Coordination control method and device for deep fault of wind power sending end, and storage medium

Abstract

The invention discloses a coordinated control method and device for a deep fault of a wind power sending end and a storage medium, and belongs to the field of safe operation of a power system. The coordination control method comprises the following steps: when a deep fault occurs at a sending end in a wind power flexible direct sending scene, S1, increasing the q-axis current amplitude of a wind power plant, reducing the d-axis current amplitude of the wind power plant, and increasing the amplitude limiting value of the q-axis current of the MMC sending end to a set value so as to meet the low-voltage fault ride-through requirement of the wind power plant; s2, detecting the voltage recovery condition of the grid-connected point of the wind power plant, and judging whether the fault is cleared or not; if yes, executing S3; if not, returning to execute S1; and S3, during the fault recovery period, adopting a suppression strategy to suppress transient overvoltage, and increasing the d-axis current of the wind power plant until the voltage of the sending end of the MMC is recovered to a steady-state value. And in case of a ride-through fault, the transient stability of the wind power plant and the MMC sending end becomes better, and the synchronization stability of the phase-locked loop is improved.

Description

Technical Field

[0001] The present invention belongs to the field of safe operation of power systems, and more specifically, relates to a coordinated control method, device and storage medium for deep faults at wind power transmission ends. Background Art

[0002] Modular multilevel converter (MMC) is a commonly used converter for long-distance, large-capacity DC transmission. It has the characteristics of large capacity, strong controllability, and high waveform quality, and has broad development prospects. As an important transmission equipment, the fault ride-through characteristics and fault recovery methods of MMC under short-circuit faults have become hot research issues. Among them, low voltage ride-through, transient stability and overvoltage suppression of MMC are three problems that need to be solved urgently.

[0003] During a short-circuit fault, the power generation and transmission sides need to achieve low voltage ride-through function. The proposed segmented voltage reduction control method mainly achieves segmented voltage reduction by setting multi-segment criteria. This method is suitable for asymmetric faults and can achieve current limiting in the steady state stage, but there is an overcurrent problem in the transient stage. The existing method uses the port fault to uniquely solve the value of the virtual impedance, and adjusts the voltage reduction amplitude in combination with the degree of over-limit of the current reference value. Since the setting value of the virtual impedance of this method is large and unique, it will cause the system voltage to be too low under mild faults, and the control effect of this method is strongly correlated with the position of the short-circuit point, so the design and setting of the virtual impedance value is more difficult.

[0004] When a short circuit fault occurs, it is necessary to ensure that the MMC does not experience transient instability. In a system containing phase-locked synchronous equipment, in order to improve transient stability, the stability is enhanced by changing the tracking performance of the phase-locked loop. However, these methods have limited effect on improving transient stability performance during faults. In the prior art, an adaptive phase-locked loop is designed, and the phase-locked loop control structure is changed to improve stability when the frequency change rate is too fast. The above method is based on the improvement of transient stability of the phase-locked loop itself, and less attention is paid to other factors affecting transient stability. Therefore, the unit control capability is not fully utilized, and transient stability is enhanced by changing the output current. Therefore, the phase-locked loop characteristics are greatly changed. After the fault is restored, the phase-locked loop needs to be switched back to the control strategy under normal conditions immediately, otherwise it is easy to cause adverse effects after the fault is restored.

[0005] In terms of overvoltage suppression, the common solution for power systems is to configure reactive power compensation devices to support voltage by compensating reactive current during faults. However, this method is less economical in terms of investment cost, construction site, and operation and maintenance.

[0006] Therefore, when a short circuit fault occurs at the sending end of the wind power system, the existing fault ride-through method will reduce the transient stability performance of the system, and the cost of configuring the reactive power compensation device is relatively high. According to the fault occurrence point and fault duration, it is necessary to design a fault ride-through method specifically for the deep fault caused by the short circuit. Summary of the invention

[0007] In view of the defects of the related art, the purpose of the present invention is to provide a coordinated control method, device and storage medium for deep faults at the wind power sending end, aiming to achieve deep fault crossing at the wind power sending end under the premise of ensuring the synchronization stability of the phase-locked loop and satisfying the MMC current limiting.

[0008] To achieve the above objectives, in a first aspect, the present invention provides a coordinated control method for deep faults at a wind power transmission end, comprising:

[0009] When a deep fault occurs at the sending end in the scenario of wind power transmission via flexible direct current, perform the following steps:

[0010] S1. Increase the q-axis current amplitude of the wind farm, reduce the d-axis current amplitude of the wind farm, and increase the q-axis current limit value at the MMC sending end to the set value to meet the low voltage fault ride-through requirements of the wind farm;

[0011] S2, detect the voltage of the wind farm station grid connection point to determine whether the fault is cleared; if so, execute S3; if not, return to execute S1;

[0012] S3. During the fault recovery period, a suppression strategy is used to suppress transient overvoltages and increase the d-axis current of the wind farm to restore the voltage at the MMC sending end to a steady-state value.

[0013] Optionally, the increasing the q-axis current amplitude of the wind farm includes:

[0014] According to the equivalent circuit diagram of the wind power transmission system through flexible direct current, the corresponding grid connection point voltage u in the synchronous stability domain of the phase-locked loop is determined ws ;

[0015] according to Calculate the q-axis component i of the wind farm current wq , and the q-axis component i of the wind farm current wq The q-axis current amplitude of the wind farm is determined by the q-axis component of the current; where ω is the system angular frequency, L f is the grounding reactance at the fault point, I mmax is the current limit value, L1 is the reactance from the wind farm to the fault point, i wd with i wq They are the d-axis and q-axis components of the wind farm current respectively.

[0016] Optionally, reducing the d-axis current amplitude of the wind farm and increasing the q-axis current limit value at the MMC sending end to a set value includes:

[0017] Get the q-axis voltage u at the wind farm grid connection point wq , with u wq = 0 is a stable boundary condition, and the power angle state equation is constructed:

[0018]

[0019] In the formula, K pPLL is the phase-locked loop ratio parameter, K iPLL is the phase-locked loop integral parameter, ω0 is the system rated angular frequency, δ is the power angle between the wind farm and the MMC sending end, is the initial phase of voltage and current, is the first derivative of δ, is the second derivative of δ;

[0020] The phase diagram method is used to perform transient stability analysis on the power angle state equation. According to the transient stability analysis results, the corresponding reduced d-axis current amplitude is determined within the synchronous stability domain of the phase-locked loop, and the q-axis current limit value at the sending end of the MMC is increased to the set value.

[0021] Optionally, detecting the voltage of the wind farm grid connection point to determine whether the fault is cleared includes:

[0022] The lowest voltage during the fault detection of the wind farm grid connection point is denoted as u wmin , and obtain the grid connection point voltage u in real time w ;

[0023] Determine the grid connection point voltage u w Is it greater than u wmin +0.1pu; if yes, the fault is determined to be cleared and the time is recorded as t2; if no, the fault is determined to be not cleared.

[0024] Optionally, during the fault recovery period, adopting a suppression strategy to suppress transient overvoltage includes:

[0025] At the fault clearing time t2, the voltage command value of the MMC sending end is reduced to 0.8pu by the voltage reduction method. After maintaining the suppression strategy for a preset time, the voltage command value of the MMC sending end is restored to 1p.u. at a rate of 2p.u. / s.

[0026] Optionally, during the fault recovery period, adopting a suppression strategy to suppress transient overvoltage includes:

[0027] Starting from the moment t2 when the fault occurs, the reset method is used to reset the output of the inner and outer loop integral controller of the d-axis on the MMC side to zero. After the suppression strategy is maintained for a preset time, the reset operation of the integral controller is canceled.

[0028] Optionally, the step of increasing the d-axis current of the wind farm to restore the voltage at the MMC sending end to a steady-state value includes:

[0029] Limit the d-axis current rising rate k = di d / dt, increasing the d-axis current of the wind farm until the voltage at the MMC sending end recovers to a steady-state value; where the value range of k is 0.2-0.6.

[0030] In a second aspect, the present invention further provides a coordinated control device for deep faults at the wind power sending end, which comprises:

[0031] The fault ride-through module is used to increase the q-axis current amplitude of the wind farm, reduce the d-axis current amplitude of the wind farm, and increase the q-axis current limit value at the MMC sending end to a set value to meet the low voltage fault ride-through requirements of the wind farm;

[0032] The fault judgment module is used to detect the voltage of the wind farm station grid connection point and determine whether the fault is cleared; if so, the overvoltage suppression module is executed; if not, the fault ride-through module is returned to execute.

[0033] The overvoltage suppression module is used to suppress transient overvoltage by adopting a suppression strategy during fault recovery, and to increase the d-axis current of the wind farm to restore the voltage at the MMC sending end to a steady-state value.

[0034] In a third aspect, the present invention further provides a computer-readable storage medium storing computer instructions, wherein the computer instructions are used to enable a processor to implement the coordinated control method for deep faults at the wind power sending end as described in any one of the first aspects when executed.

[0035] Compared with the prior art, the above technical solution conceived by the present invention can achieve the following beneficial effects:

[0036] 1. The embodiment of the present invention provides a coordinated control method for deep faults at the wind power transmission end, by reducing the q-axis current i of the new energy source during the fault period. wq , reduce the new energy d-axis current i wd , improve the voltage at the wind farm grid connection point, thereby solving the low voltage ride-through problem of the wind farm during the fault period, and ensuring that the wind turbine can operate without disconnecting from the grid within the specified time. Increase the output q-axis current limit value of the MMC sending end to the set value, so that the transient stability of the wind farm and the MMC sending end is improved, and the synchronization stability of the phase-locked loop is improved.

[0037] 2. The embodiment of the present invention provides a coordinated control method for deep faults at the wind power transmission end, and proposes two suppression strategies for transient overvoltage problems during fault recovery. Strategy 1: After a short-circuit fault occurs, the AC voltage reference value u of the MMC transmission end is reduced. sref , the short-circuit fault is removed and then restored to the initial value. At this time, the PCC point voltage established by the converter station will track the smaller reference voltage, reducing u sd The maximum value of V is solved during the fault recovery period. Strategy 2: After a short-circuit fault occurs, the integrators of the PI control module of the outer loop voltage controller and the inner loop current controller of the MMC sending end d-axis current are cleared until the power returns to the steady-state value, reducing V dref maximum value, solving the transient overvoltage problem during faults. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 This is the topology diagram of the wind power transmission system via flexible direct current;

[0039] Figure 2 It is the equivalent circuit diagram of the wind power transmission system via flexible direct current under deep fault;

[0040] Figure 3 is a schematic diagram of the steady-state operating point;

[0041] Figure 4 It is the effect of current magnitude on transient processes;

[0042] Figure 5 is the influence of dq axis limit value on transient process;

[0043] Figure 6 It is the analysis of the current and voltage change process. DETAILED DESCRIPTION

[0044] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0045] The contents involved in the above embodiment are described below in conjunction with a preferred embodiment.

[0046] Embodiment 1

[0047] In a typical wind power transmission system through flexible direct current, the wind power system uses direct-drive wind turbines and grid-following control, and uses a phase-locked loop to achieve synchronization. The sending end of the flexible direct current transmission system uses constant voltage / frequency (V / F) control to generate a constant voltage amplitude and frequency at the sending end. In this scenario, if a deep fault occurs at the sending end, if the control strategy is not applied in time, the phase-locked loop (PLL) will not have a steady-state operating point, causing the system to lose synchronization and stability. Due to the deep fault and the limited output short-circuit current capacity of the modular multilevel converter (MMC), it is impossible to achieve fault ride-through by injecting a large short-circuit current. Therefore, the fault voltage drop will be more serious and cannot meet the voltage fault ride-through requirements of the new energy machine end. After the fault is restored, due to the large number of proportional integral (PI) controllers used in power electronic equipment, there is a deviation between the reference current and the actual current. The reference voltage generated by the MMC control system will be greater than the steady-state value, causing the wind turbine to overvoltage and disconnect from the grid.

[0048] Based on the above problems, Figure 1 As shown, an embodiment of the present invention provides a coordinated control method for deep faults at a wind power transmission end, comprising:

[0049] When a deep fault occurs at the sending end in the scenario of wind power transmission via flexible direct current, perform the following steps:

[0050] S1. Increase the q-axis current amplitude of the wind farm, reduce the d-axis current amplitude of the wind farm, and increase the q-axis current limit value at the MMC sending end to the set value to meet the low voltage fault ride-through requirements of the wind farm;

[0051] S2, detect the voltage of the wind farm station grid connection point to determine whether the fault is cleared; if so, execute S3; if not, return to execute S1;

[0052] S3. During the fault recovery period, a suppression strategy is used to suppress transient overvoltages and increase the d-axis current of the wind farm to restore the voltage at the MMC sending end to a steady-state value.

[0053] like Figure 1 As shown in the figure, the basic framework of the wind power transmission system through flexible direct current. After a short circuit fault occurs at the sending end of the wind power transmission system through flexible direct current, the voltage at the PCC point of the MMC drops, and the voltage u at the wind farm grid connection point is detected and determined. ws Whether the amplitude is less than 0.8pu, detect and judge whether the current on the MMC side is i s Reaching the limit value I max If yes, it is judged that a deep short circuit fault occurs at the sending end of the wind power transmission system through flexible direct current transmission. At this time, the MMC enters the current limiting mode and presents the current source characteristic. The system equivalent circuit diagram is as follows: Figure 2 As shown. Among them, Z1 is the equivalent impedance from the fault point to the GSC grid connection point, Z2 is the equivalent impedance from the fault point to the MMC AC side line, and Z f is the fault impedance.

[0054] For the transient stability research of the system, the focus is on the synchronous stability during the fault, the voltage support characteristics and the transient overvoltage characteristics after the fault is cleared. Among them, synchronous stability is the prerequisite for the stable operation of the system.

[0055] During the fault period, in order to ensure that the PLL has a steady-state operating point, on the one hand, it is necessary to increase the terminal voltage of the new energy source after the fault to achieve low voltage ride-through, and during the fault recovery period, measures need to be taken to suppress the transient overvoltage after the fault. Considering the synchronization stability during the fault period, the fault voltage support function and the transient overvoltage suppression during the fault recovery period, a corresponding coordinated control strategy is proposed.

[0056] (1) Improve the coordinated control strategy of system transient synchronization stability. During a fault, the synchronization stability of new energy must be ensured first, so that new energy can be effectively controlled and the active and reactive power generated as required is the prerequisite for ensuring stable operation of the system. Figure 1 , when the system resistance is ignored, it can be concluded that the q-axis voltage at the grid-connected point of the wind farm grid-side converter GSC is:

[0057] u wq =ω1L f i md +ω1L f i wd +ω1L1i wd (1)

[0058] Where ω is the system angular frequency, L f is the grounding reactance of the fault point, L1 is the reactance from the wind farm to the fault point, i wd is the d-axis component of the wind farm current, i md is the d-axis current of the MMC.

[0059] According to the basic coordinate transformation relationship, the above formula is unified to the dq coordinate system of GSC, and the expression of the d-axis current output at the sending end of MMC is obtained:

[0060]

[0061] In the formula,

[0062]

[0063] Among them, I mmax is the maximum current limited by the MMC limiter, and are the dq axis limiter amplitude limits during the fault period respectively. Substituting (2) into (1) and omitting the superscript “W”, we get

[0064]

[0065] In order to ensure that the system has a steady-state operating point, according to the working principle of the phase-locked loop, it is necessary to ensure that u wq =0 has a solution, so it needs to satisfy:

[0066]

[0067] From the above formula, the steady-state operating point can be shown Figure 3 .

[0068] Substitute equations (1) and (2) into the phase angle difference formula of the GSC and MMC sending ends:

[0069] δ=θ PLL -θ MMC =∫(K PPLL ·u wsq +K PPLL ·∫u wsq dt)dt (6)

[0070] The angular motion equation can be obtained:

[0071]

[0072] In the formula, K pPLL is the phase-locked loop ratio parameter, K iPLL is the phase-locked loop integral parameter, ω0 is the system rated angular frequency, δ is the power angle between the wind farm and the MMC sending end, is the initial phase of voltage and current, is the first derivative of δ, is the second derivative of δ.

[0073] The above equations are solved in MATLAB software, and the influence of current on the transient stability of the system is analyzed with the help of the phase diagram method. The analysis results show that reducing the d-axis current of renewable energy and increasing the maximum value of the MMC output current are beneficial to improving transient stability. According to this analysis result, the corresponding reduced d-axis current amplitude is determined within the synchronous stability domain of the phase-locked loop, and the limit value of the q-axis current at the sending end of the MMC is increased to the set value. In this embodiment, the synchronous stability domain of the wind farm d-axis current and the MMC sending end current amplitude value range is set to 0.13<i wd <0.2,2<I mmax <5, the d-axis current of the wind farm is reduced within this range, and the q-axis current of the MMC sending end is increased to the set value. Specifically, the limit value of the q-axis current output by the MMC sending end is increased to 1.5pu.

[0074] like Figure 4 As shown in the figure, the smaller the d-axis current of the wind farm is, the larger the output current of the MMC sending end is, the faster the phase diagram converges, and the more stable the system is. In order to obtain the steady-state operating point, it is necessary to reduce the d-axis current of the wind farm and increase the current amplitude of the MMC sending end.

[0075] In the synchronous coordinate system of GSC, the q-axis voltage is shown in equation (4). When Figure 5 As shown in (a) in the figure, it can be seen that in this case, the distance between a and b is increased, which increases the acceleration range of the power angle and easily causes the power angle to not converge. Therefore, it is necessary to avoid this value of the limiter. When Figure 5 As shown in (b), the distance between the initial operating point a and the steady-state operating point b after the fault is shortened, indicating that the acceleration process is reduced. This value can improve the transient stability of the system. Therefore, the initial phase of the MMC output current after the fault should be greater than 0. In addition, according to the above analysis, when the operating point exceeds point c, the system cannot operate stably, so it is necessary to ensure that point a does not exceed point c. In actual engineering, the power angle operating range is 0~π / 2. In order to ensure that point a does not exceed point c, it is necessary to meet

[0076]

[0077] Therefore, it can be seen that the need to meet If you select Then the initial position after the fault may directly exceed point c, resulting in system instability, such as Figure 5 In order to enhance the synchronization stability of the system, it is advisable to select like Figure 5 As shown in (d), the dq axis limit value during the fault period can be expressed as:

[0078]

[0079] That is, the q-axis current is limited in priority, and the q-axis current is maintained at the set value during the fault. Therefore, in order to obtain a steady-state operating point, it is necessary to reduce the d-axis current of the wind farm and increase the q-axis current at the MMC sending end to the set value. In this embodiment, the d-axis current is reduced by 0.2pu, and the limit value of the MMC output q-axis current is maintained at its maximum value of 1.5pu.

[0080] (2) The strategy for improving the voltage at the wind farm grid connection point during faults is to reduce the active current of renewable energy and increase the output of reactive current while ensuring the synchronization and stability of the system in order to support the voltage at the renewable energy terminal after the fault.

[0081] When the fan is connected to the MMC, the voltage drop after a fault is more serious due to the limited output short-circuit current capacity of the MMC, and the limited short-circuit current needs to be used to obtain the maximum voltage support. Figure 2 The voltage of the new energy grid connection point can be calculated as:

[0082]

[0083] It can be clearly seen from the above formula that there is a relationship between the grid connection point voltage, the MMC current and the current of the wind farm side converter. The greater the current amplitude output by the MMC after the fault occurs, the greater the reactive current output by the GSC and the smaller the active current. The higher the grid connection point voltage of the new energy source that can be obtained, the more conducive it is for the new energy source to achieve low-voltage fault crossing. Determine the q-axis current amplitude of the wind farm according to the above formula. For the wind farm terminal voltage during the fault, when the wind turbine is connected to the MMC, the voltage drop after the fault is more serious due to the limited output short-circuit current capacity of the MMC, and it is necessary to use the limited short-circuit current to obtain the maximum voltage support. Determine the corresponding grid connection point voltage u within the synchronous stability domain of the phase-locked loop according to the equivalent circuit diagram of the wind power transmission system through flexible direct current. ws , thereby determining the q-axis component i of the wind farm current wq And increase the q-axis current amplitude; in this embodiment, the q-axis current amplitude is increased by 0.2 pu.

[0084] Optionally, the detecting of the voltage recovery status of the wind farm station grid connection point and judging whether the fault is cleared includes:

[0085] The lowest voltage during the fault detection of the wind farm grid connection point is denoted as u wmin , and obtain the grid connection point voltage u in real time w ;

[0086] Determine the grid connection point voltage u w Is it greater than u wmin +0.1pu; if yes, the fault is determined to be cleared and the time is recorded as t2; if no, the fault is determined to be not cleared.

[0087] After the fault is cleared, the fault recovery period begins. During this process, transient overvoltage will occur. In order to ensure the safety of the line, the transient overvoltage is suppressed, and the d-axis current of the wind farm is increased during the fault recovery process until the voltage at the MMC sending end is restored to a steady-state value.

[0088] The principle of transient overvoltage during fault recovery is as follows: During the fault recovery period, during the short-circuit fault, the MMC output d-axis current i sd Controlled to the minimum value of the limiter -I sdmaxAfter the fault is cleared, the external constraints of the converter station become current constraints again, and the actual current flowing into the converter station is equal to the current emitted by the renewable energy. Therefore, the reference current i output by the outer loop control at the moment of fault clearing is sdref sd , and the reference current will track the actual current under the control of the proportional integral controller. During the tracking process, there is a deviation in the input of the inner loop current controller PI control module (i sd -i sdref ), the reference voltage u generated by the MMC control system mcd will be greater than the steady-state voltage, but due to the existence of the limiter, u mcd The maximum is the limiter limit value u cdmax This will result in u s Greater than the reference voltage (1.0pu).

[0089] (3) Strategy for suppressing transient overvoltage after fault clearing. During the fault recovery phase, a suppression strategy is needed to suppress transient overvoltage. The overvoltage during the recovery process of the AC side short-circuit fault at the sending-end converter station is related to the equivalent inductance L of the transformer and bridge arm. eq , wind turbine current recovery rate (di sd / dt), the maximum value of the converter station output voltage u in the transient process after the fault is removed cdmax The main approach is to optimize the converter station control system and reduce the maximum output voltage of the converter station to suppress overvoltage.

[0090] When the circuit breaker clears the fault, the wind turbine power begins to recover. From the control logic, it can be seen that the mathematical model of the MMC inner loop current controller PI control module is:

[0091]

[0092] Among them, u dPI and u qPI They are the outputs of the d-axis and q-axis PI control links of the inner loop current controller respectively.

[0093] During a short-circuit fault, the MMC outputs a d-axis current i sd Controlled to the minimum value of the limiter -I sdmax After the fault is cleared, the external constraints of the converter station become current constraints again, and the actual current flowing into the converter station is equal to the current emitted by the renewable energy. Therefore, the reference current i output by the outer loop control at the moment of fault clearing is sdref sd , and the reference current will track the actual current. During the tracking process, there is a deviation in the input of the inner loop current controller PI control module (i sd -i sdref ),like Figure 6 ​​As shown in (a), this deviation forms a "pressurized area". The MMC output dq axis voltage is:

[0094]

[0095] From the above formula, we can see that due to the deviation between the reference current and the actual current, the reference voltage u generated by the MMC control system is mcd will be greater than the steady-state voltage, but due to the existence of the limiter, u mcd The maximum is the limiter limit value u cdmax This will result in u s Greater than the reference voltage (1.0pu), such as Figure 6 As shown in (b).

[0096] At this time, the d-axis voltage at the point of common coupling (PCC) in the transient process can be solved as:

[0097]

[0098] Among them, u sd is the d-axis voltage of MMC, i d is the d-axis current, i q is the q-axis current, u cd is the maximum d-axis voltage that can be output at the MMC sending end, L eq is the equivalent inductance of the transformer and bridge arm, R eq is the equivalent resistance of the transformer and the bridge arm; during the fault recovery period, i d To restore the active power output of new energy.

[0099] From the above formula, we can see that the change rate of the d-axis current and the maximum value of the MMC output d-axis voltage u cdmax It directly affects the magnitude of transient overvoltage at the PCC point, which will cause the voltage at the GSC grid connection point to increase.

[0100] In order to successfully suppress overvoltage during fault recovery, two MMC sending-end control strategies are proposed:

[0101] Strategy 1: When the fault is cleared, reduce the AC voltage reference value u at the sending-end converter station. sref At this time, the PCC point voltage established by the converter station will track the smaller reference voltage, thereby reducing u sd Maximum value;

[0102] Strategy 2: At the moment t2 when the fault is cleared, the integrators of the PI control modules of the outer loop voltage controller and the inner loop current controller of the D-axis of the sending-end converter station are cleared until the wind power is restored, thereby reducing the "pressurized area" and quickly achieving reference current tracking of the actual current, reducing V drefMaximum value.

[0103] The specific implementation methods include the following:

[0104] (1) At the fault clearing time t2, the voltage command value at the MMC sending end is reduced by the voltage reduction method. Specifically, the voltage command value at the MMC sending end converter station is reduced to 0.8 pu by the voltage reduction method. After the suppression strategy is maintained for a preset time, the voltage command value at the MMC sending end is restored to 1 p.u. at a rate of 2 p.u. / s.

[0105] (2) Starting from the moment t2 when the fault is cleared, the output of the inner and outer loop integral controllers of the d-axis on the MMC side is reset to zero using the zeroing method. After the suppression strategy is maintained for a preset time, the reset operation of the integral controller is canceled.

[0106] In the above two suppression strategies, the preset time length for maintaining the suppression strategy is 100ms.

[0107] In the fault recovery stage, the above method is used to suppress transient overvoltage. At the same time, the d-axis current of the wind farm is slowly increased until the voltage at the MMC sending end is restored to a steady-state value, and i is increased. d To restore the active power output of renewable energy and limit the impact of this process on u sd The impact of overvoltage, specifically, limits the d-axis current rise rate k = di d / dt, increasing the d-axis current of the wind farm to restore the voltage at the MMC sending end to the steady-state value of 1p.u., where the value range of k is 0.2-0.6.

[0108] The d-axis voltage expression in the transient process is:

[0109]

[0110] Among them, u sd is the d-axis voltage of MMC, i d is the d-axis current, i q is the q-axis current, u cdmax is the maximum d-axis voltage that can be output at the MMC sending end, L eq is the equivalent inductance of the transformer and bridge arm, R eq is the equivalent resistance of the transformer and the bridge arm; during the fault recovery period, i d To restore the active power output of renewable energy and limit the impact of this process on u sd The impact of overvoltage.

[0111] The embodiment of the present invention reduces the q-axis current i of the new energy source during the fault period. wq , reduce the new energy d-axis current i wd, increase the limit value of the output q-axis current at the sending end of the MMC to the set value, thereby solving the problem of low-voltage ride-through of the wind farm during the fault and ensuring that the wind turbine can operate without disconnecting from the grid within the specified time; during the fault, the output q-axis current at the sending end of the MMC is maintained at the set value, so that the phase diagram stability of the wind farm and the MMC sending end is improved, the synchronization stability of the phase-locked loop is improved, and the transient stability of the system is improved. The technical problems of high cost of achieving fault ride-through and complex fault ride-through control when a deep fault occurs at the sending end in the scenario of flexible direct current transmission of wind power are solved. The beneficial effects of improving the synchronization stability of the phase-locked loop and improving the transient stability of the system when riding through the fault are achieved.

[0112] Embodiment 2

[0113] The present invention also provides a coordinated control device for deep faults at the wind power sending end, which comprises:

[0114] The fault ride-through module is used to increase the q-axis current amplitude of the wind farm, reduce the d-axis current amplitude of the wind farm, and increase the q-axis current limit value at the MMC sending end to a set value to meet the low voltage fault ride-through requirements of the wind farm;

[0115] The fault judgment module is used to detect the voltage of the wind farm station grid connection point and determine whether the fault is cleared; if so, the overvoltage suppression module is executed; if not, the fault ride-through module is returned to execute.

[0116] The overvoltage suppression module is used to suppress transient overvoltage by adopting a suppression strategy during fault recovery, and to increase the d-axis current of the wind farm to restore the voltage at the MMC sending end to a steady-state value.

[0117] Embodiment 3

[0118] The present invention also provides a computer-readable storage medium, wherein the computer-readable storage medium stores computer instructions, and the computer instructions are used to enable a processor to implement the coordinated control method for deep faults at the wind power sending end as described in any one of the first embodiments when executing the computer instructions.

[0119] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A coordinated control method for deep faults at wind power transmission ends, characterized in that: include: When a deep fault occurs at the sending end in the scenario of wind power transmission via flexible direct current, perform the following steps: S1. Increase the q-axis current amplitude of the wind farm, reduce the d-axis current amplitude of the wind farm, and increase the q-axis current limit value at the MMC sending end to the set value to meet the low voltage fault ride-through requirements of the wind farm; S2, detect the voltage of the wind farm station grid connection point to determine whether the fault is cleared; if so, execute S3; if not, return to execute S1; S3. During the fault recovery period, a suppression strategy is used to suppress transient overvoltages and increase the d-axis current of the wind farm to restore the voltage at the MMC sending end to a steady-state value.

2. The method according to claim 1, characterized in that The step of increasing the q-axis current amplitude of the wind farm comprises: According to the equivalent circuit diagram of the wind power transmission system through flexible direct current, the corresponding grid connection point voltage u in the synchronous stability domain of the phase-locked loop is determined ws ; according to Calculate the q-axis component i of the wind farm current wq , and the q-axis component i of the wind farm current wq The q-axis current amplitude of the wind farm is determined by the q-axis component of the current; where ω is the system angular frequency, L f is the grounding reactance at the fault point, I mmax is the current limit value, L1 is the reactance from the wind farm to the fault point, i wd with i wq They are the d-axis and q-axis components of the wind farm current respectively.

3. The method according to claim 1, characterized in that The step of reducing the d-axis current amplitude of the wind farm and increasing the q-axis current limit value at the MMC sending end to a set value includes: Get the q-axis voltage u at the wind farm grid connection point wq , with u wq = 0 is a stable boundary condition, and the power angle state equation is constructed: In the formula, K pPLL is the phase-locked loop ratio parameter, K iPLL is the phase-locked loop integral parameter, ω0 is the system rated angular frequency, δ is the power angle between the wind farm and the MMC sending end, is the initial phase of voltage and current, is the first derivative of δ, is the second derivative of δ; The phase diagram method is used to perform transient stability analysis on the power angle state equation. According to the transient stability analysis results, the corresponding reduced d-axis current amplitude is determined within the synchronous stability domain of the phase-locked loop, and the q-axis current limit value at the sending end of the MMC is increased to the set value.

4. The method according to claim 1, characterized in that The detecting the voltage of the wind farm grid connection point and judging whether the fault is cleared includes: The lowest voltage during the fault detection of the wind farm grid connection point is denoted as u wmin , and obtain the grid connection point voltage u in real time w ; Determine the grid voltage u w Is it greater than u wmin +0.1pu; if yes, the fault is determined to be cleared and the time is recorded as t2; if no, the fault is determined to be not cleared.

5. The method according to claim 1, characterized in that During the fault recovery period, a suppression strategy is used to suppress transient overvoltage, including: At the fault clearing time t2, the voltage command value of the MMC sending end is reduced to 0.8pu by the voltage reduction method. After maintaining the suppression strategy for a preset time, the voltage command value of the MMC sending end is restored to 1p.u. at a rate of 2p.u. / s.

6. The method according to claim 1, characterized in that During the fault recovery period, a suppression strategy is used to suppress transient overvoltage, including: Starting from the moment t2 when the fault occurs, the reset method is used to reset the output of the inner and outer loop integral controller of the d-axis on the MMC side to zero. After the suppression strategy is maintained for a preset time, the reset operation of the integral controller is canceled.

7. The method according to claim 5 or 6, characterized in that The step of increasing the d-axis current of the wind farm to restore the voltage at the MMC sending end to a steady-state value includes: Limit the d-axis current rising rate k = di d / dt, increasing the d-axis current of the wind farm until the voltage at the MMC sending end recovers to a steady-state value; where the value range of k is 0.2-0.

6.

8. A coordinated control device for deep faults at wind power transmission ends, characterized in that: When a deep fault occurs at the sending end in the scenario of wind power transmission via flexible direct current, it includes: The fault ride-through module is used to increase the q-axis current amplitude of the wind farm, reduce the d-axis current amplitude of the wind farm, and increase the q-axis current limit value at the MMC sending end to a set value to meet the low voltage fault ride-through requirements of the wind farm; The fault judgment module is used to detect the voltage of the wind farm station grid connection point and determine whether the fault is cleared; if so, the overvoltage suppression module is executed; if not, the fault ride-through module is returned to execute. The overvoltage suppression module is used to suppress transient overvoltage by adopting a suppression strategy during fault recovery, and to increase the d-axis current of the wind farm to restore the voltage at the MMC sending end to a steady-state value.

9. A computer-readable storage medium, characterized in that: The computer-readable storage medium stores computer instructions, and the computer instructions are used to enable a processor to implement the coordinated control method for deep faults at the wind power transmission end according to any one of claims 1 to 7 when executed.

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