Overvoltage suppression method for offshore wind power low-frequency transmission system under M3C blocking condition

Abstract

The present invention discloses a method for suppressing overvoltage of an offshore wind power low-frequency transmission system under M3C locking conditions. Aiming at the overvoltage problem of the low-frequency power grid at the sending end of the offshore wind power low-frequency transmission system under M3C locking conditions, when the M3C converter of the offshore wind power low-frequency transmission system is locked, a disconnection signal is sent to the switchgear and low-frequency wind turbine circuit breakers on both sides of the M3C. Furthermore, through coordinated control of the M3C converter, the wind turbine converter and the crowbar circuit, the overvoltage level of the sending-end low-frequency system under the M3C fault locking conditions is effectively reduced, thereby reducing the insulation requirements and costs of the equipment. The method has good application prospects.

Description

Technical Field

[0001] The present invention belongs to the technical field of renewable energy power generation, and in particular relates to an overvoltage suppression method for an offshore wind power low-frequency transmission system under M3C locking conditions. Background Art

[0002] Currently, the construction of offshore wind farms is gradually evolving from short-distance, small-capacity installations to deep-sea, large-scale installations. When offshore wind farms exceed a certain distance from shore, the traditional industrial frequency AC transmission method presents the problem of excessive reactive power consumption by the equivalent capacitance of the submarine cable. Because the transmission frequency of the flexible low-frequency transmission method is significantly lower than that of the industrial frequency transmission method, the charging power of the low-frequency AC submarine cable is low, which can improve the transmission capacity of the AC submarine cable. Furthermore, compared with the flexible DC transmission method, flexible low-frequency transmission offers the advantages of current zero-crossing interruption and easy networking. It can directly output low-frequency power from wind turbines without the need for an offshore converter platform. Therefore, the flexible low-frequency AC transmission method for offshore wind power combines the advantages of industrial frequency AC transmission and flexible DC transmission, offering advantages in medium- and long-distance offshore wind power transmission scenarios.

[0003] In an offshore wind power low-frequency transmission system, if a system failure causes the onshore M3C converter to lock out, the offshore low-frequency grid voltage becomes uncontrolled. The offshore wind turbine will continue to charge the low-frequency transmission cable, causing the sending-end low-frequency grid voltage to rise. When the overvoltage protection threshold of the offshore wind turbine is reached, the wind turbine will disconnect from the grid. During this process, the sending-end low-frequency grid will generate a large overvoltage, which not only threatens the safe and stable operation of the entire system but also places high demands on the overvoltage insulation level of the equipment and submarine cable, significantly increasing construction costs. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for suppressing overvoltage in an offshore wind power low-frequency transmission system under an onshore M3C locked condition. The method aims to solve the problem that under an onshore M3C fault locked condition, the overvoltage of the low-frequency power grid at the sending end of the offshore wind power low-frequency transmission system threatens the safe and stable operation of the entire system through a low-cost solution.

[0005] In order to achieve the above-mentioned purpose of the invention, the present method adopts the following technical solutions:

[0006] A method for suppressing overvoltage in an offshore wind power low-frequency transmission system under M3C lockout conditions, characterized in that: when an onshore M3C converter fault lockout occurs, a disconnection signal is sent to onshore power frequency valve-side switchgear, onshore low-frequency valve-side switchgear, and low-frequency wind turbine circuit breakers, and an active power reduction signal is sent to the low-frequency wind turbines;

[0007] After receiving the active power reduction signal, the low-frequency wind turbine adjusts the active power reduction signal flag to the first flag, and adjusts the active power reduction signal flag to the second flag after the wind turbine crowbar circuit has been continuously put into operation for the maximum time. When the active power reduction signal flag is the first flag, the active power reference value of the low-frequency wind turbine side converter gradually decreases to zero, and the wind turbine crowbar circuit is actively put into operation. When the active power reduction signal flag is the second flag, the low-frequency wind turbine side converter and the wind crowbar circuit resume the control strategy operation before the M3C locking condition occurs.

[0008] The onshore power frequency valve side switchgear, onshore low frequency valve side switchgear, and low frequency wind turbine circuit breaker are disconnected after receiving the disconnection signal;

[0009] The output power of the low-frequency wind turbine side converter gradually drops to zero, and the surplus power accumulated on the low-frequency wind turbine DC bus during the power reduction process is absorbed by the wind turbine crowbar circuit.

[0010] On the basis of adopting the above technical solutions, the present invention may also adopt the following further technical solutions, or use these further technical solutions in combination:

[0011] The offshore wind power low-frequency transmission system includes: low-frequency wind turbines, low-frequency collection submarine cables, offshore low-frequency low-voltage side switchgear, offshore low-frequency step-up transformers, offshore low-frequency high-voltage side switchgear, low-frequency transmission submarine cables, onshore low-frequency grid-side switchgear, onshore low-frequency connection transformers, onshore low-frequency valve-side switchgear, onshore M3C converters, onshore power-frequency valve-side switchgear, onshore power-frequency connection transformers, and onshore power-frequency grid-side switchgear;

[0012] The low-frequency wind turbine includes: a wind turbine, a low-frequency wind turbine machine-side converter, a low-frequency wind turbine grid-side converter, a low-frequency wind turbine transformer, a low-frequency wind turbine circuit breaker, and a wind turbine crowbar circuit;

[0013] The wind turbine is connected to the low-frequency wind turbine transformer through the low-frequency wind turbine machine-side converter and the low-frequency wind turbine grid-side converter. The generated electricity is boosted by the low-frequency wind turbine transformer, connected to the low-frequency collection submarine cable through the low-frequency wind turbine circuit breaker, and then collected by the low-frequency collection submarine cable. It is connected to the offshore low-frequency step-up transformer through the offshore low-frequency low-voltage side switchgear, and then connected to the low-frequency sending submarine cable through the offshore low-frequency high-voltage side switchgear. After landing, the low-frequency sending submarine cable is connected to the onshore low-frequency grid. The side switchgear is connected to the onshore low-frequency connection transformer, and after voltage transformation, it is connected to the onshore M3C converter through the onshore low-frequency valve-side switchgear. After the onshore M3C converter is converted to the power frequency, it is connected to the onshore power frequency connection transformer through the onshore power frequency valve-side switchgear. After voltage transformation, it is connected to the onshore power frequency bus through the onshore power frequency grid-side switchgear. The wind turbine crowbar circuit is connected in parallel to the low-frequency fan DC bus between the low-frequency fan machine-side converter and the low-frequency fan grid-side converter.

[0014] The onshore M3C converter adopts a constant capacitor voltage and reactive power control strategy on the power frequency side to maintain the capacitor voltage stability of its full-bridge sub-module, and a constant low-frequency AC voltage control strategy on the low-frequency side to maintain the voltage stability of the offshore low-frequency AC power grid.

[0015] The grid-side converter of the low-frequency wind turbine adopts a constant DC bus voltage and reactive power control strategy, responsible for maintaining the stability of the DC bus voltage of the low-frequency wind turbine.

[0016] The low-frequency wind turbine side converter adopts a fixed active power and reactive power control strategy. When the active power reduction signal flag is the second flag, the active power reference value is given by the maximum power tracking module. When the active power reduction signal flag is the first flag, the active power reference value is given by the fault stage power instruction module and gradually decreases to zero.

[0017] The fan crowbar circuit is put into operation when the active power reduction signal flag is the second flag and the low-frequency fan DC bus voltage exceeds the first voltage threshold. It is shut down when the low-frequency fan DC bus voltage falls below the second voltage threshold. When the active power reduction signal flag is the first flag, the fan crowbar circuit is actively put into operation. The specific control logic is as follows:

[0018] When the low-frequency fan DC bus voltage U dc Greater than the first voltage threshold U dcr1 , or when the active power signal flag is reduced to the first flag, the fan crowbar circuit is put into operation; when the low-frequency fan DC bus voltage U dc Less than the second voltage threshold U dcr2 , and the active power reduction signal flag is the second flag, the fan crowbar circuit is cut out and runs; when the low-frequency fan DC bus voltage U dc Greater than the second voltage threshold U dcr2 and is less than the first voltage threshold U dcr1 , and when the active power reduction signal flag is the second flag, the operating state of the wind turbine crowbar circuit remains unchanged.

[0019] The low-frequency wind turbine side converter control system includes: rotor position observation module, machine-side Park conversion module, maximum power tracking module, fault phase power command module, machine-side power control module, machine-side current control module, machine-side Park inverse conversion module, and machine-side modulation module;

[0020] The rotor position observation module obtains the machine-side voltage phase, the machine-side Park transformation module transforms the machine-side current from the three-phase stationary coordinate system to the two-phase synchronous rotating coordinate system, and the machine-side power control module controls the machine-side active power and reactive power through the machine-side power PI controller. When the active power signal reduction flag is the second flag, the active power reference value is given by the maximum power tracking module. When the active power signal reduction flag is the first flag, the active power reference value is given by the fault stage power instruction module, and the active power reference value gradually decreases to zero; the machine-side current control module controls the machine-side current through the machine-side current PI controller, and the machine-side Park inverse transformation module transforms the machine-side output voltage reference value from the two-phase synchronous rotating coordinate system to the three-phase stationary coordinate system. The machine-side modulation module generates a switching signal based on the machine-side output voltage reference value to control the wind turbine machine-side converter.

[0021] In the above switchgear, the onshore low-frequency grid-side switchgear refers to the switchgear located on the low-frequency grid side of the onshore low-frequency connection transformer, the onshore low-frequency valve-side switchgear refers to the switchgear located between the onshore low-frequency connection transformer and the onshore M3C converter, the onshore power frequency valve-side switchgear refers to the switchgear located between the onshore power frequency connection transformer and the onshore M3C converter, and the onshore power frequency grid-side switchgear refers to the switchgear located on the power frequency grid side of the onshore power frequency connection transformer.

[0022] The beneficial effects of the present invention are as follows: by adopting the technical solution of the present invention, there is no need to impose excessively high requirements on the overvoltage insulation level of equipment and submarine cables. When a fault lockout occurs in the M3C converter of the offshore wind power low-frequency transmission system, a disconnection signal is simultaneously sent to the switchgear and low-frequency wind turbine circuit breakers on both sides of the M3C. Through coordinated control of the M3C converter, the wind turbine converter and the crowbar circuit, the overvoltage level of the sending-end low-frequency system under the M3C lockout condition can be effectively reduced, thereby reducing the insulation requirements and costs of the equipment. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a flow chart of the method for suppressing overvoltage in an offshore wind power low-frequency transmission system under the M3C locking condition of the present invention.

[0024] Figure 2 This is a typical topology diagram of the offshore wind power low-frequency transmission system of the present invention.

[0025] Figure 3 This is a typical topology diagram of the onshore M3C converter in the present invention.

[0026] Figure 4 This is a specific example system schematic diagram of the onshore M3C converter control system in the present invention.

[0027] Figure 5This is a specific example system schematic diagram of the low-frequency wind turbine grid-side converter control system of the present invention.

[0028] Figure 6 This is a specific example system schematic diagram of the low-frequency wind turbine side converter control system of the present invention.

[0029] Figure 7 The simulated waveform of the offshore 220kV power grid voltage when the traditional method is used under M3C fault blocking.

[0030] Figure 8 The figure shows the simulated waveform of the offshore 220 kV power grid voltage when the method of the present invention is used under M3C fault blocking. DETAILED DESCRIPTION

[0031] In order to describe the present invention more specifically, the technical solution of the present invention is described in detail below with reference to the accompanying drawings and specific embodiments. In the following implementation manner, the low voltage is 66 kV and the high voltage is 220 kV.

[0032] like Figure 2 As shown, in an embodiment of the present invention, the offshore wind power low-frequency transmission system includes: a low-frequency wind turbine, a low-frequency collection submarine cable 6, an offshore low-frequency 66kV switchgear 7, an offshore low-frequency step-up transformer 8, an offshore low-frequency 220kV switchgear 9, a low-frequency transmission submarine cable 10, an onshore low-frequency grid-side switchgear 11, an onshore low-frequency connection transformer 12, an onshore low-frequency valve-side switchgear 13, an onshore M3C converter 14, an onshore power frequency valve-side switchgear 15, an onshore power frequency connection transformer 16, and an onshore power frequency grid-side switchgear 17.

[0033] The low-frequency wind turbine includes: a wind turbine 1 , a low-frequency wind turbine machine-side converter 2 , a low-frequency wind turbine grid-side converter 3 , a low-frequency wind turbine transformer 4 , a low-frequency wind turbine circuit breaker 5 , and a wind turbine crowbar circuit 18 .

[0034] The wind turbine 1 is connected to the low-frequency wind turbine transformer 4 through the low-frequency wind turbine machine-side converter 2 and the low-frequency wind turbine grid-side converter 3. The generated electricity is boosted by the low-frequency wind turbine transformer 4, and then connected to the low-frequency collection submarine cable 6 through the low-frequency wind turbine circuit breaker 5. After being collected by the low-frequency collection submarine cable 6, it is connected to the offshore low-frequency step-up transformer 8 through the offshore low-frequency 66kV switchgear 7. After being boosted to 220kV, it is connected to the low-frequency sending submarine cable 10 through the offshore low-frequency 220kV switchgear 9. After the low-frequency sending submarine cable 10 lands, it is connected to the low-frequency sending submarine cable 10 through the land. The onshore low-frequency grid-side switchgear 11 is connected to the onshore low-frequency connection transformer 12, and after stepping down the voltage, it is connected to the onshore M3C converter 14 through the onshore low-frequency valve-side switchgear 13. After the onshore M3C converter 14 converts the frequency to the power frequency, it is connected to the onshore power frequency connection transformer 16 through the onshore power frequency valve-side switchgear 15. After stepping up the voltage, it is connected to the onshore power frequency bus through the onshore power frequency grid-side switchgear 17. The wind turbine crowbar circuit 18 is connected in parallel to the low-frequency wind turbine DC bus between the low-frequency wind turbine machine-side converter 2 and the grid-side converter 3.

[0035] like Figure 3 As shown, in this embodiment of the present invention, the onshore M3C converter 14 is composed of three frequency conversion modules, each of which contains three bridge arm branches. The entire M3C converter consists of nine bridge arm branches. Each bridge arm branch is composed of multiple cascaded full-bridge sub-modules 19 and bridge arm reactors 20 connected in series. The three-phase AC systems on both sides of the M3C converter are connected through the bridge arm branches. Each phase of the system on one side is connected to the three phases of the system on the other side through three bridge arm branches. Among them, SM represents the full-bridge sub-module, and N represents the number of full-bridge sub-modules in each bridge arm branch.

[0036] like Figure 1 As shown, in the embodiment of the present invention, the specific implementation method of the overvoltage suppression method of the offshore wind power low-frequency transmission system under the M3C locking condition is as follows:

[0037] When the onshore M3C converter 14 fails and locks out, a disconnection signal is simultaneously sent to the onshore power frequency valve side switchgear 15, the onshore low frequency valve side switchgear 13, and the low frequency wind turbine circuit breaker 5, and a signal to reduce active power is sent to the wind turbine 1.

[0038] Since the onshore M3C converter 14 is locked, it loses its control over the sending-end low-frequency grid voltage, causing the sending-end low-frequency grid voltage to become distorted.

[0039] After the low-frequency wind turbine receives the active power reduction signal, it adjusts the active power reduction signal flag to the first flag, and the active power reference value of the low-frequency wind turbine machine-side converter 2 gradually decreases to zero. The wind turbine crowbar circuit 18 actively starts operation and quickly reduces the active power output by the low-frequency wind turbine grid-side converter 3.

[0040] Since different switchgear have different breaking times and different times of receiving action signals, the onshore power frequency valve side switchgear 15, the onshore low frequency valve side switchgear 13, and the low frequency fan circuit breaker 5 are disconnected successively.

[0041] The output power of the low-frequency wind turbine side converter 2 gradually decreases to zero, and the surplus power accumulated on the low-frequency wind turbine DC bus during the power reduction process is absorbed by the wind turbine crowbar circuit 18.

[0042] After the maximum continuous input time of the wind turbine crowbar circuit, the active power reduction signal flag is adjusted to the second flag, and the low-frequency wind turbine side converter 2 and the wind turbine crowbar circuit 18 resume the control strategy operation before the M3C locking condition occurs.

[0043] like Figure 4 As shown, in an embodiment of the present invention, an onshore M3C converter control system for implementing an overvoltage suppression method for an offshore wind power low-frequency transmission system under an M3C locking condition includes: a low-frequency AC voltage control module 101, a low-frequency current control module 102, a low-frequency Park inverse transformation module 103, a low-frequency internal circulating current control module 104, a power frequency phase-locked loop module 105, a capacitor voltage and reactive power control module 106, a power frequency current control module 107, a power frequency Park inverse transformation module 108, a power frequency internal circulating current control module 109, a bridge arm voltage calculation module 110, and a modulation module 111.

[0044] In the embodiment of the present invention, the low-frequency AC voltage control module 101 controls the low-frequency d, q Shaft voltage U gdq1 The low-frequency AC voltage PI controller is used to control the voltage so that it follows the given reference value. u gd1ref and u gq1ref , the output of the low-frequency AC voltage PI controller is used as d, q Reference value of shaft current i vd1ref and i vq1ref The specific implementation of the low-frequency AC voltage control module 101 is as follows:

[0045]

[0046]

[0047] in: F PI1 ( s ) is the transfer function of the low-frequency AC voltage PI controller, k p1 is the proportionality coefficient, ki1 is the integration coefficient, i vd1ref , i vq1ref Corresponding to the current vector I vdq1ref of d axis, q Axis component.

[0048] The low frequency current control module 102 controls the low frequency d, q Shaft current I vdq1 Use low frequency current PI controller to control it so that it follows d, q Reference value of shaft current i vd1ref and i vq1ref The output of the low-frequency current PI controller passes through the limiting link and is used as the low-frequency output voltage U vdq1 The specific implementation of the low-frequency current control module is as follows:

[0049]

[0050] in: F PI2 ( s ) is the transfer function of the low-frequency current PI controller, k p2 is the proportionality coefficient, k i2 is the integration coefficient, L 1 is the equivalent inductance including the onshore low-frequency connection transformer 12 and the bridge arm reactor 20, u vd1 , u vq1 Corresponding to the voltage vector U vdq1 of d axis, q Axis component, ω r1 is the angular frequency of the low-frequency grid voltage.

[0051] The low-frequency internal circulation control module 104 controls the low-frequency internal circulation I cαβ1 As the feedback value of the low-frequency internal circulation controller, the output of the low-frequency internal circulation controller is used as the low-frequency internal circulation voltage U cαβ1 The specific implementation of the low-frequency internal circulation control module 104 is as follows:

[0052]

[0053] in, u cα1 , u cβ1 Corresponding to the voltage vector U cαβ1 of α axis, β Axis component, i cα1 , i cβ1 Corresponding to the current vector I cαβ1 of α axis, β Axis component.

[0054] The power frequency phase-locked loop module 105 is based on the power frequency grid voltage U gabc2 , calculate the power frequency grid voltage phase θ g2 .

[0055] The capacitor voltage and reactive power control module 106 controls the average value of the capacitor voltage U c and power frequency reactive power Q g2 The capacitor voltage and reactive power are controlled by PI controllers to follow the given reference values respectively. U cref and Q g2ref , the outputs of capacitor voltage and reactive power PI controllers are respectively used as d, q Reference value of shaft current i vd2ref and i vq2ref The specific implementation of the capacitor voltage and reactive power control module 106 is as follows:

[0056]

[0057]

[0058] in: F PI3 ( s ) is the transfer function of the capacitor voltage and reactive power PI controller, k p3 is the proportionality coefficient, k i3 is the integration coefficient, i vd2ref , i vq2refCorresponding to the current vector I vdq2ref of d axis, q Axis component.

[0059] The power frequency current control module 107 controls the power frequency d, q Shaft current I vdq2 Use the power frequency current PI controller to control it to follow the reference value I vdq2ref The output of the power frequency current PI controller is used as the power frequency output voltage U vdq2 The specific implementation of the power frequency current control module 107 is as follows:

[0060]

[0061] in: F PI4 ( s ) is the transfer function of the power frequency current PI controller, k p4 is the proportionality coefficient, k i4 is the integration coefficient, L 2 is the equivalent inductance including the onshore power frequency connection transformer 16 and the bridge arm reactor 20, u vd2 , u vq2 Corresponding to the voltage vector U vdq2 of d axis, q Axis component, ω g2 is the angular frequency of the power grid voltage.

[0062] The power frequency internal circulation control module 109 controls the power frequency internal circulation I cαβ2 As the feedback value of the power frequency internal circulation controller, there is no need to perform rotation coordinate transformation on it, and control can be achieved in the stationary coordinate system. The output of the power frequency internal circulation controller is used as the power frequency internal circulation voltage U cαβ2 The specific implementation of the power frequency internal circulation control module 109 is as follows:

[0063]

[0064] in, u cα2 , u cβ2Corresponding to the voltage vector U cαβ2 of α axis, β Axis component, i cα2 , i cβ2 Corresponding to the current vector I cαβ2 of α axis, β Axis component.

[0065] The bridge arm voltage calculation module 110 uses the low frequency output voltage U vαβ1 , low-frequency internal circulating voltage U cαβ1 , power frequency output voltage U vαβ2 , power frequency internal circulating current voltage U cαβ2 , the reference voltages of the nine bridge arms of the M3C frequency-changing valve are calculated.

[0066] The modulation module 111 generates a modulation instruction according to the reference voltage of the nine bridge arms of the M3C frequency-changing valve to realize the control of the M3C frequency-changing valve.

[0067] like Figure 5 As shown, in an embodiment of the present invention, a low-frequency wind turbine grid-side converter control system for implementing an overvoltage suppression method for an offshore wind power low-frequency transmission system under M3C locking conditions includes: a phase-locked loop module 201, a grid-side Park conversion module 202, a DC bus voltage and reactive power control module 203, a grid-side current control module 204, a grid-side Park inverse conversion module 205, a grid-side modulation module 206, and a crowbar circuit control module 207.

[0068] The phase-locked loop module 201 obtains the grid voltage phase based on the grid voltage. The grid-side Park transformation module 202 transforms the grid-side current from a three-phase stationary coordinate system to a two-phase synchronous rotating coordinate system. The DC bus voltage and reactive power control module 203 controls the DC bus voltage and grid-side output reactive power of the low-frequency wind turbine through a DC bus voltage and reactive power PI controller. The grid-side current control module 204 controls the grid-side current through a grid-side current PI controller. The grid-side Park inverse transformation module 205 transforms the grid-side output voltage reference value from a two-phase synchronous rotating coordinate system to a three-phase stationary coordinate system. The grid-side modulation module 206 generates a switching signal based on the grid-side output voltage reference value to control the grid-side converter of the low-frequency wind turbine.

[0069] The crowbar circuit control module 207 actively puts the fan crowbar circuit into operation when the active power reduction signal flag is the first flag. When the active power reduction signal flag is the second flag, the crowbar circuit is put into operation when the low-frequency fan DC bus voltage exceeds a first voltage threshold, and is shut down when the low-frequency fan DC bus voltage falls below a second voltage threshold. The specific control logic is as follows:

[0070] When the low-frequency fan DC bus voltage U dc Greater than the first voltage threshold U dcr1 , or when the active power signal flag is reduced to the first flag, the fan crowbar circuit is put into operation; when the low-frequency fan DC bus voltage U dc Less than the second voltage threshold U dcr2 , and the active power reduction signal flag is the second flag, the fan crowbar circuit is cut out and runs; when the low-frequency fan DC bus voltage U dc Greater than the second voltage threshold U dcr2 and is less than the first voltage threshold U dcr1 , and when the active power reduction signal flag is the second flag, the operating state of the wind turbine crowbar circuit remains unchanged.

[0071] like Figure 6 As shown, in this embodiment of the present invention, a low-frequency wind turbine-side converter control system for implementing an overvoltage suppression method for an offshore wind power low-frequency transmission system under M3C blocking conditions includes: a rotor position observation module 301, a machine-side Park conversion module 302, a maximum power tracking module 303, a fault-phase power instruction module 304, a machine-side power control module 305, a machine-side current control module 306, a machine-side Park inverse conversion module 307, and a machine-side modulation module 308. A PMSG is a permanent magnet synchronous wind generator.

[0072] The rotor position observation module 301 obtains the machine-side voltage phase, the machine-side Park transformation module 302 transforms the machine-side current from a three-phase stationary coordinate system to a two-phase synchronous rotating coordinate system, and the machine-side power control module 305 controls the machine-side active power and reactive power through a machine-side power PI controller. When the active power reduction signal flag is the second flag, the active power reference value is given by the maximum power tracking module 303. When the active power reduction signal flag is the first flag, the active power reference value is given by the fault stage power instruction module 304, and the active power reference value gradually decreases to zero; the machine-side current control module 306 controls the machine-side current through a machine-side current PI controller, and the machine-side Park inverse transformation module 307 transforms the machine-side output voltage reference value from a two-phase synchronous rotating coordinate system to a three-phase stationary coordinate system. The machine-side modulation module 308 generates a switching signal based on the machine-side output voltage reference value to control the wind turbine machine-side converter.

[0073] In an embodiment of the present invention, a simulation model of an offshore wind power low-frequency transmission system is built based on the PSCAD simulation platform, and the effectiveness of the method is verified through simulation research. Figure 7 Figure 3. Simulated waveform of the offshore 220kV low-frequency grid voltage when the traditional method is used under M3C fault blocking. The red, blue, and green waveforms represent the voltage waveforms of phases A, B, and C, respectively. At 5.047s, the onshore M3C converter is blocked due to a fault. At 5.107s, the onshore power-frequency valve-side switchgear is disconnected. At 5.146s, the onshore low-frequency valve-side switchgear is disconnected. At 5.185s, the offshore wind turbine-side converter is blocked due to overvoltage. The overvoltage phase amplitude of the 220kV low-frequency grid reaches 385kV. Figure 8 The following is a simulated waveform of the offshore 220kV low-frequency grid voltage when the method of the present invention is used. The red, blue, and green waveforms represent the voltage waveforms of phases A, B, and C, respectively. At 5.047s, the onshore M3C converter was locked due to a fault. At 5.051s, the wind turbine crowbar circuit was put into operation. At 5.107s, the onshore power frequency valve-side switchgear was disconnected. At 5.146s, the onshore low-frequency valve-side switchgear was disconnected. At 5.151s, the low-frequency wind turbine circuit breaker was disconnected. During the entire process, the overvoltage of the 220kV low-frequency grid was effectively suppressed, with a maximum phase amplitude of 335kV. The above simulation results verify the effectiveness of the proposed method for suppressing overvoltage in offshore wind power low-frequency transmission systems under the M3C locked operating condition.

[0074] The above description of the embodiments is intended to facilitate understanding and application of the present invention by those skilled in the art. It will be apparent that those skilled in the art can readily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without requiring inventive effort. Therefore, the present invention is not limited to the above embodiments, and improvements and modifications made by those skilled in the art based on the disclosure of the present invention should fall within the scope of protection of the present invention.

Claims

1. A method for suppressing overvoltage in an offshore wind power low-frequency transmission system under M3C locking conditions, characterized by: When the onshore M3C converter fault lockout occurs, a disconnection signal is sent to the onshore power frequency valve-side switchgear, onshore low-frequency valve-side switchgear, and low-frequency wind turbine circuit breaker, and a signal to reduce active power is sent to the low-frequency wind turbine; After the low-frequency wind turbine receives the active power reduction signal, it adjusts the active power reduction signal flag to the first flag, and adjusts the active power reduction signal flag to the second flag after the maximum continuous input time of the wind turbine crowbar circuit; when the active power reduction signal flag is the first flag, the active power reference value of the low-frequency wind turbine side converter gradually decreases to zero, and the wind turbine crowbar circuit is actively put into operation; when the active power reduction signal flag is the second flag, the low-frequency wind turbine side converter and the wind crowbar circuit resume the control strategy operation before the M3C locking condition occurs; when the low-frequency wind turbine DC bus voltage U dc Greater than the first voltage threshold U dcr1 , or when the active power signal flag is reduced to the first flag, the fan crowbar circuit is put into operation; when the low-frequency fan DC bus voltage U dc Less than the second voltage threshold U dcr2 , and when the active power reduction signal flag is the second flag, the fan crowbar circuit is cut out and runs; when the low-frequency fan DC bus voltage U dc Greater than the second voltage threshold U dcr2 And is less than the first voltage threshold U dcr1 , and when the active power reduction signal flag is the second flag, the wind turbine crowbar circuit operating state remains unchanged; the onshore power frequency valve side switchgear, the onshore low-frequency valve side switchgear, and the low-frequency wind turbine circuit breaker are disconnected after receiving the disconnection signal; The output power of the low-frequency wind turbine side converter gradually drops to zero. During the power reduction process, the surplus power accumulated on the low-frequency wind turbine DC bus is absorbed by the wind turbine crowbar circuit.

2. The method for suppressing overvoltage in an offshore wind power low-frequency transmission system under M3C locking conditions according to claim 1, characterized in that: The offshore wind power low-frequency transmission system includes: low-frequency wind turbines, low-frequency collection submarine cables, offshore low-frequency low-voltage side switchgear, offshore low-frequency step-up transformers, offshore low-frequency high-voltage side switchgear, low-frequency transmission submarine cables, onshore low-frequency grid-side switchgear, onshore low-frequency connection transformers, onshore low-frequency valve-side switchgear, onshore M3C converters, onshore power-frequency valve-side switchgear, onshore power-frequency connection transformers, and onshore power-frequency grid-side switchgear; The low-frequency wind turbine includes: a wind turbine, a low-frequency wind turbine machine-side converter, a low-frequency wind turbine grid-side converter, a low-frequency wind turbine transformer, a low-frequency wind turbine circuit breaker, and a wind turbine crowbar circuit; The wind turbine is connected to the low-frequency wind turbine transformer through the low-frequency wind turbine machine-side converter and the low-frequency wind turbine grid-side converter. The generated electricity is boosted by the low-frequency wind turbine transformer, connected to the low-frequency collection submarine cable through the low-frequency wind turbine circuit breaker, and then collected by the low-frequency collection submarine cable. It is connected to the offshore low-frequency step-up transformer through the offshore low-frequency low-voltage side switchgear, and then connected to the low-frequency sending submarine cable through the offshore low-frequency high-voltage side switchgear. After landing, the low-frequency sending submarine cable is connected to the onshore low-frequency grid. The side switchgear is connected to the onshore low-frequency connection transformer, and after voltage transformation, it is connected to the onshore M3C converter through the onshore low-frequency valve-side switchgear. After the onshore M3C converter is converted to the power frequency, it is connected to the onshore power frequency connection transformer through the onshore power frequency valve-side switchgear. After voltage transformation, it is connected to the onshore power frequency bus through the onshore power frequency grid-side switchgear. The wind turbine crowbar circuit is connected in parallel to the low-frequency fan DC bus between the low-frequency fan machine-side converter and the low-frequency fan grid-side converter.

3. The method for suppressing overvoltage in an offshore wind power low-frequency transmission system under M3C locking conditions according to claim 1, characterized in that: The onshore M3C converter adopts a constant capacitor voltage and reactive power control strategy on the power frequency side to maintain the capacitor voltage stability of its full-bridge sub-module, and a constant low-frequency AC voltage control strategy on the low-frequency side to maintain the voltage stability of the offshore low-frequency AC power grid.

4. The method for suppressing overvoltage in an offshore wind power low-frequency transmission system under M3C locking conditions according to claim 2, characterized in that: The grid-side converter of the low-frequency wind turbine adopts a constant DC bus voltage and reactive power control strategy to maintain the stability of the DC bus voltage of the low-frequency wind turbine; The low-frequency wind turbine side converter adopts a fixed active power and reactive power control strategy. When the active power reduction signal flag is the second flag, the active power reference value is given by the maximum power tracking module. When the active power reduction signal flag is the first flag, the active power reference value is given by the fault stage power instruction module and gradually decreases to zero.

5. The method for suppressing overvoltage in an offshore wind power low-frequency transmission system under M3C locking conditions according to claim 1, characterized in that: The low-frequency wind turbine side converter control system includes: rotor position observation module, machine-side Park conversion module, maximum power tracking module, fault phase power command module, machine-side power control module, machine-side current control module, machine-side Park inverse conversion module, and machine-side modulation module; The rotor position observation module obtains the machine-side voltage phase, the machine-side Park transformation module transforms the machine-side current from the three-phase stationary coordinate system to the two-phase synchronous rotating coordinate system, and the machine-side power control module controls the machine-side active power and reactive power through the machine-side power PI controller. When the active power signal reduction flag is the second flag, the active power reference value is given by the maximum power tracking module. When the active power signal reduction flag is the first flag, the active power reference value is given by the fault stage power instruction module, and the active power reference value gradually decreases to zero; the machine-side current control module controls the machine-side current through the machine-side current PI controller, and the machine-side Park inverse transformation module transforms the machine-side output voltage reference value from the two-phase synchronous rotating coordinate system to the three-phase stationary coordinate system. The machine-side modulation module generates a switching signal based on the machine-side output voltage reference value to control the wind turbine machine-side converter.

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