A method and device for voltage balance control between phases of a converter chain of a star SVG

Abstract

The present application belongs to the technical field of star SVG control, and particularly relates to a commutation chain phase-to-phase voltage balance control method and device for star SVG. A first negative sequence current instruction value and a second negative sequence current instruction value obtained by phase-to-phase voltage balance closed-loop control are added together as a final negative sequence current instruction value for commutation chain phase-to-phase voltage balance control. In a power grid voltage imbalance fault condition, a negative sequence current feedforward instruction value is obtained by negative sequence current feedforward control, and the negative sequence current feedforward instruction value is taken as the first negative sequence current instruction value. The negative sequence current feedforward control can quickly and accurately solve the negative sequence current theoretical value required for commutation chain phase-to-phase voltage balance in a power grid imbalance condition through theoretical calculation, and the solution error of the theoretical value is compensated by phase-to-phase voltage balance closed-loop control, so as to improve the response speed and balance effect of phase-to-phase voltage balance, avoid triggering power module overvoltage protection, and improve the power grid fault ride-through capability of the SVG.

Description

Technical Field

[0001] This invention belongs to the field of star-type SVG control technology, specifically relating to a method and device for controlling phase-to-phase voltage balancing in the converter chain of a star-type SVG. Background Technology

[0002] A star-shaped SVG is connected in parallel to the power grid, and its main circuit topology is as follows: Figure 1 As shown, dynamically adjusting its output reactive power can improve the power factor and voltage stability, and it has been widely used in new energy power generation fields such as photovoltaic power generation and wind power generation. In order to improve the grid adaptability of SVG and provide effective dynamic reactive power support during grid imbalance faults, SVG is required to have grid imbalance fault ride-through capability.

[0003] Currently, star-type SVG commonly adopts a phase-to-phase voltage balancing control method based on negative-sequence current in the converter chain. Under grid imbalance fault conditions, while providing dynamic positive-sequence reactive current support to the grid, a certain amount of negative-sequence current also needs to be output to achieve phase-to-phase voltage balancing in the three-phase converter chain. The negative-sequence current is rapidly output during grid imbalance faults and rapidly withdrawn during fault recovery to reduce phase-to-phase voltage imbalance during fault occurrence and recovery, thus avoiding triggering overvoltage protection of the power module.

[0004] Common interphase voltage balancing control methods adjust the negative sequence current based on the interphase voltage deviation, and the control block diagram is as follows: Figure 2 As shown, the disadvantage of this method is that it only has a regulating effect after the phase-to-phase voltage deviation occurs, and there is a certain lag in the voltage equalization response speed. It is a lagging regulation, which leads to a large degree of phase-to-phase voltage imbalance when the grid imbalance fault occurs and is restored, and may even trigger the power module overvoltage protection, resulting in the failure of grid imbalance fault ride-through. Summary of the Invention

[0005] The purpose of this invention is to provide a method and apparatus for phase-to-phase voltage equalization control of a star-type SVG converter chain, in order to solve the problem that the phase-to-phase voltage equalization control method based solely on adjusting the negative sequence current according to the phase-to-phase voltage deviation has a certain lag in the voltage equalization response speed, which leads to a large degree of phase-to-phase voltage imbalance when a grid imbalance fault occurs.

[0006] To address the aforementioned technical problems, this invention provides a converter chain phase-to-phase voltage balancing control method for a star-type SVG. The method involves adding a first negative-sequence current command value and a second negative-sequence current command value obtained through phase-to-phase voltage balancing closed-loop control to obtain the final negative-sequence current command value for converter chain phase-to-phase voltage balancing control. This final negative-sequence current command value is then used to generate a control signal for the star-type SVG to control it. Specifically, under grid fault conditions, if the grid imbalance is greater than or equal to a set threshold, a negative-sequence current feedforward command value is obtained using negative-sequence current feedforward control. This negative-sequence current feedforward command value is then used as the first negative-sequence current command value. Furthermore, the negative-sequence current feedforward control is calculated from the positive and negative sequence components of the grid line voltage and the positive-sequence reactive current command value of the converter chain.

[0007] Its beneficial effects are as follows: Under the fault condition of grid voltage imbalance, the negative sequence current feedforward command value generated by the negative sequence current feedforward control is used as the first negative sequence current command value, and it is added to the second negative sequence current command value obtained by the phase-to-phase voltage equalization closed-loop control as the final negative sequence current command value of the converter chain phase-to-phase voltage equalization control. That is, the negative sequence current feedforward control can quickly and accurately solve the theoretical value of the negative sequence current required for the phase-to-phase voltage equalization of the converter chain under the grid imbalance condition through theoretical calculation. Then, the phase-to-phase voltage equalization closed-loop control is used to compensate for the solution error of the above theoretical value, thereby improving the response speed and equalization effect of phase-to-phase voltage equalization, avoiding triggering the power module overvoltage protection, and improving the grid fault ride-through capability of SVG.

[0008] Furthermore, the formula for calculating the negative-sequence current feedforward command value, derived from the positive and negative sequence components of the grid line voltage and the positive-sequence reactive current command value of the converter chain, is as follows:

[0009]

[0010] In the formula, I nd_ffd I nq_ffd These are the negative sequence current feedforward d-axis and q-axis command values, respectively; U spp_pd U spp_pq These are the positive-sequence d-axis and q-axis components of the grid line voltage, respectively; U spp_nd U spp_nq These are the negative-sequence d-axis and q-axis components of the grid line voltage, respectively; I pq_ref This is the positive sequence reactive current command value for the converter chain.

[0011] Furthermore, under grid fault conditions, if the grid imbalance is less than a set threshold, the first negative sequence current command value is 0 until the grid fault state is exited.

[0012] Its beneficial effects are as follows: considering that the positive and negative sequence dq components of voltage will cause disturbance to the negative sequence current feedforward command value when the power grid is restored, the first negative sequence current command does not use the negative sequence current feedforward command value, but is directly set to 0, thereby ensuring the stable and reliable operation of the system.

[0013] Furthermore, under steady-state conditions of the power grid, the first negative sequence current command value is the negative sequence current feedforward command value.

[0014] Its beneficial effect is that, considering the timeliness of the negative sequence current feedforward command value input, the first negative sequence current command value is set as the negative sequence current feedforward command value under the steady-state operating conditions of the power grid.

[0015] Furthermore, under grid fault conditions, if the first negative sequence current command value is not 0, the phase-to-phase voltage equalization coefficient in the phase-to-phase voltage equalization closed-loop control will be adjusted to the intermediate value; if the first negative sequence current command value is 0, the phase-to-phase voltage equalization coefficient will be adjusted to the maximum value when the phase-to-phase voltage deviation is greater than the set deviation threshold.

[0016] Its beneficial effect is that when the first negative sequence current command is 0 and the phase-to-phase voltage deviation is greater than the set deviation threshold, the phase-to-phase voltage equalization coefficient is adjusted to the maximum value so that the phase-to-phase voltage equalization closed-loop control can play a complete phase-to-phase voltage equalization role.

[0017] Furthermore, after the power grid fault is restored and continues for a certain period of time, the phase-to-phase voltage equalization coefficient is adjusted to the minimum value.

[0018] Furthermore, under steady-state conditions of the power grid, the phase-to-phase voltage equalization coefficient in the phase-to-phase voltage equalization closed-loop control is adjusted to the minimum value.

[0019] Its beneficial effects are as follows: Under steady-state conditions of the power grid, the dynamic adjustment of reactive current has a certain disturbance to the voltage of the three-phase capacitors and will also cause a slight phase-to-phase voltage deviation. Therefore, the phase-to-phase voltage equalization coefficient is adjusted to the minimum value at this time in order to maintain the dynamic characteristics of reactive current.

[0020] To address the aforementioned technical problems, this invention also provides a phase-to-phase voltage equalization control device for the converter chain of a star-type SVG, comprising a memory and a processor. The processor is used to execute computer program instructions stored in the memory to implement the phase-to-phase voltage equalization control method for the converter chain of the star-type SVG described above, and achieve the same beneficial effects as the method. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the star-shaped SVG main circuit topology targeted by this invention;

[0022] Figure 2 It is an existing technology for interphase voltage equalization control method based on interphase voltage deviation to adjust negative sequence current;

[0023] Figure 3 This is a block diagram of the phase-to-phase voltage equalization control method for the converter chain of the star-shaped SVG of the present invention;

[0024] Figure 4 These are the initial positions of the positive and negative dq coordinate systems of the grid line voltage and the positive and negative sequence dq coordinate systems of the converter chain current provided by this invention.

[0025] Figure 5-1 This is a comparison chart of the negative sequence current command obtained by using a common method based on phase-to-phase voltage deviation adjustment under the condition of a single-phase power grid voltage drop to 0.2 pu, and the theoretically calculated negative sequence current feedforward d-axis command (pu).

[0026] Figure 5-2 This is a comparison chart of the negative sequence current command obtained by using a common method based on phase-to-phase voltage deviation adjustment under the condition of a single-phase power grid voltage drop to 0.2 pu, and the theoretically calculated negative sequence current feedforward q-axis command (pu).

[0027] Figure 6-1 This is a comparison chart of the negative sequence current command obtained by using a common method based on phase-to-phase voltage deviation adjustment when the two phases of the power grid drop to 0.2 pu, and the theoretically calculated negative sequence current feedforward d-axis command (pu).

[0028] Figure 6-2 This is a comparison chart of the negative sequence current command obtained by using a common method based on phase-to-phase voltage deviation adjustment when the two phases of the power grid drop to 0.2pu, and the theoretically calculated negative sequence current feedforward q-axis command (pu).

[0029] Figure 7-1 This is a simulation diagram of the phase-to-phase voltage balancing effect (commutator chain current, unit pu) under the condition that the two phases of the power grid drop to 0.2 pu.

[0030] Figure 7-2 This is a simulation result of the phase-to-phase voltage balancing using the method of this invention under the condition that the two phases of the power grid drop to 0.2 pu. (The highest and lowest voltages of the three-phase power modules, in V) is shown in the figure.

[0031] Figure 8-1 The figure shows the simulation effect (commutator chain current, unit pu) of phase-to-phase voltage balancing under the condition that the two phases of the power grid drop to 0.2 pu using a common phase-to-phase voltage deviation adjustment method.

[0032] Figure 8-2 This is a simulation diagram of the phase-to-phase voltage balancing effect (highest and lowest voltages of the three-phase power modules, in V) under the condition that the two phases of the power grid drop to 0.2 pu. Detailed Implementation

[0033] This invention combines negative-sequence current feedforward control and phase-to-phase voltage balancing closed-loop control to form a converter chain phase-to-phase voltage balancing control. The negative-sequence current feedforward command value obtained from the negative-sequence current feedforward control is used as the first negative-sequence current command value, and the phase-to-phase voltage balancing closed-loop control obtains the second negative-sequence current command value. The first and second negative-sequence current command values ​​are superimposed to obtain the final negative-sequence current command value for achieving converter chain phase-to-phase voltage balancing control. The negative-sequence current feedforward control can quickly and accurately solve for the theoretical value of the negative-sequence current required for converter chain phase-to-phase voltage balancing under grid imbalance conditions through theoretical calculation. Then, the phase-to-phase voltage balancing closed-loop control method compensates for the calculation error of the above theoretical value, thereby improving the response speed and balancing effect of phase-to-phase voltage balancing, avoiding triggering power module overvoltage protection, and improving the grid fault ride-through capability of the SVG.

[0034] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0035] Method Implementation Examples:

[0036] The schematic diagram of the star-shaped SVG main circuit topology used in this embodiment is shown below. Figure 1 As shown, several power modules are connected in series to form a converter chain, and the three-phase converter chain is connected to the power grid through a connecting reactor. Under the condition of power grid imbalance fault, a converter chain phase-to-phase voltage balancing control method based on negative sequence current is adopted. While providing dynamic positive sequence reactive current support to the power grid, a certain amount of negative sequence current needs to be output to achieve phase-to-phase voltage balancing of the three-phase converter chain.

[0037] The converter chain phase-to-phase voltage equalization control method of the star SVG of the present invention is as follows: the first negative sequence current command value and the second negative sequence current command value obtained by phase-to-phase voltage equalization closed-loop control are added together as the final negative sequence current command value of the converter chain phase-to-phase voltage equalization control, and the control signal of the star SVG is generated by the final negative sequence current command value to control the star SVG.

[0038] Under different operating conditions, the first negative sequence current command value and the phase-to-phase voltage equalization coefficient in the commutator chain phase-to-phase voltage equalization control (i.e., Figure 3 The adjustment method for the proportional gain of the PI controller is as follows:

[0039] 1) The first negative sequence current command value under the fault condition of grid voltage imbalance.

[0040] When the grid is in a fault condition and the grid imbalance is greater than or equal to a set threshold, the negative sequence current feedforward control is used to obtain the negative sequence current feedforward command value, which is then used as the first negative sequence current command value. The negative sequence current feedforward control calculates the negative sequence current feedforward command value from the positive and negative sequence components of the grid line voltage and the positive sequence reactive current command value of the converter chain. Its control block diagram is as follows: Figure 3 As shown, the specific calculation formula is as follows:

[0041]

[0042] In the formula, I nd_ffd I nq_ffd These are the negative sequence current feedforward d-axis and q-axis command values, respectively; U spp_pd U spp_pq U represents the positive-sequence d- and q-axis components of the grid line voltage; spp_nd U spp_nq The negative sequence d- and q-axis components of the grid line voltage; I pq_ref This represents the positive-sequence reactive current command value for the converter link. The initial positions of the grid line voltage (positive and negative sequence dq coordinate system) and the initial positions of the converter link current (positive and negative sequence dq coordinate system) are shown below. Figure 4 As shown, the initial positions of the positive and negative sequence dq coordinate systems of the grid line voltage coincide, the initial position of the positive sequence dq coordinate system of the converter chain current lags behind the initial position of the positive sequence dq coordinate system of the grid line voltage by π / 6, and the initial position of the negative sequence dq coordinate system of the converter chain current leads the initial position of the positive sequence dq coordinate system of the grid line voltage by π / 6.

[0043] Negative sequence current feedforward control can quickly and accurately solve the theoretical value of negative sequence current required for phase-to-phase voltage balancing in the converter chain under unbalanced grid conditions. Then, the calculation error of the above theoretical value is compensated by phase-to-phase voltage balancing closed-loop control, thereby improving the response speed and balancing effect of phase-to-phase voltage balancing, avoiding triggering the overvoltage protection of the power module, and improving the grid fault ride-through capability of the SVG.

[0044] The initial value of the positive-sequence reactive current is 0.2 pu for inductive applications. Simulations are performed under grid conditions where a single-phase current drops to 0.2 pu and a two-phase current drops to 0.2 pu. A common phase-to-phase voltage balancing control method based on phase-to-phase voltage deviation to regulate the negative-sequence current is used for simulation. The simulated negative-sequence current command value is compared with the theoretically calculated negative-sequence current feedforward command value. Figure 5-1 , Figure 5-2 , Figure 6-1 and Figure 6-2 As shown, the steady-state values ​​of the two are consistent, proving the correctness of the theoretical calculation. It can also be seen that the theoretically calculated negative sequence current feedforward command value is relatively more advanced and stable during fault occurrence and recovery. It can be predicted that the response speed and equalization effect of the phase-to-phase voltage equalization control using negative sequence current feedforward control will also be better.

[0045] 2) First negative sequence current command value under the fault condition of power grid voltage balance.

[0046] Under grid voltage balance fault conditions, on the one hand, the theoretical value of the negative sequence current command used to achieve phase-to-phase voltage balance is considered to be 0; on the other hand, the positive and negative sequence dq components of the voltage will disturb the negative sequence current feedforward command value when the grid fault is restored. Therefore, the first negative sequence current command value does not use the negative sequence current feedforward command value. Thus, the negative sequence current feedforward control also includes negative sequence current feedforward enabling / disabling logic: under grid fault conditions, if the voltage imbalance is less than a set value, such as 0.05pu for 15ms, the first negative sequence current command value is equal to 0 until the grid fault state is exited; otherwise, the first negative sequence current command value is equal to the negative sequence current feedforward command value.

[0047] 3) The first negative sequence current command value under steady-state power grid conditions.

[0048] In reality, most power grid faults are voltage asymmetry faults. Considering the timeliness of the negative sequence current feedforward command value input, the negative sequence current feedforward control input and output logic also includes: under the steady-state operating condition of the power grid, the first negative sequence current command value is equal to the negative sequence current feedforward command value.

[0049] 4) Phase-to-phase voltage equalization coefficient in converter chain phase-to-phase voltage equalization control under power grid fault conditions.

[0050] If the first negative sequence current command value is the negative sequence current feedforward command value, then the phase-to-phase voltage equalization closed-loop control mainly plays the role of compensating for the steady-state error of the negative sequence current feedforward command value, and its phase-to-phase voltage equalization coefficient is not likely to take a large value; if the first negative sequence current command value is 0, then the phase-to-phase voltage equalization closed-loop control plays a complete phase-to-phase voltage equalization role, and its phase-to-phase voltage equalization coefficient can be adjusted according to the phase-to-phase voltage deviation. Therefore, the phase-to-phase voltage equalization closed-loop control includes the phase-to-phase voltage equalization coefficient adjustment logic: under grid fault conditions, if the first negative sequence current command value is not 0, then the phase-to-phase voltage equalization coefficient is adjusted to the intermediate value; if the first negative sequence current command value is 0, then when the phase-to-phase voltage deviation is greater than a set value, the phase-to-phase voltage equalization coefficient is adjusted to the maximum value, otherwise the current value is maintained; after the grid fault recovery lasts for a certain period of time, the phase-to-phase voltage equalization coefficient is adjusted to the minimum value.

[0051] 5) Phase-to-phase voltage equalization coefficient in converter chain phase-to-phase voltage equalization control under steady-state power grid conditions.

[0052] Under steady-state grid conditions, dynamic regulation of reactive current causes some disturbance to the three-phase capacitor voltage and also creates a slight phase-to-phase voltage deviation. However, if the phase-to-phase voltage equalization closed-loop control uses a large phase-to-phase voltage equalization coefficient, its output negative-sequence current command value will worsen the dynamic characteristics of the reactive current. Therefore, the voltage equalization coefficient adjustment logic of the phase-to-phase voltage equalization closed-loop control also includes: under steady-state grid conditions, the phase-to-phase voltage equalization coefficient should be minimized.

[0053] The initial value of the positive-sequence reactive current is capacitive 0.2 pu. Under the condition that the two-phase reactive current of the power grid drops to 0.2 pu, the simulation effect of the method of this invention is as follows: Figure 7-1 and Figure 7-2 As shown in the figure, the simulation results of using a common method based on inter-phase voltage deviation adjustment are as follows: Figure 8-1 and Figure 8-2 As shown, judging from the commutator current and the highest and lowest voltage waveforms of the three-phase power modules, the method of the present invention is significantly superior in terms of voltage equalization response speed, equalization effect, and current maximum / minimum values.

[0054] In summary, the negative sequence current feedforward control of this invention can quickly and accurately solve the theoretical value of the negative sequence current required for phase-to-phase voltage balancing of the converter chain under unbalanced grid conditions through theoretical calculation. Then, by using phase-to-phase voltage balancing closed-loop control to compensate for the calculation error of the above theoretical value, it can improve the response speed and balancing effect of phase-to-phase voltage balancing, avoid triggering the overvoltage protection of the power module, and improve the grid fault ride-through capability of the SVG.

[0055] Device Example:

[0056] An embodiment of a star-type SVG converter chain phase-to-phase voltage equalization control device according to the present invention includes a memory, a processor, and an internal bus. The processor and the memory communicate and exchange data with each other through the internal bus. The memory is used to execute computer program instructions stored in the memory to implement the star-type SVG converter chain phase-to-phase voltage equalization control method described in the method embodiment of the present invention. The processor can be a microprocessor (MCU), a programmable logic device (FPGA), or other processing device. The memory can be any type of memory that stores information using electrical energy, such as RAM or ROM.

Claims

1. A method for phase-to-phase voltage equalization control of the converter chain in a star-type SVG, characterized in that, The first negative sequence current command value and the second negative sequence current command value obtained by phase-to-phase voltage equalization closed-loop control are added together to obtain the final negative sequence current command value of the converter chain phase-to-phase voltage equalization control. The final negative sequence current command value is used to generate the control signal of the star SVG to control the star SVG. In the case of grid fault, if the grid imbalance is greater than or equal to the set threshold, the negative sequence current feedforward command value is obtained by using negative sequence current feedforward control, and the negative sequence current feedforward command value is used as the first negative sequence current command value. The negative sequence current feedforward control is calculated by the positive and negative sequence components of the grid line voltage and the positive sequence reactive current command value of the converter chain. The formula for calculating the negative sequence current feedforward command value is as follows: In the formula, I nd_ffd , I nq_ffd These are the negative sequence current feedforward d-axis and q-axis command values, respectively; U spp_pd , U spp_pq These are the positive sequence d-axis and q-axis components of the grid line voltage, respectively. U spp_nd , U spp_nq These are the negative sequence d-axis and q-axis components of the grid line voltage, respectively. I pq_ref This is the positive sequence reactive current command value for the converter chain.

2. The method for phase-to-phase voltage equalization control of the converter chain of a star-type SVG according to claim 1, characterized in that, The initial positions of the positive-sequence dq coordinate system and the negative-sequence dq coordinate system of the power grid line voltage coincide; The initial position of the positive sequence dq coordinate system of the commutator current lags behind the initial position of the positive sequence dq coordinate system of the grid line voltage by π / 6; the initial position of the negative sequence dq coordinate system of the commutator current leads the initial position of the positive sequence dq coordinate system of the grid line voltage by π / 6.

3. The method for phase-to-phase voltage equalization control of the converter chain of a star-type SVG according to claim 1, characterized in that, Under grid fault conditions, if the grid imbalance is less than a set threshold, the first negative sequence current command value is 0 until the grid fault state is exited.

4. The method for phase-to-phase voltage equalization control of the converter chain of a star-type SVG according to claim 1, characterized in that, Under steady-state conditions of the power grid, the first negative sequence current command value is the negative sequence current feedforward command value.

5. The method for phase-to-phase voltage equalization control of the converter chain of a star-type SVG according to claim 3, characterized in that, Under power grid fault conditions, if the first negative sequence current command value is not 0, the phase-to-phase voltage equalization coefficient in the phase-to-phase voltage equalization closed-loop control is adjusted to the intermediate value; if the first negative sequence current command value is 0, the phase-to-phase voltage equalization coefficient is adjusted to the maximum value when the phase-to-phase voltage deviation is greater than the set deviation threshold; wherein, the phase-to-phase voltage equalization coefficient is the proportional coefficient of the PI controller in the phase-to-phase voltage equalization closed-loop control.

6. The method for phase-to-phase voltage equalization control of the converter chain of a star-type SVG according to claim 5, characterized in that, After the power grid fault is restored and remains so for a certain period of time, the phase-to-phase voltage equalization coefficient will be adjusted to its minimum value.

7. The method for phase-to-phase voltage equalization control of the converter chain of a star-type SVG according to claim 1, characterized in that, Under steady-state conditions of the power grid, the phase-to-phase voltage equalization coefficient in the phase-to-phase voltage equalization closed-loop control is adjusted to a minimum value; wherein, the phase-to-phase voltage equalization coefficient is the proportional coefficient of the PI controller in the phase-to-phase voltage equalization closed-loop control.

8. A converter chain phase-to-phase voltage equalization control device for a star-type SVG, characterized in that, Includes a memory and a processor, the processor being used to execute computer program instructions stored in the memory to implement the following method for interphase voltage equalization control of the commutation chain of a star-type SVG: The first negative sequence current command value and the second negative sequence current command value obtained by phase-to-phase voltage equalization closed-loop control are added together to obtain the final negative sequence current command value of the converter chain phase-to-phase voltage equalization control. The final negative sequence current command value is used to generate the control signal of the star SVG to control the star SVG. In the case of grid fault, if the grid imbalance is greater than or equal to the set threshold, the negative sequence current feedforward command value is obtained by using negative sequence current feedforward control, and the negative sequence current feedforward command value is used as the first negative sequence current command value. The negative sequence current feedforward control is calculated by the positive and negative sequence components of the grid line voltage and the positive sequence reactive current command value of the converter chain. The formula for calculating the negative sequence current feedforward command value is as follows: In the formula, I nd_ffd , I nq_ffd These are the negative sequence current feedforward d-axis and q-axis command values, respectively; U spp_pd , U spp_pq These are the positive sequence d-axis and q-axis components of the grid line voltage, respectively. U spp_nd , U spp_nq These are the negative sequence d-axis and q-axis components of the grid line voltage, respectively. I pq_ref This is the command value for the positive sequence reactive current of the converter chain.

9. The converter chain phase-to-phase voltage equalization control device for a star-type SVG according to claim 8, characterized in that: The initial positions of the positive-sequence dq coordinate system and the negative-sequence dq coordinate system of the power grid line voltage coincide; The initial position of the positive sequence dq coordinate system of the commutator current lags behind the initial position of the positive sequence dq coordinate system of the grid line voltage by π / 6; the initial position of the negative sequence dq coordinate system of the commutator current leads the initial position of the positive sequence dq coordinate system of the grid line voltage by π / 6.

10. The converter chain phase-to-phase voltage equalization control device for a star-shaped SVG according to claim 8, characterized in that, Under grid fault conditions, if the grid imbalance is less than a set threshold, the first negative sequence current command value is 0 until the grid fault state is exited.

11. The converter chain phase-to-phase voltage equalization control device for a star-shaped SVG according to claim 8, characterized in that, Under steady-state conditions of the power grid, the first negative sequence current command value is the negative sequence current feedforward command value.

12. The converter chain phase-to-phase voltage equalization control device for a star-shaped SVG according to claim 10, characterized in that, Under power grid fault conditions, if the first negative sequence current command value is not 0, the phase-to-phase voltage equalization coefficient in the phase-to-phase voltage equalization closed-loop control is adjusted to the intermediate value; if the first negative sequence current command value is 0, the phase-to-phase voltage equalization coefficient is adjusted to the maximum value when the phase-to-phase voltage deviation is greater than the set deviation threshold; wherein, the phase-to-phase voltage equalization coefficient is the proportional coefficient of the PI controller in the phase-to-phase voltage equalization closed-loop control.

13. The converter chain phase-to-phase voltage equalization control device for a star-shaped SVG according to claim 12, characterized in that, After the power grid fault is restored and remains so for a certain period of time, the phase-to-phase voltage equalization coefficient will be adjusted to its minimum value.

14. The converter chain phase-to-phase voltage equalization control device for a star-shaped SVG according to claim 8, characterized in that, Under steady-state conditions of the power grid, the phase-to-phase voltage equalization coefficient in the phase-to-phase voltage equalization closed-loop control is adjusted to a minimum value; wherein, the phase-to-phase voltage equalization coefficient is the proportional coefficient of the PI controller in the phase-to-phase voltage equalization closed-loop control.

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