An offshore station ac fault control method and related device of an offshore wind power through flexible dc grid-connected system

Abstract

The application provides an offshore station AC fault control method and related device of an offshore wind power flexible DC grid-connected system, comprising obtaining a dq-axis current amplitude during a fault, and performing amplitude limiting control according to an outer loop current limiting strategy, namely, when the current amplitude exceeds a maximum short-circuit current, reducing the amplitude limiting value by a proportion of the excess; after the fault is cleared, monitoring the outer loop voltage and the active power recovery rate of the wind turbine converter, and an AC fault clearing flag, to ensure that the active power recovery rate does not exceed the voltage recovery rate; in inner loop control, the integral of the integral element is cleared during the fault, and is restarted after the fault is cleared, to reduce the influence of the fault; the modulation wave element exits the oscillation suppression function during the fault, and is restored after the fault is cleared, to stabilize the AC voltage. The application effectively improves the safety and reliability of the offshore wind power grid-connected system, reduces the risk of DC tripping and wind turbine disconnection, and provides an important reference for engineering control system design.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of offshore wind power flexible DC grid-connected system control, and particularly relates to an offshore station AC fault control method for offshore wind power through a flexible DC grid-connected system and a related device. BACKGROUND

[0002] In recent years, with the development of new energy power generation technology and power electronic technology, the technical scheme of using VSC-HVDC (flexible DC) to send large-scale long-distance offshore wind power to the land power grid has been widely used. At present, dozens of offshore wind power through flexible DC grid-connected projects have been put into operation worldwide.

[0003] Compared with the traditional two-end networking flexible DC, the offshore wind power through flexible DC grid-connected system has a great difference in DC control function. Due to the particularity of offshore wind farms, such as being far away from the land and having a harsh environment, the grid-connected system needs to have higher reliability and stability to cope with various complex working conditions and fault conditions. AC fault is a common and high-probability fault type, and is also a working condition that needs to be focused on in the design of offshore wind power through flexible DC grid-connected system. For flexible DC, it is generally required to realize AC fault ride-through and not trip under various AC fault conditions, so as to ensure the stable operation of the system and the reliable supply of power.

[0004] However, the existing offshore station AC fault control strategy of offshore wind power through flexible DC grid-connected system has some problems and deficiencies. In the offshore station AC fault control strategy of offshore wind power through flexible DC grid-connected system, if the cooperation with the low-penetration control of the wind turbine converter of the offshore wind farm is not considered, and only the AC voltage control of the flexible DC offshore station is considered, serious consequences such as DC trip and wind turbine off-grid may occur due to improper cooperation between the flexible DC and the wind farm, which affects the safety and reliability of the system and increases the maintenance cost and economic loss. SUMMARY

[0005] Therefore, the present application provides an offshore station AC fault control method for offshore wind power through a flexible DC grid-connected system and a related device, which aims to realize smooth start of the flexible DC through coordinated control between the offshore station converter station of the offshore wind power flexible DC grid-connected system and the wind turbine converter of the wind farm, avoid overvoltage or overcurrent of the equipment caused by the start-up process, and improve the operation safety and reliability of the flexible DC.

[0006] In order to achieve the above purpose, the technical scheme provided by the present application is as follows:

[0007] In a first aspect, the present application provides an offshore station AC fault control method for offshore wind power through a flexible DC grid-connected system, comprising the following steps:

[0008] During the AC fault period of the offshore station, the dq-axis current amplitude in the dq-axis outer loop control of the offshore station is obtained, and outer loop current limiting control is performed according to a preset outer loop current limiting strategy; the outer loop current limiting strategy is that when the dq-axis current amplitude exceeds the maximum short-circuit current allowed by the offshore station, the current limiting amplitude of the dq-axis is proportionally reduced according to the exceeding degree.

[0009] During the recovery stage after the AC fault of the offshore station is cleared, the outer loop voltage recovery rate in the dq-axis outer loop control of the offshore station, the active power recovery rate of the fan converter and the AC fault clearing flag are obtained;

[0010] According to a preset coordinated control strategy, coordinated control between the converter station of the offshore station and the fan converter is performed; the coordinated control strategy is that the active power recovery rate is less than the outer loop voltage recovery rate;

[0011] The integral element of the dq-axis inner loop control of the offshore station performs inner loop integral control according to a preset inner loop integral clear control strategy; the inner loop integral clear control strategy is that during the AC fault period, the integral is enabled to be cleared, and after the AC fault clearing flag is obtained, the integral is restarted;

[0012] The modulation wave element of the dq-axis inner loop control of the offshore station performs AC voltage low-frequency oscillation suppression control function control according to a preset AC oscillation suppression additional modulation wave strategy; the AC oscillation suppression additional modulation wave strategy is that during the AC fault period, the AC low-frequency oscillation suppression function is automatically exited, and after the AC fault clearing flag is obtained, the AC low-frequency oscillation suppression function is automatically put into.

[0013] Further, the outer loop current limiting strategy includes:

[0014] When the dq-axis current amplitude during the fault period is within the range of the maximum short-circuit current allowed by the offshore station, the limiting amplitude is performed according to the normal limiting amplitude;

[0015] When the dq-axis current amplitude during the fault period exceeds the maximum short-circuit current allowed by the offshore station, the d-axis current limiting amplitude and the q-axis current limiting amplitude are proportionally reduced according to the proportion that the dq-axis current amplitude exceeds the maximum short-circuit current.

[0016] Further, the outer loop current limiting strategy is executed by adding an outer loop current limiter in the dq-axis outer loop control of the offshore station; the outer loop current limiter includes a multiplier, a square root operator, a comparator and a divider;

[0017] The outer loop current limiter executes the outer loop current limiting strategy, including:

[0018] The multiplier is used to square the d-axis reference current and the q-axis reference current, respectively;

[0019] The square root operator is used to square the sum of the two square operation results to obtain the dq-axis synthesized current amplitude;

[0020] The comparator is used to compare the dq-axis synthesized current amplitude with the maximum allowable short-circuit current, and the larger current value is taken as the output;

[0021] The divider is used to divide the current value output by the comparator by the dq-axis synthesized current amplitude to obtain a proportional factor for limiting the current, and the proportional factor is input to the current limiting link in the offshore station dq-axis outer loop control.

[0022] Further, the coordinated control strategy is implemented by adding an active power recovery rate controller to the fan converter and adding an outer loop voltage recovery rate controller to the offshore station dq-axis outer loop control;

[0023] The coordinated control strategy is implemented by using the active power recovery rate controller and the outer loop voltage recovery rate controller, including:

[0024] The time required for the input AC voltage of the d-axis control to recover to the rated value after the fault is cleared is obtained from the outer loop voltage recovery rate controller;

[0025] The time required for the active power of the fan converter to recover to the value before the fault after the fault is cleared is set to be less than the time required in the outer loop voltage recovery rate controller; the times required in the active power recovery rate controller and the outer loop voltage recovery rate controller respectively represent the active power recovery rate and the outer loop voltage recovery rate.

[0026] Further, the inner loop integral zeroing controller is added to the integral link of the offshore station dq-axis inner loop control to implement the inner loop integral zeroing control strategy;

[0027] The inner loop integral zeroing control strategy is implemented by using the inner loop integral zeroing controller, including:

[0028] The inner loop integral zeroing controller obtains the offshore station AC fault flag; when the offshore station AC fault flag is 1, it indicates a fault, and when it is 0, it indicates that the fault is cleared;

[0029] When the offshore station AC fault flag is 1, the inner loop integral zeroing controller clears the integral result of the integral link and takes it as the output of the integral link; when the offshore station AC fault flag is 0, the inner loop integral zeroing controller makes the integral link start integrating again.

[0030] Further, the AC low-frequency oscillation suppression function is implemented by extracting the low-frequency component of the AC voltage from the filter link of the offshore station AC oscillation suppression control, then multiplying the low-frequency component by a damping coefficient, and superimposing it on the modulation wave of the d-axis control to suppress the low-frequency component of the AC voltage.

[0031] Further, the comparison link is further arranged before the filtering link, and the comparison link obtains a deviation signal by comparing the reference voltage on the AC side of the offshore station with the actual voltage on the DC side, and takes the deviation signal as the input of the filtering link.

[0032] In a second aspect, the present application provides an offshore station AC fault control device of a flexible DC grid-connected system of offshore wind power, comprising:

[0033] The first control module is configured to obtain the dq-axis current amplitude in the dq-axis outer loop control of the offshore station during the offshore station AC fault, and perform outer loop current limiting control according to a preset outer loop current limiting strategy; the outer loop current limiting strategy is to proportionally reduce the current limiting value of the dq-axis according to the exceeding degree when the dq-axis current amplitude exceeds the maximum short-circuit current allowed by the offshore station.

[0034] The second control module is configured to obtain the outer loop voltage recovery rate in the dq-axis outer loop control of the offshore station and the active power recovery rate of the wind turbine converter during the offshore station AC fault clearance and recovery stage, and perform coordinated control between the offshore station converter station and the wind turbine converter according to a preset coordinated control strategy; the coordinated control strategy is to make the active power recovery rate less than the outer loop voltage recovery rate.

[0035] The third control module is configured to obtain an AC fault clearance flag during the offshore station AC fault clearance and recovery stage, and perform inner loop integral control of the offshore station dq-axis inner loop control according to a preset inner loop integral clear control strategy; the inner loop integral clear control strategy is to enable integral clear during the AC fault, and to restart the integral after obtaining the AC fault clearance flag.

[0036] The fourth control module is configured to obtain an AC fault clearance flag during the offshore station AC fault clearance and recovery stage, and perform AC voltage low-frequency oscillation suppression control function control of the offshore station dq-axis inner loop control according to a preset AC oscillation suppression additional modulation wave strategy; the AC oscillation suppression additional modulation wave strategy is to automatically exit the AC low-frequency oscillation suppression function during the AC fault, and to automatically enter the AC low-frequency oscillation suppression function after obtaining the AC fault clearance flag.

[0037] In a third aspect, the present application provides a computer device, the device comprising a processor and a memory:

[0038] The memory is configured to store a computer program and send instructions of the computer program to the processor;

[0039] The processor executes the offshore station AC fault control method of a flexible DC grid-connected system of offshore wind power according to the instructions of the computer program.

[0040] In a fourth aspect, the present application provides a computer-readable storage medium, and the computer-readable storage medium stores a computer program, and the computer program is executed by a processor to implement the offshore station AC fault control method of the offshore wind power through the flexible DC grid system according to the first aspect.

[0041] To sum up, the present application provides an offshore station AC fault control method of the offshore wind power through the flexible DC grid system and related devices, including obtaining the dq-axis current amplitude in the dq-axis outer loop control of the offshore station during the offshore station AC fault, and performing the outer loop current limiting control according to the preset outer loop current limiting strategy; the outer loop current limiting strategy is to proportionally reduce the current limiting value of the dq-axis according to the exceeding degree when the dq-axis current amplitude exceeds the maximum short-circuit current allowed by the offshore station; obtaining the outer loop voltage recovery rate, the active power recovery rate of the wind turbine converter and the AC fault clearing flag in the dq-axis outer loop control of the offshore station in the recovery stage after the offshore station AC fault is cleared; performing the coordinated control between the offshore station converter station and the wind turbine converter according to the preset coordinated control strategy; the coordinated control strategy is to make the active power recovery rate less than the outer loop voltage recovery rate; performing the inner loop integral control in the integral link of the dq-axis inner loop control of the offshore station according to the preset inner loop integral clear control strategy; the inner loop integral clear control strategy is to enable the integral clear during the AC fault, and to restart the integral after the AC fault clearing flag is obtained; performing the AC voltage low-frequency oscillation suppression control function control according to the preset AC oscillation suppression additional modulation wave strategy in the modulation wave link of the dq-axis inner loop control of the offshore station; the AC oscillation suppression additional modulation wave strategy is to automatically exit the AC low-frequency oscillation suppression function during the AC fault, and to automatically put into the AC low-frequency oscillation suppression function after the AC fault clearing flag is obtained. The present application can realize the offshore station AC fault successful crossing of the offshore wind power through the flexible DC grid system, provide reference and guidance for the offshore wind power flexible DC grid engineering control system design, reduce the DC tripping and wind turbine off-grid risk of the wind farm, and improve the safety and reliability of the offshore wind power through the flexible DC grid system. BRIEF DESCRIPTION OF DRAWINGS

[0042] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or prior art description. Obviously, the drawings in the following description are only some embodiments of the present application, and those skilled in the art can also obtain other drawings according to these drawings without any creative effort.

[0043] Figure 1 A flow chart of the offshore station AC fault control method of the offshore wind power through the flexible DC grid system provided by the present application is provided.

[0044] Figure 2 This is a schematic diagram of the topology of a dual-end offshore wind power system connected to a flexible DC grid, provided by the present invention.

[0045] Figure 3 This is a schematic diagram illustrating the working principle of dq transformation of voltage and current on the valve side of the offshore station flexible DC converter, provided in an embodiment of the present invention.

[0046] Figure 4 This is a schematic diagram of the working principle of the outer loop controller of the offshore converter station provided in an embodiment of the present invention;

[0047] Figure 5 This is a schematic diagram of the working principle of the outer loop current limiter for an offshore converter station provided in an embodiment of the present invention.

[0048] Figure 6 A schematic diagram of the working principle of the active power recovery rate limiter for the wind turbine converter in an offshore wind farm, provided in an embodiment of the present invention;

[0049] Figure 7 A schematic diagram of the working principle of the inner loop controller and integral zeroing control of the offshore converter station provided in an embodiment of the present invention;

[0050] Figure 8 This is a schematic diagram of the AC oscillation suppression control principle for offshore stations provided in an embodiment of the present invention;

[0051] Figure 9 This is a schematic diagram of the overall technical solution for the offshore wind power startup method via a flexible DC grid-connected system provided in an embodiment of the present invention;

[0052] Figure 10 A block diagram of an AC fault control device for an offshore wind power station connected to a flexible DC grid system, provided as an embodiment of the present invention;

[0053] Figure 11 This is a block diagram of a computer device provided in an embodiment of the present invention. Detailed Implementation

[0054] To make the objectives, features, and advantages of this invention more apparent and understandable, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the embodiments described below are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0055] Please see Figures 1-2 , Figure 1 A method for AC fault control at offshore wind power stations connected to a flexible DC grid system is shown. Figure 2 The topology of a dual-ended offshore wind power system connected to a flexible DC grid is shown.

[0056] As shown in the formula (1), the dq-axis current amplitude of the offshore converter station is limited by the dq-axis current amplitude limit value, and the dq-axis current amplitude limit value is determined by the dq-axis current amplitude limit value of the offshore converter station and the dq-axis current amplitude limit value of the wind turbine converter. Figure 2 As shown in the formula (1), the dq-axis current amplitude of the offshore converter station is limited by the dq-axis current amplitude limit value, and the dq-axis current amplitude limit value is determined by the dq-axis current amplitude limit value of the offshore converter station and the dq-axis current amplitude limit value of the wind turbine converter.

[0057] Therefore, in view of the above problems, the present application provides a kind of offshore wind power through flexible DC grid-connected system offshore station AC fault control method and related device. The following detailed description of an embodiment of the offshore wind power through flexible DC grid-connected system offshore station AC fault control method of the present application.

[0058] As shown in the formula (1), the dq-axis current amplitude of the offshore converter station is limited by the dq-axis current amplitude limit value, and the dq-axis current amplitude limit value is determined by the dq-axis current amplitude limit value of the offshore converter station and the dq-axis current amplitude limit value of the wind turbine converter. Figure 1 As shown in the formula (1), the dq-axis current amplitude of the offshore converter station is limited by the dq-axis current amplitude limit value, and the dq-axis current amplitude limit value is determined by the dq-axis current amplitude limit value of the offshore converter station and the dq-axis current amplitude limit value of the wind turbine converter.

[0059] S1: during offshore station AC fault, the dq-axis current amplitude in offshore station dq-axis outer ring control is obtained, and outer ring current limiting control is carried out according to the preset outer ring current limiting strategy; the outer ring current limiting strategy is that when the dq-axis current amplitude exceeds the maximum short-circuit current allowed by the offshore station, the current limiting value of the dq-axis is reduced in proportion to the extent of exceeding.

[0060] It should be noted that in this step, the outer ring current limiting strategy is adopted, and when the current amplitude exceeds the preset maximum short-circuit current, the current limiting value is reduced in proportion to the extent of exceeding. By this kind of dynamic current limiting adjustment mode, the current can be effectively limited, so that the risk of overcurrent of equipment caused by excessive short-circuit current of offshore station under serious AC fault is prevented, and the safety and reliability of the system are improved.

[0061] S2: in the recovery stage after offshore station AC fault is cleared, the outer ring voltage recovery rate in offshore station dq-axis outer ring control, the active power recovery rate of wind turbine converter and the AC fault clearing flag are obtained.

[0062] It should be noted that the outer ring voltage recovery rate refers to the rate at which the DC voltage in the dq-axis outer ring control of the offshore converter station recovers to the normal level after the offshore station AC fault is cleared. The active power recovery rate of the wind turbine converter refers to the rate at which the active power of the wind turbine recovers to the level before the fault in the recovery process after the fault is cleared. The AC fault clearing flag is a flag or signal indicating that the AC fault has been successfully eliminated.

[0063] S3: The coordinated control between the offshore station converter station and the wind turbine converter is performed according to a preset coordinated control strategy; the coordinated control strategy is to make the active power recovery rate less than the outer loop voltage recovery rate.

[0064] It should be noted that, in this step, the active power recovery rate of the wind turbine converter is adjusted according to the coordinated control strategy, so that it is less than the outer loop voltage recovery rate. In this way, the coordinated control between the offshore station converter station and the wind turbine converter of the offshore wind power flexible DC grid-connected system can be realized, voltage instability caused by excessive power recovery during the recovery process can be avoided, smooth recovery can be achieved, and wind turbine disconnection from the grid can be prevented.

[0065] S4: The integral element of the offshore station dq-axis inner loop control is controlled according to a preset inner loop integral zeroing control strategy; the inner loop integral zeroing control strategy is to enable integral zeroing during the AC fault period, and to restart the integral after the AC fault clearing flag is obtained.

[0066] It should be noted that, in this step, the integral element of the inner loop control is zeroed during the fault period by using the inner loop integral zeroing control strategy, so as to eliminate the influence of current fluctuation during the fault. After the fault is cleared, the integral element restarts the integral. In this way, the influence of the fault on the control accuracy of the system can be reduced, the accuracy of the control strategy can be ensured, and the dynamic response capability of the system can be improved.

[0067] S5: The generation modulation wave element of the offshore station dq-axis inner loop control is controlled according to a preset AC oscillation suppression additional modulation wave strategy; the AC oscillation suppression additional modulation wave strategy is to automatically exit the AC low-frequency oscillation suppression function during the AC fault period, and to automatically put into the AC low-frequency oscillation suppression function after the AC fault clearing flag is obtained.

[0068] It should be noted that, in this step, the AC oscillation suppression additional modulation wave strategy is used, the AC low-frequency oscillation suppression function is exited during the fault, and the function is put into again after the fault is cleared. In this way, unnecessary control interference can be avoided during the fault, the voltage can be quickly and stably recovered during the recovery phase, and wind turbine disconnection from the grid can be prevented.

[0069] The embodiment provides an offshore station AC fault control method of a marine wind power flexible DC grid connection system, comprehensive protection and control of the marine wind power grid connection system are realized by adopting different control strategies during the fault period and the recovery stage. Through amplitude limiting control, coordinated control, integral zeroing and oscillation suppression and the like, the voltage stability during the AC fault period and the recovery stage after the clearing is improved, reliable and stable AC voltage is provided for the wind farm, wind turbine off-grid is prevented, and the stability and the rapid recovery capability of the system are ensured. The control method has the advantages of strong operability and simple and convenient implementation.

[0070] In one embodiment, an outer loop current limiting strategy design is provided. The strategy includes:

[0071] When the dq-axis current amplitude during the fault period is within the range of the maximum short-circuit current allowed by the offshore station, the amplitude limiting is performed according to the normal amplitude limiting;

[0072] When the dq-axis current amplitude during the fault period exceeds the maximum short-circuit current allowed by the offshore station, the d-axis current amplitude limiting value and the q-axis current amplitude limiting value are reduced in the same proportion according to the proportion of the dq-axis current amplitude exceeding the maximum short-circuit current.

[0073] Please refer to Figure 3 , Figure 3 The dq transformation of the valve side voltage and current of the offshore station flexible DC converter is shown. During the AC fault of the offshore station, the valve side voltage and current of the offshore station are subjected to dq transformation (as shown in Figure 3 ), to generate the dq-axis voltage and current. When the dq-axis current amplitude during the fault period is within the range of the allowed maximum short-circuit current Imax, the amplitude limiting is performed according to the normal amplitude limiting; when the dq-axis current amplitude during the fault period exceeds Imax, the current amplitude limiting value of the dq-axis is reduced in the same proportion, so that the current during the fault period is limited, and the flexible DC is prevented from tripping due to improper control of the flexible DC offshore station during the fault period.

[0074] The valve side current direction before the AC fault of the offshore station is from the wind turbine to the DC converter (forward direction), and after the AC fault, the valve side current direction is from the DC converter to the wind power AC field fault point (negative direction). Especially during the serious fault such as three-phase metallic grounding, if no additional outer loop current limiter is provided, the dq-axis current may generate a large fault current, which exceeds the breaking capacity of the AC switch, and is not conducive to the clearing of the AC fault.

[0075] Assume that in the high power operating condition, the valve side current id and iq before the AC fault of the offshore station are 0.8p.u. and 0.1p.u. respectively; during the fault, if there is no additional outer loop current limiter, the valve side current id and iq are -0.8p.u. and -0.1p.u. respectively, and the dq axis current amplitude is 0.81p.u., which will exceed the limit of the maximum short circuit current Imax=0.5p.u. allowed by the offshore station, then the proposed outer loop current limiter will work, and the valve side current id and iq during the fault will be limited to -0.49p.u. and -0.062p.u. respectively, thus meeting the requirement of the fault current limit of the offshore station.

[0076] In a further embodiment, the outer loop current limiting strategy is implemented by adding an outer loop current limiter in the dq axis outer loop control of the offshore station. Please refer to Figure 4 and Figure 5 , Figure 4 shows the working principle of the outer loop controller of the offshore converter station, Figure 5 shows the working principle of the outer loop current limiter of the offshore converter station. In the dq axis outer loop control of the offshore station (such as Figure 4 ), an outer loop current limiter (such as Figure 5 ) is added: the outer loop current limiter includes a multiplier, a square root operator, a comparator and a divider. The outer loop current limiter implements the outer loop current limiting strategy, including using the multiplier to square the d-axis reference current idref and the q-axis reference current iqref respectively; using the square root operator to take the square root of the sum of the two square operation results to obtain the dq axis combined current amplitude Idq; using the comparator to compare the dq axis combined current amplitude Idq and the allowed maximum short circuit current Imax, and taking the smaller current value as the output; using the divider to divide the current value output by the comparator by the dq axis combined current amplitude to obtain the proportion factor Klim for limiting the current, and inputting the proportion factor Klim into the current limiting link in the dq axis outer loop control of the offshore station.

[0077] In one embodiment, the coordinated control strategy is implemented by adding an active power recovery rate controller in the wind turbine converter and an outer loop voltage recovery rate controller in the dq axis outer loop control of the offshore station; the active power recovery rate controller and the outer loop voltage recovery rate controller are used to implement the coordinated control strategy, including:

[0078] the time required for the input AC voltage of the d-axis control to recover to the rated value after the fault is cleared from the outer loop voltage recovery rate controller;

[0079] The time required for the active power to recover to the value before the fault after the fault is cleared is less than the time required in the outer loop voltage recovery rate controller; the time required in the active power recovery rate controller and the outer loop voltage recovery rate controller respectively represents the active power recovery rate and the outer loop voltage recovery rate.

[0080] Please refer to Figure 6 , Figure 6 The working principle of the active power recovery rate limiter of the offshore wind farm fan converter is shown. For the recovery stage after the AC fault of the offshore station is cleared, in the dq axis outer loop control of the offshore station, an AC voltage amplitude recovery rate link (such as Figure 4 ) is added to the input AC voltage link of the d-axis control after the AC fault, and the time required for the voltage to recover to the rated value after the fault is cleared is T1. At the same time, an active power recovery rate link (such as Figure 6 ) is added to the recovery link of the fan converter after the AC fault is cleared, and the time required for the power to recover to the value before the fault after the fault is cleared is T2, so that by setting a suitable recovery rate, T2 is less than T1, so that during the fault recovery period, the wind farm fan output power is recovered according to a certain rate after the AC voltage is recovered to stable, which can effectively prevent the risk of overvoltage of the flexible DC voltage or overcurrent of the bridge arm caused by too fast recovery of the fan power, resulting in DC trip.

[0081] In one embodiment, an inner loop integral zero controller is added to the integral link of the dq axis inner loop control of the offshore station to execute the inner loop integral zero control strategy; the inner loop integral zero controller executing the inner loop integral zero control strategy comprises:

[0082] The inner loop integral zero controller obtains an offshore station AC fault flag; when the offshore station AC fault flag is 1, it indicates a fault, and when it is 0, it indicates that the fault is cleared;

[0083] When the offshore station AC fault flag is 1, the inner loop integral zero controller clears the integral result of the integral link as the output of the integral link; when the offshore station AC fault flag is 0, the inner loop integral zero controller makes the integral link start integrating again.

[0084] Please refer to Figure 7 , Figure 7 The inner loop controller and integral zero control principle diagram of the offshore converter station is shown. For the recovery stage after the AC fault of the offshore station is cleared, in the integral link of the dq axis inner loop control of the offshore station, an inner loop integral zero control according to the flag of the AC fault being cleared is added (such as Figure 7), that is, during AC fault (ACfault = 1), the integral is cleared, and after the end of the fault (ACfault = 0), the AC voltage returns to the rated value, and the integral is restarted. Through the integral clearing control, the stability of the AC voltage of the offshore station is improved, and the influence of current fluctuation during the fault on the voltage amplitude is reduced.

[0085] In one embodiment, the AC low-frequency oscillation suppression function suppresses the low-frequency component of the AC voltage by extracting the low-frequency component of the AC voltage from the filter link of the AC oscillation suppression control of the offshore station, and then superimposes the low-frequency component multiplied by a damping coefficient on the modulation wave of the d-axis control.

[0086] The AC low-frequency oscillation suppression function of the present embodiment is mainly used to suppress the low-frequency component of the AC voltage of the offshore station, because low-frequency oscillation may affect the stability of the power system, leading to voltage fluctuation, increased equipment loss, etc. In the power electronic control system, the d-axis control is an important part of controlling key parameters such as active power of the system, and through this superimposition method, the characteristics of the modulation wave can be changed, thereby suppressing the low-frequency component of the AC voltage.

[0087] In a further embodiment, the filter link further includes a comparison link before the filter link, which obtains a deviation signal by comparing the AC side reference voltage of the offshore station with the actual DC side voltage, and takes the deviation signal as the input of the filter link.

[0088] Please refer to Figure 8 , Figure 8 The principle of AC oscillation suppression control of the offshore station is shown. For the recovery stage after the AC fault of the offshore station is cleared, a modulation wave link is generated in the dq-axis inner loop control of the offshore station, and an AC voltage low-frequency oscillation suppression control function (such as Figure 8 ) is added according to the flag of AC fault clearing, that is, during the AC fault, the AC low-frequency oscillation suppression function is automatically exited; after the end of the fault, the AC voltage returns to the rated value, and then the AC low-frequency oscillation suppression function is automatically put into operation, which is beneficial to quickly stabilize the AC voltage after the fault and provide stable and strong AC grid-connected voltage for the wind farm, preventing the wind turbine from repeatedly entering low-voltage ride-through and off-grid.

[0089] Please refer to Figure 9 , Figure 9The offshore wind power through flexible DC grid connection system starting method combined with the above-mentioned embodiments is shown. It mainly includes: 1) during the AC fault of the offshore station: in the dq axis outer ring control of the offshore station, an outer ring current limiter is added: when the dq axis current amplitude during the fault is within the allowed maximum current Imax range, the normal limiting is still performed; when the dq current amplitude during the fault exceeds Imax, the current limiting value of the dq axis is reduced in the same proportion, so as to limit the current during the fault and prevent the flexible DC from tripping due to improper control of the flexible DC offshore station during the fault. 2) After the AC fault of the offshore station is cleared, the recovery stage: in the dq axis outer ring control of the offshore station, an AC voltage amplitude recovery rate link is added in the input AC voltage link of the d-axis control after the AC fault, and the time required for the voltage to recover to the rated value after the fault is cleared is T1. At the same time, an active power recovery rate link is added in the recovery link of the wind turbine converter after the AC fault is cleared, and the time required for the power to recover to the pre-fault value after the fault is cleared is T2, so that by setting appropriate recovery rates, T2 is less than T1, so as to ensure that during the fault recovery period, the AC voltage is recovered to stable, and then the wind farm wind turbine output power is recovered at a certain rate, which can effectively prevent the risk of DC tripping caused by overvoltage or bridge arm overcurrent of the flexible DC due to too fast recovery of the wind turbine power. 3) After the AC fault of the offshore station is cleared, the recovery stage: in the integral link of the dq axis inner ring control of the offshore station, an inner ring integral clear control is added according to the flag of clearing the AC fault, that is, during the AC fault, the integral clear is enabled, and after the fault is over and the AC voltage recovers to the rated value, the integral is restarted. Through the integral clear control, the stability of the AC voltage of the offshore station is improved, and the influence of the current fluctuation during the fault on the voltage amplitude is reduced. 4) After the AC fault of the offshore station is cleared, the recovery stage: in the modulation wave link of the dq axis inner ring control of the offshore station, an AC voltage low-frequency oscillation suppression control function is added according to the flag of clearing the AC fault, that is, during the AC fault, the AC low-frequency oscillation suppression function is automatically exited; after the fault is over and the AC voltage recovers to the rated value, the low-frequency oscillation suppression function is automatically put into, which is beneficial to quickly stabilize the AC voltage after the fault and provide stable and strong AC grid connection voltage for the wind farm, preventing the wind turbine from repeatedly entering low-voltage ride-through and off-grid.

[0090] Based on the same inventive concept, this application also provides an AC fault control device for offshore wind power stations connected to a flexible DC grid, used to implement the aforementioned AC fault control method for offshore wind power stations connected to a flexible DC grid. The solution provided by this device is similar to the solution described in the above method. Therefore, the specific limitations in the embodiments of the AC fault control device for offshore wind power stations connected to a flexible DC grid provided below can be found in the limitations of the AC fault control method for offshore wind power stations connected to a flexible DC grid above, and will not be repeated here.

[0091] Please see Figure 10 This invention also provides an AC fault control device for offshore wind power stations connected to a flexible DC grid system, comprising:

[0092] The first control module is used to acquire the dq axis current amplitude in the outer loop control of the dq axis of the offshore station during an AC fault, and to perform outer loop current limiting control according to a preset outer loop current limiting strategy. The outer loop current limiting strategy is to reduce the dq axis current limiting value proportionally to the extent of the excess when the dq axis current amplitude exceeds the maximum allowable short-circuit current of the offshore station.

[0093] The second control module is used to obtain the outer loop voltage recovery rate and the active power recovery rate of the wind turbine converter in the outer loop control of the dq axis of the offshore station during the recovery phase after the AC fault is cleared at the offshore station. It then performs coordinated control between the offshore station converter station and the wind turbine converter according to a preset coordinated control strategy. The coordinated control strategy is to make the active power recovery rate less than the outer loop voltage recovery rate.

[0094] The third control module is used to acquire the AC fault clearing flag during the recovery phase after the AC fault is cleared at the offshore station, and to perform inner-loop integral control in the integral link of the dq axis inner loop control at the offshore station according to the preset inner-loop integral clearing control strategy. The inner-loop integral clearing control strategy is to enable integral clearing during the AC fault period, and then restart the integral after acquiring the AC fault clearing flag.

[0095] The fourth control module is used to acquire the AC fault clearance flag during the recovery phase after the AC fault is cleared at the offshore station, and to perform AC voltage low-frequency oscillation suppression control function in the modulation wave generation link of the dq axis inner loop control at the offshore station according to the preset AC oscillation suppression additional modulation wave strategy. The AC oscillation suppression additional modulation wave strategy is to automatically exit the AC low-frequency oscillation suppression function during the AC fault, and automatically re-engage the AC low-frequency oscillation suppression function after acquiring the AC fault clearance flag.

[0096] Those skilled in the art can clearly understand that, for the convenience and brevity of description, only the division of the above functional units and modules is taken as an example, and in actual application, the above functions can be completed by different functional units and modules according to needs, that is, the internal structure of the system is divided into different functional units or modules to complete all or part of the functions described above. Each functional unit or module in the embodiment can be integrated in one processing unit, or each unit can exist physically, or two or more units can be integrated in one unit. The integrated unit can be realized in the form of hardware or in the form of software functional unit. In addition, the specific name of each functional unit or module is only for the convenience of mutual distinction, and does not limit the protection scope of the present application. The specific working process of the unit or module in the system can refer to the corresponding process in the foregoing method embodiment, and will not be repeated here.

[0097] With reference to Figure 11 The embodiment of the present application also provides a computer device, including a memory and a processor and a computer program stored in the memory, when the computer program is executed on the processor, the offshore station AC fault control method of the offshore wind power through flexible DC grid-connected system is realized.

[0098] The computer device can be a desktop computer, a notebook computer, a palm computer and a cloud server and the like. The computer device can include, but is not limited to, a processor and a memory. Those skilled in the art can understand that, Figure 11 It is only an example of the computer device and does not constitute a limitation on the computer device, and can include more or fewer components than the illustration, or combine certain components or different components, for example, can also include input and output devices, network access devices and the like.

[0099] The processor can be a central processing unit (CPU), and the processor can also be other general-purpose processors, digital signal processors (DSP), application specific integrated circuits (ASIC), field-programmable gate arrays (FPGA) or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. The general-purpose processor can be a microprocessor or the processor can also be any conventional processor.

[0100] The memory can be an internal storage unit of the computer device in some embodiments, such as a hard disk or a memory of the computer device. The memory can also be an external storage device of the computer device in other embodiments, such as a plug-in hard disk, a smart media card (SMC), a secure digital (SD) card, a flash card, and the like. Further, the memory can include both an internal storage unit and an external storage device of the computer device. The memory is used to store an operating system, an application program, a boot loader, data, and other programs, such as program codes of the computer program. The memory can also be used to temporarily store data that has been output or is to be output.

[0101] The embodiment of the present application further provides a computer readable storage medium, which stores a computer program. The computer program is run by a processor to implement the offshore station AC fault control method of the offshore wind power through flexible DC grid system.

[0102] In the embodiment, the integrated unit, if implemented in the form of a software functional unit and sold or used as an independent product, can be stored in a computer readable storage medium. Based on such understanding, the present application can implement all or part of the processes in the above-mentioned embodiment methods by a computer program to instruct related hardware, and the computer program can be stored in a computer readable storage medium. The computer program can implement the steps of each method embodiment when executed by a processor. The computer program includes computer program codes, which can be in the form of source code, object code, executable files, or some intermediate forms. The computer readable medium at least includes any entity or device capable of carrying the computer program code to a photographing device / terminal equipment, a recording medium, a computer memory, a read-only memory (ROM), a random access memory (RAM), an electrical carrier signal, a telecommunications signal, and a software distribution medium. For example, a U disk, a mobile hard disk, a magnetic disk or an optical disk, and the like. In some jurisdictions, according to legislation and patent practice, the computer readable medium can not be an electrical carrier signal and a telecommunications signal.

[0103] In the above embodiments, the description of each embodiment has its own focus, and the parts not described or recorded in detail in a certain embodiment can be referred to the relevant description of other embodiments.

[0104] Those skilled in the art can understand that the units and algorithm steps of each example described in combination with the embodiments disclosed herein can be realized by electronic hardware or a combination of computer software and electronic hardware. Whether the functions are realized in hardware or software depends on the specific application and design constraints of the technical solution. Those skilled in the art can use different methods to realize the described functions for each specific application, but such implementation should not be considered beyond the scope of the present application.

[0105] In the embodiments disclosed in the present application, it should be understood that the disclosed apparatus / terminal device and method can be implemented in other ways. For example, the apparatus / terminal device embodiments described above are merely schematic, for example, the division of the modules or units is merely a logical function division, and actual implementation can have another division manner, for example, a plurality of units or components can be combined or integrated into another system, or some features can be ignored or not executed. In addition, the coupling or direct coupling or communication connection between the displayed or discussed each other can be indirect coupling or communication connection through some interfaces, devices or units, and can be electrical, mechanical or other forms.

[0106] The above embodiments are only used to illustrate the technical solutions of the present application, rather than limit them; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that the technical solutions recorded in the foregoing embodiments can be modified, or some technical features can be replaced by equivalents; and these modifications or replacements do not make the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A method for AC fault control of an offshore station of a marine wind power system connected to a grid via a flexible HVDC system, characterized in that, The method comprises the following steps: During the AC fault of the offshore station, the dq-axis current amplitude in the dq-axis outer loop control of the offshore station is obtained, and outer loop current limiting control is performed according to a preset outer loop current limiting strategy; the outer loop current limiting strategy is that when the dq-axis current amplitude exceeds the maximum short-circuit current allowed by the offshore station, the d-axis current limiting value and the q-axis current limiting value are reduced in proportion to the proportion that the dq-axis current amplitude exceeds the maximum short-circuit current. After the AC fault of the offshore station is cleared, the outer loop voltage recovery rate in the dq-axis outer loop control of the offshore station, the active power recovery rate of the wind turbine converter, and an AC fault clearing flag are obtained; According to a preset coordinated control strategy, coordinated control between the converter station of the offshore station and the wind turbine converter is performed; the coordinated control strategy is that the active power recovery rate is less than the outer loop voltage recovery rate; The integral element of the dq-axis inner loop control of the offshore station performs inner loop integral control according to a preset inner loop integral clearing control strategy; the inner loop integral clearing control strategy is that during the AC fault, integral clearing is enabled, and after the AC fault clearing flag is obtained, integral is restarted; The modulation wave element of the dq-axis inner loop control of the offshore station performs AC voltage low-frequency oscillation suppression control function control according to a preset AC oscillation suppression additional modulation wave strategy; the AC oscillation suppression additional modulation wave strategy is that during the AC fault, the AC low-frequency oscillation suppression function is automatically exited, and after the AC fault clearing flag is obtained, the AC low-frequency oscillation suppression function is automatically put into.

2. The offshore station AC fault control method of the offshore wind power flexible DC grid-connected system according to claim 1, characterized in that, The outer loop current limiting strategy comprises: When the dq-axis current amplitude during the fault is within the range of the maximum short-circuit current allowed by the offshore station, normal limiting is performed; When the dq-axis current amplitude during the fault exceeds the maximum short-circuit current allowed by the offshore station, the d-axis current limiting value and the q-axis current limiting value are reduced in proportion to the proportion that the dq-axis current amplitude exceeds the maximum short-circuit current. 3.The offshore station AC fault control method of the offshore wind power flexible HVDC grid-connected system according to claim 1 or 2, characterized in that, The outer loop current limiting strategy is implemented by adding an outer loop current limiter in the dq-axis outer loop control of the offshore station; The outer loop current limiter comprises a multiplier, a square root operator, a comparator, and a divider; The outer loop current limiter implements the outer loop current limiting strategy, which comprises: The d-axis reference current and the q-axis reference current are squared by the multiplier respectively; The sum of the two square operation results is square-rooted by the square root operator to obtain a dq-axis combined current amplitude; The dq-axis combined current amplitude and the allowed maximum short-circuit current are compared by the comparator, and the larger current value is taken as the output; The current value output by the comparator is divided by the dq-axis combined current amplitude by the divider to obtain a proportion factor for limiting current, and the proportion factor is input to the current limiting element in the dq-axis outer loop control of the offshore station.

4. The offshore station AC fault control method of the offshore wind power flexible DC grid-connected system according to claim 1, characterized in that, The coordinated control strategy is implemented by adding an active power recovery rate controller in the wind turbine converter and adding an outer loop voltage recovery rate controller in the dq-axis outer loop control of the offshore station; The coordinated control strategy is implemented by the active power recovery rate controller and the outer loop voltage recovery rate controller, which comprises: The input AC voltage of the d-axis control is obtained from the outer loop voltage recovery rate device; the time required for the recovery of the active power control of the wind turbine converter to the value before the fault is less than the time required for the recovery of the outer loop voltage to the rated value after the fault is cleared; The time required for the recovery of the active power control of the wind turbine converter to the value before the fault is less than the time required for the recovery of the outer loop voltage to the rated value after the fault is cleared; the time required for the active power recovery rate controller and the outer loop voltage recovery rate controller respectively represents the active power recovery rate and the outer loop voltage recovery rate.

5. The offshore station AC fault control method of the offshore wind power flexible DC grid-connected system according to claim 1, characterized in that, The integral element of the dq-axis inner loop control of the offshore station is provided with an inner loop integral zeroing controller to execute the inner loop integral zeroing control strategy; The inner loop integral zeroing controller executes the inner loop integral zeroing control strategy, comprising: The inner loop integral zeroing controller obtains an offshore station AC fault flag; when the offshore station AC fault flag is 1, it indicates a fault, and when the offshore station AC fault flag is 0, it indicates that the fault is cleared; When the offshore station AC fault flag is 1, the inner loop integral zeroing controller clears the integral result of the integral element and takes it as the output of the integral element; when the offshore station AC fault flag is 0, the inner loop integral zeroing controller makes the integral element start integration again.

6. The offshore station AC fault control method of the offshore wind power flexible DC grid-connected system according to claim 1, characterized in that, The AC low-frequency oscillation suppression function suppresses the low-frequency component of the AC voltage by extracting the low-frequency component of the AC voltage from the filter element of the AC oscillation suppression control of the offshore station, and then superimposes the low-frequency component multiplied by a damping coefficient on the modulation wave of the d-axis control, thereby suppressing the low-frequency component of the AC voltage.

7. The offshore station AC fault control method of the offshore wind power flexible DC grid-connected system according to claim 6, characterized in that, The filter element further comprises a comparison element, which obtains a deviation signal by comparing the AC side reference voltage of the offshore station with the actual DC side voltage, and takes the deviation signal as the input of the filter element.

8. An offshore station AC fault control device of an offshore wind power flexible DC grid interconnection system, characterized in that, Comprising: The first control module is configured to obtain the dq-axis current amplitude in the dq-axis outer loop control of the offshore station during the AC fault of the offshore station, and perform outer loop current limiting control according to a preset outer loop current limiting strategy; the outer loop current limiting strategy is to proportionally reduce the current limiting value of the dq-axis according to the degree of exceeding when the dq-axis current amplitude exceeds the maximum short-circuit current allowed by the offshore station; The second control module is configured to obtain the outer loop voltage recovery rate in the dq-axis outer loop control of the offshore station and the active power recovery rate of the wind turbine converter during the recovery stage after the AC fault of the offshore station is cleared, and perform coordinated control between the offshore converter station and the wind turbine converter according to a preset coordinated control strategy; the coordinated control strategy is to make the active power recovery rate less than the outer loop voltage recovery rate; The third control module is configured to obtain an AC fault clearing flag during the recovery stage after the AC fault of the offshore station is cleared, and perform inner loop integral control of the integral element of the dq-axis inner loop control of the offshore station according to a preset inner loop integral zeroing control strategy; the inner loop integral zeroing control strategy is to enable integral zeroing during the AC fault, and to start integration again after the AC fault clearing flag is obtained. The fourth control module is configured to acquire an AC fault clearing flag in a post-AC fault clearing recovery stage of the offshore station, and control an AC voltage low-frequency oscillation suppression control function according to a preset AC oscillation suppression additional modulation wave strategy in a dq-axis inner loop control of the offshore station.

9. A computer device, comprising: The device comprises a processor and a memory: The memory is configured to store a computer program and send instructions of the computer program to the processor; The processor executes the instructions of the computer program to perform the offshore station AC fault control method of the offshore wind power through the flexible DC grid system according to any one of claims 1-7.

10. A computer-readable storage medium, characterized in that, The computer program is stored on the computer readable storage medium and is executed by the processor to perform the offshore station AC fault control method of the offshore wind power through the flexible DC grid system according to any one of claims 1-7.

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