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

Abstract

The invention discloses a coordination control method and device for a shallow fault of a wind power sending end and a storage medium, and belongs to the field of safe operation of a power system. Comprising the following steps: when a sending end has a shallow fault in a wind power flexible direct sending scene, S1, increasing the q-axis current amplitude of a wind power plant so as to improve the voltage of a grid-connected point; the d-axis current amplitude of the wind power plant is reduced, and the amplitude limiting value of the d-axis voltage of the MMC sending end is increased to a set value, so that the synchronous stability of a phase-locked loop is improved, and the low-voltage fault ride-through requirement of the wind power plant is met; s2, detecting the voltage of a grid-connected point of the wind power station, 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 electric power systems, and more specifically, relates to a coordinated control method, device and storage medium for shallow 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 mainly considers the influence of the phase-locked loop parameters and structure on transient stability, while paying less attention to other factors affecting transient stability. Therefore, the unit control capability is not fully utilized. The 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 shallow 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 shallow faults at the wind power sending end, aiming to achieve shallow fault riding 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 shallow faults at a wind power transmission end, comprising:

[0009] When a shallow 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 to increase the voltage at the grid connection point; reduce the d-axis current amplitude of the wind farm and maintain the d-axis voltage limit value output by the MMC sending end at the set value to improve the synchronization stability of the phase-locked loop and meet the low-voltage fault ride-through requirements of the wind farm;

[0011] S2, detect the voltage recovery status of the wind farm station grid connection point, and 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 of the fault point, L1 is the reactance from the wind farm to the fault point, L2 is the reactance from the fault point to the MMC, u mcmax is the maximum voltage of the MMC sending end, i wq is the q-axis component of the wind farm current.

[0016] Optionally, the step of reducing the d-axis current amplitude of the wind farm and increasing the d-axis voltage 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 limit value of the d-axis voltage 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 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 instant t2 when the fault is cleared, the voltage command value at the MMC sending end is reduced to 0.8pu 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 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 is cleared, the output of the inner and outer loop integral controller 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.

[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] From the fault clearing time t2, the d-axis current rising rate k is limited to 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 shallow 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 to increase the voltage at the grid connection point; reduce the d-axis current amplitude of the wind farm and increase the d-axis voltage limit value at the MMC sending end to the set value to improve the synchronization stability of the phase-locked loop and 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 to 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 shallow faults at the wind power transmission end as described in any one of the first aspects when executing the computer instructions.

[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 shallow 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 of 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 limit value of the d-axis voltage output at 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. The present invention proposes a fault ride-through control method for shallow faults caused by short circuits at the sending end without adding additional hardware equipment.

[0037] 2. The embodiment of the present invention provides a coordinated control method for shallow faults at the wind power transmission end, and proposes two suppression strategies for the transient overvoltage problem 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 This is the equivalent circuit diagram of the wind power transmission system via flexible direct current under shallow fault conditions;

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

[0041] Figure 4 Schematic diagram of the effect of current magnitude on transient processes, where (a) is the effect of the maximum voltage of the MMC and (b) is the effect of the active current of the new energy source;

[0042] Figure 5 Schematic diagram of the influence of the dq axis limit value on the transient process, where (a) is a schematic diagram of the influence of the dq axis limit value on the transient process when φ1<0, and (b) is a schematic diagram of the influence of the dq axis limit value on the transient process when φ1>0;

[0043] Figure 6 This is a schematic diagram of the analysis of the current and voltage change process, where (a) is the current change curve and (b) is u cd (c) is the voltage change curve of the PCC point. 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] In a typical wind power transmission system via 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 shallow 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. When the wind turbine is connected to a modular multilevel converter (MMC), due to the limited output short-circuit current capacity of the MMC, the voltage drop after the fault is more serious and cannot meet the voltage fault ride-through requirements of the new energy machine end. It is necessary to use the limited short-circuit current to obtain the maximum voltage support. 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.

[0047] Based on the above problems, Figure 1 As shown, an embodiment of the present invention provides a coordinated control method for shallow faults at a wind power transmission end, comprising:

[0048] When a shallow fault occurs at the sending end in the scenario of wind power transmission via flexible direct current, perform the following steps:

[0049] S1. Increase the q-axis current amplitude of the wind farm to increase the voltage at the grid connection point; reduce the d-axis current amplitude of the wind farm and increase the d-axis voltage limit value at the MMC sending end to the set value to improve the synchronization stability of the phase-locked loop and meet the low-voltage fault ride-through requirements of the wind farm;

[0050] 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;

[0051] 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.

[0052] 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 characteristics; if no, it is judged that a shallow 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 full modulation mode and presents the voltage source characteristics. 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.

[0053] 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.

[0054] 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.

[0055] (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

[0056]

[0057] 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, L2 is the reactance from the fault point to the MMC, u mcmax is the maximum voltage of the MMC sending end, i wq is the q-axis component of the wind farm current.

[0058] In order to ensure that the system has a steady-state operating point, it is also necessary to ensure that u wsq=0 has a solution. Need to satisfy:

[0059]

[0060] According to the relationship between the dq coordinate system of the converter on the new energy grid side and the MMC coordinate system:

[0061]

[0062] Among them, to unify to the GSC synchronous coordinate system, the following conditions must be met:

[0063]

[0064] Therefore, the condition for the existence of a steady-state operating point needs to satisfy the relationship shown in equation (5). It can be seen that the existence of a steady-state operating point is related to the active current of the new energy source, the fault impedance, the fault location and the maximum output voltage of the MMC.

[0065] By unifying equation (1) to the dq coordinate system of the converter on the new energy grid side, the q-axis voltage of the new energy can be obtained as:

[0066]

[0067] in,

[0068] According to formula (6), the dynamic process of transient power angle can be obtained as follows: Figure 3 The dynamic process analysis is the same as that of the current limiting mode, so it will not be repeated here. Similarly, point b is the steady-state operation point, and point c is the unstable point.

[0069] Using the same method, we can get the power angle motion equation:

[0070]

[0071] 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.

[0072] 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 output current of the MMC are beneficial to improving transient stability. According to this analysis result, the corresponding reduced d-axis current amplitude and the set value of the d-axis voltage limit value at the MMC sending end are determined within the synchronous stability domain of the phase-locked loop. 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, within this range, the d-axis current of the wind farm is reduced, and the limit value of the d-axis voltage at the sending end of the MMC is increased to a set value. Specifically, the limit value of the d-axis voltage at the sending end of the MMC is increased to 1.5pu.

[0073] like Figure 4 As shown in the figure, the larger the AC voltage that the MMC can output, the faster the phase curve converges, and the easier it is for the system to achieve synchronous stability. 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 at the MMC sending end. Figure 3 The larger the power angle adjustment range b~c shown in , the easier it is to achieve transient stability.

[0074] In the synchronous coordinate system of GSC, the q-axis voltage is shown in equation (6). Since the range of δ0 is 0 to π / 2, similarly, in order to ensure that point a does not exceed point c, the following conditions must be met:

[0075]

[0076] exist When δ0 is large, the starting point a may exceed point c, resulting in an unbalanced system. Figure 5 As shown in (a). When Figure 5 As shown in (b). Increase The size of will cause the power angle acceleration phase of the a~b section to become longer, which is not conducive to the stability of the system. That is, in full modulation mode, according to u cd =u mcmax ,u cq =0 principle to distribute the dq axis voltage.

[0077] 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 limit value of the d-axis voltage output at the MMC sending end to a set value. In combination with the synchronous stability domain of the d-axis current of the wind farm and the amplitude range of the MMC sending end current, in this embodiment, the d-axis current is reduced by 0.2 pu, and the limit value of the d-axis voltage output by the MMC is maintained at its maximum value of 1.5 pu.

[0078] (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.

[0079] 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:

[0080]

[0081] 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, L2 is the reactance from the fault point to the MMC, u mcmax is the maximum voltage of the MMC sending end, i wq is the q-axis component of the wind farm current.

[0082] It can be clearly seen from the above formula that the relationship between the grid connection point voltage, the MMC current and the current of the wind farm side converter is that the smaller the current amplitude of the MMC output after the fault occurs, the larger the GSC grid connection point voltage is, which indicates that the greater the reactive power output of the GSC, the higher the voltage at the new energy machine end. And the initial phase of the MMC voltage, that is, the distribution principle of the dq axis voltage will not affect the magnitude of the new energy machine end voltage. The magnitude of the new energy machine end voltage is only affected by the amplitude of the MMC output voltage. The higher the new energy grid connection point voltage that can be obtained, the more conducive it is for new energy to achieve low-voltage fault crossing. According to the equivalent circuit diagram of the wind power flexible direct current transmission system, the corresponding grid connection point voltage u in the phase-locked loop synchronous stability domain is determined. 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.

[0083] Optionally, detecting the voltage of the wind farm grid connection point to determine whether the fault is cleared includes:

[0084] 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 ;

[0085] 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.

[0086] 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 so that the voltage at the MMC sending end is restored to the steady-state value before the fault.

[0087] 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 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 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).

[0088] (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.

[0089] 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:

[0090]

[0091] 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.

[0092] 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

[0093]

[0094] 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 is the maximum limit value u of the limiter cdmax This will result in u s Greater than the reference voltage (1.0pu), such as Figure 6 As shown in (b).

[0095] At this time, the d-axis voltage at the point of common coupling (PCC) in the transient process can be solved as:

[0096]

[0097] 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.

[0098] 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 This directly affects the magnitude of the transient overvoltage at the PCC point. This voltage will cause the voltage at the GSC grid connection point to increase. At this time, the voltage at the converter station PCC point will be greater than the steady-state value, such as Figure 6 ​As shown in (c). This transient overvoltage will also bring risks to the safe operation of new energy, and measures need to be taken to suppress the overvoltage. The overvoltage mechanism in full modulation mode is the same as that in current limiting mode, but because the "boost area" is significantly increased at this time, the overvoltage lasts longer.

[0099] In order to successfully suppress overvoltage during fault recovery, two MMC sending-end control strategies are proposed:

[0100] 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;

[0101] 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 dref Maximum value.

[0102] The specific implementation methods include the following:

[0103] (1) At the instant t2 when the fault is cleared, 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.

[0104] (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.

[0105] In the above two suppression strategies, the preset time length for maintaining the suppression strategy is 100ms.

[0106] 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.

[0107] The embodiment of the present invention reduces the q-axis current i of the new energy source during the fault period. wq , improve the voltage at the wind farm grid connection point, thereby solving the problem of low-voltage ride-through of the wind farm during faults and ensuring that the wind turbine can operate without disconnecting from the grid within the specified time; during the fault, the d-axis voltage output by the MMC sending end is maintained at 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. The technical problem of high cost of achieving fault ride-through and complex fault ride-through control when a shallow fault occurs at the sending end in the scenario of flexible direct current transmission of wind power is solved. The beneficial effect of improving the synchronization stability of the phase-locked loop and improving the transient stability of the system is achieved when riding through the fault.

[0108] Embodiment 2

[0109] The present invention also provides a coordinated control device for shallow faults at the wind power sending end, which comprises:

[0110] The fault ride-through module is used to increase the q-axis current amplitude of the wind farm to increase the voltage at the grid connection point; reduce the d-axis current amplitude of the wind farm and increase the d-axis voltage limit value at the MMC sending end to the set value to improve the synchronization stability of the phase-locked loop and meet the low-voltage fault ride-through requirements of the wind farm;

[0111] The fault judgment module is used to detect the voltage recovery status 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;

[0112] 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.

[0113] Embodiment 3

[0114] 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 shallow faults at the wind power sending end as described in any one of the first embodiments when executing the computer instructions.

[0115] 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 shallow faults at wind power transmission ends, characterized in that: include: When a shallow 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 to increase the voltage at the grid connection point; reduce the d-axis current amplitude of the wind farm and increase the d-axis voltage limit value at the MMC sending end to the set value to improve the synchronization stability of the phase-locked loop and 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 of the fault point, L1 is the reactance from the wind farm to the fault point, L2 is the reactance from the fault point to the MMC, u mcmax is the maximum voltage of the MMC sending end, i wq is the q-axis component of the wind farm current.

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 d-axis voltage 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 limit value of the d-axis voltage 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 2, characterized in that: During the fault recovery period, a suppression strategy is used to suppress transient overvoltage, including: At the instant t2 when the fault is cleared, the voltage command value at 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 at 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 is cleared, the output of the inner and outer loop integral controller 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.

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: From the fault clearing time t2, the d-axis current rising rate k is limited to 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 shallow faults at wind power transmission ends, characterized in that: When a shallow 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 to increase the voltage at the grid connection point; reduce the d-axis current amplitude of the wind farm and increase the d-axis voltage limit value at the MMC sending end to the set value to improve the synchronization stability of the phase-locked loop and 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 to 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 shallow faults at the wind power transmission end according to any one of claims 1 to 7 when executed.

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