Voltage control method and device for coordinating active power and reactive power of inverter under asymmetric fault

Abstract

The application relates to a voltage control method and equipment for active and reactive power coordination of an inverter under asymmetric fault, and relates to the technical field of power systems. The control of the inverter output current is realized through a positive and negative sequence current inner loop and a phase-locked loop, the support for the grid connection point voltage is completed, and the low voltage ride through control of the inverter is realized. The low voltage ride through control method always considers the positive and negative sequence voltages of the grid connection point, coordinates the active power and the reactive power output by the inverter, comprehensively considers the limitations of the phase voltage, the phase current and the capacity of the inverter, achieves the support effect of the inverter on the grid connection point voltage, minimizes the active power shortage of the inverter, and guarantees the stability of the system frequency.

Description

Technical Field

[0001] This invention relates to the field of power system technology, specifically to a voltage control method and device for coordinating active and reactive power in an inverter under asymmetrical faults. Background Technology

[0002] With the increasing penetration rate of new energy sources, the grid connection / off-grid operation of new energy units has a significant impact on the stability of the power system. my country's wind power grid connection guideline GB / T19963-2011 explicitly requires that grid-connected new energy units possess low-voltage ride-through capability and provide a certain amount of reactive current to the grid for voltage support during faults. The low-voltage ride-through method for new energy units under grid faults is basically achieved by applying control commands to the grid-side inverter. Most voltage dips in the power system are caused by asymmetrical faults; therefore, researching inverter low-voltage ride-through control technology under asymmetrical fault conditions is of great significance for the stable operation of the power system.

[0003] The inventors discovered the following problems in current asymmetric fault ride-through control methods for wind farms: (1) In current research on low-voltage ride-through control methods for new energy units under asymmetrical faults, in order to maintain the symmetry of the three-phase current output by the inverter, positive-sequence reactive current is usually injected according to the grid connection guidelines, while the negative-sequence current is set to zero. At this time, the grid side needs to provide a larger negative-sequence current, which causes the negative-sequence voltage at the grid connection point to rise, which is detrimental to the safety and stability of the system. In addition, some studies design the negative-sequence current injection value based on line parameters, but in actual operation, it is difficult for new energy power plants to obtain line impedance parameters in real time, which can easily lead to inaccurate low-voltage ride-through control commands and consequences such as inverter low-voltage ride-through failure.

[0004] (2) Because the inverter generates a large amount of reactive power during the low-voltage period, the active power output of the inverter is reduced accordingly due to the inverter capacity limitation, which can easily lead to a shortage of active power in the system. If the active power shortage in the system is higher than the frequency regulation reserve, it will cause a more serious frequency safety accident. Therefore, the size of the active power shortage is also an important issue to be considered during the low-voltage period of the inverter. However, most existing studies only inject the minimum reactive current according to the grid connection guidelines and use the remaining capacity to maintain the active power output of the inverter, which cannot achieve a fully coordinated support effect of active and reactive power. Therefore, the inverter capacity should be reasonably allocated during the low-voltage period to coordinate the active and reactive power output of the inverter and keep the voltage and frequency of the system within a safe range. Summary of the Invention

[0005] In order to overcome the shortcomings of the above technologies, this invention provides a voltage control method and device that maximizes the difference between the positive and negative sequence voltages at the grid connection point, thereby achieving the inverter's support effect on the grid connection point voltage, while minimizing the inverter's active power deficit and ensuring the stability of the system frequency.

[0006] The technical solution adopted by this invention to overcome its technical problems is: A voltage control method for coordinating active and reactive power in an inverter under asymmetrical fault conditions includes the following steps: S01. Real-time monitoring of system asymmetric faults to determine whether an asymmetric fault has occurred on the network side. If an asymmetric fault has occurred, proceed to step S02. S02. Calculate the positive sequence voltage amplitude at the grid connection point. and the negative sequence voltage amplitude at the grid connection point ; S03. Calculate the positive-sequence reactive current command at the inverter output. and the negative sequence reactive current command output by the inverter ; S04. Construct positive-sequence reactive current control parameters, negative-sequence reactive current control parameters, and positive-sequence active current command. The optimization problem is solved using the formula. Calculate the optimal solution, where To increase the weight of the positive-sequence voltage difference at the grid connection point in this optimization problem, To reduce the weight of the active power deficit in this optimization problem, This is the positive-sequence active current command output by the inverter. This represents the amplitude of the positive-sequence voltage at the grid connection point before the asymmetrical fault. This refers to the positive-sequence active current output by the inverter before an asymmetrical fault. S05. Transfer the positive-sequence active current command. Positive sequence reactive current command and the negative sequence reactive current command output by the inverter The input is fed into the control module inside the grid-side inverter, and the output current of the inverter is controlled through the positive and negative sequence current inner loop and the phase-locked loop.

[0007] Furthermore, in step S01, the grid-side voltage is decomposed into sequence components. If there is a negative sequence component in the grid-side voltage, it is determined that an asymmetric fault has occurred on the grid side.

[0008] Furthermore, in step S02, the formula is used... The positive sequence voltage amplitude at the grid connection point was calculated. In the formula This represents the magnitude of the positive-sequence component of the grid-side voltage. The positive sequence impedance is the line connecting the grid to the grid-side transmission line. The magnitude of the positive-sequence component of the current on the transmission line from the grid connection point to the grid side is given by the formula. The negative sequence voltage amplitude at the grid connection point was calculated. In the formula This represents the magnitude of the negative sequence component of the grid-side voltage. The negative sequence impedance is the line connecting the grid to the grid side transmission line. It represents the amplitude of the negative sequence component of the current on the transmission line from the grid connection point to the grid side.

[0009] Furthermore, in step S03, the formula is used... The inverter output positive-sequence reactive current command is calculated. In the formula These are the positive-sequence reactive current control parameters. The minimum phase voltage of the three phases at the grid connection point is obtained through the formula. The negative sequence reactive current command of the inverter output is calculated. In the formula These are the control parameters for negative sequence reactive current. This represents the maximum phase voltage of the three phases at the grid connection point.

[0010] Furthermore, the optimization problem in step S04 The constraints to be satisfied are: ,in , The voltage amplitude of phase a at the grid connection point. The voltage amplitude of phase b at the grid connection point. The voltage amplitude of phase c at the grid connection point. The initial phase difference between the positive and negative sequence voltages at the grid connection point. The initial phase difference between the positive and negative sequence currents output by the inverter. The current of phase a output by the inverter. This refers to the b-phase current output by the inverter. This refers to the c-phase current output by the inverter. This refers to the inverter output phase current limit. This represents the positive sequence voltage value at the grid connection point when the inverter is not under control after a fault. This is a positive-sequence active current command. It is a negative sequence active current command.

[0011] Furthermore, in step S04, the inverter outputs a negative-sequence active current command. Set to 0.

[0012] A voltage control device for coordinating active and reactive power in an inverter under asymmetrical fault conditions includes: At least one processor, and A memory communicatively connected to the at least one processor; wherein the memory stores instructions executable by the at least one processor, the instructions being executed by the at least one processor to enable the at least one processor to: Real-time monitoring of system asymmetric faults is performed to determine whether an asymmetric fault has occurred on the network side. If an asymmetric fault has occurred, step S02 is executed. Calculate the positive sequence voltage amplitude at the grid connection point and the negative sequence voltage amplitude at the grid connection point ; Calculate the positive sequence reactive current command of the inverter output. and the negative sequence reactive current command output by the inverter ; Construct positive-sequence reactive current control parameters, negative-sequence reactive current control parameters, and positive-sequence active current commands. The optimization problem is solved using the formula. Calculate the optimal solution, where To increase the weight of the positive-sequence voltage difference at the grid connection point in this optimization problem, To reduce the weight of the active power deficit in this optimization problem, This is the positive-sequence active current command output by the inverter. This represents the amplitude of the positive-sequence voltage at the grid connection point before the asymmetrical fault. This refers to the positive-sequence active current output by the inverter before an asymmetrical fault. The positive-sequence active current command Positive sequence reactive current command and the negative sequence reactive current command output by the inverter The input is fed into the control module inside the grid-side inverter, and the output current of the inverter is controlled through the positive and negative sequence current inner loop and the phase-locked loop.

[0013] The beneficial effects of this invention are as follows: By considering the reactive current injection standard during low-voltage circuit breaking and the adjustable reactive power capacity of the grid-side inverter, a flexible control method for positive and negative sequence voltages under asymmetrical faults is proposed. This method simultaneously issues positive and negative sequence current commands to achieve the effects of increasing the positive sequence voltage at the grid connection point and decreasing the negative sequence voltage at the grid connection point. By simultaneously considering the impact of voltage and active power on the system and optimizing the current control parameters, with the goal of maximizing the difference between the positive and negative sequence voltages at the grid connection point and minimizing the active power deficit of the inverter, and comprehensively considering the constraints of inverter phase voltage, phase current, and capacity, an optimization problem for the positive and negative sequence current control parameters is established, achieving the effect of supporting the grid connection point voltage while minimizing the reduction of the inverter's active power output. Attached Figure Description

[0014] Figure 1 This is a control block diagram of the voltage control method for coordinating active and reactive power in an inverter under asymmetric faults according to the present invention. Figure 2 The flowchart shows the voltage control method for coordinating active and reactive power in an inverter under asymmetric faults according to the present invention. Figure 3This is a comparison curve of the positive and negative sequence voltage amplitude at the grid connection point under different control methods under asymmetric faults, provided in an embodiment of the present invention. Figure 4 The present invention provides a comparison curve of the instantaneous active power output of the inverter under different control methods in the case of asymmetric faults, as provided in an embodiment of the invention. Detailed Implementation

[0015] The following is in conjunction with the appendix Figure 1 To be continued Figure 4 The present invention will be further described below.

[0016] As attached Figure 1 and attached Figure 2 As shown, a voltage control method for coordinating active and reactive power output of an inverter under asymmetrical fault conditions is proposed. First, the impact of the positive and negative sequence currents output by the inverter under asymmetrical fault conditions on the grid side is analyzed, and a flexible control method for the positive and negative sequence voltages is proposed to increase the positive sequence voltage and decrease the negative sequence voltage at the grid connection point. Then, to coordinate the active and reactive power output of the inverter, considering the constraints of the inverter's phase voltage, phase current, and capacity, an optimization problem for the positive and negative sequence current control parameters is established to maximize the difference between the positive and negative sequence voltages at the grid connection point, achieving the inverter's support effect on the grid connection point voltage, while minimizing the inverter's active power deficit to ensure system frequency stability. Specifically, the method includes the following steps: S01. Monitor the system for asymmetric faults in real time and determine whether an asymmetric fault has occurred on the network side. If an asymmetric fault has occurred, proceed to step S02.

[0017] S02. Calculate the positive sequence voltage amplitude at the grid connection point. and the negative sequence voltage amplitude at the grid connection point .

[0018] S03. Calculate the positive-sequence reactive current command at the inverter output. and the negative sequence reactive current command output by the inverter .

[0019] S04. Construct positive-sequence reactive current control parameters, negative-sequence reactive current control parameters, and positive-sequence active current command. The optimization problem is solved using the formula. Calculate the optimal solution, where To increase the weight of the positive-sequence voltage difference at the grid connection point in this optimization problem, To reduce the weight of the active power deficit in this optimization problem, This is the positive-sequence active current command output by the inverter. This represents the amplitude of the positive-sequence voltage at the grid connection point before the asymmetrical fault. This refers to the positive-sequence active current output by the inverter before an asymmetrical fault.

[0020] S05. Transfer the positive-sequence active current command. Positive sequence reactive current command and the negative sequence reactive current command output by the inverter The input is fed into the control module inside the grid-side inverter, and the output current of the inverter is controlled through the positive and negative sequence current inner loop and the phase-locked loop.

[0021] Appendix Figure 3 The magnitudes of the positive and negative sequence voltages at the grid connection point under different control methods for simulation verification of the proposed method under asymmetric faults are presented. It can be seen that, without considering the inverter's active power output, the inverter can output more reactive power to support the grid connection point voltage, maximizing the difference between the positive and negative sequence voltage magnitudes. However, for the control method considering active power output, the difference between the positive and negative sequence voltage magnitudes is reduced due to the introduction of active current in the optimization problem.

[0022] Appendix Figure 4 The instantaneous active power output of the inverter under different control modes for simulation verification of the method proposed in this invention is given. It can be seen that compared with the control mode that uses all the inverter capacity to support voltage, the control mode that considers active power output only reduces the active power by about 30%, and the voltage support effect of the two control modes is not much different. Therefore, the control mode that considers active power output can achieve better results.

[0023] In one embodiment of the present invention, in step S01, the grid-side voltage is decomposed into sequence components. If a negative sequence component exists in the grid-side voltage, an asymmetric fault is determined to have occurred on the grid side. The specific method for determining whether the grid connection point voltage is qualified is as follows: the grid connection point voltage is decomposed into positive and negative sequences. If the positive sequence voltage drops below 0.9 pu, the grid connection point voltage is determined to be unqualified.

[0024] In one embodiment of the present invention, step S02 is performed using the formula The positive sequence voltage amplitude at the grid connection point was calculated. In the formula This represents the magnitude of the positive-sequence component of the grid-side voltage. The positive sequence impedance is the line connecting the grid to the grid-side transmission line. The magnitude of the positive-sequence component of the current on the transmission line from the grid connection point to the grid side is given by the formula. The negative sequence voltage amplitude at the grid connection point was calculated. In the formula This represents the magnitude of the negative sequence component of the grid-side voltage. The negative sequence impedance is the line connecting the grid to the grid side transmission line. This represents the amplitude of the negative-sequence current component on the transmission line from the grid connection point to the grid side. From the above formula, it can be deduced that under asymmetrical fault conditions, the amplitude of the positive-sequence voltage at the grid connection point is directly proportional to the amplitude of the positive-sequence current on the transmission line, and the amplitude of the negative-sequence voltage at the grid connection point is inversely proportional to the amplitude of the negative-sequence current on the transmission line. Therefore, during low-voltage ride-through, increasing the positive-sequence current while decreasing the negative-sequence current through control commands can achieve better support for the grid connection point voltage.

[0025] In one embodiment of the present invention, active and reactive currents are decoupled based on dq transformation, meaning the magnitude of the active power output by the inverter is determined by and directly proportional to the current component on the d-axis. Similarly, the magnitude of the reactive power output by the inverter is also directly proportional to the q-axis current. Decoupling control of the inverter's active and reactive power can be achieved by adjusting the positive-sequence active and reactive current commands. Since the magnitude of the positive-sequence voltage at the grid connection point is proportional to the magnitude of the positive-sequence current on the transmission line, and the magnitude of the negative-sequence voltage at the grid connection point is inversely proportional to the magnitude of the negative-sequence current on the transmission line, and the magnitude of the minimum phase voltage is proportional to the magnitude of the positive-sequence voltage, while the magnitude of the maximum phase voltage is proportional to the magnitude of the negative-sequence voltage, step S03, under the premise of satisfying the inverter's adjustable reactive capacity, uses the formula... The inverter output positive-sequence reactive current command is calculated. In the formula These are the positive-sequence reactive current control parameters. The minimum phase voltage of the three phases at the grid connection point is obtained through the formula. The negative sequence reactive current command of the inverter output is calculated. In the formula These are the control parameters for negative sequence reactive current. This represents the maximum phase voltage of the three phases at the grid connection point.

[0026] In one embodiment of the present invention, the optimization problem in step S04 The constraints to be satisfied are: ,in , The voltage amplitude of phase a at the grid connection point. The voltage amplitude of phase b at the grid connection point. The voltage amplitude of phase c at the grid connection point. The initial phase difference between the positive and negative sequence voltages at the grid connection point. The initial phase difference between the positive and negative sequence currents output by the inverter. The current of phase a output by the inverter. This refers to the b-phase current output by the inverter. This refers to the c-phase current output by the inverter. This refers to the inverter output phase current limit. This represents the positive sequence voltage value at the grid connection point when the inverter is not under control after a fault. This is a positive-sequence active current command. This is a negative-sequence active current command. This optimization problem achieves the coordination of active and reactive power output from the inverter, and can solve for the optimal current control parameters under different fault types and different grid-side voltage sags, thereby establishing an offline control parameter library for different fault types and voltage sags. When an asymmetrical fault occurs, the offline control parameter library module can select control parameters from the offline control parameter library based on the fault type and grid connection point voltage sag depth provided by the inverter's low-voltage ride-through module, and then pass them to the grid-side inverter's inner-loop control module.

[0027] In one embodiment of the present invention, the negative-sequence active current command output by the inverter in step S04 Set to 0.

[0028] This invention also relates to a voltage control device for coordinating active and reactive power in an inverter under asymmetrical fault conditions, comprising: At least one processor, and A memory communicatively connected to the at least one processor; wherein the memory stores instructions executable by the at least one processor, the instructions being executed by the at least one processor to enable the at least one processor to: Real-time monitoring of system asymmetric faults is performed to determine whether an asymmetric fault has occurred on the network side. If an asymmetric fault has occurred, step S02 is executed. Calculate the positive sequence voltage amplitude at the grid connection point and the negative sequence voltage amplitude at the grid connection point ; Calculate the positive sequence reactive current command of the inverter output. and the negative sequence reactive current command output by the inverter ; Construct positive-sequence reactive current control parameters, negative-sequence reactive current control parameters, and positive-sequence active current commands. The optimization problem is solved using the formula. Calculate the optimal solution, where To increase the weight of the positive-sequence voltage difference at the grid connection point in this optimization problem, To reduce the weight of the active power deficit in this optimization problem, This is the positive-sequence active current command output by the inverter. This represents the amplitude of the positive-sequence voltage at the grid connection point before the asymmetrical fault. This refers to the positive-sequence active current output by the inverter before an asymmetrical fault. The positive-sequence active current command Positive sequence reactive current command and the negative sequence reactive current command output by the inverter The input is fed into the control module inside the grid-side inverter, and the output current of the inverter is controlled through the positive and negative sequence current inner loop and the phase-locked loop.

[0029] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A voltage control method for coordinating active and reactive power in an inverter under asymmetrical fault conditions, characterized in that, Includes the following steps: S01. Real-time monitoring of system asymmetric faults to determine whether an asymmetric fault has occurred on the network side. If an asymmetric fault has occurred, proceed to step S02. S02. Calculate the positive sequence voltage amplitude at the grid connection point. and the negative sequence voltage amplitude at the grid connection point ; S03. Calculate the positive-sequence reactive current command at the inverter output. and the negative sequence reactive current command output by the inverter ; S04. Construct positive-sequence reactive current control parameters, negative-sequence reactive current control parameters, and positive-sequence active current command. The optimization problem is solved using the formula. Calculate the optimal solution, where To increase the weight of the positive-sequence voltage difference at the grid connection point in this optimization problem, To reduce the weight of the active power deficit in this optimization problem, This is the positive-sequence active current command output by the inverter. This represents the amplitude of the positive-sequence voltage at the grid connection point before the asymmetrical fault. This refers to the positive-sequence active current output by the inverter before an asymmetrical fault. S05. Transfer the positive-sequence active current command. Positive sequence reactive current command and the negative sequence reactive current command output by the inverter The input is fed into the control module inside the grid-side inverter, and the output current of the inverter is controlled through the positive and negative sequence current inner loop and the phase-locked loop. In step S03, the formula is used. The inverter output positive-sequence reactive current command is calculated. In the formula These are the positive sequence reactive current control parameters. The minimum phase voltage of the three phases at the grid connection point is obtained through the formula. The negative sequence reactive current command of the inverter output is calculated. In the formula These are the negative sequence reactive current control parameters. This refers to the maximum phase voltage of the three phases at the grid connection point; Optimization problem in step S04 The constraints to be satisfied are: ,in , The voltage amplitude of phase a at the grid connection point. The voltage amplitude of phase b at the grid connection point. The voltage amplitude of phase c at the grid connection point. The initial phase difference between the positive and negative sequence voltages at the grid connection point. The initial phase difference between the positive and negative sequence currents output by the inverter. This refers to the phase a current output by the inverter. This refers to the b-phase current output by the inverter. This refers to the c-phase current output by the inverter. This refers to the inverter output phase current limit. This represents the positive sequence voltage value at the grid connection point when the inverter is not under control after a fault. This is a positive-sequence active current command. It is a negative sequence active current command.

2. The voltage control method for coordinating active and reactive power in an inverter under asymmetrical faults according to claim 1, characterized in that: In step S01, the grid-side voltage is decomposed into sequence components. If there is a negative sequence component in the grid-side voltage, it is determined that an asymmetric fault has occurred on the grid side.

3. The voltage control method for coordinating active and reactive power in an inverter under asymmetrical faults according to claim 1, characterized in that: In step S02, the formula is used. The positive sequence voltage amplitude at the grid connection point was calculated. In the formula This represents the magnitude of the positive-sequence component of the grid-side voltage. The positive sequence impedance is the line connecting the grid to the grid-side transmission line. The magnitude of the positive-sequence component of the current on the transmission line from the grid connection point to the grid side is given by the formula. The negative sequence voltage amplitude at the grid connection point was calculated. In the formula This represents the magnitude of the negative sequence component of the grid-side voltage. The negative sequence impedance is the line connecting the grid to the grid side transmission line. It represents the amplitude of the negative sequence component of the current on the transmission line from the grid connection point to the grid side.

4. The voltage control method for coordinating active and reactive power in an inverter under asymmetrical faults according to claim 1, characterized in that: The negative sequence active current command output by the inverter in step S04 Set to 0.

5. An apparatus for implementing a voltage control method for coordinating active and reactive power in an inverter under any one of claims 1 to 4, characterized in that, include: At least one processor, and A memory communicatively connected to the at least one processor; wherein the memory stores instructions executable by the at least one processor, the instructions being executed by the at least one processor to enable the at least one processor to: Real-time monitoring of system asymmetric faults is performed to determine whether an asymmetric fault has occurred on the network side. If an asymmetric fault has occurred, step S02 is executed. Calculate the positive sequence voltage amplitude at the grid connection point and the negative sequence voltage amplitude at the grid connection point ; Calculate the positive sequence reactive current command of the inverter output. and the negative sequence reactive current command output by the inverter ; Construct positive-sequence reactive current control parameters, negative-sequence reactive current control parameters, and positive-sequence active current commands. The optimization problem is solved using the formula. Calculate the optimal solution, where To increase the weight of the positive-sequence voltage difference at the grid connection point in this optimization problem, To reduce the weight of the active power deficit in this optimization problem, This is the positive-sequence active current command output by the inverter. This represents the amplitude of the positive-sequence voltage at the grid connection point before the asymmetrical fault. This refers to the positive-sequence active current output by the inverter before an asymmetrical fault. The positive-sequence active current command Positive sequence reactive current command and the negative sequence reactive current command output by the inverter The input is fed into the control module inside the grid-side inverter, and the output current of the inverter is controlled through the positive and negative sequence current inner loop and the phase-locked loop.

Images