Back-to-back flexible direct current system and bilateral network construction type frequency support control method thereof

Abstract

The invention discloses a back-to-back flexible direct current system and a bilateral network construction type frequency support control method thereof, and relates to the technical field of flexible direct current power transmission control. The method comprises the following steps: two back-to-back flexible direct current converter stations both adopt network construction type control based on power synchronization so as to realize symmetrical frequency active support for alternating current systems on two sides; the method adopts network construction type control based on power synchronization. Calculating an active power instruction of the flexible DC converter station according to a preset power instruction, the frequency feedback power instruction and the voltage balance power instruction; wherein the frequency feedback power instruction is calculated based on the frequency difference of the alternating current systems on the two sides, and the voltage balance power instruction is calculated based on the direct current voltage. According to the invention, the symmetrical frequency active supporting capability of the back-to-back flexible direct current system to the alternating current systems on the two sides can be improved.

Description

Technical Field

[0001] This invention belongs to the field of flexible DC transmission control technology, specifically relating to a back-to-back flexible DC system and its bilateral grid-type frequency support control method. Background Technology

[0002] Back-to-back projects promote mutual power complementarity and resource allocation within interconnected power grids, and serve as backup power sources for each other in emergencies. Back-to-back DC technology can be divided into two categories: conventional back-to-back DC and flexible back-to-back DC. Conventional back-to-back DC uses semi-controlled converter devices, relying on grid voltage for commutation. Flexible back-to-back DC uses fully controlled converter devices, possessing four-quadrant power operation capability, and can simultaneously provide active and reactive power support to the interconnected grid. Currently, back-to-back DC projects are widely used both domestically and internationally, such as the Fujian-Guangdong back-to-back (Fujian / Guangdong interconnection), the Chongqing-Hubei back-to-back (Sichuan-Chongqing / Central China interconnection), the Greater Bay Area back-to-back (Guangdong internal interconnection), and the Shandong-Western back-to-back (Yunnan / Southern Power Grid main grid interconnection).

[0003] In the face of the problem of low grid inertia caused by the large-scale grid connection of heterogeneous power electronic equipment such as new energy and ultra-high voltage DC, which crowds out the start-up of conventional hydro and thermal power sources, it is necessary to utilize the rapid adjustment capability of back-to-back flexible DC active power to achieve power support and frequency support between interconnected grids in order to improve system stability.

[0004] To address practical engineering challenges, existing research has proposed strategies such as additional frequency control, quasi-synchronous control, and heterosynchronous power flow control to resolve frequency and angle-of-attack stability issues in interconnected power grids. These strategies aim to achieve simultaneous operation of the AC systems on both sides at the same frequency or to proactively and rapidly adjust transmission power based on changes in the phase angle between the two sides. Academically, current research often employs techniques such as power-frequency droop control, DC capacitor energy synchronization control, and virtual synchronization control to enhance the inertia and damping support of flexible DC converter stations for the AC system. However, in back-to-back flexible DC grid-based control methods, current approaches primarily focus on optimizing the grid control design for the inverter-side flexible DC converter station, while traditional control methods are still retained on the rectifier side, failing to achieve proactive frequency support for the two-sided system.

[0005] Therefore, it is necessary to propose a solution to the problem of bilateral frequency support in back-to-back flexible DC systems. Summary of the Invention

[0006] The purpose of this invention is to provide a back-to-back flexible DC system and its bilateral grid-type frequency support control method, so as to improve the back-to-back flexible DC system's ability to actively support the frequency of the AC systems on both sides symmetrically.

[0007] To achieve the above objectives, the technical solution provided in this application is as follows:

[0008] This application provides a back-to-back flexible DC system and its bilateral grid-type frequency support control method. The back-to-back flexible DC system includes two sides, each side including a flexible DC converter station; the DC sides of the flexible DC converter stations on both sides are interconnected, and the AC side of the flexible DC converter station on each side is connected to an AC system, forming an interconnected power grid.

[0009] The back-to-back flexible DC system bilateral grid-type frequency support control method is as follows: both flexible DC converter stations adopt grid-type control based on power synchronization to achieve symmetrical active frequency support for the AC systems on both sides; the grid-type control based on power synchronization includes: calculating the active power command of the flexible DC converter station according to the preset power command, frequency feedback power command, and voltage balance power command; wherein, the frequency feedback power command is calculated based on the frequency difference of the AC systems on both sides, and the voltage balance power command is calculated based on the DC voltage.

[0010] In one possible implementation, the active power command of the flexible DC converter station is calculated based on a preset power command, a frequency feedback power command, and a voltage balance power command. The calculation formula is as follows:

[0011] ;

[0012] In the formula, and They represent Side and Active power command for the flexible DC converter station on the side; and They represent Side and Angular frequency of the side-mounted AC system; , and These represent the preset power command, voltage balance power command, and frequency feedback power command, respectively. and These represent the proportional and integral coefficients of the DC voltage proportional-integral controller, respectively. and These represent the DC-side voltage and its rated value of the flexible DC converter station, respectively. Indicates the frequency adjustment coefficient. This represents a variable in the complex frequency domain.

[0013] In one possible implementation, by changing the frequency adjustment coefficient The degree to which back-to-back flexible DC systems participate in power support for interconnected power grids can be flexibly adjusted to achieve dynamic sharing of frequency regulation resources between the two AC systems; frequency regulation coefficient The larger the value, the greater the degree to which the back-to-back flexible DC system participates in bilateral frequency regulation; the frequency regulation coefficient The smaller the value, the less the back-to-back flexible DC system participates in bilateral frequency regulation.

[0014] In one possible implementation, the transfer function of the power synchronization-based grid-type control is:

[0015] ;

[0016] In the formula, and These represent the internal potential angular frequency and phase angle of the flexible DC converter station, respectively. and These represent the virtual inertia coefficient and the virtual damping coefficient, respectively. and These represent the active power command of the flexible DC converter station and the active power output to the AC system, respectively. and These represent the angular frequency of the AC system and its rated value, respectively.

[0017] In one possible implementation, with Using the phase reference, the DC voltage of the dq axis is converted into the AC voltage of the abc three-phase system through coordinate transformation from the dq rotating coordinate system to the abc three-phase stationary coordinate system; the abc three-phase AC voltage is used as the modulation voltage of the converter valve in the flexible DC converter station.

[0018] Compared with the prior art, the present invention has the following beneficial effects.

[0019] 1) In the back-to-back flexible DC system control mode, there is no differentiated design between the rectifier and inverter sides. Both flexible DC converter stations at both ends adopt a grid-type control based on power synchronization to achieve symmetrical frequency support for the AC systems on both sides. 2) There are no additional equipment requirements. Operationally, there is no longer a distinction between balanced and unbalanced stations. While achieving stable DC voltage control of the flexible DC system, the back-to-back bidirectional power flow operation becomes more flexible. 3) Through frequency difference feedback and parameter adjustment between the two AC systems, the strength of the back-to-back flexible DC system's participation in frequency support and power support for the bilateral system can be dynamically adjusted. The interconnection power can be adaptively changed according to the degree of power disturbance in the two AC systems, actively supporting the frequency stability of the two AC systems. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below.

[0021] Figure 1 This is a block diagram of a control method in one embodiment of this application;

[0022] Figure 2This is a schematic diagram of a system in one embodiment of this application;

[0023] Figure 3 This is a simulation curve of the frequency of the i-side AC system under the control method of one embodiment of this application;

[0024] Figure 4 This is a frequency simulation curve of the j-side AC system under the control method of one embodiment of this application;

[0025] Figure 5 This is a simulation curve of the output active power of the i-side flexible DC converter station under the control method of one embodiment of this application. Detailed Implementation

[0026] To enable those skilled in the art to better understand the present application, the technical solution of the present application will be further described in detail below with reference to the embodiments and accompanying drawings.

[0027] This application innovatively designs both the rectifier-side flexible DC converter station and the inverter-side flexible DC converter station as a grid-type control mode based on power synchronization, and achieves symmetrical frequency active support of the back-to-back flexible DC system for the AC systems on both sides by adaptively changing the active power command.

[0028] The following will refer to the appendix. Figure 1 and Figure 2 Specific embodiments of this application are described below.

[0029] This application provides a back-to-back flexible DC system and its bilateral grid-type frequency support control method. The back-to-back flexible DC system includes two sides, each side including a flexible DC converter station; the DC sides of the flexible DC converter stations on both sides are interconnected, and the AC side of the flexible DC converter station on each side is connected to an AC system, forming an interconnected power grid.

[0030] The back-to-back flexible DC system bilateral grid-type frequency support control method is as follows: both flexible DC converter stations adopt grid-type control based on power synchronization to achieve symmetrical active frequency support for the AC systems on both sides; the grid-type control based on power synchronization includes: calculating the active power command of the flexible DC converter station according to the preset power command, frequency feedback power command, and voltage balance power command; wherein, the frequency feedback power command is calculated based on the frequency difference of the AC systems on both sides, and the voltage balance power command is calculated based on the DC voltage.

[0031] In some embodiments, the active power command of the flexible DC converter station is calculated based on a preset power command, a frequency feedback power command, and a voltage balance power command, using the following formula:

[0032] ;

[0033] In the formula, and They represent Side and Active power command for the flexible DC converter station on the side; and They represent Side and Angular frequency of the side-mounted AC system; , and These represent preset power command, voltage balance power command, and frequency feedback power command, respectively. and These represent the proportional and integral coefficients of a DC voltage proportional-integral controller (PI controller), used to regulate voltage balance power commands. and These represent the DC-side voltage and its rated value of the flexible DC converter station, respectively. It is the actual DC voltage of the back-to-back flexible DC system, used to calculate the voltage balance power command; Side and Energy storage components (including capacitors, modular multilevel converter sub-modules, etc.) are connected to the DC bus between the flexible DC converter stations on both sides to stabilize DC voltage and buffer power fluctuations. Indicates the frequency adjustment coefficient. This represents a variable in the complex frequency domain.

[0034] Under the above control method, the frequency feedback power command (component) is used to drive bilateral power support: calculation based on the frequency difference between the two AC systems is the core of achieving active frequency support. When a power disturbance occurs in one side of the AC system, It will automatically trigger the other side to send power to the disturbance side, dynamically sharing frequency modulation resources.

[0035] Voltage balance power command is used to ensure DC-side stability: it calculates and compensates for DC voltage deviations in real time through a DC voltage PI controller. This avoids voltage fluctuations in the DC capacitor caused by bidirectional power flow.

[0036] The preset power command is used to provide a basic scheduling interface: it can be directly derived from scheduling commands and serve as a reference value for bilateral power exchange; when the frequency difference is 0 or the frequency adjustment coefficient is 0... When =0, the interconnect power automatically reverts to its normal value. This ensures that routine scheduling requirements are met.

[0037] In some embodiments, by changing the frequency adjustment coefficient The degree to which back-to-back flexible DC systems participate in power support for interconnected power grids can be flexibly adjusted to achieve dynamic sharing of frequency regulation resources between the two AC systems; frequency regulation coefficient The larger the value, the greater the degree to which the back-to-back flexible DC system participates in bilateral frequency regulation; the frequency regulation coefficient The smaller the value, the less the back-to-back flexible DC system participates in bilateral frequency regulation.

[0038] In some embodiments, the transfer function of the power synchronization-based grid-type control is:

[0039] ;

[0040] In the formula, and These represent the internal potential angular frequency and phase angle of the flexible DC converter station, respectively. and These represent the virtual inertia coefficient and the virtual damping coefficient, respectively. and These represent the active power command of the flexible DC converter station and the active power output to the AC system, respectively. and These represent the angular frequency of the AC system and its rated value, respectively.

[0041] The grid-based control based on power synchronization in this application is a targeted optimization design within the framework of virtual synchronous machine technology. It simulates the "inertia, damping, and frequency / voltage regulation characteristics" of a synchronous generator, enabling the flexible DC converter station to possess grid support capabilities similar to those of a traditional synchronous generator. The grid-based control based on power synchronization utilizes the power difference (the active power command from the flexible DC converter station and the active power output to the AC system, i.e., ...) and The power difference (the difference between the voltage and the power output) serves as the core input for virtual synchronous regulation, rather than relying on voltage or other signals. This is a specific application of virtual synchronous machine technology in the "back-to-back flexible DC bilateral support scenario." The power difference directly reflects the deviation between the command target and the actual operating state of the flexible DC converter station: when... Greater than This indicates that the flexible DC converter station needs to increase its output power to match the command target; when Less than When this occurs, the output power needs to be reduced. The core characteristic of a synchronous generator is that power imbalance leads to frequency changes. This application uses a grid-type control based on power synchronization to replicate this characteristic through power difference, thereby driving frequency synchronization adjustment through power difference.

[0042] The control method in this application, compared with the traditional grid-following control (which relies on grid voltage / frequency signals for control and passively follows grid operation), enables the flexible DC converter station to actively build and support grid frequency and voltage stability, and belongs to the grid-building control.

[0043] like Figure 1 As shown, with Taking a flexible DC converter station as an example, a grid-based control system based on power synchronization is adopted, and its transfer function is:

[0044] ;

[0045] The relevant parameters are explained as follows: and They represent Side and The internal potential angular frequency of the flexible DC converter station; and They represent Side and The angular frequency of the side AC system is Side and The actual operating angular frequency of the AC system is the core basis for calculating the frequency feedback power command and directly reflects the frequency status of the AC system. and They represent Side and The internal potential phase angle of the flexible DC converter station; and They represent Side and The virtual inertia coefficient of the flexible DC converter station is Side and The control parameters of the flexible DC converter station are used to simulate the inertia characteristics of a synchronous generator and affect the frequency response speed. and They represent Side and The virtual damping coefficient of the flexible DC converter station is Side and The control parameters of the flexible DC converter station are used to suppress frequency oscillations and improve system stability; and They represent Side and The active power output from the flexible DC converter station to the AC system is Side and The active power actually output to the AC system by the flexible DC converter station is the result of the execution of the active power command, which directly reflects the power support and frequency support effect. and They represent Side and The dq-axis internal potential and voltage of the side-flexible DC converter station and They represent Side and Flexible DC converter station on the side.

[0046] In some embodiments, with Using the phase reference, the DC voltage of the dq axis is converted into the AC voltage of the abc three-phase system by coordinate transformation from the dq rotating coordinate system to the abc three-phase stationary coordinate system; the abc three-phase AC voltage is used as the modulation voltage of the converter valve (such as a modular multilevel converter composed of IGBTs) in the flexible DC converter station.

[0047] by Taking the flexible DC converter station as an example, Using the phase reference, to ensure that the three-phase voltage phase is synchronized with the AC system, the dq-axis voltage is converted into the abc three-phase AC voltage through coordinate transformation from the dq rotating coordinate system to the abc three-phase stationary coordinate system.

[0048] Based on the control method in the above embodiments, if the initial angular frequency of the AC systems on both sides is equal to the rated value, and the secondary frequency modulation effect is not considered, then when the AC system experiences a power disturbance, the steady-state angular frequency deviation of the AC systems on both sides satisfies the following expression:

[0049] ;

[0050] In the formula, and They represent Side and The equivalent primary frequency modulation coefficient of the side-channel AC system; and They represent Side and The steady-state angular frequency deviation of the AC system is Side and The steady-state frequency deviation of the AC system after being subjected to power disturbance is the core evaluation indicator of the frequency support effect; the smaller the deviation, the better the support effect. and They represent Side and Active power disturbance of the AC system (load power is positive).

[0051] In some embodiments, a preset power command is provided. It can originate from scheduling instructions.

[0052] Based on the control method in the above embodiments, when the frequency adjustment coefficient... When the angular frequency difference between the two AC systems is equal to 0 or equal to 0 (meaning there is no frequency difference between the two sides, indicating no power deficit or surplus on either side, and no need for power support via back-to-back DC), the power command is due to voltage balance. To compensate for DC voltage deviations and maintain DC side stability, the power of the back-to-back flexible DC interconnect (i.e., the power transmitted between the two converter stations) is equal to the preset power command.

[0053] This invention, in its back-to-back flexible DC system control method, does not differentiate between the rectifier and inverter sides. Both flexible DC converter stations at both ends adopt a grid-based control system based on power synchronization, achieving symmetrical frequency support for the AC systems on both sides. No additional equipment is required, eliminating the need to distinguish between balanced and unbalanced stations during operation. While achieving stable DC voltage control of the flexible DC system, the back-to-back bidirectional power flow operation becomes more flexible. Through frequency difference feedback and parameter adjustment between the two AC systems, the strength of the back-to-back flexible DC system's participation in frequency support and power support for the bilateral system can be dynamically adjusted. The interconnection power adaptively changes according to the degree of power disturbance in the two AC systems, actively supporting the frequency stability of the two AC systems.

[0054] An application example of this application is as follows.

[0055] Reference Appendix Figure 2 In a power system scenario, a back-to-back flexible DC system connects two asynchronous power grids. Both AC systems employ a typical primary frequency regulation model considering the prime mover speed regulation process, and the governor parameters are shown in the diagram. Specifically, the rated frequencies of the i-side and j-side AC systems are both 50Hz. The rated capacity of the flexible DC converter station is 600MVA, and the rated DC voltage is ±200kV. The rated AC voltage on the valve side is 208kV, and the rated AC voltage on the grid side is 525kV.

[0056] The system parameters are normalized using the rated capacity of the flexible DC converter station, the rated frequency of the AC system, and the rated voltage of each voltage level as baseline values. The equivalent capacitance of the DC system is 0.192 pu. The equivalent impedance X of the AC system on the i-side is... eqi Equivalent impedance X of AC system on side j eqj Both are 0.35 pu. The equivalent inertia M of the i-side AC system. sysi And the equivalent primary frequency modulation coefficient D sysi The equivalent inertia M of the j-side AC system is 50 and 90 respectively. sysj And the equivalent primary frequency modulation coefficient D sysj They are 40 and 80 respectively.

[0057] Reference Appendix Figure 1 A frequency support control method for a back-to-back flexible DC system with a bilateral grid configuration includes:

[0058] S1. Both flexible DC converter stations adopt a grid-based control system based on power synchronization. The virtual inertia coefficient M of the flexible DC converter station on side i. i and virtual damping coefficient D i The virtual inertia coefficient M of the flexible DC converter station on the j-side is taken as 2 and 25 respectively. j and virtual damping coefficient D j Take 2 and 20 respectively.

[0059] S2. Calculate the active power command of the flexible DC converter station based on the preset power command, frequency feedback power command, and voltage balance power command. Frequency regulation coefficient K F Take 80. The proportional gain K of the DC voltage PI controller. VP Take 0.2, integral coefficient K VI Take 0.5.

[0060] Specifically, by changing the frequency adjustment coefficient K F The degree to which back-to-back flexible DC transmission participates in power support for interconnected power grids can be flexibly adjusted. Assume the initial frequency of the AC systems on both sides i and j is 50Hz, and the active power disturbance P of the AC system on side i is... disi The active power disturbance P of the J-side AC system is 0.5 pu. disi If the frequency adjustment coefficient K is 0, then... F When the values ​​are 0, 40, 80, and 160, the steady-state frequency deviations of the AC system on the i-side are -0.278Hz, -0.214Hz, -0.192Hz, and -0.174Hz, respectively, and the steady-state frequency deviations of the AC system on the j-side are 0Hz, -0.071Hz, -0.096Hz, and -0.116Hz, respectively.

[0061] Specifically, let the preset power command P be... D When the frequency adjustment coefficient K is 0, F When the power of the back-to-back flexible DC interconnect is equal to 0 or the frequency difference between the two AC systems is equal to 0, the power of the back-to-back flexible DC interconnect is equal to 0.

[0062] Construct the back-to-back flexible DC system as described above using PSCAD / EMTDC software. (See attached reference.) Figure 3 To be continued Figure 5 P was obtained disi From 0 to 0.5, the simulation curves of the AC system frequency on the i-side and j-side, and the simulation curve of the output active power of the flexible DC converter station on the i-side, under the control method of the present invention.

[0063] According to the simulation curves, a power disturbance occurs in the AC system on the i-side, and the frequency regulation coefficient K... FWhen the frequency is 0, the active power of the back-to-back flexible DC output recovers to 0 after a short, slight fluctuation. All power disturbances are borne by the AC system on the i-side. The maximum frequency deviation exceeds -0.69Hz, and the steady-state frequency deviation is close to -0.28Hz. Frequency adjustment coefficient K F When the frequency difference is not equal to 0, under the bilateral frequency difference feedback, the AC system on the j-side absorbs part of the power disturbance occurring in the AC system on the i-side. Frequency adjustment coefficient K F The larger the value, the smaller the frequency drop and steady-state deviation at the disturbed end, and the larger the frequency drop and steady-state deviation at the other end. This also leads to oscillations in back-to-back power. Frequency adjustment coefficient K F When the frequency is 80, the lowest frequency point of the AC system on side i increases from 49.31Hz to 49.57Hz, while the lowest frequency point of the AC system on side j drops from 49.98Hz to 49.68Hz. This effectively raises the lowest frequency point and prevents the risk of low-frequency load shedding. At this time, the steady-state voltage deviation of the AC system on side j is half of the steady-state frequency deviation of side i. The steady-state frequency deviations of system i and system j are found to be -0.192Hz and -0.096Hz, respectively, which are basically consistent with the simulation results of -0.1925Hz and -0.0962Hz.

[0064] As can be seen from the specific simulation cases above, by designing both flexible DC converter stations at the back-to-back ends as a grid-type control mode based on power synchronization and adaptively changing the active power command, the back-to-back flexible DC system can achieve symmetrical frequency active support for the AC systems on both sides.

[0065] 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 back-to-back flexible DC system, characterized in that, It includes two sides, each side including a flexible DC converter station; the DC sides of the flexible DC converter stations on both sides are interconnected, and the AC side of the flexible DC converter station on each side is connected to an AC system to form an interconnected power grid. Both flexible DC converter stations employ grid-based control based on power synchronization to achieve symmetrical frequency active support for the AC systems on both sides. The grid-based control based on power synchronization includes: calculating the active power command of the flexible DC converter station according to a preset power command, a frequency feedback power command, and a voltage balance power command; wherein the frequency feedback power command is calculated based on the frequency difference between the AC systems on both sides, and the voltage balance power command is calculated based on the DC voltage.

2. The system according to claim 1, characterized in that, The active power command of the flexible DC converter station is calculated based on the preset power command, frequency feedback power command, and voltage balance power command. The calculation formula is as follows: ； In the formula, and They represent Side and Active power command for the flexible DC converter station on the side; and They represent Side and Angular frequency of the side-mounted AC system; , and These represent the preset power command, voltage balance power command, and frequency feedback power command, respectively. and These represent the proportional and integral coefficients of the DC voltage proportional-integral controller, respectively. and These represent the DC-side voltage and its rated value of the flexible DC converter station, respectively. Indicates the frequency adjustment coefficient; This represents a variable in the complex frequency domain.

3. The system according to claim 2, characterized in that, By changing the frequency adjustment coefficient The degree to which back-to-back flexible DC systems participate in power support for interconnected power grids can be flexibly adjusted to achieve dynamic sharing of frequency regulation resources between the two AC systems; frequency regulation coefficient The larger the value, the greater the degree to which the back-to-back flexible DC system participates in bilateral frequency regulation; the frequency regulation coefficient The smaller the value, the less the back-to-back flexible DC system participates in bilateral frequency regulation.

4. The system according to claim 2, characterized in that, The transfer function of the power synchronization-based network control is: ； In the formula, and These represent the internal potential angular frequency and phase angle of the flexible DC converter station, respectively. and These represent the virtual inertia coefficient and the virtual damping coefficient, respectively. and These represent the active power command of the flexible DC converter station and the active power output to the AC system, respectively. and These represent the angular frequency of the AC system and its rated value, respectively.

5. The system according to any one of claims 1 to 4, characterized in that, by Using the phase reference, the DC voltage of the dq axis is converted into the AC voltage of the abc three-phase system through coordinate transformation from the dq rotating coordinate system to the abc three-phase stationary coordinate system; the abc three-phase AC voltage is used as the modulation voltage of the converter valve in the flexible DC converter station.

6. A method for frequency support control of a back-to-back flexible DC system with a bilateral grid configuration, characterized in that, The back-to-back flexible DC system includes two sides, each side including a flexible DC converter station; the DC sides of the flexible DC converter stations on both sides are interconnected, and the AC side of the flexible DC converter station on each side is connected to an AC system to form an interconnected power grid. Both flexible DC converter stations employ grid-based control based on power synchronization to achieve symmetrical frequency active support for the AC systems on both sides. The grid-based control based on power synchronization includes: calculating the active power command of the flexible DC converter station according to a preset power command, a frequency feedback power command, and a voltage balance power command; wherein the frequency feedback power command is calculated based on the frequency difference between the AC systems on both sides, and the voltage balance power command is calculated based on the DC voltage.

7. The method according to claim 6, characterized in that, The active power command of the flexible DC converter station is calculated based on the preset power command, frequency feedback power command, and voltage balance power command. The calculation formula is as follows: ； In the formula, and They represent Side and Active power command for the flexible DC converter station on the side; and They represent Side and Angular frequency of the side-mounted AC system; , and These represent the preset power command, voltage balance power command, and frequency feedback power command, respectively. and These represent the proportional and integral coefficients of the DC voltage proportional-integral controller, respectively. and These represent the DC-side voltage and its rated value of the flexible DC converter station, respectively. Indicates the frequency adjustment coefficient. This represents a variable in the complex frequency domain.

8. The method according to claim 7, characterized in that, By changing the frequency adjustment coefficient The degree to which back-to-back flexible DC systems participate in power support for interconnected power grids can be flexibly adjusted to achieve dynamic sharing of frequency regulation resources between the two AC systems; frequency regulation coefficient The larger the value, the greater the degree to which the back-to-back flexible DC system participates in bilateral frequency regulation; the frequency regulation coefficient The smaller the value, the less the back-to-back flexible DC system participates in bilateral frequency regulation.

9. The method according to claim 7, characterized in that, The transfer function of the power synchronization-based network control is: ； In the formula, and These represent the internal potential angular frequency and phase angle of the flexible DC converter station, respectively. and These represent the virtual inertia coefficient and the virtual damping coefficient, respectively. and These represent the active power command of the flexible DC converter station and the active power output to the AC system, respectively. and These represent the angular frequency of the AC system and its rated value, respectively.

10. The method according to claims 6-9, characterized in that, by Using the phase reference, the DC voltage of the dq axis is converted into the AC voltage of the abc three-phase system through coordinate transformation from the dq rotating coordinate system to the abc three-phase stationary coordinate system; the abc three-phase AC voltage is used as the modulation voltage of the converter valve in the flexible DC converter station.

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