regulator to output the instantaneous angular frequency. The vector product of the instantaneous angular frequency and the preset angular frequency is integrated to obtain the phase angle of the target transformer converter.

7. The three-phase imbalance control method for a flexible interconnection system in a transformer substation according to claim 6, characterized in that, Controlling the active and reactive power of converters in each target distribution area includes: Based on the direct-axis voltage modulation signal of each target transformer area converter, the direct-axis current reference value of each target transformer area converter is obtained; based on the quadrature-axis voltage modulation signal of each target transformer area converter, the quadrature-axis current reference value of each target transformer area converter is obtained. Based on the phase angle of each target transformer area converter, the direct-axis current reference value and quadrature-axis current reference value of each target transformer area converter are transformed by coordinate transformation to obtain the three-phase current reference value of each target transformer area converter; Based on the reference values ​​and actual values ​​of the three-phase current of each target transformer area converter, the error value of the three-phase current of each target transformer area converter is calculated. The actual values ​​of direct-axis current and quadrature-axis current of each target transformer area converter are filtered by a moving average filter. The filtered actual values ​​of direct-axis current and quadrature-axis current of each target transformer area converter are then input into a PI controller, which outputs the correction values ​​of direct-axis current and quadrature-axis current of each target transformer area converter. Based on the phase angle of each target transformer, the direct-axis current correction and quadrature-axis current correction of each target transformer are transformed by coordinate transformation to obtain the three-phase current compensation value of each target transformer. The three-phase current compensation value of each target transformer area converter is superimposed with its three-phase current error value to obtain the three-phase modulation error of each target transformer area converter; the three-phase modulation error of each target transformer area converter is subjected to triangular carrier modulation to generate the PWM drive signal of each target transformer area converter, thereby controlling the active power and reactive power of each target transformer area converter.

8. The three-phase imbalance control method for a flexible interconnection system in a transformer substation according to claim 7, characterized in that, After obtaining the three-phase current compensation values ​​for each target transformer area, the following is also included: Based on the averaging coefficient of the actual three-phase current of each target transformer area converter, the current sharing reference value of each target transformer area converter is obtained. The three-phase current compensation value of each target transformer area converter is superimposed with its current sharing reference value to obtain the new three-phase current compensation value of each target transformer area converter.

9. A three-phase imbalance control device for a flexible interconnection system of transformer substations, characterized in that, include: The signal acquisition module is used to select the transformer converters in the flexible interconnection system controlled by the PQ control algorithm as the target transformer converters. Collect the first control parameter output by the active power PI controller in the PQ control framework of each target transformer area; The limiting setting module is used to limit and set the first control parameter of each target transformer area based on the upper and lower limits of the DC bus voltage target value, so as to obtain the voltage target value of each target transformer area. The voltage regulation module is used to calculate the difference between the target voltage value of each target transformer area converter and the actual voltage value of the DC bus, input the difference into the PI controller, and output the second control parameter of each target transformer area converter. The current regulation module is used to take the second control parameter of each target transformer area converter as the target value of the direct axis current of each target transformer area converter, and calculate the difference with the actual value of the direct axis current of each target transformer area converter. The difference is then used as the input of the direct axis current PI controller in the PQ control framework of each target transformer area converter. The power control module is used to control the active and reactive power of each target transformer area converter based on the voltage modulation signal output by the PQ control framework of each target transformer area converter.

10. A computer-readable storage medium, characterized in that, The computer-readable storage medium stores a computer program, which, when executed by a processor, implements the steps of the three-phase imbalance control method for a flexible interconnection system of transformer substations as described in any one of claims 1 to 8.

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