Offshore station alternating current fault control method for offshore wind power through flexible direct current grid-connected system and related device

Abstract

The invention provides an offshore station alternating-current fault control method and related device for offshore wind power through a flexible direct-current grid-connected system, and the method comprises the steps: obtaining the dq-axis current amplitude during a fault period, and carrying out the amplitude limiting control according to an outer-loop current limiting strategy, namely, reducing the amplitude limiting value according to an exceeding proportion when the current amplitude exceeds the maximum short-circuit current; after the fault is cleared, monitoring the active power recovery rate of the outer loop voltage and the fan converter and an alternating current fault clearing mark, and ensuring that the active power recovery rate does not exceed the voltage recovery rate; in the inner loop control, the integral of the integral link is reset during the fault period, and is restarted after the fault is cleared, so as to reduce the influence of the fault; and the modulation wave link quits the oscillation suppression function during the fault period and recovers after the fault is cleared so as to stabilize the AC voltage. According to the method, the safety and reliability of the offshore wind power grid-connected system are effectively improved, the risks of direct current tripping and fan off-grid are reduced, and an important reference is provided for the design of an engineering control system.

Description

Technical Field

[0001] The present invention belongs to the technical field of control of flexible DC grid-connected systems for offshore wind power, and particularly relates to a method for controlling AC faults at an offshore station of an offshore wind power grid-connected system via a flexible DC grid-connected system and related devices. Background Art

[0002] In recent years, with the development of new energy power generation technology and power electronics technology, the technical solution of using VSC-HVDC (flexible DC) to transmit large-scale long-distance offshore wind power and connect it to the onshore power grid has been widely applied. Currently, dozens of offshore wind power grid-connected projects via flexible DC have been put into operation worldwide.

[0003] The offshore wind power grid-connected system via flexible DC has significant differences in DC control functions compared with traditional two-terminal grid-connected flexible DC. Due to the particularity of offshore wind farms, such as being far from land and having a harsh environment, its grid-connected system needs to have higher reliability and stability to cope with various complex working conditions and fault situations. AC faults are a common and highly probable fault type, and are also the working conditions that need to be focused on when designing the offshore wind power grid-connected system via flexible DC. For flexible DC, it is generally required to achieve AC fault ride-through and keep the DC from tripping under various AC fault conditions to ensure the stable operation of the system and the reliable supply of electricity.

[0004] However, there are some problems and deficiencies in the existing AC fault control strategies for offshore stations of offshore wind power grid-connected systems via flexible DC. In the AC fault control strategy for offshore stations of offshore wind power grid-connected systems via flexible DC, if the coordination with the low-voltage ride-through control of the wind turbine converters in the offshore wind farm is not considered and only the AC voltage control of the flexible DC offshore station is considered, there may be serious consequences such as DC tripping and wind turbine disconnection due to improper coordination between the flexible DC and the wind farm, affecting the safety and reliability of the system and increasing the maintenance cost and economic losses. Summary of the Invention

[0005] In view of this, the present invention proposes a method for controlling AC faults at an offshore station of an offshore wind power grid-connected system via a flexible DC grid-connected system and related devices, aiming to achieve the smooth start of the flexible DC through the coordinated control between the converter station of the offshore station of the offshore wind power flexible DC grid-connected system and the wind turbine converters in the wind farm, avoid overvoltage or overcurrent of equipment during the start-up process, and improve the operation safety and reliability of the flexible DC.

[0006] To achieve the above object, the technical solutions provided by the present invention are as follows:

[0007] In the first aspect, the present invention provides a method for controlling AC faults at an offshore station of an offshore wind power grid-connected system via a flexible DC grid-connected system, including the following steps:

[0008] During the AC fault of the offshore station, obtain the dq-axis current amplitudes in the dq-axis outer-loop control of the offshore station, and perform outer-loop current limiting control according to the preset outer-loop current limiting strategy; the outer-loop current limiting strategy is that when the dq-axis current amplitudes exceed the maximum short-circuit current allowed by the offshore station, reduce the dq-axis current limiting values proportionally according to the exceeding degree;

[0009] During the recovery stage after the AC fault of the offshore station is cleared, obtain 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;

[0010] Perform coordinated control between the converter station of the offshore 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;

[0011] In the integral link of the dq-axis inner-loop control of the offshore station, perform inner-loop integral control according to the preset inner-loop integral reset control strategy; the inner-loop integral reset control strategy is to enable integral reset during the AC fault and restart integration after obtaining the AC fault clearing flag;

[0012] In the modulation wave generation link of the dq-axis inner-loop control of the offshore station, perform AC voltage low-frequency oscillation suppression control function according to the preset additional modulation wave strategy for AC oscillation suppression; the additional modulation wave strategy for AC oscillation suppression is to automatically exit the AC low-frequency oscillation suppression function during the AC fault and automatically re-enter the AC low-frequency oscillation suppression function after obtaining the AC fault clearing flag.

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

[0014] When the dq-axis current amplitudes during the fault are within the maximum short-circuit current allowed by the offshore station, perform limiting according to the normal limiting;

[0015] When the dq-axis current amplitudes during the fault exceed the maximum short-circuit current allowed by the offshore station, reduce the d-axis current limiting value and the q-axis current limiting value proportionally according to the ratio of the dq-axis current amplitudes exceeding the maximum short-circuit current.

[0016] Furthermore, execute the outer-loop current limiting strategy 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] Use the multiplier to perform square operations on the d-axis reference current and the q-axis reference current respectively;

[0019] The square root operation is performed on the sum of the two square operation results by a square root calculator to obtain the amplitude of the dq-axis synthetic current;

[0020] The comparator is used to compare the amplitude of the dq-axis synthetic current 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 amplitude of the dq-axis synthetic current to obtain a proportionality factor for current limitation, and the proportionality factor is input to the current limiting link in the dq-axis outer loop control of the offshore station.

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

[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] Obtain 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;

[0025] Set the time required for the active power of the wind turbine converter controlled by the active power recovery rate controller to recover to the value before the fault after the fault is cleared 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 are used to represent the active power recovery rate and the outer loop voltage recovery rate respectively.

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

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

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

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

[0030] Furthermore, the AC low-frequency oscillation suppression function extracts the low-frequency component of the AC voltage from the filtering link of the AC oscillation suppression control of the offshore station, and then multiplies the low-frequency component by the damping coefficient and superimposes it on the modulation wave of the d-axis control to suppress the low-frequency component of the AC voltage.

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

[0032] In a second aspect, the present invention provides an AC fault control device for an offshore substation of a flexible DC grid-connected system for offshore wind power, including:

[0033] A first control module, configured to obtain the dq-axis current amplitudes in the dq-axis outer-loop control of the offshore substation during an AC fault of the offshore substation, and perform outer-loop current limiting control according to a preset outer-loop current limiting strategy; the outer-loop current limiting strategy is that when the dq-axis current amplitudes exceed the maximum short-circuit current allowed by the offshore substation, the current limiting values of the dq-axis are reduced proportionally according to the exceeded degree;

[0034] A second control module, configured to obtain the outer-loop voltage recovery rate in the dq-axis outer-loop control of the offshore substation and the active power recovery rate of the wind turbine converter during the recovery stage after the AC fault of the offshore substation is cleared, and perform coordinated control between the converter station of the offshore substation 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] A third control module, configured to obtain an AC fault clearing flag during the recovery stage after the AC fault of the offshore substation is cleared, and perform inner-loop integration control on the integration link in the dq-axis inner-loop control of the offshore substation according to a preset inner-loop integration clearing control strategy; the inner-loop integration clearing control strategy is to enable integration clearing during the AC fault, and restart integration after obtaining the AC fault clearing flag;

[0036] A fourth control module, configured to obtain an AC fault clearing flag during the recovery stage after the AC fault of the offshore substation is cleared, and perform AC voltage low-frequency oscillation suppression control function control on the modulation wave generation link in the dq-axis inner-loop control of the offshore substation according to a preset additional modulation wave strategy for AC oscillation suppression; the additional modulation wave strategy for AC oscillation suppression is to automatically exit the AC low-frequency oscillation suppression function during the AC fault, and automatically input the AC low-frequency oscillation suppression function after obtaining the AC fault clearing flag.

[0037] In a third aspect, the present invention provides a computer device, which includes a processor and a memory:

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

[0039] The processor executes an AC fault control method for an offshore substation of a flexible DC grid-connected system for offshore wind power as described in the first aspect according to the instructions of the computer program.

[0040] In a fourth aspect, the present invention provides a computer-readable storage medium, on which a computer program is stored. When the computer program is executed by a processor, it implements a method for controlling an AC fault of an offshore station in an offshore wind power integrated grid system via a flexible DC as described in the first aspect.

[0041] In summary, the present invention provides a method for controlling an AC fault of an offshore station in an offshore wind power integrated grid system via a flexible DC and related devices, including: during an AC fault of the offshore station, obtaining the dq-axis current amplitudes in the dq-axis outer loop control of the offshore station, and performing outer loop current limiting control according to a preset outer loop current limiting strategy; the outer loop current limiting strategy is that when the dq-axis current amplitudes exceed the maximum short-circuit current allowed by the offshore station, reducing the current limiting values of the dq-axis in proportion to the exceeding degree; during the recovery stage after the AC fault of the offshore station is cleared, 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; performing coordinated control between the converter station of the offshore 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; performing inner loop integral control according to a preset inner loop integral clearing control strategy in the integral link of the dq-axis inner loop control of the offshore station; the inner loop integral clearing control strategy is to enable integral clearing during the AC fault and restart the integral after obtaining the AC fault clearing flag; performing control function for suppressing low-frequency AC voltage oscillation according to a preset additional modulation wave strategy for suppressing AC oscillation in the modulation wave generation link of the dq-axis inner loop control of the offshore station; the additional modulation wave strategy for suppressing AC oscillation is to automatically exit the AC low-frequency oscillation suppression function during the AC fault and automatically input the AC low-frequency oscillation suppression function after obtaining the AC fault clearing flag. The present invention can achieve successful crossing of the AC fault of the offshore station in the offshore wind power integrated grid system via a flexible DC, provide reference and guidance for the design of the control system of the offshore wind power flexible DC grid connection project, reduce the risk of DC tripping and wind farm turbine disconnection, and improve the safety and reliability of the offshore wind power integrated grid system via a flexible DC. Description of the Drawings

[0042] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or in the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained according to these drawings.

[0043] Figure 1 It is a flowchart of a method for controlling an AC fault of an offshore station in an offshore wind power integrated grid system via a flexible DC provided by an embodiment of the present invention;

[0044] Figure 2 Topological schematic diagram of a dual - terminal offshore wind power integrated into the grid via a flexible DC system provided by the present invention;

[0045] Figure 3 Principle diagram of dq transformation of the valve - side voltage and current of the flexible DC converter at the offshore station provided by the embodiment of the present invention;

[0046] Figure 4 Principle diagram of the outer - loop controller of the offshore converter station provided by the embodiment of the present invention;

[0047] Figure 5 Principle diagram of the outer - loop current limiter of the offshore converter station provided by the embodiment of the present invention;

[0048] Figure 6 Principle diagram of the active power recovery rate limiter of the wind turbine converter in the offshore wind farm provided by the embodiment of the present invention;

[0049] Figure 7 Principle diagram of the inner - loop controller and integral reset control in the offshore converter station provided by the embodiment of the present invention;

[0050] Figure 8 Principle diagram of the AC oscillation suppression control of the offshore station provided by the embodiment of the present invention;

[0051] Figure 9 Overall technical scheme schematic diagram of the startup method of the offshore wind power integrated into the grid via a flexible DC system provided by the embodiment of the present invention;

[0052] Figure 10 Block diagram of the composition of an AC fault control device for an offshore station in an offshore wind power integrated into the grid via a flexible DC system provided by the embodiment of the present invention;

[0053] Figure 11 Block diagram of the composition of a computer device provided by the embodiment of the present invention. Specific embodiments

[0054] To make the objectives, features, and advantages of the present invention more obvious and understandable, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the embodiments described below are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0055] Please refer to Figure 1-2 , Figure 1 which shows an AC fault control method for an offshore station in an offshore wind power integrated into the grid via a flexible DC system; Figure 2 which shows the topology of a dual - terminal offshore wind power integrated into the grid via a flexible DC system.

[0056] As Figure 2 shown, before the alternating current output by the wind turbines in the offshore wind farm enters the converter valve of the offshore station, an AC fault may occur, and at this time, the offshore converter station is in a fault state. However, in the existing AC fault control strategies for the offshore station of the flexible DC grid-connected system for offshore wind power, if the coordination with the low-ride-through control of the wind turbine converters in the offshore station is not considered and only the AC voltage control of the flexible DC offshore station is considered, there may be serious consequences such as DC tripping and turbine disconnection due to improper coordination between the flexible DC and the wind farm, affecting the safety and reliability of the system, and increasing the maintenance cost and economic losses.

[0057] Therefore, in view of the above problems, the present invention provides an AC fault control method and related device for the offshore station of the flexible DC grid-connected system for offshore wind power. The following is a detailed introduction to the embodiments of an AC fault control method for the offshore station of the flexible DC grid-connected system for offshore wind power according to the present invention.

[0058] As Figure 1 shown, this embodiment provides an AC fault control method for the offshore station of the flexible DC grid-connected system for offshore wind power, including the following steps:

[0059] S1: During the AC fault of the offshore station, obtain the dq-axis current amplitudes in the dq-axis outer-loop control 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 that when the dq-axis current amplitudes exceed the maximum short-circuit current allowed by the offshore station, the current limit values of the dq-axis are reduced in the same proportion according to the exceeding degree.

[0060] It should be noted that in this step, by adopting the outer-loop current limiting strategy, when the current amplitude exceeds the preset maximum short-circuit current, the current limit values are reduced in the same proportion according to the exceeding degree. Through this dynamic current limiting adjustment method, the current can be effectively limited, preventing the short-circuit current of the offshore station from being too large and exceeding the switching breaking capacity under severe AC faults, causing risks such as equipment overcurrent, and improving the safety and reliability of the system.

[0061] S2: During the recovery stage after the AC fault of the offshore station is cleared, obtain 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 the AC fault clearance flag.

[0062] It should be noted that the outer-loop voltage recovery rate refers to the rate at which the DC voltage in the dq-axis outer-loop control of the offshore converter station returns to the normal level after the AC fault of the offshore station 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 unit returns to the level before the fault during the recovery process after the fault is cleared. The AC fault clearance flag is a flag or signal used to indicate that the AC fault has been successfully eliminated.

[0063] S3: Conduct coordinated control between the offshore 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.

[0064] It should be noted that in this step, according to the coordinated control strategy, the active power recovery rate of the wind turbine converter is adjusted to be less than the outer-loop voltage recovery rate. This can achieve the coordinated control between the offshore converter station of the offshore wind power flexible DC grid-connected system and the wind farm wind turbine converter, avoid voltage instability caused by too fast power recovery during the recovery process, achieve smooth recovery, and prevent the wind turbines from tripping off the grid.

[0065] S4: Conduct inner-loop integral control in the integral link of the dq-axis inner-loop control of the offshore converter station according to the preset inner-loop integral reset control strategy; the inner-loop integral reset control strategy is to enable integral reset during the AC fault and restart the integral after obtaining the AC fault clearing flag.

[0066] It should be noted that in this step, by adopting the inner-loop integral reset control strategy, during the fault, the integral link of the inner-loop control is reset to eliminate the influence of current fluctuations during the fault. After the fault is cleared, the integral link restarts the integral. This can reduce the influence of the fault on the system control accuracy, ensure the accuracy of the control strategy, and improve the dynamic response ability of the system.

[0067] S5: Conduct AC voltage low-frequency oscillation suppression control function in the modulation wave generation link of the dq-axis inner-loop control of the offshore converter station according to the preset additional modulation wave strategy for AC oscillation suppression; the additional modulation wave strategy for AC oscillation suppression is to automatically withdraw from the AC low-frequency oscillation suppression function during the AC fault and automatically resume the AC low-frequency oscillation suppression function after obtaining the AC fault clearing flag.

[0068] It should be noted that in this step, the additional modulation wave strategy for AC oscillation suppression is adopted. During the fault, the AC low-frequency oscillation suppression function is withdrawn; after the fault is cleared, the function is resumed. This can avoid unnecessary control interference during the fault, quickly stabilize the voltage during the recovery stage, and prevent the wind turbines from tripping off the grid.

[0069] This embodiment provides a method for controlling AC faults in an offshore station of an offshore wind power integrated system via a flexible DC grid connection. By adopting different control strategies during the fault period and the recovery stage, comprehensive protection and control of the offshore wind power grid connection system are achieved. Through means such as amplitude limiting control, coordinated control, integral reset, and oscillation suppression, it is beneficial to improve the voltage stability during AC faults and the recovery stage after clearance, provide a reliable and stable AC voltage for the wind farm, prevent the wind turbines from tripping off the grid, and ensure the stability and rapid recovery ability of the system. The control method proposed by the present invention also 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 maximum short-circuit current allowed by the offshore station, perform amplitude limiting 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, proportionally reduce the d-axis current limit value and the q-axis current limit value according to the ratio of the dq-axis current amplitude exceeding the maximum short-circuit current.

[0073] Please refer to Figure 3 , Figure 3 which shows the dq transformation of the voltage and current on the valve side of the flexible DC converter at the offshore station. During the AC fault at the offshore station, perform dq transformation on the voltage and current on the valve side of the flexible DC converter at the offshore station (such as Figure 3 ), generating the voltage and current on the dq axis. When the dq-axis current amplitude during the fault period is within the allowed maximum short-circuit current Imax, still perform amplitude limiting according to the normal amplitude limiting; when the dq current amplitude during the fault period exceeds Imax, proportionally reduce the current limit values on the dq axis, 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.

[0074] Before the AC fault at the offshore station, the current direction on the valve side is from the wind turbine to the DC converter (positive). When an AC fault occurs, the current direction on the valve side is from the DC converter to the fault point in the wind power AC field (negative). Especially during severe faults such as three-phase metallic grounding, if there is no additional outer-loop current limiter for the dq-axis current, a large fault current may be generated, exceeding the breaking capacity of the AC switch, which is not conducive to the clearance of AC faults.

[0075] Hypothesis: Under high-power operating conditions, the valve-side currents \(i_d\) and \(i_q\) before the occurrence of an AC fault at the offshore station are 0.8 p.u. and 0.1 p.u. respectively; during the fault, without additional outer-loop current limitation, the valve-side currents \(i_d\) and \(i_q\) are -0.8 p.u. and -0.1 p.u. respectively, and the generated dq-axis current amplitude is 0.81 p.u., which will exceed the limit of the maximum short-circuit current \(I_{max}=0.5\) p.u. allowed by the offshore station. Then the proposed outer-loop current limiter will come into effect, limiting the valve-side currents \(i_d\) and \(i_q\) during the fault to -0.49 p.u. and -0.062 p.u. respectively, thus meeting the requirements of the offshore station's fault current limitation.

[0076] In a further embodiment, the outer-loop current limitation strategy is implemented by adding an outer-loop current limiter to the dq-axis outer-loop control of the offshore station. Please refer to Figure 4 and Figure 5 , Figure 4 which shows the working principle of the outer-loop controller of the offshore converter station, Figure 5 which 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 limitation strategy, including performing square operations on the d-axis reference current \(i_{dref}\) and the q-axis reference current \(i_{qref}\) respectively using the multiplier; performing a square root operation on the sum of the two square operation results using the square root operator to obtain the dq-axis synthesized current amplitude \(I_{dq}\); comparing the dq-axis synthesized current amplitude \(I_{dq}\) with the allowed maximum short-circuit current \(I_{max}\) using the comparator and taking the smaller current value as the output; dividing the current value output by the comparator by the dq-axis synthesized current amplitude using the divider to obtain the proportionality factor \(K_{lim}\) for current limitation, and inputting the proportionality factor \(K_{lim}\) into the current limiting link in the dq-axis outer-loop control of the offshore station.

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

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

[0079] Set the time required for the active power of the wind turbine converter controlled by the active power recovery rate controller to recover from after fault clearing to before the fault 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 are used to represent the active power recovery rate and the outer loop voltage recovery rate respectively.

[0080] Please refer to Figure 6 , Figure 6 which shows the working principle of the active power recovery rate limiter of the wind turbine converter in the offshore wind farm. For the recovery stage after the AC fault clearing in the offshore substation, in the dq-axis outer loop control of the offshore substation, in the input AC voltage link of the d-axis control, add a link for the recovery rate of the AC voltage amplitude after the AC fault (such as Figure 4 ), and the time required for the voltage to recover from after fault clearing to the rated value is T1. At the same time, in the recovery link of the wind turbine converter after the AC fault clearing, add an active power recovery rate link (such as Figure 6 ), and the time required for the power to recover from after fault clearing to before the fault is T2. Then, by setting an appropriate recovery rate, make T2 less than T1 to ensure that during the fault recovery period, after the AC voltage recovers to stability, the output power of the wind farm turbines is recovered at a certain rate, which can effectively prevent the risk of DC tripping caused by overvoltage of the flexible DC voltage or overcurrent of the bridge arm due to too fast recovery of the turbine power.

[0081] In one embodiment, add an inner loop integral reset controller in the integral link of the dq-axis inner loop control of the offshore substation to execute the inner loop integral reset control strategy; using the inner loop integral reset controller to execute the inner loop integral reset control strategy includes:

[0082] The inner loop integral reset controller obtains the AC fault flag of the offshore substation; when the AC fault flag of the offshore substation is 1, it indicates a fault, and when it is 0, it indicates fault clearing;

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

[0084] Please refer to Figure 7 , Figure 7 which shows the schematic diagram of the inner loop controller and the integral reset control in the offshore converter station. For the recovery stage after the AC fault clearing in the offshore substation, in the integral link of the dq-axis inner loop control of the offshore substation, add an inner loop integral reset control that is turned on and off according to the flag of AC fault clearing (such as Figure 7), that is, during an AC fault (ACfault = 1), the integration is cleared. After the fault ends (ACfault = 0) and the AC voltage returns to the rated value, the integration starts again. By controlling the integration clearing, the stability of the AC voltage at the offshore station is improved, and the influence of current fluctuations during the fault on the voltage amplitude is reduced.

[0085] In one embodiment, the AC low-frequency oscillation suppression function extracts the low-frequency component of the AC voltage from the filtering link of the AC oscillation suppression control of the offshore station, multiplies the low-frequency component by a damping coefficient, and then superimposes it on the modulation wave of the d-axis control, thereby suppressing the low-frequency component of the AC voltage.

[0086] The AC low-frequency oscillation suppression function of this embodiment is mainly to suppress the low-frequency component in the AC voltage of the offshore station, because low-frequency oscillations may affect the stability of the power system, resulting in problems such as voltage fluctuations and increased equipment losses. In the power electronic control system, the d-axis control is an important part of controlling key parameters such as the active power of the system. Through this superimposition method, the characteristics of the modulation wave can be changed, and then the low-frequency component in the AC voltage can be suppressed.

[0087] In a further embodiment, a comparison link is also included before the filtering link. 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 uses the deviation signal as the input of the filtering link.

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

[0089] Please refer to Figure 9 , Figure 9The overall technical solution of the start-up method for the offshore wind power integrated grid system via flexible DC is shown in combination with the above embodiments. It mainly includes: 1) During the AC fault at the offshore station: In the dq-axis outer loop control of the offshore station, an outer loop current limiter is added. When the dq-axis current amplitude during the fault is within the allowable maximum current Imax range, the normal limiting is still carried out. When the dq-axis current amplitude during the fault exceeds Imax, the current limit 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) During the recovery stage after the AC fault at the offshore station is cleared: In the dq-axis outer loop control of the offshore station, in the input AC voltage link of the d-axis control, a recovery rate link of the AC voltage amplitude after the AC fault is added. The time required for the voltage to recover from the fault clearance to the rated value is T1. At the same time, in the recovery link of the wind turbine converter after the AC fault is cleared, an active power recovery rate link is added. The time required for the power to recover from the fault clearance to the value before the fault is T2. Then, by setting an appropriate recovery rate, T2 is made less than T1, ensuring that during the fault recovery period, after the AC voltage recovers to stability, the output power of the wind farm turbines is recovered at a certain rate, which can effectively prevent the risk of DC tripping caused by overvoltage of the flexible DC voltage or overcurrent of the bridge arm due to too fast recovery of the turbine power. 3) During the recovery stage after the AC fault at the offshore station is cleared: In the integral link of the dq-axis inner loop control of the offshore station, an inner loop integral reset control that is switched on and off according to the flag of the AC fault clearance is added, that is, during the AC fault, the integral reset is enabled, and after the fault ends and the AC voltage recovers to the rated value, the integration starts again. Through the integral reset control, the stability of the AC voltage at the offshore station is improved, and the influence of current fluctuations during the fault on the voltage amplitude is reduced. 4) During the recovery stage after the AC fault at the offshore station is cleared: In the modulation wave generation link of the dq-axis inner loop control of the offshore station, an AC voltage low-frequency oscillation suppression control function that is switched on and off according to the flag of the AC fault clearance is added, that is, during the AC fault, the AC low-frequency oscillation suppression function is automatically withdrawn; after the fault ends and the AC voltage recovers to the rated value, the low-frequency oscillation suppression function is automatically switched on, which is beneficial to quickly stabilizing the AC voltage after the fault and providing a stable and strong AC grid connection voltage for the wind farm, preventing the turbines from repeatedly entering the low voltage ride-through and tripping off the grid.

[0090] Based on the same inventive concept, an embodiment of the present application further provides a control device for an offshore substation AC fault of an offshore wind power integrated grid system via flexible DC to implement the above-mentioned offshore substation AC fault control method for an offshore wind power integrated grid system via flexible DC. The solution provided by this device to solve the problem is similar to the solution described in the above method. Therefore, the specific limitations in the embodiment of the control device for an offshore substation AC fault of an offshore wind power integrated grid system via flexible DC provided below can refer to the limitations on the offshore substation AC fault control method for an offshore wind power integrated grid system via flexible DC in the foregoing, and will not be repeated here.

[0091] Please refer to Figure 10 , an embodiment of the present invention further provides a control device for an offshore substation AC fault of an offshore wind power integrated grid system via flexible DC, including:

[0092] A first control module, configured to obtain the dq-axis current amplitude in the dq-axis outer loop control of the offshore substation during an offshore substation 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 that when the dq-axis current amplitude exceeds the maximum short-circuit current allowed by the offshore substation, the current limiting value of the dq-axis is reduced proportionally according to the exceeding degree;

[0093] A second control module, configured to obtain the outer loop voltage recovery rate in the dq-axis outer loop control of the offshore substation and the active power recovery rate of the wind turbine converter during the recovery stage after the offshore substation AC fault is cleared, and perform coordinated control between the offshore substation 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] A third control module, configured to obtain an AC fault clearing flag during the recovery stage after the offshore substation AC fault is cleared, and perform inner loop integral control on the integral link of the dq-axis inner loop control of the offshore substation according to a preset inner loop integral clearing control strategy; the inner loop integral clearing control strategy is to enable integral clearing during the AC fault, and then restart the integral after obtaining the AC fault clearing flag;

[0095] A fourth control module, configured to obtain an AC fault clearing flag during the recovery stage after the offshore substation AC fault is cleared, and perform AC voltage low-frequency oscillation suppression control function control on the modulation wave generation link of the dq-axis inner loop control of the offshore substation according to a preset additional modulation wave strategy for AC oscillation suppression; the additional modulation wave strategy for AC oscillation suppression is to automatically exit the AC low-frequency oscillation suppression function during the AC fault, and then automatically input the AC low-frequency oscillation suppression function after obtaining the AC fault clearing flag.

[0096] Those skilled in the art can clearly understand that, for the convenience and conciseness of description, only the division of the above-mentioned functional units and modules is used as an example. In actual applications, the above-mentioned functions can be allocated to different functional units and modules according to needs, that is, the internal structure of the system can be divided into different functional units or modules to complete all or part of the functions described above. Each functional unit and module in the embodiment can be integrated into a processing unit, or each unit can exist physically alone, or two or more units can be integrated into one unit. The above-mentioned integrated unit can be implemented in the form of hardware or in the form of a software functional unit. In addition, the specific names of the functional units and modules are only for the convenience of mutual distinction and do not limit the protection scope of this application. The specific working processes of the units and modules in the above-mentioned system can refer to the corresponding processes in the foregoing method embodiments and will not be elaborated here.

[0097] Referring to Figure 11 , an embodiment of the present invention further provides a computer device, including: a memory, a processor, and a computer program stored on the memory. When the computer program is executed on the processor, it implements the offshore AC fault control method of the offshore wind power integrated into the grid system via a flexible DC as described in any one of the above methods.

[0098] The computer device may be a computing device such as a desktop computer, a notebook, a palm computer, and a cloud server. The computer device may include, but is not limited to, a processor and a memory. Those skilled in the art can understand that Figure 11 merely an example of a computer device, which does not constitute a limitation on the computer device, and may include more or fewer components than shown in the figure, or combine some components, or different components. For example, it may also include input / output devices, network access devices, etc.

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

[0100] In some embodiments, the memory may be an internal storage unit of the computer device, such as the hard disk or memory of the computer device. In other embodiments, the memory may also be an external storage device of the computer device, such as a plug-in hard disk, a Smart Media Card (SMC), a Secure Digital (SD) card, a Flash Card, etc. equipped on the computer device. Further, the memory may also include both the internal storage unit and the external storage device of the computer device. The memory is used to store an operating system, application programs, a BootLoader, data, and other programs, such as the program code of the computer program. The memory may also be used to temporarily store data that has been output or will be output.

[0101] An embodiment of the present invention also provides a computer-readable storage medium, on which a computer program is stored. When the computer program is run by a processor, it implements the offshore AC fault control method of the offshore wind power integrated into the grid system via flexible DC as described in any one of the above methods.

[0102] In this embodiment, if the integrated unit is implemented in the form of a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on such an understanding, to implement all or part of the processes in the above embodiment methods of the present application, a computer program can be used to instruct relevant hardware to complete. The computer program can be stored in a computer-readable storage medium. When the computer program is executed by a processor, it can implement the steps of the above various method embodiments. Among them, the computer program includes computer program code, and the computer program code can be in the form of source code, object code, executable file, or some intermediate form, etc. The computer-readable medium can at least include: any entity or device that can carry the computer program code to the photographing device / terminal device, a recording medium, a computer memory, a read-only memory (ROM), a random access memory (RAM), an electrical carrier signal, a telecommunication signal, and a software distribution medium. For example, a USB flash drive, a mobile hard disk, a magnetic disk, or an optical disc, etc. In some jurisdictions, according to legislation and patent practice, the computer-readable medium cannot be an electrical carrier signal and a telecommunication signal.

[0103] In the above embodiments, the descriptions of the various embodiments have their own emphases. For the parts not detailed or recorded in a certain embodiment, reference can be made to the relevant descriptions of other embodiments.

[0104] Those of ordinary skill in the art will appreciate that the units and algorithm steps of each example described in combination with the embodiments disclosed herein can be implemented in electronic hardware, or in a combination of computer software and electronic hardware. Whether these functions are executed in hardware or software depends on the specific application and design constraints of the technical solution. A person skilled in the art can use different methods to implement the described functions for each specific application, but such implementation should not be considered to exceed the scope of this application.

[0105] In the embodiments disclosed in this 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 illustrative. For example, the division of the modules or units is only a logical function division. In actual implementation, there may be other division methods. For example, multiple units or components can be combined or integrated into another system, or some features can be ignored or not executed. Another point is that the displayed or discussed coupling or direct coupling or communication connection between each other can be through some interfaces. The indirect coupling or communication connection of the apparatus or unit can be in an electrical, mechanical or other form.

[0106] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements for some of the technical features. And these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for controlling AC faults at an offshore station of an offshore wind power flexible DC grid-connected system, characterized in that: The steps include: During an AC fault at the offshore station, the dq axis current amplitude in the dq axis outer loop control of the offshore station is obtained, and the outer loop current limiting control is performed according to a preset outer loop current limiting strategy; the outer loop current limiting strategy is to reduce the dq axis current limiting value in proportion to the degree of excess when the dq axis current amplitude exceeds the maximum short-circuit current allowed by the offshore station; In the recovery phase 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 the AC fault clearing flag are obtained; Coordinated control is performed 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; In the integral link of the dq axis inner loop control at the offshore station, the inner loop integral control is performed according to a 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 restart the integration after obtaining the AC fault clearing flag; In the modulation wave generation link of the dq axis inner loop control of the offshore station, the AC voltage low-frequency oscillation suppression control function is controlled 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 activate the AC low-frequency oscillation suppression function after obtaining the AC fault clearing flag.

2. The method for controlling AC faults at offshore stations of an offshore wind power flexible DC grid-connected system according to claim 1, characterized in that: The outer loop current limiting strategy includes: When the dq axis current amplitude during the fault period is within the maximum short-circuit current allowed by the offshore station, the amplitude is limited according to the normal amplitude limitation; When the dq axis current amplitude during the fault exceeds the maximum short-circuit current allowed by the offshore station, the d axis current amplitude limit value and the q axis current amplitude limit value are reduced in the same proportion according to the ratio in which the dq axis current amplitude exceeds the maximum short-circuit current.

3. The method for controlling AC faults at offshore stations of an offshore wind power flexible DC 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 outer loop control of the dq axis of the offshore station; The outer loop current limiter includes a multiplier, a square root operator, a comparator and a divider; The outer loop current limiter executes the outer loop current limiting strategy, including: Using the multiplier to perform square operations on the d-axis reference current and the q-axis reference current respectively; Using the square root operator to perform a square root operation on the sum of the two square operation results to obtain the dq axis composite current amplitude; Using the comparator to compare the dq axis composite current amplitude with the maximum allowed short-circuit current, and taking the larger current value as output; The divider is used to divide the current value output by the comparator by the dq axis composite current amplitude to obtain a proportional factor for limiting the current, and the proportional factor is input into the current limiting link in the dq axis outer loop control of the offshore station.

4. The method for controlling AC faults at offshore stations of an 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 to the wind turbine converter and adding an outer loop voltage recovery rate controller to the dq axis outer loop control of the offshore station; Utilizing the active power recovery rate controller and the outer loop voltage recovery rate device to execute the coordinated control strategy includes: Obtaining from the outer loop voltage recovery rate device the time required for the input AC voltage of the d-axis control to recover to the rated value after the fault is cleared; The time required for the active power recovery rate controller to restore the active power of the wind turbine converter to the state before the fault is cleared is set to be 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 are respectively used to represent the active power recovery rate and the outer loop voltage recovery rate.

5. The method for controlling AC faults at offshore stations of an offshore wind power flexible DC grid-connected system according to claim 1, characterized in that: In the integral link of the inner loop control of the dq axis at the offshore station, an inner loop integral clearing controller is added to execute the inner loop integral clearing control strategy; The inner loop integral clearing controller is used to execute the inner loop integral clearing control strategy, including: The inner loop integral clearing 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; When the offshore station AC fault flag is 1, the inner loop integral clearing controller clears the integral result of the integral link and uses it as the output of the integral link; when the offshore station AC fault flag is 0, the inner loop integral clearing controller restarts the integral link.

6. The method for controlling AC faults at offshore stations of an offshore wind power flexible DC grid-connected system according to claim 1, characterized in that: The AC low-frequency oscillation suppression function extracts the low-frequency component of the AC voltage from the filtering link of the AC oscillation suppression control of the offshore station, multiplies the low-frequency component by the damping coefficient and then superimposes it on the modulation wave of the d-axis control, thereby suppressing the low-frequency component of the AC voltage.

7. The method for controlling AC faults at offshore stations of an offshore wind power flexible DC grid-connected system according to claim 6, characterized in that: The filtering link also includes a comparison link, which 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 uses the deviation signal as the input of the filtering link.

8. An offshore station AC fault control device for an offshore wind power flexible DC grid-connected system, characterized in that: include: The first control module is used 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 reduce the dq axis current limiting value in proportion to the degree of excess when the dq axis current amplitude exceeds the maximum short-circuit current allowed by the offshore station; 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 offshore station dq axis and perform coordinated control between the offshore station converter station and the wind turbine converter according to a preset coordinated control strategy in the recovery stage after the AC fault of the offshore station is cleared; 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 used to obtain an AC fault clearing flag in the recovery stage after the AC fault of the offshore station is cleared, and perform inner loop integral control in the integral link of the dq axis inner loop control of the offshore station according to a 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 restart the integration after obtaining the AC fault clearing flag; The fourth control module is used to obtain the AC fault clearing flag in the recovery stage after the AC fault of the offshore station is cleared, and to control the AC voltage low-frequency oscillation suppression control function according to the preset AC oscillation suppression additional modulation wave strategy in the modulation wave generation 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 automatically start the AC low-frequency oscillation suppression function after obtaining the AC fault clearing flag.

9. A computer device, characterized in that: The device comprises a processor and a memory: The memory is used to store a computer program and send instructions of the computer program to the processor; The processor executes an offshore station AC fault control method for an offshore wind power flexible DC grid-connected system according to instructions of the computer program as described in any one of claims 1-7.

10. A computer-readable storage medium, characterized in that: The computer-readable storage medium stores a computer program, and when the computer program is executed by the processor, an offshore station AC fault control method for an offshore wind power flexible DC grid-connected system according to any one of claims 1 to 7 is implemented.

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