A method and device for coordinated control of wind power sending end shallow fault and a storage medium

Abstract

The application discloses a kind of wind power sending end shallow fault coordination control method, device and storage medium, belong to the field of safe operation of electric power system. Including: in the scene of wind power through flexible sending, when shallow fault occurs at sending end, the following steps are executed: S1, the q-axis current amplitude of wind farm is raised, to improve the voltage of grid connection point;The d-axis current amplitude of wind farm is reduced, and the amplitude limit of d-axis voltage of MMC sending end is raised to set value, to improve the synchronization stability of phase-locked loop, meet the low voltage fault ride-through requirement of wind farm;S2, detect the voltage of wind farm station grid connection point, judge whether the fault is cleared;If yes, execute S3;If not, return to execute S1;S3, during fault recovery, transient overvoltage is inhibited by inhibition strategy, and the d-axis current of wind farm is raised to the voltage of MMC sending end recovers to steady state value.In the process of crossing fault, make wind farm and MMC sending end transient stability better, improve the synchronization stability of phase-locked loop.

Description

Technical Field

[0001] This 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 shallow faults at the wind power transmission end. Background Technology

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

[0003] During short-circuit faults, both the generation and transmission sides need to achieve low-voltage ride-through capability. The proposed segmented voltage reduction control method mainly achieves segmented voltage reduction by setting multiple criteria. This method is suitable for asymmetrical faults and can achieve current limiting in the steady-state stage, but it suffers from overcurrent problems in the transient stage. Existing methods use the port fault to uniquely solve for the virtual impedance value and adjust the voltage reduction amplitude in combination with the degree of current reference exceeding the limit. Because the virtual impedance setting value of this method is large and unique, it can lead to excessively low system voltage under mild faults. Furthermore, the control effect of this method is strongly correlated with the location of the short-circuit point, making the design and setting of the virtual impedance value quite difficult.

[0004] When a short-circuit fault occurs, it is necessary to ensure that the MMC (Modular Locking Controller) does not experience transient instability. In systems containing phase-locked loop (PLL) synchronization equipment, stability is enhanced by modifying the tracking performance of the PLL to improve transient stability. However, these methods have limited effectiveness in improving transient stability during faults. Existing technologies have designed an adaptive PLL that modifies the PLL control structure to improve stability when the frequency change rate is too rapid. The above methods improve transient stability based on the PLL itself, mainly considering the influence of PLL parameters and structure on transient stability, while paying less attention to other factors affecting transient stability. Therefore, they fail to fully utilize the unit's control capabilities. Enhancing transient stability by changing the output current requires significant modifications to the PLL characteristics. After fault recovery, the PLL must be immediately switched back to the control strategy under normal operating conditions; otherwise, adverse effects may easily occur after fault recovery.

[0005] In terms of overvoltage suppression, a common solution in power systems is to configure reactive power compensation devices, which compensate for reactive current during faults to provide voltage support. However, this method is less economical in terms of investment costs, construction site requirements, and operation and maintenance.

[0006] Therefore, when a short-circuit fault occurs at the sending end of a wind power system, existing fault ride-through methods will reduce the transient stability of the system, and the cost of configuring reactive power compensation devices is high. Depending on the fault location and duration, fault ride-through methods need to be designed specifically for shallow faults caused by short circuits. Summary of the Invention

[0007] In view of the deficiencies of related technologies, the purpose of this invention is to provide a coordinated control method, device and storage medium for shallow faults at the wind power transmission end, which aims to achieve shallow fault ride-through at the wind power transmission end while ensuring the synchronization stability of the phase-locked loop and meeting the current limiting of the MMC.

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

[0009] When a shallow fault occurs at the sending end in a scenario where wind power is transmitted via flexible direct current, the following steps should be performed:

[0010] S1. Increase the q-axis current amplitude of the wind farm to improve the voltage at the grid connection point; decrease the d-axis current amplitude of the wind farm to maintain the limiting value of the d-axis voltage output by the MMC at the set value, so as 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. Check the voltage recovery status at the wind farm's grid connection point to determine if the fault has been cleared; if yes, proceed to S3; if no, return to S1.

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

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

[0014] Based on the equivalent circuit diagram of the wind power transmission system via flexible direct current, determine the corresponding grid connection point voltage u within the synchronous stability region of the phase-locked loop. ws ;

[0015] according to Calculate the q-axis component of the wind farm current i wq And composed of the q-axis component i of the wind farm current wq The q-axis current amplitude of the wind farm is determined by the current q-axis component; where ω is the system angular frequency, and L... f L1 is the grounding reactance at the fault point, L2 is the reactance from the wind farm to the fault point, and L3 is the reactance from the fault point to the MMC. mcmax i is the maximum value of the MMC sending terminal voltage. wq This represents the q-axis component of the wind farm current.

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

[0017] Obtain the q-axis voltage u at the grid connection point of the wind farm wq , with u wq =0 is the stable boundary condition. Construct the work angle state equation:

[0018]

[0019] In the formula, K pPLL K is the proportional parameter of the phase-locked loop. iPLL Here are the phase-locked loop integral parameters, ω0 is the system's rated angular frequency, and δ is the power angle of the wind farm and the MMC sending end. The initial phase of voltage and current. Let δ be the first derivative. The second derivative of δ;

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

[0021] Optionally, detecting the voltage at the wind farm's grid connection point to determine whether the fault has been cleared includes:

[0022] The lowest voltage detected during a grid connection point fault at the wind farm 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, then the fault is cleared and the time is recorded as t2; if no, then the fault is not cleared.

[0024] Optionally, during fault recovery, the suppression strategy for transient overvoltage includes:

[0025] At the moment t2 when the fault is cleared, the voltage command value of the MMC sending end is reduced to 0.8pu using the step-down method. After the suppression strategy is maintained for a preset time, the voltage command value of the MMC sending end is restored to 1pu at a rate of 2p.u. / s.

[0026] Optionally, during fault recovery, the suppression strategy for transient overvoltage includes:

[0027] At the moment t2 when the fault is cleared, the output of the inner and outer loop integral controllers on the d-axis of the MMC side is zeroed using the zeroing method. After the suppression strategy is maintained for a preset time, the zeroing 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] Starting from the fault clearing time t2, the d-axis current rise rate is limited to k = di. d / dt, which increases the d-axis current of the wind farm until the voltage at the MMC sending end returns to a steady-state value; where k ranges from 0.2 to 0.6.

[0030] Secondly, the present invention also provides a coordinated control device for shallow faults at the wind power transmission end, which includes the following components when a shallow fault occurs at the transmission end in a scenario where wind power is transmitted via flexible direct current:

[0031] The fault ride-through module is used to increase the q-axis current amplitude of the wind farm to improve the voltage at the grid connection point; and to decrease the d-axis current amplitude of the wind farm and increase the limit value of the d-axis voltage at the MMC sending end to the set value, so as 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 diagnosis module is used to detect the voltage at the grid connection point of the wind farm and determine whether the fault has been cleared. If yes, the overvoltage suppression module is executed; if not, the process returns to execute the fault ride-through module.

[0033] The overvoltage suppression module is used to suppress transient overvoltages during fault recovery by employing suppression strategies, thereby increasing the d-axis current of the wind farm until the voltage at the MMC sending end recovers to a steady-state value.

[0034] Thirdly, the present invention also provides a computer-readable storage medium storing computer instructions for causing a processor to execute a coordinated control method for shallow faults at the wind power transmission end as described in any one of the first aspects.

[0035] Compared with the prior art, the above-described technical solutions conceived in this invention can achieve the following beneficial effects:

[0036] 1. This invention provides a coordinated control method for shallow faults at the wind power transmission end, which reduces the q-axis current i of the new energy source during the fault period. wq Reduce the d-axis current i of new energy wdThis invention improves the grid connection voltage of the wind farm, thereby resolving the low-voltage ride-through problem during faults and ensuring that the wind turbines can operate without disconnecting from the grid for a specified period. Increasing the limiting value of the d-axis voltage output from the MMC transmitter to a set value improves the transient stability of the wind farm and the MMC transmitter, enhancing the synchronization stability of the phase-locked loop. This invention proposes a fault ride-through control method for shallow faults caused by transmitter short circuits without requiring additional hardware.

[0037] 2. This invention provides a coordinated control method for shallow faults at the wind power transmission end, proposing two suppression strategies to address transient overvoltage issues during fault recovery. Strategy 1: After a short-circuit fault occurs, the AC voltage reference value u at the MMC transmission end is reduced. sref After the short-circuit fault is cleared and the voltage returns to its initial value, the PCC point voltage established by the converter station will track the smaller reference voltage, reducing u sd The maximum value of this strategy solves the transient overvoltage problem during fault recovery. Strategy two involves resetting the integrators of the outer loop voltage controller and inner loop current controller PI control modules of the MMC sending-end d-axis current outer loop voltage controller and inner loop current controller to zero until the power recovers to its steady-state value, thus reducing V... dref The maximum value of [the value] solved the problem of transient overvoltage during faults. Attached Figure Description

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

[0039] Figure 2 This is the equivalent circuit diagram of a wind power transmission system via flexible direct current under shallow fault conditions.

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

[0041] Figure 4 This is a schematic diagram showing the effect of current magnitude on the transient process, where (a) represents the effect of the maximum voltage of the MMC, and (b) represents the effect of the active current of the new energy source.

[0042] Figure 5 These are schematic diagrams illustrating the effect of the dq-axis limiting value on the transient process. (a) shows the effect of the dq-axis limiting value on the transient process when φ1 < 0, and (b) shows the effect of the dq-axis limiting value on the transient process when φ1 > 0.

[0043] Figure 6 These are schematic diagrams illustrating the changes in current and voltage. (a) shows the current change curve, and (b) shows the voltage change curve. cd Voltage variation curves, (c) is the voltage variation curve at point PCC. Detailed Implementation

[0044] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0045] The following description, in conjunction with a preferred embodiment, illustrates the content involved in the above embodiments.

[0046] In a typical wind power transmission system via flexible DC transmission, the wind power system uses direct-drive wind turbines with grid-connected control, employing a phase-locked loop (PLL) for synchronization. The sending end of the flexible DC transmission system uses constant voltage / frequency (V / F) control to generate a constant voltage amplitude and frequency. In this scenario, if a shallow fault occurs at the sending end, without timely control measures, the PLL will lack a steady-state operating point, causing the system to lose synchronization stability. When the wind turbine is connected to a modular multilevel converter (MMC), the limited short-circuit current capability of the MMC results in a more severe voltage drop after the fault, failing to meet the voltage fault ride-through requirements of the new energy turbine. This necessitates utilizing the limited short-circuit current to achieve maximum voltage support. After fault recovery, due to the extensive use of proportional-integral (PI) controllers within the power electronic equipment, a deviation exists between the reference current and the actual current. The reference voltage generated by the MMC control system will exceed the steady-state value, causing overvoltage and grid disconnection of the wind turbine.

[0047] Based on the above issues, such as Figure 1 As shown, this embodiment of the invention provides a coordinated control method for shallow faults at the wind power transmission end, including:

[0048] When a shallow fault occurs at the sending end in a scenario where wind power is transmitted via flexible direct current, the following steps should be performed:

[0049] S1. Increase the q-axis current amplitude of the wind farm to improve the voltage at the grid connection point; decrease the d-axis current amplitude of the wind farm and increase the limiting value of the d-axis voltage 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 at the grid connection point of the wind farm to determine if the fault has been cleared; if yes, proceed to S3; if no, return to S1.

[0051] S3. During fault recovery, a suppression strategy is adopted to suppress transient overvoltage and increase the d-axis current of the wind farm until the voltage at the MMC sending end recovers to the steady-state value.

[0052] like Figure 1 The diagram illustrates the basic framework of a wind power transmission system via flexible direct current (FDDC). After a short-circuit fault occurs at the sending end of the FDDC system, the voltage at the PCC point of the MMC drops. The voltage u at the grid connection point of the wind farm is then detected and determined. ws Check if the amplitude is less than 0.8 pu, and determine if the current on the MMC side is i. s Reaching the limit value I max If so, it is determined that a deep short-circuit fault has occurred at the sending end of the wind power transmission system via flexible DC transmission. In this case, the MMC enters current-limiting mode and exhibits current source characteristics. If not, it is determined that a shallow short-circuit fault has occurred at the sending end of the wind power transmission system via flexible DC transmission. In this case, the MMC enters full modulation mode and exhibits voltage source characteristics. The equivalent circuit diagram of the system is as follows: Figure 2 As shown. Where 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 AC side line of the MMC, Z... f This is the fault impedance.

[0053] For the study of system transient stability, the main focus is on synchronous stability during a fault, voltage support characteristics, and transient overvoltage characteristics after the fault is cleared. Among these, synchronous stability is a prerequisite for stable system operation.

[0054] During a fault, to ensure the PLL has a steady-state operating point, it is necessary to increase the terminal voltage of the new energy source after the fault to achieve low-voltage ride-through. During fault recovery, measures need to be taken to suppress transient overvoltages after the fault. Taking into account the synchronization stability during the fault, the fault voltage support effect, and the transient overvoltage suppression during fault recovery, a corresponding coordinated control strategy is proposed.

[0055] (1) Improve the system's transient synchronization stability coordination control strategy. During a fault, ensuring the synchronization stability of new energy sources is paramount before effectively controlling them and ensuring the generation of active and reactive power as required. This is a prerequisite for ensuring stable system operation. Figure 1 Neglecting system resistance, the q-axis voltage at the grid connection point of the wind farm's grid-side converter GSC can be obtained as follows:

[0056]

[0057] Where ω is the system angular frequency, L f L1 is the grounding reactance at the fault point, L2 is the reactance from the wind farm to the fault point, and L3 is the reactance from the fault point to the MMC. mcmax i is the maximum value of the MMC sending terminal voltage. wq This represents the q-axis component of the wind farm current.

[0058] To ensure the existence of a steady-state operating point in the system, it is also necessary to ensure u wsq=0 has a solution. It needs to satisfy:

[0059]

[0060] Based on the relationship between the dq coordinate system and the MMC coordinate system of the new energy grid-side converter, we can know that:

[0061]

[0062] To unify to the GSC synchronized coordinate system, the following must be satisfied:

[0063]

[0064] Therefore, the condition for the existence of the steady-state operating point is that the relationship shown in equation (5) must be satisfied. It can be seen that the existence of the 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] Unifying equation (1) to the dq coordinate system of the new energy grid-side converter, the q-axis voltage of the new energy can be obtained as:

[0066]

[0067] in,

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

[0069] Using the same method, the equations of motion with respect to work angle can be obtained:

[0070]

[0071] In the formula, K pPLL K is the proportional parameter of the phase-locked loop. iPLL Here are the phase-locked loop integral parameters, ω0 is the system's rated angular frequency, and δ is the power angle of the wind farm and the MMC sending end. The initial phase of voltage and current. Let δ be the first derivative. It is the second derivative of δ.

[0072] The above equations were solved using MATLAB software, and the impact of current on the transient stability of the system was analyzed using the phase diagram method. The analysis results indicate that reducing the d-axis current of the renewable energy source and increasing the maximum value of the MMC output current are beneficial to improving transient stability. Based on this analysis, the corresponding reduction in the d-axis current amplitude and the setting value for increasing the d-axis voltage limit at the MMC transmitter were determined within the synchronous stability region of the phase-locked loop. In this embodiment, the synchronous stability region for the amplitude range of the wind farm d-axis current and the MMC transmitter current is set to 0.13 < i. wd <0.2, 2<I mmax <5, within this range, reduce the d-axis current of the wind farm and increase the limit value of the d-axis voltage at the MMC transmitter to the set value. Specifically, increase the limit value of the d-axis voltage at the MMC transmitter to 1.5pu.

[0073] like Figure 4 As shown, the larger the AC voltage that the MMC can output, the faster the phase diagram curve converges, and the easier it is for the system to achieve synchronous stability. To obtain the steady-state operating point, it is necessary to reduce the d-axis current of the wind farm and increase the amplitude of the MMC's sending-end current. Figure 3 The larger the power angle adjustment range b to c shown, 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 δ0 ranges from 0 to π / 2, similarly, to ensure that point a does not exceed point c, the following must be satisfied:

[0075]

[0076] exist If δ0 is large, it may cause the starting point a to exceed point c, leading to system imbalance, such as... Figure 5 As shown in (a). When At times, such as Figure 5 As shown in (b). Add The magnitude of this value will lengthen the acceleration phase of the power angle in segment a to b, which is detrimental to system stability. Therefore, selecting... That is, in full modulation mode, according to u cd =u mcmax u cq The dq axis voltages are allocated according to the principle of 0.

[0077] To obtain the steady-state operating point, it is necessary to reduce the d-axis current of the wind farm and increase the limiting value of the d-axis voltage output by the MMC transmitter to the set value. Combining the synchronous stability domain of the d-axis current of the wind farm and the amplitude range of the MMC transmitter current, in this embodiment, the d-axis current is reduced by 0.2 pu, and the limiting value of the MMC output d-axis voltage is maintained at its maximum value of 1.5 pu.

[0078] (2) Improve the voltage strategy of the wind farm grid connection point during the fault. On the basis of ensuring system synchronization and stability, in order to support the voltage of the new energy generator after the fault, it is necessary to reduce the active current of the new energy during the fault and increase the output of reactive current.

[0079] When the wind turbine is connected to the MMC, the voltage drop after a fault is more severe because the MMC has limited short-circuit current output capability. Therefore, it is necessary to utilize the limited short-circuit current to obtain the maximum voltage support. According to... Figure 2 The voltage at the grid connection point of the new energy source can be calculated as follows:

[0080]

[0081] Where ω is the system angular frequency, L f L1 is the grounding reactance at the fault point, L2 is the reactance from the wind farm to the fault point, and L3 is the reactance from the fault point to the MMC. mcmax i is the maximum value of the MMC sending terminal voltage. wq This represents the q-axis component of the wind farm current.

[0082] The above equation clearly shows the relationship between the grid connection point voltage, MMC current, and the current of the wind farm-side converter. The smaller the amplitude of the MMC output current after a fault occurs, the larger the obtained GSC grid connection point voltage, indicating that the larger the reactive power output of the GSC, the higher the voltage at the renewable energy generator terminal. Furthermore, the initial phase of the MMC voltage, i.e., the distribution principle of the dq-axis voltage, does not affect the magnitude of the renewable energy generator terminal voltage. The magnitude of the renewable energy generator terminal voltage is only affected by the amplitude of the MMC output voltage. A higher renewable energy grid connection point voltage is more conducive to the renewable energy achieving low-voltage fault ride-through. The corresponding grid connection point voltage u within the synchronous stability domain of the phase-locked loop is determined based on the equivalent circuit diagram of the wind power transmission system via flexible direct current. ws Thus, the q-axis component i of the wind farm current is determined. wq In addition, the q-axis current amplitude is increased; in this embodiment, the q-axis current amplitude is increased by 0.2 pu.

[0083] Optionally, detecting the voltage at the wind farm's grid connection point to determine whether the fault has been cleared includes:

[0084] The lowest voltage detected during a grid connection point fault at the wind farm is denoted as u. wmin And obtain the grid connection point voltage u in real time. w ;

[0085] Determine the grid connection point voltage u w Is it greater than u? wmin +0.1pu; if yes, then the fault is cleared and the time is recorded as t2; if no, then the fault is not cleared.

[0086] After the fault is cleared, the fault recovery period begins. During this process, transient overvoltages may occur. In order to ensure line safety and suppress transient overvoltages, the d-axis current of the wind farm is increased to restore the voltage at the MMC sending end to the steady-state value before the fault during the fault recovery process.

[0087] The principle behind transient overvoltage during fault recovery is as follows: During fault recovery, during a short-circuit fault, the MMC outputs the d-axis current i sd Controlled to the minimum value limited by the limiter -I sdmax After the fault is cleared, the external constraints of the converter station revert to current constraints, and the actual current flowing into the converter station equals the current generated by the new energy source. Therefore, the reference current i output by the outer loop control at the instant the fault is cleared... sdref sd Furthermore, 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 It will be greater than the steady-state voltage, but due to the presence of the limiter, u mcd The maximum is the limit value u of the limiter. cdmax This will lead to u s Greater than the reference voltage (1.0 pu).

[0088] (3) Suppression strategy for transient overvoltage after fault clearance: During the fault recovery phase, a suppression strategy is needed to suppress transient overvoltage. The overvoltage during the recovery process of a short-circuit fault on the AC side of 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 output voltage u of the converter station during the transient process after fault clearance. cdmax Related. The main approach is to optimize the converter station control system, thereby suppressing overvoltage by reducing the maximum output voltage of the converter station.

[0089] After the circuit breaker trips and clears the fault, the wind turbine power begins to recover. According to the control logic, the mathematical model of the MMC inner loop current controller PI control module is:

[0090]

[0091] Among them, u dPI and u qPI These are the output quantities of the d-axis and q-axis PI control loops of the inner loop current controller, respectively.

[0092] During a short-circuit fault, the MMC outputs the d-axis current i sd Controlled to the minimum value limited by the limiter -I​sdmax After the fault is cleared, the external constraints of the converter station revert to current constraints, and the actual current flowing into the converter station equals the current generated by the new energy source. Therefore, the reference current i output by the outer loop control at the instant the fault is cleared... sdref sd Furthermore, 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 the "pressure area". The MMC output dq-axis voltage is

[0093]

[0094] As can be seen from the above formula, due to the deviation between the reference current and the actual current, the reference voltage u generated by the MMC control system... mcd It will be greater than the steady-state voltage, but due to the presence of the limiter, u mcd The maximum limiting value u of the limiter cdmax This will lead to u s Greater than the reference voltage (1.0 pu), such as Figure 6 As shown in (b).

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

[0096]

[0097] Among them, u sd i is the d-axis voltage of the MMC. d Let i be the d-axis current. q Let u be the q-axis current. cd The maximum d-axis voltage that can be output by the MMC terminal is L. eq R is the equivalent inductance of the transformer and the bridge arm. eq The equivalent resistance of the transformer and bridge arm; during fault recovery, increase i. d To restore the active power output of new energy sources.

[0098] From the above equation, we can see the rate of change of the d-axis current and the maximum value of the d-axis voltage output by the MMC, u. cdmax This directly affects the magnitude of the transient overvoltage at the PCC point. This voltage will cause the GSC grid connection point voltage to rise, at which point the converter station PCC point voltage will exceed the steady-state value, such as... Figure 6 ​As shown in (c). This transient overvoltage also poses a risk to the safe operation of new energy sources, requiring measures to suppress it. The overvoltage mechanism in full modulation mode is the same as in current-limiting mode; however, because the "voltage boosting area" is significantly larger, the overvoltage duration is longer.

[0099] To ensure that overvoltage can be successfully suppressed during fault recovery, two MMC sending-end control strategies are proposed:

[0100] Strategy 1: At the moment the fault is cleared (t2), reduce the AC voltage reference value u of the sending-end converter station. sref At this time, the PCC point voltage established by the converter station will track a smaller reference voltage, thereby reducing u sd Maximum value;

[0101] Strategy 2: At the moment t2 when the fault is cleared, reset the integrators of the PI control modules of the outer loop voltage controller and inner loop current controller of the d-axis of the sending-end converter station to zero until the wind power is restored. This reduces the "pressure area," quickly enables the reference current to track the actual current, and lowers V. dref Maximum value.

[0102] Specifically, the following implementation methods are included:

[0103] (1) At the moment t2 when the fault is cleared, the voltage command value of the MMC sending end is reduced by step-down method. Specifically, the voltage command value of the MMC sending end converter station is reduced to 0.8pu by step-down method. After the suppression strategy is maintained for a preset time, the voltage command value of the MMC sending end is restored to 1pu at a rate of 2p.u. / s.

[0104] (2) Starting from the moment t2 when the fault is cleared, the output of the inner and outer loop integral controllers on the d-axis of the MMC side is zeroed using the zeroing method. After the suppression strategy is maintained for a preset time, the zeroing operation of the integral controller is canceled.

[0105] In both of the above suppression strategies, the preset duration for maintaining the suppression strategy is 100ms.

[0106] During the fault recovery phase, the above methods are used to suppress transient overvoltages. Simultaneously, the d-axis current of the wind farm is slowly increased until the voltage at the MMC sending end returns to its steady-state value, and i is increased. d To restore the active power output of new energy sources while simultaneously limiting the impact of this process on u sd The effect of overvoltage, specifically, limits the rate of rise of the d-axis current, k = di. d / dt, increases the d-axis current of the wind farm to restore the voltage at the MMC sending end to the steady-state value 1p.u., where the value of k ranges from 0.2 to 0.6.

[0107] This invention reduces the q-axis current i of the new energy source during a fault. wq This method increases the grid connection voltage of the wind farm, thereby solving the low-voltage ride-through problem during faults and ensuring that the wind turbines can operate without disconnecting from the grid for a specified period. During faults, it maintains the d-axis voltage output from the MMC (Multi-Channel Controller) at a set value, improving the transient stability of the wind farm and the MMC, and enhancing the synchronization stability of the phase-locked loop (PLL). It addresses the technical challenges of high cost and complex fault ride-through control when shallow faults occur at the sending end in wind power transmission via flexible direct current (FTDC). It achieves the beneficial effects of improving PLL synchronization stability and enhancing system transient stability during fault ride-through.

[0108] Example 2

[0109] This invention also provides a coordinated control device for shallow faults at the wind power transmission end, which includes the following when a shallow fault occurs at the transmission end in a scenario where wind power is transmitted via flexible direct current:

[0110] The fault ride-through module is used to increase the q-axis current amplitude of the wind farm to improve the voltage at the grid connection point; and to decrease the d-axis current amplitude of the wind farm and increase the limit value of the d-axis voltage at the MMC sending end to the set value, so as 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 diagnosis module is used to detect the voltage recovery status at the grid connection point of the wind farm and determine whether the fault has been cleared. If yes, the overvoltage suppression module is executed; if no, the process returns to execute the fault ride-through module.

[0112] The overvoltage suppression module is used to suppress transient overvoltages during fault recovery by employing suppression strategies, thereby increasing the d-axis current of the wind farm until the voltage at the MMC sending end recovers to a steady-state value.

[0113] Example 3

[0114] The present invention also provides a computer-readable storage medium storing computer instructions for causing a processor to execute a coordinated control method for shallow faults at the wind power transmission end as described in any one of Embodiments 1.

[0115] Those skilled in the art will readily understand that the above description is merely 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 within the scope of protection of the present invention.

Claims

1. A coordinated control method for shallow faults at the transmission end of a wind power plant, characterized in that, include: When a shallow fault occurs at the sending end in a scenario where wind power is transmitted via flexible direct current, the following steps should be performed: S1. Increase the q-axis current amplitude of the wind farm to improve the voltage at the grid connection point; decrease the d-axis current amplitude of the wind farm and increase the limiting value of the d-axis voltage 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 at the grid connection point of the wind farm to determine if the fault has been cleared; if so, proceed to S3. If not, return to execute S1; S3. During the fault recovery period, a suppression strategy is adopted to suppress transient overvoltage and increase the d-axis current of the wind farm until the voltage at the MMC sending end recovers to the steady-state value. The increase in the q-axis current amplitude of the wind farm includes: Based on the equivalent circuit diagram of the wind power transmission system via flexible direct current, determine the corresponding grid connection point voltage within the synchronous stability region of the phase-locked loop. u ws ; according to Calculate the q-axis component of the wind farm current i wq And composed of the q-axis component of the wind farm current. i wq The magnitude of the q-axis current that raises the wind farm is determined by the current q-axis component; where, ω The system angular frequency, L f For the grounding reactance at the fault point, L 1 represents the reactance from the wind farm to the fault point. L 2 represents the reactor from the fault point to the MMC. u mcmax This represents the maximum value of the MMC sending terminal voltage. i wq For the q-axis component of the wind farm current; The reduction of the d-axis current amplitude of the wind farm and the increase of the limiting value of the d-axis voltage at the MMC sending end to the set value include: Obtain the q-axis voltage at the grid connection point of the wind farm u wq ,by u wq = 0 is the stable boundary condition, and the work angle state equation is constructed as follows: In the formula, K pPLL These are the proportional parameters of the phase-locked loop. K iPLL These are the integral parameters of the phase-locked loop. ω 0 represents the system's rated angular frequency. For the power angle of wind farm and MMC sending end, φ 1 represents the initial phase of the voltage and current. for The first derivative, for The second derivative, i wd The d-axis component of the wind farm current; The phase diagram method is used to perform transient stability analysis on the power angle state equation. Based on the transient stability analysis results, the corresponding reduction of the d-axis current amplitude and the limit value of increasing the d-axis voltage at the MMC sending end are determined within the synchronous stability domain of the phase-locked loop.

2. The method as described in claim 1, characterized in that, The process of detecting the voltage at the grid connection point of the wind farm to determine whether the fault has been cleared includes: The lowest voltage detected during a grid connection point fault at the wind farm is denoted as... u wmin And obtain the grid connection point voltage in real time. u w ; Determine the voltage at the grid connection point u w Is it greater than u wmin +0.1pu; if so, then the fault is cleared, and this moment is recorded as... t 2; If not, then the fault has not been cleared.

3. The method as described in claim 2, characterized in that, The method for suppressing transient overvoltages during fault recovery includes: At the moment of fault clearing t 2. The voltage command value of the MMC sending end is reduced to 0.8 pu using the step-down method. After the suppression strategy is maintained for a preset time, the voltage command value of the MMC sending end is restored to 1 pu at a rate of 2 p.u. / s.

4. The method as described in claim 2, characterized in that, The method for suppressing transient overvoltages during fault recovery includes: At the moment of fault clearing t Starting from point 2, the zeroing method is used to zero the output of the inner and outer loop integral controllers on the d-axis of the MMC side. After maintaining this suppression strategy for a preset time, the zeroing operation of the integral controller is canceled.

5. The method as described in claim 3 or 4, characterized in that, The process 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 moment of fault clearing t Starting from point 2, the rate of rise of the d-axis current is limited to k=d i d / d t The d-axis current of the wind farm is increased to restore the voltage at the MMC sending end to a steady-state value; where the value of k ranges from 0.2 to 0.

6.

6. A coordinated control device for shallow faults at the transmission end of a wind power plant, characterized in that, When a shallow fault occurs at the sending end in a scenario where wind power is transmitted 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 improve the voltage at the grid connection point; and to decrease the d-axis current amplitude of the wind farm and increase the limit value of the d-axis voltage at the MMC sending end to the set value, so as 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 diagnosis module is used to detect the voltage at the grid connection point of the wind farm and determine whether the fault has been cleared. If yes, the overvoltage suppression module is executed; if not, the process returns to execute the fault ride-through module. The overvoltage suppression module is used to suppress transient overvoltages during fault recovery, thereby increasing the d-axis current of the wind farm until the voltage at the MMC sending end recovers to its steady-state value. The increase in the q-axis current amplitude of the wind farm includes: Based on the equivalent circuit diagram of the wind power transmission system via flexible direct current, determine the corresponding grid connection point voltage within the synchronous stability region of the phase-locked loop. u ws ; according to Calculate the q-axis component of the wind farm current i wq And composed of the q-axis component of the wind farm current. i wq The magnitude of the q-axis current that raises the wind farm is determined by the current q-axis component; where, ω The system angular frequency, L f For the grounding reactance at the fault point, L 1 represents the reactance from the wind farm to the fault point. L 2 represents the reactor from the fault point to the MMC. u mcmax This represents the maximum value of the MMC sending terminal voltage. i wq For the q-axis component of the wind farm current; The reduction of the d-axis current amplitude of the wind farm and the increase of the limiting value of the d-axis voltage at the MMC sending end to the set value include: Obtain the q-axis voltage at the grid connection point of the wind farm u wq ,by u wq = 0 is the stable boundary condition, and the work angle state equation is constructed as follows: In the formula, K pPLL These are the proportional parameters of the phase-locked loop. K iPLL These are the integral parameters of the phase-locked loop. ω 0 represents the system's rated angular frequency. For the power angle of wind farm and MMC sending end, φ 1 represents the initial phase of the voltage and current. for The first derivative, for The second derivative, i wd The d-axis component of the wind farm current; The phase diagram method is used to perform transient stability analysis on the power angle state equation. Based on the transient stability analysis results, the corresponding reduction of the d-axis current amplitude and the limit value of increasing the d-axis voltage at the MMC sending end are determined within the synchronous stability domain of the phase-locked loop.

7. A computer-readable storage medium, characterized in that, The computer-readable storage medium stores computer instructions that, when executed by a processor, implement the coordinated control method for shallow faults at the wind power transmission end as described in any one of claims 1-5.

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