Flexible DC power transmission system, receiving end AC fault ride-through method and device thereof, and VSC

Abstract

The invention belongs to the technical field of flexible direct-current power transmission, and particularly relates to a flexible direct-current power transmission system, a receiving end alternating-current fault ride-through method and device thereof and a VSC. The control method of the receiving end VSC in the method comprises the following steps: when a receiving end AC fault occurs, taking a receiving end sub-module capacitor voltage balance controller as an outer loop controller, and obtaining a receiving end VSC valve group modulation voltage DC component according to the output of a sub-module capacitor overvoltage controller; the receiving terminal module capacitor voltage balance controller outputs an active current reference value used for controlling an inner loop current controller of alternating current according to the sub-module capacitor voltage reference value and the sub-module capacitor voltage average value; and obtaining a trigger pulse for controlling the receiving end VSC according to the active current reference value and the direct current voltage instruction value of the receiving end VSC valve group. The technical problem that the control effect of a control system is not ideal due to the fact that parameter setting of a voltage regulation function is difficult in the prior art is solved.

Description

Technical Field

[0001] This invention belongs to the field of flexible DC transmission technology, specifically relating to a flexible DC transmission system and its receiving-end AC fault ride-through method, device, and VSC. Background Technology

[0002] A type of ultra-high voltage flexible DC transmission connection, such as Figure 1 As shown, both the sending and receiving ends are voltage source converters (VSCs). The VSC adopts a modular multilevel converter topology composed of full and half-bridge hybrid submodules, which has advantages such as no commutation failure risk and decoupled control of active and reactive power, and is suitable for application scenarios such as large-scale new energy base transmission in desert areas and weak AC system power transmission.

[0003] When an AC system fault occurs at the receiving end, the active power output of the receiving-end VSC is limited, resulting in DC power exceeding AC power and causing the VSC submodule capacitor voltage to rise. At the sending end, in steady state, it operates in constant active power control mode. When the DC voltage exceeds a certain level, it switches to DC voltage deviation control to reduce the active power output at the sending end. Due to the long length and high inductive reactance of the UHVDC lines, DC voltage deviation control cannot reduce the DC power delivered to the receiving end to a sufficiently low level in a timely manner, causing the receiving submodule capacitor voltage to exceed the voltage protection setting.

[0004] In the prior art, the method to suppress overvoltage of the receiving-end VSC submodule is to detect the AC voltage of the receiving end, reduce the DC voltage of the VSC through a segmented voltage regulation function, reduce the DC power injected into the receiving-end VSC, and thus suppress the overvoltage of the VSC submodule capacitor.

[0005] The controller structure of the receiving-end VSC in the prior art is as follows: Figure 2 As shown. VSC valve group DC voltage command value U dcord The voltage regulation function is based on the amplitude U of the AC positive sequence fundamental voltage on the VSC valve group converter transformer side. acpos DC voltage limiting value U after limiting dcordlim U dcordlim The DC voltage command is input to the DC voltage controller to control the DC voltage U of the VSC valve group. dcGi (i=1 represents the high-pressure valve group, i=2 represents the low-pressure valve group) control is performed, and the VSC valve group active current reference value i is output. dref To the inner loop current controller; i dref and i qref These are the active current reference value and reactive current reference value of the VSC valve group, u dq For the d-axis and q-axis components of the VSC valve group network-side voltage, i dq For the d-axis and q-axis components of the AC current on the valve side of the VSC valve group converter transformer, Udcmod For the DC component of the modulated voltage of the VSC valve group, i pj and i nj These represent the upper and lower bridge arm currents of phase j, respectively. The inner loop current controller, circulating current suppressor, and valve controller are the basic controllers for flexible DC transmission, and will not be described in detail here.

[0006] Under steady-state conditions, the receiving-end VSC valve group operates in DC voltage control mode, using a DC voltage controller for closed-loop control to generate an active current reference value, thus achieving active power balance in the DC transmission system. When a fault occurs in the receiving-end AC system, U... acpos Decrease, the voltage regulation function is based on U acpos Calculated U dcordlim The voltage is lower than the steady-state voltage reference value, which reduces the DC voltage of the VSC and decreases the surplus power of the VSC.

[0007] During a fault in the receiving-end AC system, the existing control methods for the sending end are as follows: upon detecting a sudden increase in DC current, switch to DC current control mode to maintain the DC current at the pre-fault level. Alternatively, through inter-station communication, transmit the amplitude of the receiving-end AC positive-sequence fundamental voltage to the sending end, reducing the active power of the sending end and maintaining the DC current at the pre-fault level.

[0008] However, the above scheme has some problems: 1) For the receiving-end VSC, the parameter tuning of the voltage regulation function is very difficult. The tuned parameters of the voltage regulation function should suppress the submodule capacitor voltage below the overvoltage protection setting, while transmitting as much DC power as possible. However, AC power grids and DC transmission systems have many operating modes. The parameters tuned under certain operating modes may not be optimal for other operating modes or different power levels, resulting in unsatisfactory overvoltage suppression and thus unsatisfactory control performance of the control system. 2) For the sending-end VSC, switching control modes or receiving voltage signals from the receiving end through inter-station communication will cause delays, resulting in untimely suppression of overvoltage in the receiving-end VSC submodule. Summary of the Invention

[0009] The purpose of this invention is to provide a flexible DC transmission system and its receiving-end AC fault ride-through method, device, and VSC to solve the technical problem that the difficulty in setting the parameters of the voltage regulation function in the prior art leads to unsatisfactory control effect of the control system.

[0010] To solve the above-mentioned technical problems, the present invention provides a technical solution for a flexible DC transmission receiving-end AC fault ride-through method: a flexible DC transmission receiving-end AC fault ride-through method.

[0011] The control method of the receiving-end VSC in this method includes:

[0012] When an AC fault occurs at the receiving end, the receiving end module capacitor voltage balance controller is used as the outer loop controller, and the DC component of the receiving end VSC valve group modulation voltage is obtained based on the output of the sub-module capacitor overvoltage controller.

[0013] The terminal module capacitor voltage balance controller outputs the active current reference value of the inner loop current controller for controlling AC current based on the sub-module capacitor voltage reference value and the sub-module capacitor voltage average value, so as to stabilize the sub-module capacitor voltage average value at the sub-module capacitor voltage reference value.

[0014] The submodule capacitor overvoltage controller amplifies the deviation between the submodule capacitor voltage reference value and the submodule capacitor voltage average value, adds it to the DC voltage command value of the receiving end VSC valve group, and outputs it to ensure that the submodule capacitor voltage does not exceed the submodule overvoltage protection setting value.

[0015] The trigger pulse for controlling the receiving-end VSC is obtained based on the active current reference value and the DC component of the modulation voltage of the receiving-end VSC valve group.

[0016] The beneficial effects of the above technical solution are as follows: The technical solution of the flexible DC transmission receiving-end AC fault ride-through method of the present invention belongs to an improved invention. When a fault occurs in the receiving-end AC system of a flexible DC transmission, the overvoltage controller of the receiving-end VSC submodule capacitor in this invention reduces the DC voltage of the receiving-end VSC, thereby reducing the DC power injected into the receiving-end VSC. The receiving-end VSC submodule capacitor voltage balance controller controls the VSC to output the maximum active current, maintaining a balance between the DC power and active power of the receiving-end VSC. The receiving-end AC fault ride-through method of the present invention does not use a preset voltage regulation function, and can maximize the DC power level during the fault. The present invention solves the technical problem in the prior art where the parameter tuning of the voltage regulation function is difficult, resulting in unsatisfactory control effect of the control system.

[0017] Furthermore, the control method for the receiving-end VSC also includes: when the AC fault at the receiving end is cleared, switching the receiving-end terminal module capacitor voltage balance controller to a DC voltage controller; and switching the sub-module capacitor overvoltage controller to the receiving-end VSC valve group DC voltage command value of the DC voltage controller;

[0018] The DC voltage controller outputs the active current reference value of the inner loop current controller based on the deviation between the DC voltage command value and the DC voltage measurement value of the receiving-end VSC valve group, so as to stabilize the DC voltage measurement value of the receiving-end VSC valve group at the DC voltage command value.

[0019] Furthermore, the criterion for receiving-end AC fault is: when the average value of the submodule capacitor voltage is greater than or equal to the submodule overvoltage setting value and the duration reaches the overvoltage judgment delay time, then the receiving-end AC fault is considered to have occurred.

[0020] Furthermore, the criterion for clearing the AC fault at the receiving end is as follows: when the average value of the submodule capacitor voltage drops below the fault clearing judgment value and the duration reaches the fault clearing judgment delay time, the AC fault at the receiving end is considered cleared.

[0021] Furthermore, the output of the submodule capacitor overvoltage controller is the output after amplitude limiting processing; the amplitude limiting processing of the submodule capacitor overvoltage controller includes:

[0022] When no AC fault occurs at the receiving end, the maximum and minimum limit values ​​of the submodule capacitor overvoltage controller are both the DC voltage command values ​​of the VSC valve group;

[0023] When a receiving-end AC fault occurs and an AC system emergency fault occurs, the maximum and minimum limit values ​​of the submodule capacitor overvoltage controller are both 0.

[0024] When a receiving-end AC fault occurs but no AC system emergency fault occurs, the maximum limit value of the submodule capacitor overvoltage controller is the VSC valve group DC voltage command value, and the minimum limit value is 0.

[0025] The criterion for an emergency fault in the AC system is as follows: when the amplitude of the AC positive sequence fundamental voltage on the converter substation side is less than or equal to the AC system emergency fault discrimination setpoint and the duration reaches the fault judgment delay time, an AC system emergency fault is considered to have occurred.

[0026] Furthermore, the active current reference value output by the receiving terminal module capacitor voltage balance controller is the active current reference value after limiting processing; the limiting of the receiving terminal module capacitor voltage balance controller includes:

[0027] When no AC fault occurs at the receiving end, the maximum and minimum limit values ​​of the capacitor voltage balance controller of the receiving terminal module are both the active current reference values ​​output by the DC voltage controller.

[0028] When a receiving-end AC fault occurs, the maximum and minimum limit values ​​of the receiving-end terminal module capacitor voltage balance controller are the maximum and minimum limit values ​​of the active current of the VSC valve group, respectively.

[0029] Furthermore, the control method of the sending-end VSC in this method includes:

[0030] The terminal module capacitor voltage balance controller is used as the outer loop controller;

[0031] The terminal module capacitor voltage balance controller is used to output the active current reference value of the inner loop current controller for controlling the AC current based on the sub-module capacitor voltage reference value and the sub-module capacitor voltage average value, so as to stabilize the sub-module capacitor voltage average value at the sub-module capacitor voltage reference value.

[0032] The DC component of the modulation voltage of the VSC valve group at the sending end is obtained from the DC current controller.

[0033] The DC current controller is used to output a control quantity based on the DC current reference value and the DC current measurement value, so as to stabilize the DC current measurement value at the DC current reference value.

[0034] The trigger pulse for controlling the sending-end VSC is obtained based on the active current reference value and the DC component of the modulation voltage of the sending-end VSC valve group.

[0035] Furthermore, the method of obtaining the DC component of the modulation voltage of the VSC valve group at the sending end according to the DC current controller includes: taking half of the superposition value of the output of the DC current controller and the output of the DC overvoltage controller as the DC component of the modulation voltage of the VSC valve group at the sending end.

[0036] The DC overvoltage controller includes: inputting the deviation between the DC high voltage reference value and the DC voltage measurement value into the DC overvoltage regulator; and outputting a control quantity as the output of the DC overvoltage controller after the DC overvoltage regulator is limited to a maximum limit value of 0.

[0037] Furthermore, the DC current reference value is obtained by obtaining the DC current reference value through an active power controller;

[0038] The active power controller includes: inputting the deviation between the active power reference value and the positive sequence active power measurement value into the active power regulator; the output of the active power regulator is limited and superimposed with the active power reference value to obtain a power reference value; and the power reference value is divided by the DC voltage to obtain the DC current reference value.

[0039] Furthermore, the DC voltage in the active power controller is a DC voltage smoothed by a first-order inertial element.

[0040] The present invention also provides a technical solution for a flexible DC transmission receiving-end AC fault ride-through device: a flexible DC transmission receiving-end AC fault ride-through device, comprising a processor, the processor being used to execute a computer program to implement the steps of the flexible DC transmission receiving-end AC fault ride-through method as described above.

[0041] The present invention also provides a VSC technical solution: a VSC for installation at the receiving end of a flexible DC transmission system, including a VSC controller, the VSC controller including a processor, the processor being used to execute a computer program to implement the steps of the AC fault ride-through method at the receiving end of the flexible DC transmission system as described above.

[0042] The present invention also provides a technical solution for a flexible DC transmission system: a flexible DC transmission system including a receiving-end VSC, the receiving-end VSC including a receiving-end VSC controller, the receiving-end VSC controller including a processor, the processor of the receiving-end VSC controller being used to execute a computer program to implement the steps of the flexible DC transmission receiving-end AC fault ride-through method as described above.

[0043] The beneficial effects of the above technical solution are as follows: The technical solution of the flexible DC transmission system of the present invention belongs to an improved invention. When a fault occurs in the receiving-end AC system of the flexible DC transmission system, the overvoltage controller of the receiving-end VSC submodule capacitor in the present invention reduces the DC voltage of the receiving-end VSC, thereby reducing the DC power injected into the receiving-end VSC. The receiving-end VSC submodule capacitor voltage balance controller controls the VSC to output the maximum active current, maintaining a balance between the DC power and active power of the receiving-end VSC. The receiving-end AC fault ride-through method of the present invention does not use a preset voltage regulation function, and can maximize the DC power level during the fault. The present invention solves the technical problem in the prior art where the parameter tuning of the voltage regulation function is difficult, resulting in unsatisfactory control effect of the control system.

[0044] Furthermore, the sending-end VSC in the flexible DC transmission system includes a sending-end VSC controller, which includes a processor. The processor of the sending-end VSC controller is used to execute a computer program to implement the control method of the sending-end VSC as described below:

[0045] The terminal module capacitor voltage balance controller is used as the outer loop controller;

[0046] The terminal module capacitor voltage balance controller is used to output the active current reference value of the inner loop current controller for controlling the AC current based on the sub-module capacitor voltage reference value and the sub-module capacitor voltage average value, so as to stabilize the sub-module capacitor voltage average value at the sub-module capacitor voltage reference value.

[0047] The DC component of the modulation voltage of the VSC valve group at the sending end is obtained from the DC current controller.

[0048] The DC current controller is used to output a control quantity based on the DC current reference value and the DC current measurement value, so as to stabilize the DC current measurement value at the DC current reference value.

[0049] The trigger pulse for controlling the sending-end VSC is obtained based on the active current reference value and the DC component of the modulation voltage of the sending-end VSC valve group.

[0050] Furthermore, the method of obtaining the DC component of the modulation voltage of the VSC valve group at the sending end according to the DC current controller includes: taking half of the superposition value of the output of the DC current controller and the output of the DC overvoltage controller as the DC component of the modulation voltage of the VSC valve group at the sending end.

[0051] The DC overvoltage controller includes: inputting the deviation between the DC high voltage reference value and the DC voltage measurement value into the DC overvoltage regulator; and outputting a control quantity as the output of the DC overvoltage controller after the DC overvoltage regulator is limited to a maximum limit value of 0.

[0052] Furthermore, the DC current reference value is obtained by obtaining the DC current reference value through an active power controller;

[0053] The active power controller includes: inputting the deviation between the active power reference value and the positive sequence active power measurement value into the active power regulator; the output of the active power regulator is limited and superimposed with the active power reference value to obtain a power reference value; and the power reference value is divided by the DC voltage to obtain the DC current reference value. Attached Figure Description

[0054] Figure 1 This is a schematic diagram of an ultra-high voltage flexible DC transmission line in the prior art;

[0055] Figure 2 This is a control block diagram of the receiving-end VSC in the prior art;

[0056] Figure 3 This is a control block diagram of the receiving-end VSC in an embodiment of the AC fault ride-through method for flexible DC transmission of the present invention;

[0057] Figure 4 This is a block diagram of the enablement of the overvoltage controller for the submodule capacitor in the embodiment of the AC fault ride-through method for the receiving end of the flexible DC transmission of the present invention.

[0058] Figure 5 This is a block diagram of the overvoltage controller limiting the submodule capacitor in the embodiment of the AC fault ride-through method for flexible DC transmission receiving end of the present invention;

[0059] Figure 6 This is a block diagram of the limiting controller for the capacitor voltage balance controller in the submodule of the flexible DC transmission receiving-end AC fault ride-through method of the present invention.

[0060] Figure 7 This is a control block diagram of the sending-end VSC in an embodiment of the AC fault ride-through method for flexible DC transmission at the receiving end of the present invention.

[0061] Figure 8 This is a block diagram of the active power controller in an embodiment of the AC fault ride-through method for flexible DC transmission receiving end of the present invention;

[0062] Figure 9 This is a flowchart illustrating the implementation of the AC fault ride-through method at the receiving end of the flexible DC transmission line according to the present invention. Detailed Implementation

[0063] In the event of a fault in the receiving-end AC system of a flexible DC transmission system, the overvoltage controller of the receiving-end VSC submodule capacitor in this invention reduces the DC voltage of the receiving-end VSC, thereby decreasing the DC power injected into the receiving-end VSC. The receiving-end VSC submodule capacitor voltage balance controller controls the VSC to output the maximum active current, maintaining a balance between the DC power and active power of the receiving-end VSC. This invention's receiving-end AC fault ride-through method does not use a preset voltage regulation function, maximizing the DC power level during the fault. This invention solves the technical problem in the prior art where the parameter tuning of the voltage regulation function is difficult, resulting in unsatisfactory control performance of the control system.

[0064] Implementation method of AC fault ride-through method at the receiving end of flexible DC transmission:

[0065] The flexible DC transmission receiving-end AC fault ride-through method of this embodiment is based on, for example, Figure 1 The illustrated UHV flexible DC transmission system topology uses modular multilevel converters (VSCs) composed of hybrid full-and-half-bridge submodules at both the receiving and sending ends as voltage source converters (VSCs). The AC fault ride-through method at the receiving end of this embodiment includes VSC control strategies at both the receiving and sending ends, requiring the VSCs at both ends to cooperate in completing the AC fault ride-through at the receiving end.

[0066] The control block diagram of the receiving-end VSC in this embodiment is as follows: Figure 3 As shown, compared to the prior art, the control strategy of the receiving-end VSC in this embodiment enables the submodule capacitor overvoltage controller when a receiving-end AC fault occurs, and determines the DC component U of the VSC valve group modulation voltage based on the submodule capacitor overvoltage controller. dcmod This reduces the DC voltage of the VSC and the DC power injected into the VSC; and switches the DC voltage controller of the outer loop to the submodule capacitor voltage balance controller in steady state.

[0067] exist Figure 3 Middle,U cavg i represents the average capacitor voltage of the VSC valve group submodule. cdmax and i cdmin These are the maximum and minimum limit values ​​for the output of the submodule capacitor voltage balance controller, respectively. en For submodule control enable signal, i dref_UCD U is the output value of the DC voltage controller, k is the overvoltage control coefficient of the submodule capacitor, and U is the output value of the DC voltage controller. dcmax and U dcminThese are the maximum and minimum limit values ​​for the output of the submodule capacitor overvoltage controller. The receiving-end VSC valve group controller mainly includes a DC voltage controller, a submodule capacitor voltage balance controller, a submodule capacitor overvoltage controller, an inner loop current controller, a circulating current suppressor, and a valve controller. Under steady-state conditions, SMC en =0, the VSC valve group is in DC voltage control mode, the same as the conventional flexible DC control method. Under AC system fault conditions, SMC en =1, the VSC valve group switches to submodule capacitor voltage balance control and submodule capacitor overvoltage control modes.

[0068] Specifically, the submodule capacitor voltage balancing controller includes: balancing the average value U of the submodule capacitor voltage. cavg The difference between the voltage reference value of the VSC valve group submodule capacitor (per unit value 1 in the figure) and the voltage reference value of the PI controller is input to the PI controller. The output of the PI controller is limited and used as the inner loop VSC valve group active current reference value i. dref .

[0069] The submodule capacitor overvoltage controller includes: averaging the submodule capacitor voltage U... cavg The difference between the voltage reference value of the VSC valve group submodule capacitor (per unit value 1 in the figure) and the voltage reference value of the VSC valve group DC voltage command value U is amplified by the submodule capacitor overvoltage control coefficient k and then superimposed. dcord The output of the submodule capacitor overvoltage controller is then obtained, and the DC component U of the VSC valve group modulation voltage is obtained based on the output of the submodule capacitor overvoltage controller. dcmod .

[0070] Submodule capacitor overvoltage controller enable block diagram (i.e., SMC) en Enable block diagram (as shown) Figure 4 As shown, U cset1 The overvoltage setpoint is set for the submodule, and t1 is the overvoltage detection delay time; U cset2 t2 is the fault clearing determination delay time, where t1 is the fault clearing determination delay time. When U cavg ≥U cset1 When the duration reaches t1, an AC fault is considered to have occurred at the receiving end, a pulse signal appears at the S port of the RS flip-flop, and the SMC... en =1, the submodule capacitor voltage balance controller and the submodule capacitor overvoltage controller are enabled. When the AC fault at the receiving end is cleared, U cavg Descending to U cset2 The following (i.e., U) cavg ≤U cset2 When the duration reaches t2, the AC fault at the receiving end is considered cleared, a pulse signal appears at the R port of the RS flip-flop, and the SMC... en =0, the VSC valve group switches to DC voltage control mode.

[0071] Tuned U cset1 The voltage of the submodule capacitor should not exceed the overvoltage protection setting (referring to the overvoltage protection setting for trigger valve lockout) under a three-phase ground fault in the AC system. A typical value is: U cset1＝ 1.1pu, t1=0.001s; U cset2 =1.05pu, t2=1.2s.

[0072] The block diagram of the capacitor overvoltage controller limiting submodule is as follows: Figure 5 As shown, U dcmax and U dcmin U represents the maximum and minimum limit values ​​of the output value of the capacitor overvoltage controller in the submodule. acpos U represents the amplitude of the AC positive sequence fundamental voltage on the grid side of the VSC valve group converter transformer. cset3 For emergency fault detection settings in the AC system, t3 is the fault detection delay time, and U dcord This is the DC voltage command value for the VSC valve group. When U... acpos ≥U cset3 If the duration reaches t3, an emergency fault is considered to have occurred in the AC system. Typical values ​​for the emergency fault determination criteria of the AC system: U cset3 =0.2, t3=0.005s. U acpos A voltage of less than 0.2 pu indicates an emergency fault in the AC system, causing extremely high overvoltage in the submodule capacitors. This is the most severe fault condition among AC faults at the receiving end.

[0073] When no receiving-end AC fault (i.e., SMC) occurs en When =0), the maximum amplitude U of the submodule capacitor overvoltage controller dcmax and minimum amplitude value U dcmin All are U dcord When a receiving-end AC fault (SMC) occurs... en =1) and when an emergency failure occurs in the AC system, U dcmax U is 0 dcmin The value is 0. When a receiving-end AC fault (SMC) occurs... en =1) and when no emergency failure occurs in the AC system, U dcmax For U dcord U dcmin It is 0.

[0074] The limiting block diagram of the submodule capacitor voltage balance controller is as follows: Figure 6 As shown, i cdmax and i cdmin These are the maximum and minimum limiting values ​​for the output of the capacitor voltage balance controller in the VSC valve group submodule, i. dref_UCD This is the output value of the DC voltage controller. When SMC... enWhen = 0, the maximum and minimum limit values ​​of the submodule capacitor voltage balance controller are both i. dref_UCD When SMC en When = 1, the maximum and minimum limit values ​​of the submodule capacitor voltage balance controller are i and i respectively. dmax and i dmin , where i dmax and i dmin These are the maximum and minimum limits for the active current of the VSC valve group, respectively, which are preset values.

[0075] Average value U of the submodule capacitor voltage in the initial stage of a fault in the receiving-end AC system cavg Increase. When U cavg ≥U cset1 At that time, the submodule capacitor overvoltage controller is enabled, and the VSC valve group modulates the DC component of the voltage U. dcmod It will not be equal to 0.5U dcord Due to the submodule capacitor voltage U cavg >1.0 pu, the submodule capacitor overvoltage control coefficient k of the submodule capacitor overvoltage controller amplifies the submodule capacitor deviation, causing the output value of the submodule capacitor overvoltage controller to begin to decrease, and the DC component U of the VSC valve group modulates the voltage. dcmod It decreases accordingly. U dcmod Reducing the number of sub-modules in the upper and lower arms of the VSC phase unit decreases the total number of sub-modules, reduces the VSC DC voltage, and reduces the DC power injected into the VSC.

[0076] When U cavg ≥U cset1 When the submodule capacitor voltage balance controller is enabled, the active current reference value i of the inner loop current controller... dref The output is switched from the DC voltage controller output to the submodule capacitor voltage balancing controller output. During a fault, U cavg >1.0 pu, therefore the submodule capacitor voltage balance controller is in a limiting state, i dref =i dmin = -1×i dmax The VSC outputs the maximum active current. The parameter k of the submodule capacitor overvoltage controller should be set to ensure that when the AC voltage drops to 20%, the submodule capacitor overvoltage does not exceed the submodule overvoltage protection setting, and the transmission power reaches the maximum transmission capacity. Through simulation tests, the typical value of k is 2.3.

[0077] During the steady-state period of a fault in the receiving-end AC system, the submodule capacitor overvoltage controller keeps the submodule capacitor voltage within the submodule overvoltage protection setting, maintaining the balance between VSC active power and DC power. At this time, U... cavg>1.0 pu, the submodule capacitor voltage balance controller is still in the limiting state, and the VSC outputs the maximum active current. In traditional methods for suppressing submodule overvoltage based on voltage regulation functions, U dcmod Due to the voltage regulation function, the preset voltage regulation curve is difficult to control the appropriate DC voltage during dynamic changes in AC voltage, making it almost impossible for the DC power injected into the VSC to be optimal. Traditional methods may result in insufficient DC power injected into the VSC during fault steady state due to excessive reduction of DC voltage by the voltage regulation function, leading to inadequate active power support for the AC system.

[0078] After the AC system fault at the receiving end is cleared, the AC voltage recovers, the active power output of the VSC increases, and the capacitor voltage of the VSC submodule begins to decrease. The submodule capacitor overvoltage controller enters the limiting state, U dcmod =0.5U dcord When the total number of submodules engaged in the upper and lower arms of the VSC phase unit reaches the pre-fault level, the VSC DC voltage rises, and the DC power injected into the VSC increases. At this time, the submodule capacitor voltage balance controller controls the submodule capacitor voltage to decrease to 1.0 pu to maintain the balance between VSC DC power and AC power. Traditional methods for suppressing submodule overvoltage based on voltage regulation functions may cause significant fluctuations in submodule capacitor voltage during fault recovery due to excessively high or low DC voltage, adversely affecting the power recovery speed and stability.

[0079] The control block diagram of the sending-end VSC in this embodiment is as follows: Figure 7 As shown, in Figure 7 middle, I dcref and I dc These are the reference and measured DC current values ​​for the VSC valve assembly, U. dcGi U is the DC voltage of the VSC valve group. dcHref This is the reference value for the DC high voltage of the VSC valve group. The outer loop controller is the same sub-module capacitor voltage balancing controller as the receiving end controller. The DC voltage command value is obtained by superimposing the outputs of the DC current controller and the DC overvoltage controller.

[0080] The DC current controller includes: a DC current reference value I dcref and DC current measurement value I dc The deviation between the two is input into the PI controller, and the output of the PI controller is limited before being output.

[0081] The DC overvoltage controller includes: a DC voltage reference value U dcHref and DC voltage measurement value U dcGi The deviation between U and U is input to the PI controller, and the PI controller output is limited by a maximum limit of 0 and a minimum limit of -1. That is, when U... dcHref ≥U dcGiWhen U is in operation, the DC overvoltage controller always outputs 0; when U is in operation... dcGi >U dcHref Only when the DC overvoltage controller outputs a negative number not less than -1 will the DC overvoltage controller output such a number.

[0082] At the sending end, both the DC current controller and the submodule capacitor voltage balance controller are operational during both transient and steady-state processes. In the initial stage of a fault in the receiving end's AC system, the receiving end submodule capacitor overvoltage controller reduces the DC voltage of the receiving end VSC valve group, causing a sudden increase in the sending end's DC current. The sending end DC current controller rapidly reduces the U of the sending end VSC valve group. dcmod This reduces the DC voltage of the sending-end VSC valve group, suppressing DC current overshoot. The submodule capacitor voltage balance controller controls the active current of the VSC valve group, maintaining a balance between the active power and DC power of the VSC valve group, and controlling the stability of the submodule capacitor voltage. Traditional methods require detecting overcurrent and switching to DC current control mode or reducing the active power of the sending end through inter-station communication. Traditional methods have time delays, large DC current overshoots, and increase the surplus power of the receiving-end VSC.

[0083] If a minor fault occurs at the receiving end for an extended period, and the overvoltage controller of the receiving terminal module fails to activate, an overvoltage will occur at the sending end VSC. If the DC voltage U of VSC... dcGi Greater than U dcHref The DC overvoltage controller activates, reducing the U value of the VSC valve group at the sending end. dcmod This reduces the DC voltage of the sending-end VSC, thereby reducing the DC power at the sending end and suppressing the power surplus of the receiving-end VSC.

[0084] The sending end VSC is also configured with, for example, Figure 8 The active power controller shown serves as the DC current reference value I for the DC current controller. dcref . Figure 8 In the middle, P acref and P acpos These are the reference value for active power of the VSC valve group and the measured value for positive sequence active power, U. dcGi I is the DC voltage of the VSC valve group. dcref This is the DC current reference value. The DC current reference value of the VSC valve group is equal to the power reference value divided by the DC voltage. To eliminate active power control deviation, the active power reference value P... acref The output value of the active power PI controller is superimposed. To suppress DC current disturbances under transient faults, the DC voltage is smoothed through a first-order inertial element. Under normal circumstances, the time constant of the first-order inertial element is typically 10s, while during valve group activation / deactivation and power increase / decrease processes, the time constant of the first-order inertial element is typically 0.02s.

[0085] In other embodiments, the control method of the sending-end VSC that cooperates with the receiving end can also adopt other control methods in the prior art that take into account AC fault ride-through at the receiving end, such as the control method of the sending-end VSC in the background art.

[0086] Figure 9 This is a flowchart illustrating the implementation of a fault ride-through control method for an ultra-high voltage flexible DC transmission receiving-end AC system provided by the present invention. The specific process is as follows:

[0087] Step 1: Determine the input and output signals. Input signals include: average voltage of VSC submodule capacitors, DC voltage, DC current, converter transformer grid-side voltage, converter transformer valve-side current, and VSC bridge arm current. Output signals are VSC valve group trigger pulses.

[0088] Step Two: According to Figures 3-8 The control block diagram shown is used to write a VSC valve group controller program, including: inner loop current controller, circulating current suppressor, valve controller, DC voltage controller, DC current controller, submodule capacitor overvoltage control, submodule capacitor voltage balance controller, DC overvoltage controller and controller limiting program.

[0089] Step 3: According to Figure 1 Based on the topology shown, a simulation platform for an ultra-high voltage flexible DC transmission system was built to test the fault ride-through performance of the receiving-end AC system.

[0090] Implementation method of AC fault ride-through device at the receiving end of flexible DC transmission line:

[0091] A flexible DC transmission receiving-end AC fault ride-through device includes a processor for executing a computer program to implement the steps of the flexible DC transmission receiving-end AC fault ride-through method as described above. The specific flexible DC transmission receiving-end AC fault ride-through method has been described in sufficient detail in the above-described embodiments and will not be repeated here.

[0092] Specifically, a processor can be a CPU, or other general-purpose processors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. A general-purpose processor can be a microprocessor or any conventional processor. A processor can also be a processor that supports the Advanced Reduced Instruction Set Machine (ARM) architecture.

[0093] VSC Implementation Method:

[0094] A VSC (Variable Residual Current Carrier) is provided for installation at the receiving end of a flexible DC transmission system. The VSC includes a VSC controller and a processor for executing a computer program to implement the steps of the AC fault ride-through method for the receiving end of a flexible DC transmission system as described above. The specific AC fault ride-through method for the receiving end of a flexible DC transmission system has been described in sufficient detail in the above-described embodiments and will not be repeated here.

[0095] Specifically, a processor can be a CPU, or other general-purpose processors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. A general-purpose processor can be a microprocessor or any conventional processor. A processor can also be a processor that supports the Advanced Reduced Instruction Set Machine (ARM) architecture.

[0096] Implementation methods for flexible DC transmission systems:

[0097] A flexible DC transmission system includes a receiving-end VSC, which includes a receiving-end VSC controller. The receiving-end VSC controller includes a processor, which executes a computer program to implement the steps of the flexible DC transmission receiving-end AC fault ride-through method as described above. The specific flexible DC transmission receiving-end AC fault ride-through method has been described in sufficient detail in the above-described embodiments and will not be repeated here.

[0098] Furthermore, the sending-end VSC in the flexible DC transmission system includes a sending-end VSC controller, which includes a processor. The processor of the sending-end VSC controller executes a computer program to implement the following control method for the sending-end VSC: using a sending-end sub-module capacitor voltage balancing controller as an outer loop controller; the sending-end sub-module capacitor voltage balancing controller outputs an active current reference value for controlling the AC current of an inner loop current controller based on the sub-module capacitor voltage reference value and the average sub-module capacitor voltage; to stabilize the average sub-module capacitor voltage at the sub-module capacitor voltage reference value; obtaining the sending-end VSC valve group DC voltage command value based on the DC current controller; the DC current controller outputs a control quantity as the output of the DC current controller based on the DC current reference value and the DC current measurement value, to stabilize the DC current measurement value at the DC current reference value; and obtaining a trigger pulse for controlling the sending-end VSC based on the active current reference value and the sending-end VSC valve group DC voltage command value. The specific control method for the sending-end VSC has been described in sufficient detail in the above-described implementation of the flexible DC transmission receiving-end AC fault ride-through method, and will not be repeated here.

[0099] Specifically, a processor can be a CPU, or other general-purpose processors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. A general-purpose processor can be a microprocessor or any conventional processor. A processor can also be a processor that supports the Advanced Reduced Instruction Set Machine (ARM) architecture.

[0100] This invention has the following characteristics:

[0101] In flexible DC transmission, when a fault occurs in the receiving-end AC system, the overvoltage controller of the receiving-end VSC submodule capacitor in this invention reduces the DC voltage of the receiving-end VSC, thereby reducing the DC power injected into the receiving-end VSC. The receiving-end VSC submodule capacitor voltage balance controller controls the maximum active current output of the VSC, maintaining a balance between the DC power and active power of the receiving-end VSC. When the receiving-end VSC DC voltage decreases, the sending-end VSC DC current controller reduces the sending-end VSC DC voltage, suppressing DC current overshoot in the initial stage of the fault. In the event of a prolonged minor fault at the receiving end, the sending-end DC overvoltage controller reduces the sending-end DC voltage. Through the coordination of the sending and receiving-end VSCs, the surplus power of the receiving-end VSC is reduced, suppressing the overvoltage level of the VSC submodule capacitor. The receiving-end AC system fault ride-through control method of this invention does not use a preset voltage regulation function, and can maximize the DC power level during the fault; after the fault is cleared, it can quickly and smoothly restore active power; it does not rely on inter-station communication, and the DC current overshoot in the initial stage of the fault is small.

[0102] Finally, it should be noted that the above description is merely a preferred embodiment of the present invention and is 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 make modifications to the technical solutions described in the foregoing embodiments without creative effort, 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 method for AC fault ride-through at the receiving end of a flexible DC transmission line, characterized in that, The control method of the receiving-end VSC in this method includes: When an AC fault occurs at the receiving end, the receiving end module capacitor voltage balance controller is used as the outer loop controller, and the DC component of the receiving end VSC valve group modulation voltage is obtained based on the output of the sub-module capacitor overvoltage controller. The terminal module capacitor voltage balance controller outputs the active current reference value of the inner loop current controller for controlling AC current based on the sub-module capacitor voltage reference value and the sub-module capacitor voltage average value, so as to stabilize the sub-module capacitor voltage average value at the sub-module capacitor voltage reference value. The submodule capacitor overvoltage controller amplifies the deviation between the submodule capacitor voltage reference value and the submodule capacitor voltage average value, adds it to the DC voltage command value of the receiving end VSC valve group, and outputs it to ensure that the submodule capacitor voltage does not exceed the submodule overvoltage protection setting value. The trigger pulse for controlling the receiving-end VSC is obtained based on the active current reference value and the DC component of the modulation voltage of the receiving-end VSC valve group.

2. The AC fault ride-through method for the receiving end of a flexible DC transmission line according to claim 1, characterized in that, The control method for the receiving-end VSC also includes: when the AC fault at the receiving end is cleared, switching the receiving-end terminal module capacitor voltage balance controller to a DC voltage controller; and switching the sub-module capacitor overvoltage controller to the receiving-end VSC valve group DC voltage command value of the DC voltage controller; The DC voltage controller outputs the active current reference value of the inner loop current controller based on the deviation between the DC voltage command value and the DC voltage measurement value of the receiving-end VSC valve group, so as to stabilize the DC voltage measurement value of the receiving-end VSC valve group at the DC voltage command value.

3. The AC fault ride-through method for the receiving end of a flexible DC transmission line according to claim 1, characterized in that, The criterion for receiving-end AC fault is: when the average value of the submodule capacitor voltage is greater than or equal to the submodule overvoltage setting value and the duration reaches the overvoltage judgment delay time, then the receiving-end AC fault is considered to have occurred.

4. The AC fault ride-through method for the receiving end of a flexible DC transmission line according to claim 2, characterized in that, The criterion for clearing AC faults at the receiving end is as follows: when the average value of the submodule capacitor voltage drops below the fault clearing judgment value and the duration reaches the fault clearing judgment delay time, the AC fault at the receiving end is considered cleared.

5. The AC fault ride-through method for flexible DC transmission receiving end according to claim 1, characterized in that, The output of the submodule capacitor overvoltage controller is the output after amplitude limiting. The limiting processing of the submodule capacitor overvoltage controller includes: When no AC fault occurs at the receiving end, the maximum and minimum limit values ​​of the submodule capacitor overvoltage controller are both the DC voltage command values ​​of the VSC valve group; When a receiving-end AC fault occurs and an AC system emergency fault occurs, the maximum and minimum limit values ​​of the submodule capacitor overvoltage controller are both 0. When a receiving-end AC fault occurs but no AC system emergency fault occurs, the maximum limit value of the submodule capacitor overvoltage controller is the VSC valve group DC voltage command value, and the minimum limit value is 0. The criterion for an emergency fault in the AC system is as follows: when the amplitude of the AC positive sequence fundamental voltage on the converter substation side is less than or equal to the AC system emergency fault discrimination setpoint and the duration reaches the fault judgment delay time, an AC system emergency fault is considered to have occurred.

6. The AC fault ride-through method for the receiving end of a flexible DC transmission line according to claim 2, characterized in that, The active current reference value output by the capacitor voltage balance controller of the receiving terminal module is the active current reference value after the amplitude limiting process. The limiting function of the capacitor voltage balance controller of the receiving terminal module includes: When no AC fault occurs at the receiving end, the maximum and minimum limit values ​​of the capacitor voltage balance controller of the receiving terminal module are both the active current reference values ​​output by the DC voltage controller. When a receiving-end AC fault occurs, the maximum and minimum limit values ​​of the receiving-end terminal module capacitor voltage balance controller are respectively the maximum and minimum limit values ​​of the active current of the VSC valve group.

7. The AC fault ride-through method for flexible DC transmission receiving end according to any one of claims 1 to 6, characterized in that, The control method of the sending-end VSC in this method includes: The terminal module capacitor voltage balance controller is used as the outer loop controller; The terminal module capacitor voltage balance controller is used to output the active current reference value of the inner loop current controller for controlling the AC current based on the sub-module capacitor voltage reference value and the sub-module capacitor voltage average value, so as to stabilize the sub-module capacitor voltage average value at the sub-module capacitor voltage reference value. The DC component of the modulation voltage of the VSC valve group at the sending end is obtained from the DC current controller. The DC current controller is used to output a control quantity based on the DC current reference value and the DC current measurement value, so as to stabilize the DC current measurement value at the DC current reference value. The trigger pulse for controlling the sending-end VSC is obtained based on the active current reference value and the DC component of the modulation voltage of the sending-end VSC valve group.

8. The AC fault ride-through method for the receiving end of a flexible DC transmission line according to claim 7, characterized in that, The method of obtaining the DC component of the modulation voltage of the VSC valve group at the sending end according to the DC current controller includes: taking half of the superposition value of the output of the DC current controller and the output of the DC overvoltage controller as the DC component of the modulation voltage of the VSC valve group at the sending end. The DC overvoltage controller includes: inputting the deviation between the DC high voltage reference value and the DC voltage measurement value into the DC overvoltage regulator; and outputting a control quantity as the output of the DC overvoltage controller after the DC overvoltage regulator is limited to a maximum limit value of 0.

9. The AC fault ride-through method for the receiving end of a flexible DC transmission line according to claim 7, characterized in that, The DC current reference value is obtained as follows: the DC current reference value is obtained through an active power controller; The active power controller includes: inputting the deviation between the active power reference value and the positive sequence active power measurement value into the active power regulator; the output of the active power regulator is limited and superimposed with the active power reference value to obtain a power reference value; and the power reference value is divided by the DC voltage to obtain the DC current reference value.

10. The AC fault ride-through method for the receiving end of a flexible DC transmission line according to claim 9, characterized in that, The DC voltage in the active power controller is a DC voltage that has been smoothed by a first-order inertial element.

11. A flexible DC transmission receiving-end AC fault ride-through device, comprising a processor, characterized in that, The processor is used to execute a computer program to implement the steps of the flexible DC transmission receiving-end AC fault ride-through method as described in any one of claims 1 to 6.

12. A VSC for installation at the receiving end of a flexible DC transmission system, comprising a VSC controller, the VSC controller including a processor, characterized in that, The processor is used to execute a computer program to implement the steps of the flexible DC transmission receiving-end AC fault ride-through method as described in any one of claims 1 to 6.

13. A flexible DC transmission system, comprising a receiving-end VSC, wherein the receiving-end VSC includes a receiving-end VSC controller, and the receiving-end VSC controller includes a processor, characterized in that, The processor of the receiving-end VSC controller is used to execute a computer program to implement the steps of the flexible DC transmission receiving-end AC fault ride-through method as described in any one of claims 1 to 6.

14. The flexible DC transmission system according to claim 13, characterized in that, The sending-end VSC in a flexible DC transmission system includes a sending-end VSC controller, which includes a processor. The processor of the sending-end VSC controller is used to execute a computer program to implement the control method of the sending-end VSC as described below: The terminal module capacitor voltage balance controller is used as the outer loop controller; The terminal module capacitor voltage balance controller is used to output the active current reference value of the inner loop current controller for controlling the AC current based on the sub-module capacitor voltage reference value and the sub-module capacitor voltage average value, so as to stabilize the sub-module capacitor voltage average value at the sub-module capacitor voltage reference value. The DC component of the modulation voltage of the VSC valve group at the sending end is obtained from the DC current controller. The DC current controller is used to output a control quantity based on the DC current reference value and the DC current measurement value, so as to stabilize the DC current measurement value at the DC current reference value. The trigger pulse for controlling the sending-end VSC is obtained based on the active current reference value and the DC component of the modulation voltage of the sending-end VSC valve group.

15. The flexible DC transmission system according to claim 14, characterized in that, The method of obtaining the DC component of the modulation voltage of the VSC valve group at the sending end according to the DC current controller includes: taking half of the superposition value of the output of the DC current controller and the output of the DC overvoltage controller as the DC component of the modulation voltage of the VSC valve group at the sending end. The DC overvoltage controller includes: inputting the deviation between the DC high voltage reference value and the DC voltage measurement value into the DC overvoltage regulator; and outputting a control quantity as the output of the DC overvoltage controller after the DC overvoltage regulator is limited to a maximum limit value of 0.

16. The flexible DC transmission system according to claim 14, characterized in that, The DC current reference value is obtained as follows: the DC current reference value is obtained through an active power controller; The active power controller includes: inputting the deviation between the active power reference value and the positive sequence active power measurement value into the active power regulator; the output of the active power regulator is limited and superimposed with the active power reference value to obtain a power reference value; and the power reference value is divided by the DC voltage to obtain the DC current reference value.

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