Method for assisting fault ride-through of permanent magnet synchronous wind turbine generator
By using a fault current limiter with adjustable resistance in the permanent magnet synchronous wind turbine, the resistance size is controlled to balance the input and output active power of the DC bus, the problem of unstable DC bus voltage during the fault crossing of the permanent magnet synchronous wind turbine is solved, and the voltage stability and dynamic balance are achieved, ensuring the normal operation of the unit.
Patent Information
- Application Number
- CN202510501005.0
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-04-21
- Publication Date
- 2025-07-25
AI Technical Summary
In the prior art, the DC bus voltage stability of the permanent magnet synchronous wind turbine is difficult to maintain during the fault crossing, resulting in offset and oscillation of the DC bus voltage of the back-to-back converter. The existing unloading circuit cannot achieve dynamic balance between the DC bus input and output active power.
The fault current limiter with adjustable resistance is connected in parallel with the DC bus capacitor of the back-to-back converter, and its input and cut-out are controlled through a series on-off switch. Based on the active power difference between the machine side and the grid side converter, the resistance of the fault current limiter is regulated to maintain the stability of the DC bus voltage.
Dynamic balance of DC bus voltage is achieved, periodic oscillation of voltage is avoided, stability during fault travel is improved, and the normal operation of the permanent magnet synchronous wind turbine is ensured.
Smart Images
Figure CN120377353A_ABST
Abstract
Description
Technical Field
[0001] The present invention relates to the technical field of wind power generation control, and in particular to a method for assisting a permanent magnet synchronous wind turbine to ride through faults. Background Technique
[0002] A grid-connected permanent magnet synchronous wind turbine needs to maintain operation for a certain period of time during a grid fault to support grid stability, that is, it has the fault ride-through ability specified in the wind turbine grid connection guidelines. The permanent magnet synchronous wind turbine is connected to the grid through a full-power back-to-back converter. The back-to-back converter can decouple the permanent magnet synchronous generator from the grid fault to a certain extent during the fault ride-through. However, during the fault ride-through, the power output capacity of the grid-side converter is limited due to the grid voltage dip, and the active power imbalance between the machine-side converter and the grid-side converter will cause the DC bus voltage of the back-to-back converter to shift. Therefore, the key to the successful fault ride-through of the permanent magnet synchronous wind turbine lies in maintaining the stability of the DC bus voltage of the back-to-back converter during the fault ride-through.
[0003] Regarding the DC voltage stabilization control problem during the fault ride-through of a permanent magnet synchronous wind turbine, the existing mainstream technical solution is to connect a discharge circuit in parallel at both ends of the DC capacitor of the back-to-back converter. During the fault ride-through, when the DC bus voltage exceeds its upper allowable value, the discharge circuit is switched on, and the electrical energy stored in the DC bus is dissipated through the discharge circuit, and the DC bus voltage gradually decreases; when the DC bus voltage drops to its lower allowable value, the discharge circuit is switched off. The main shortcoming of the existing discharge circuit technical solution is that its discharge circuit uses a fixed resistance value, so it cannot actively balance the input and output active power of the DC bus during the switching-on process. The discharge circuit will be switched on and off repeatedly during the fault ride-through process, resulting in periodic oscillation of the DC bus voltage. Therefore, a method for assisting a permanent magnet synchronous wind turbine to ride through faults is proposed. Summary of the Invention
[0004] The purpose of the present invention is to solve the problems in the prior art, and a method for assisting a permanent magnet synchronous wind turbine to ride through faults is proposed.
[0005] A method for assisting a permanent magnet synchronous wind turbine to ride through faults includes the following steps:
[0006] S1. Install a fault current limiter with adjustable resistance value at the back-to-back converter of the permanent magnet synchronous wind turbine. The fault current limiter is connected in parallel with the DC bus capacitor of the back-to-back converter. The fault current limiter is controlled to be switched on and off through a series-connected switch. Under the normal operating condition of the permanent magnet synchronous wind turbine, the switch is turned off and the fault current limiter is switched off;
[0007] S2. After the permanent magnet synchronous wind turbine enters the fault ride-through state, detect the active power input of the machine-side converter and the active power output of the grid-side converter. When the difference between the two is greater than the set threshold criterion for the fault current limiter to be put into operation, close the on-off switch of the fault current limiter, and based on the difference between the two, adjust the resistance value of the fault current limiter to maintain the stability of the DC bus voltage during the fault ride-through period;
[0008] S3. After the permanent magnet synchronous wind turbine exits the fault ride-through state, continue to adjust the resistance value of the fault current limiter based on the difference between the active power input of the machine-side converter and the active power output of the grid-side converter. When the difference between the two is less than the set threshold criterion for the fault current limiter to cut out, open the on-off switch of the fault current limiter, and the fault current limiter exits operation. Based on the active power consumed by the fault current limiter at the moment of exiting operation, adjust the reference value of the active current control of the grid-side converter so that the additional active power of the grid-side converter is equal to the active power consumed by the fault current limiter at the moment of exiting operation, avoiding the impact on the DC bus voltage caused by the exit of the fault current limiter.
[0009] Preferably, step S1 includes the following steps:
[0010] The resistance of the fault current limiter is adjusted by regulating the duty cycle of the on-off signal applied to the switch of the switch branch. The resistance value of the fault current limiter is calculated based on Equation (1):
[0011]
[0012] In the formula, d is the duty cycle of the on-off signal applied to the switch of the switch branch, is the maximum resistance value that the fault current limiter can be adjusted to reach.
[0013] Preferably, step S2 includes the following steps:
[0014] The upper allowable value of the DC bus voltage of the back-to-back converter during the operation of the permanent magnet synchronous wind turbine is The rated value of the DC bus voltage is On the premise of assuming that the fault current limiter is not put into operation during the fault ride-through period, based on Equation (2), determine the upper allowable value ΔP of the difference ΔP between the active power input of the machine-side converter and the active power output of the grid-side converter upper , and set it as the threshold criterion for the fault current limiter to be put into operation:
[0015]
[0016] In the formula, C is the size of the DC bus capacitor, and t is the duration of the fault ride-through;
[0017] When the difference ΔP between the active power input of the machine-side converter and the active power output of the grid-side converter exceeds the upper limit allowable value ΔP upper After that, the on-off switch of the fault current limiter is closed, and the fault current limiter is put into operation.
[0018] Preferably, the step S2 includes the following steps:
[0019] During the fault ride-through period, after the fault current limiter is put into operation, the resistance value R of the fault current limiter FCL is regulated. The inputs of the resistance value regulation system of the fault current limiter include the difference ΔP between the active power input of the machine-side converter and the active power output of the grid-side converter, and the DC bus voltage V of the back-to-back converter DC , and the regulation system is shown in Equation (3):
[0020]
[0021] In the formula, k p and k i are respectively the gain coefficient of the proportional link and the gain coefficient of the integral link in the resistance value regulation system of the fault current limiter. Among them, the proportional link is used for the fast response regulation of the large deviation of the DC bus voltage, and only takes effect under the condition that the deviation of the DC bus voltage is greater than 20% of the maximum allowable deviation of the DC bus voltage. The integral link is used for the correction of the small deviation of the DC bus voltage, and only takes effect under the condition that the deviation of the DC bus voltage is not greater than 20% of the maximum allowable deviation of the DC bus voltage.
[0022] Preferably, the method for determining the adjustable range of the resistance value regulation system of the fault current limiter is as follows:
[0023] The resistance value regulation system of the fault current limiter needs to set the lower limit of the resistance value during the fault ride-through process The setting idea is: when the fault current limiter reaches the lower limit of this resistance value, the difference between the active power input of the machine-side converter and the active power output of the grid-side converter reaches its lower limit allowable value ΔP lower , as shown in Equation (4):
[0024]
[0025] Solve Equation (4) to obtain the lower limit of the resistance value of the fault current limiter, as shown in Equation (5):
[0026]
[0027] The resistance value regulation system sets the upper limit of the resistance value for the fault current limiter The setting idea is: when the fault current limiter reaches the set upper limit of the resistance value When the active power consumed by the fault current limiter is exactly equal to the upper limit allowable value ΔP of the difference ΔP between the active power input of the machine-side converter and the active power output of the grid-side converter upper , as shown in Equation (6):
[0028]
[0029] Solving Equation (6) gives the upper limit of the fault current limiter resistance value, as shown in Equation (7):
[0030]
[0031] The selection of the fault current limiter needs to refer to the upper limit of the fault current limiter resistance value, that is, the maximum resistance value that the fault current limiter can be adjusted to reach should not be less than the upper limit of the resistance value
[0032] Preferably, the step S3 includes the following steps:
[0033] After the permanent magnet synchronous wind turbine exits the fault ride-through state, the cut-out threshold criterion ΔP adopted by the fault current limiter min , as shown in Equation (8):
[0034]
[0035] When the difference between the active power input of the machine-side converter and the active power output of the grid-side converter is less than the cut-out threshold criterion ΔP min , the on-off switch of the fault current limiter is turned off, the fault current limiter is cut out, and at the same time, an increment is applied to the active current control reference value of the grid-side converter to make the active power increase of the grid-side converter equal to the active power consumed by the fault current limiter at the cut-out moment, and the increment of the active current control reference value of the grid-side converter is calculated based on Equation (9):
[0036]
[0037] where V g is the modulus of the grid-side converter voltage
[0038] Compared with the existing technology, the advantages of the present invention are as follows:
[0039] 1. Compared with the existing DC bus voltage stabilization control scheme based on a fixed resistance value unloading circuit, by adopting a fault current limiter with an adjustable resistance value, the regulation of the active power dissipated on the resistance can be realized, and then the dynamic balance between the active power input and output of the DC bus capacitor can be realized, so that the DC bus voltage approaches its control reference value, and the stability of the DC bus voltage during the fault ride-through period is improved
[0040] 2. The method for regulating the resistance value of the fault current limiter designed by the present invention can set the upper and lower limits of the resistance value of the fault current limiter based on the current fault ride-through scenario, avoiding the occurrence of a rapid drop in the DC bus capacitor voltage caused by too small a resistance value. BRIEF DESCRIPTION OF THE DRAWINGS
[0041] Figure 1 It is the flowchart of the method of the present invention.
[0042] Figure 2 It is the topological structure diagram of the permanent magnet synchronous wind turbine generator set with a fault current limiter installed at the DC bus of the present invention.
[0043] Figure 3 It is the curve graph of the difference between the active power input of the machine-side converter and the active power output of the grid-side converter in the present invention.
[0044] Figure 4 It is the control block diagram of the fault current limiter resistance value regulation system in the present invention.
[0045] Figure 5 It is the curve graph of the resistance value of the fault current limiter and its upper and lower limits in the present invention.
[0046] Figure 6 It is the curve graph of the DC bus voltage of the back-to-back converter of the permanent magnet synchronous wind turbine generator set in the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS
[0047] In order to make the technical means, creative features, achieved purposes and effects of the present invention easy to understand, the present invention will be further described below in conjunction with specific embodiments.
[0048] Refer to Figure 1 As shown, a method for assisting a permanent magnet synchronous wind turbine generator set to ride through a fault includes the following steps:
[0049] S1: Install a fault current limiter with adjustable resistance at the back-to-back converter of the permanent magnet synchronous wind turbine generator set. The fault current limiter is connected in parallel with the DC bus capacitor of the back-to-back converter. The fault current limiter is controlled to be switched on and off through a series-connected switch. Under the normal operating condition of the permanent magnet synchronous wind turbine generator set, the switch is turned off and the fault current limiter is switched out.
[0050] Refer to Figure 2 , the fault current limiter can be controlled to be switched on or off through the switch. After the fault current limiter is switched on, the electric energy stored in the DC bus capacitor can be dissipated through the resistance of the fault current limiter to avoid the DC bus voltage exceeding the upper limit of its safety constraint.
[0051] The resistance of the fault current limiter is regulated by adjusting the duty cycle of the on-off signal applied to the switch in the switching branch. The resistance value R of the fault current limiter FCL is calculated based on Equation (1):
[0052]
[0053] where d is the duty cycle of the on-off signal applied to the switch in the switching branch, and is the maximum resistance value that the fault current limiter can be regulated to reach.
[0054] S2: After the permanent magnet synchronous wind turbine generator set enters the fault ride-through state, detect the active power input of the machine-side converter and the active power output of the grid-side converter. When the difference between the two is greater than the set threshold criterion for the fault current limiter to be put into operation, close the on-off switch of the fault current limiter, and regulate the resistance of the fault current limiter based on the difference between the two to maintain the stability of the DC bus voltage during the fault ride-through.
[0055] The upper allowable value of the DC bus voltage V of the back-to-back converter during the operation of the permanent magnet synchronous wind turbine generator set DC is The rated value of the DC bus voltage is On the premise of the assumption that the fault current limiter is not put into operation during the fault ride-through, determine the upper allowable value ΔP of the difference ΔP between the active power input of the machine-side converter and the active power output of the grid-side converter based on Equation (2) upper , and set it as the threshold criterion for the fault current limiter to be put into operation:
[0056]
[0057] where C is the size of the DC bus capacitor and t is the duration of the fault ride-through;
[0058] When the difference ΔP between the active power input of the machine-side converter and the active power output of the grid-side converter exceeds the upper allowable value ΔP upper , close the on-off switch of the fault current limiter, and the fault current limiter is put into operation.
[0059] Refer to Figure 4 As shown, during the fault ride-through, after the fault current limiter is put into operation, regulate the resistance value R of the fault current limiter FCL . The inputs of the resistance regulation system of the fault current limiter include the difference ΔP between the active power input of the machine-side converter and the active power output of the grid-side converter, and the DC bus voltage V of the back-to-back converter DC , and the regulation system is as shown in Equation (3):
[0060]
[0061] where k p and k i are respectively the gain coefficient of the proportional link and the gain coefficient of the integral link in the resistance value regulation system of the fault current limiter, where the proportional link is used for the fast response regulation of the large-scale deviation of the DC bus voltage, and it only takes effect under the condition that the deviation of the DC bus voltage is greater than 20% of the maximum allowable deviation of the DC bus voltage (i.e., ); the integral link is used for the correction of the small-scale deviation of the DC bus voltage, and it only takes effect under the condition that the deviation of the DC bus voltage is not greater than 20% of the maximum allowable deviation of the DC bus voltage (i.e., ).
[0062] To avoid the situation that the rapid reduction of the resistance value of the fault current limiter causes the rapid release of the energy of the DC bus capacitor, and then leads to the reduction of the DC bus voltage below the rated value, the resistance value regulation system of the fault current limiter needs to set a lower limit of the resistance value The setting idea is as follows: when the fault current limiter reaches the lower limit of the resistance value, the difference ΔP between the active power input of the machine-side converter and the active power output of the grid-side converter reaches its lower limit allowable value ΔP lower (under the corresponding working condition of the lower limit allowable value ΔP lower , the DC bus voltage just drops to the lower limit allowable value of the DC bus voltage ) at the end of the fault ride-through process, as shown in Equation (4):
[0063]
[0064] Solving Equation (4) gives the lower limit of the resistance value of the fault current limiter, as shown in Equation (5):
[0065]
[0066] The resistance value regulation system sets an upper limit of the resistance value for the fault current limiter The setting idea is as follows: when the fault current limiter reaches the upper limit of the set resistance value , the active power consumed by the fault current limiter is exactly equal to the upper limit allowable value ΔP of the difference ΔP between the active power input of the machine-side converter and the active power output of the grid-side converter upper , as shown in Equation (6):
[0067]
[0068] Solving Equation (6) gives the upper limit of the resistance value of the fault current limiter, as shown in Equation (7):
[0069]
[0070] The selection of the fault current limiter needs to refer to the upper limit of the resistance value of the fault current limiter, that is, the maximum resistance value that the fault current limiter can be adjusted to reach. It should not be less than the upper limit of the resistance value.
[0071] S3: After the permanent magnet synchronous wind turbine exits the fault ride-through state, continue to adjust the resistance value of the fault current limiter based on the difference between the active power input of the machine-side converter and the active power output of the grid-side converter. When the difference between the two is less than the set cut-out threshold criterion of the fault current limiter, disconnect the on-off switch of the fault current limiter, and the fault current limiter exits the operation. Based on the active power consumed by the fault current limiter at the moment of exiting the operation, adjust the reference value of the active current control of the grid-side converter so that the additional active power of the grid-side converter is equal to the active power consumed by the fault current limiter at the moment of exiting the operation, and avoid the impact on the DC bus voltage caused by the exit of the fault current limiter.
[0072] After the permanent magnet synchronous wind turbine exits the fault ride-through state, the cut-out threshold criterion ΔP adopted by the fault current limiter min is as shown in Equation (8):
[0073]
[0074] When the difference between the active power input of the machine-side converter and the active power output of the grid-side converter is less than the cut-out threshold criterion ΔP min , the on-off switch of the fault current limiter is turned off, the fault current limiter is cut out, and at the same time, an increment is applied to the reference value of the active current control of the grid-side converter so that the additional active power of the grid-side converter is equal to the active power consumed by the fault current limiter at the cut-out moment. The increment of the reference value of the active current control of the grid-side converter is calculated based on Equation (9):
[0075]
[0076] where V g is the modulus of the voltage of the grid-side converter.
[0077] Embodiment
[0078] Set the maximum resistance value that the fault current limiter can be adjusted to reach as 8 pu. After the fault current limiter is put into operation, its resistance value can be adjusted in the range of 0 pu to 8 pu. The size of the DC bus capacitor of the back-to-back converter is 0.2 pu. The fault ride-through starts at 0.2 s and lasts for 0.5 s. The rated value of the DC bus voltage is 1 pu, and the upper allowable value of the DC bus voltage is 1.2 pu. The upper allowable value ΔP of the difference between the active power input of the machine-side converter and the active power output of the grid-side converter calculated based on Equation (2)upper is 0.176 pu.
[0079] As Figure 3 shown, the permanent magnet synchronous wind turbine enters the fault ride-through state at 0.2 s. The difference between the active power input of the machine-side converter and the active power output of the grid-side converter gradually increases from 0 pu to 0.2 pu. When it exceeds its upper limit allowable value of 0.176 pu at 0.285 s, the fault current limiter is put into operation.
[0080] As Figure 5 shown, at the initial moment (0.285 s) when the fault current limiter is put into operation, its resistance value is 5.5 pu. At this resistance value, the active power consumed by the fault current limiter is exactly equal to the difference between the input power of the machine-side converter and the output power of the grid-side converter at this moment; as Figure 6 shown, at the moment when the fault current limiter is put into operation (0.285 s), the DC bus voltage has risen to 1.048 pu. At this time, the proportional link in the fault current limiter resistance value regulation system takes effect (the gain coefficient of the proportional link in the embodiment of the present invention is set to 30), and the resistance value of the fault current limiter is further reduced to increase the active power consumed by the fault current limiter, so that the DC bus voltage stops rising and begins to fall. Figure 6 In, the DC bus voltage drops to 1.04 pu at 0.405 s. At this time, the integral link in the fault current limiter resistance value regulation system takes effect (the gain coefficient of the integral link in the embodiment of the present invention is set to 30000). Due to the relatively large gain coefficient value of the integral link, the output of the fault current limiter resistance value regulation system is limited to its lower limit allowable value in the time period of 0.405 s - 0.463 s (refer to Figure 5 ), avoiding the occurrence of a rapid drop in the DC bus voltage caused by too rapid a decrease in the resistance value of the fault current limiter. As can be seen from Figure 6 , while achieving a rapid drop of the DC bus voltage to its rated value, the DC bus voltage is prevented from dropping to its lower limit allowable value (0.9 pu).
[0081] As Figure 3 shown, when the fault ride-through process ends at 0.7 s, the difference between the active power input of the machine-side converter and the active power output of the grid-side converter gradually decreases. At this time, the resistance value of the fault current limiter gradually increases to reduce its active power loss; at 0.772 s, the difference between the active power input of the machine-side converter and the active power output of the grid-side converter is less than the cut-out threshold criterion, and the fault current limiter exits operation (as Figure 5 shown). At the same time, the increment of the active current control reference value of the grid-side converter calculated based on Equation (9) is 0.138 pu; as can be seen from Figure 6It can be seen that by instantaneously increasing the active power output of the grid-side converter at the cut-off moment (0.772 s) of the fault current limiter, the DC bus voltage does not continue to rise after the cut-off of the fault current limiter, but gradually drops back to its rated value under the control of the DC bus voltage of the grid-side converter. From Figure 6 It can be seen that during the entire fault ride-through process, by adopting the technical solution of regulating the resistance value of the fault current limiter, the DC bus voltage can always be limited within its safe constraint range (0.9 pu - 1.2 pu), thereby assisting the permanent magnet synchronous wind turbine to successfully perform fault ride-through.
[0082] As is known by technical common sense, the present invention can be implemented by other embodiments that do not depart from its spiritual essence or essential features. Therefore, the above-disclosed embodiments are illustrative in all aspects and not exclusive. All changes within the scope of the present invention or within the scope equivalent to the present invention are encompassed by the present invention.
Claims
1. A method for assisting a permanent magnet synchronous wind turbine to ride through faults, characterized in that: It includes the following steps: S1. Install a fault current limiter with adjustable resistance at the back-to-back converter of the permanent magnet synchronous wind turbine. The fault current limiter is connected in parallel with the DC bus capacitor of the back-to-back converter. The fault current limiter is controlled to be switched on and off via a series-connected on-off switch. Under normal operating conditions of the permanent magnet synchronous wind turbine, the on-off switch is off and the fault current limiter is switched out; S2. After the permanent magnet synchronous wind turbine enters the fault ride-through state, detect the active power input of the machine-side converter and the active power output of the grid-side converter. When the difference between the two is greater than the set threshold criterion for the fault current limiter to be switched on, close the on-off switch of the fault current limiter, and based on the difference between the two, adjust the resistance value of the fault current limiter to maintain the stability of the DC bus voltage during the fault ride-through; S3. After the permanent magnet synchronous wind turbine exits the fault ride-through state, continue to adjust the resistance value of the fault current limiter based on the difference between the active power input of the machine-side converter and the active power output of the grid-side converter. When the difference between the two is less than the set threshold criterion for the fault current limiter to be switched out, open the on-off switch of the fault current limiter, and the fault current limiter exits operation. Based on the active power consumed by the fault current limiter at the moment of exiting operation, adjust the reference value of the active current control of the grid-side converter so that the additional active power generated by the grid-side converter is equal to the active power consumed by the fault current limiter at the moment of exiting operation, to avoid the impact on the DC bus voltage caused by the exit of the fault current limiter from operation.
2. The method for assisting a permanent magnet synchronous wind turbine to ride through faults according to claim 1, wherein: The step S1 includes the following steps: The resistance of the fault current limiter is adjusted by regulating the duty cycle of the on-off signal applied to the switch in the switch branch. The resistance value of the fault current limiter is calculated based on Equation (1): Where d is the duty cycle of the on / off signal applied to the switch in the switching branch, is the maximum resistance value that the fault current limiter can be adjusted to reach.
3. A method for assisting a permanent magnet synchronous wind turbine to ride through faults according to claim 1, characterized in that: The step S2 includes the following steps: During the operation of the permanent magnet synchronous wind turbine, the upper limit allowable value of the DC bus voltage of the back-to-back converter is The rated value of the DC bus voltage is On the premise of the assumption that the fault current limiter is not put into operation during the fault ride-through, based on Equation (2), the upper limit allowable value ΔP of the difference ΔP between the active power input of the machine-side converter and the active power output of the grid-side converter is determined upper , which is set as the input threshold criterion of the fault current limiter: In the formula, C is the size of the DC bus capacitor, and t is the duration of the fault ride-through; When the difference ΔP between the active power input of the converter on the generator side and the active power output of the converter on the grid side exceeds the upper limit allowable value ΔP upper After that, close the on-off switch of the fault current limiter, and the fault current limiter is put into operation.
4. A method for assisting a permanent magnet synchronous wind turbine to ride through faults according to claim 1, characterized in that: The step S2 includes the following steps: During the fault ride-through period, after the fault current limiter is put into operation, the resistance value R of the fault current limiter FCL is regulated. The inputs of the resistance value regulation system of the fault current limiter include the difference ΔP between the active power input of the machine-side converter and the active power output of the grid-side converter, and the DC bus voltage V of the back-to-back converter DC . The regulation system is shown in Equation (3): where k p and k i are the gain coefficients of the proportional link and the integral link in the resistance value regulation system of the fault current limiter respectively. The proportional link is used for the fast response regulation of large-scale DC bus voltage deviation and only takes effect when the DC bus voltage deviation is greater than 20% of the maximum allowable DC bus voltage deviation. The integral link is used for the correction of small-scale DC bus voltage deviation and only takes effect when the DC bus voltage deviation is not greater than 20% of the maximum allowable DC bus voltage deviation.
5. A method for assisting a permanent magnet synchronous wind turbine to ride through faults according to claim 4, characterized in that: The method for determining the adjustable range of the fault current limiter resistance value regulation system is as follows: The resistance value regulation system of the fault current limiter needs to set a lower limit of the resistance value during the fault ride-through process The setting idea is as follows: when the fault current limiter reaches the lower limit of the resistance value, the difference between the active power input of the machine-side converter and the active power output of the grid-side converter reaches its lower limit allowable value ΔP lower , as shown in Equation (4): Solve Equation (4) to obtain the lower limit of the fault current limiter resistance value, as shown in Equation (5): The resistance value regulation system sets the upper limit of the resistance value for the fault current limiter The setting idea is that when the fault current limiter reaches the upper limit of the set resistance value the active power consumed by the fault current limiter is exactly equal to the upper limit allowable value ΔP of the difference ΔP between the active power input of the machine-side converter and the active power output of the grid-side converter upper , as shown in Equation (6): Solve Equation (6) to obtain the upper limit of the fault current limiter resistance value, as shown in Equation (7): The selection of the fault current limiter needs to refer to the upper limit of the resistance value of the fault current limiter, that is, the maximum resistance value that the fault current limiter can be adjusted to reach should not be less than the upper limit of the resistance value 6. A method for assisting a permanent magnet synchronous wind turbine to ride through faults according to claim 1, characterized in that: The S3 includes the following steps: After the permanent magnet synchronous wind turbine generator set exits the fault ride-through state, the cut-off threshold criterion ΔP adopted by the fault current limiter min is shown in Equation (8): When the difference between the active power input of the converter on the turbine side and the active power output of the converter on the grid side is less than the cut-out threshold criterion ΔP min , the on-off switch of the fault current limiter is turned off, the fault current limiter is cut out, and at the same time, an increment is applied to the active current control reference value of the converter on the grid side to make the additional active power of the converter on the grid side equal to the active power consumed by the fault current limiter at the cut-out moment, and the increment of the active current control reference value of the converter on the grid side is calculated based on Equation (9): where, V g is the magnitude of the grid-side converter voltage.