Overvoltage suppression method for M3C locking working condition offshore wind power low-frequency power transmission system

Abstract

The invention discloses an M3C locking working condition offshore wind power low-frequency power transmission system overvoltage suppression method, which aims at the overvoltage problem of a low-frequency power grid at the sending end of an offshore wind power low-frequency power transmission system under an M3C locking working condition, and comprises the following steps of: when an M3C converter of the offshore wind power low-frequency power transmission system is locked, sending a disconnection signal to switch equipment on two sides of the M3C and a low-frequency fan circuit breaker; and through coordinated control of the M3C converter, the fan converter and the crowbar circuit, the overvoltage level of a sending-end low-frequency system under the M3C fault locking working condition is effectively reduced, so that the equipment insulation requirement and cost are reduced, and the method has a good application prospect.

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] At present, the construction of offshore wind farms is gradually developing from short distance and small capacity to deep sea and large scale. When the offshore wind farm is more than a certain distance from the shore, the traditional industrial frequency AC transmission method has the problem of excessive reactive power consumption of the equivalent capacitance of the submarine cable. Since the transmission frequency of the flexible low-frequency transmission method is significantly lower than the industrial frequency transmission method, the charging power of the low-frequency AC submarine cable is small, which can improve the transmission capacity of the AC submarine cable. At the same time, compared with the flexible DC transmission method, the flexible low-frequency transmission has the advantages of current zero-crossing breaking and easy networking. It can use wind turbines to directly output low-frequency electricity without the need for an offshore converter platform. Therefore, the offshore wind power flexible low-frequency AC transmission method has the advantages of both industrial frequency AC transmission and flexible DC transmission, and has advantages in medium and long-distance offshore wind power transmission scenarios.

[0003] In the offshore wind power low-frequency transmission system, when a system failure causes the onshore M3C converter to lock, the offshore low-frequency grid voltage is out of control, and the offshore wind turbines will continue to charge the low-frequency transmission submarine cable, causing the voltage of the low-frequency grid at the sending end to increase. When the overvoltage protection threshold of the offshore wind turbine is reached, the wind turbine will be disconnected from the grid. In the above process, the low-frequency grid at the sending end will generate a large overvoltage, which will not only threaten the safe and stable operation of the entire system, but also put forward high requirements on the overvoltage insulation level of the equipment and submarine cables, greatly increasing the cost of engineering construction. Summary of the invention

[0004] The purpose of the present invention is to provide a method for suppressing overvoltage of an offshore wind power low-frequency transmission system under an onshore M3C locked condition, and to solve the problem that 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 under an onshore M3C fault locked condition through a low-cost solution.

[0005] In order to achieve the above-mentioned invention object, the present method adopts the following technical scheme: An overvoltage suppression method for an offshore wind power low-frequency transmission system under M3C locking conditions, characterized in that: when an onshore M3C converter fault lockout occurs, a disconnection signal is sent to an onshore power frequency valve side switchgear, an onshore low-frequency valve side switchgear, and a low-frequency wind turbine circuit breaker, and an active power reduction signal is sent to the low-frequency wind turbine; 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 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 turbine crowbar circuit resume the control strategy operation before the M3C locking condition occurs; The onshore power frequency valve side switchgear, onshore low frequency valve side switchgear, and low frequency fan circuit breaker are disconnected after receiving the disconnection signal; The output power of the low-frequency fan side converter gradually drops to zero, and the surplus power accumulated on the low-frequency fan DC bus during the power reduction process is absorbed by the fan crowbar circuit.

[0006] 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: The offshore wind power low-frequency transmission system includes: low-frequency wind turbines, low-frequency power 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 comprises: 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 generator 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. After the generated electricity is stepped up by the low-frequency wind turbine transformer, it is connected to the low-frequency power collection cable through the low-frequency wind turbine circuit breaker. After being collected by the low-frequency power collection cable, it is connected to the offshore low-frequency step-up transformer through the offshore low-frequency low-voltage side switchgear. After stepping up, it is connected to the low-frequency sending submarine cable through the offshore low-frequency high-voltage side switchgear. After the low-frequency sending submarine cable lands, it is connected to the low-frequency power transmission cable through the onshore low-frequency grid. The side switch device 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 switch device. After the onshore M3C converter is converted to industrial frequency, it is connected to the onshore industrial frequency connection transformer through the onshore industrial frequency valve-side switch device, and after voltage transformation, it is connected to the onshore industrial frequency bus through the onshore industrial frequency grid-side switch device; 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.

[0007] The onshore M3C converter adopts a constant capacitor voltage and reactive power control strategy on the industrial frequency side to maintain the capacitor voltage stability of its full-bridge submodule, and a fixed low-frequency AC voltage control strategy on the low-frequency side to maintain the voltage stability of the offshore low-frequency AC power grid.

[0008] 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.

[0009] The low-frequency wind turbine side converter adopts a fixed active power and reactive power control strategy. When the active power signal flag is reduced to the second flag, the active power reference value is given by the maximum power tracking module. When the active power signal flag is reduced to the first flag, the active power reference value is given by the fault stage power instruction module and gradually decreases to zero.

[0010] The fan crowbar circuit, when the active power reduction signal flag is the second flag, when the low-frequency fan DC bus voltage exceeds the first voltage threshold, is put into operation, and when the low-frequency fan DC bus voltage is lower than the second voltage threshold, it is cut out of operation; when the active power reduction signal flag is the first flag, the fan crowbar circuit is actively put into operation, and the specific control logic is as follows: When the DC bus voltage of the low-frequency fan 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 operating state of the wind turbine crowbar circuit remains unchanged.

[0011] The low-frequency wind turbine machine-side converter control system includes: a rotor position observation module, a machine-side Park conversion module, a maximum power tracking module, a fault stage power command module, a machine-side power control module, a machine-side current control module, a machine-side Park inverse conversion module, and a 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 flag is reduced to the second flag, the active power reference value is given by the maximum power tracking module, and when the active power signal flag is reduced to 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 according to the machine-side output voltage reference value to control the wind turbine machine-side converter.

[0012] 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 is the switchgear located on the power-frequency grid side of the onshore power-frequency connection transformer.

[0013] The beneficial effect of the present invention is that: by adopting the technical solution of the present invention, there is no need to make excessively high requirements on the overvoltage insulation level of the equipment and the submarine cable. When the fault lockout of the M3C converter of the offshore wind power low-frequency transmission system occurs, a disconnection signal is sent to the switch equipment and low-frequency wind turbine circuit breakers on both sides of the M3C at the same time, and through the coordinated control of the M3C converter, the wind turbine converter and the crowbar circuit, the overvoltage level of the low-frequency system at the sending end under the M3C lockout condition can be effectively reduced, thereby reducing the insulation requirements and costs of the equipment. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] 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.

[0015] Figure 2 This is a typical topological diagram of the offshore wind power low-frequency transmission system of the present invention.

[0016] Figure 3 This is a typical topological diagram of the onshore M3C converter in the present invention.

[0017] Figure 4 It is a specific example system schematic diagram of the onshore M3C converter control system in the present invention.

[0018] Figure 5This is a specific example system schematic diagram of the low-frequency wind turbine grid-side converter control system of the present invention.

[0019] Figure 6 This is a specific example system schematic diagram of the low-frequency wind turbine side converter control system of the present invention.

[0020] Figure 7 This is the simulated waveform of the offshore 220kV power grid voltage when the traditional method is used under M3C fault blocking.

[0021] Figure 8 It is the simulated waveform of the offshore 220kV power grid voltage when the method of the present invention is adopted under M3C fault blocking. DETAILED DESCRIPTION

[0022] In order to describe the present invention more specifically, the technical solution of the present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. In the following implementation manner, the low voltage is 66 kV and the high voltage is 220 kV.

[0023] 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 switch device 7, an offshore low-frequency step-up transformer 8, an offshore low-frequency 220kV switch device 9, a low-frequency transmission submarine cable 10, an onshore low-frequency grid-side switch device 11, an onshore low-frequency connection transformer 12, an onshore low-frequency valve-side switch device 13, an onshore M3C converter 14, an onshore power frequency valve-side switch device 15, an onshore power frequency connection transformer 16, and an onshore power frequency grid-side switch device 17.

[0024] The low-frequency wind turbine comprises: 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.

[0025] 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, connected to the low-frequency power collection cable 6 through the low-frequency wind turbine circuit breaker 5, collected by the low-frequency power collection cable 6, connected to the offshore low-frequency boosting transformer 8 through the offshore low-frequency 66kV switchgear 7, and connected to the low-frequency sending submarine cable 10 through the offshore low-frequency 220kV switchgear 9 after boosting to 220kV; the low-frequency sending submarine cable 10 is landed and connected to the low-frequency sending submarine cable 10 through the land The upper low-frequency grid-side switch device 11 is connected to the onshore low-frequency connection transformer 12, and after reducing the voltage, it is connected to the onshore M3C converter 14 through the onshore low-frequency valve-side switch device 13. After the onshore M3C converter 14 is converted to the industrial frequency, it is connected to the onshore industrial frequency connection transformer 16 through the onshore industrial frequency valve-side switch device 15. After boosting, it is connected to the onshore industrial frequency bus through the onshore industrial frequency grid-side switch device 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.

[0026] like Figure 3 As shown, in the embodiment of the present invention, the onshore M3C converter 14 is composed of three frequency conversion modules, each of which includes three bridge arm branches. The entire M3C converter is composed of nine bridge arm branches, each of which 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, and 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. 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: When the onshore M3C converter 14 fails and is locked, a disconnection signal is 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 at the same time, and an active power reduction signal is sent to the wind turbine 1.

[0027] Since the onshore M3C converter 14 is locked, the control function on the sending-end low-frequency grid voltage is lost, and the sending-end low-frequency grid voltage becomes distorted.

[0028] After the low-frequency wind turbine receives the active power reduction signal, it adjusts the active power reduction signal flag to the first flag, the active power reference value of the low-frequency wind turbine side converter 2 gradually decreases to zero, and the wind turbine crowbar circuit 18 actively starts operation to quickly reduce the active power output by the low-frequency wind turbine grid-side converter 3.

[0029] Since the breaking time of different switch devices and the time of receiving the action signal are different, the onshore power frequency valve side switch device 15, the onshore low frequency valve side switch device 13, and the low frequency fan circuit breaker 5 are disconnected successively.

[0030] The output power of the low-frequency fan side converter 2 gradually decreases to zero, and the surplus power accumulated on the low-frequency fan DC bus during the power reduction process is absorbed by the fan crowbar circuit 18.

[0031] 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.

[0032] 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 circulation 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 circulation control module 109, a bridge arm voltage calculation module 110, and a modulation module 111.

[0033] 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 to follow 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:

[0034]

[0035] in: F PI1 ( s ) is the transfer function of the low-frequency AC voltage PI controller, k p1 is the proportionality coefficient, k i1is the integration coefficient, i vd1ref , i vq1ref Corresponding to the current vector I vdq1ref of d axis, q Axis component.

[0036] The low frequency current control module 102 controls the low frequency d, q Shaft current I vdq1 A low-frequency current PI controller is used to control it to follow 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:

[0037] 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.

[0038] 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:

[0039] in, ucα1 , 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.

[0040] 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 .

[0041] 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 controller 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:

[0042]

[0043] in: F PI3 ( s ) is the transfer function of capacitor voltage and reactive power PI controller, k p3 is the proportionality coefficient, k i3 is the integration coefficient, i vd2ref , i vq2ref Corresponding to the current vector I vdq2ref of d axis, q Axis component.

[0044] The power frequency current control module 107 is used for 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:

[0045] 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 industrial frequency grid voltage.

[0046] 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 transform the rotating coordinates, and the control can be realized 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:

[0047] in, u cα2 , u cβ2 Corresponding to the voltage vector U cαβ2 of α axis, β Axis component, i cα2 , icβ2 Corresponding to the current vector I cαβ2 of α axis, β Axis component.

[0048] 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 voltage U cαβ2 , the reference voltage of the nine bridge arms of the M3C frequency-changing valve is calculated.

[0049] 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.

[0050] like Figure 5 As shown, in an embodiment of the present invention, a low-frequency wind turbine grid-side converter control system that implements 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.

[0051] The phase-locked loop module 201 obtains the grid voltage phase according to the grid voltage, the grid-side Park transformation module 202 transforms the grid-side current from the three-phase stationary coordinate system to the 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 the DC bus voltage and reactive power PI controller, the grid-side current control module 204 controls the grid-side current through the grid-side current PI controller, the grid-side Park inverse transformation module 205 transforms the grid-side output voltage reference value from the two-phase synchronous rotating coordinate system to the three-phase stationary coordinate system, and the grid-side modulation module 206 generates a switching signal according to the grid-side output voltage reference value to control the grid-side converter of the low-frequency wind turbine.

[0052] 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 low-frequency fan DC bus voltage is put into operation when it exceeds the first voltage threshold, and the low-frequency fan DC bus voltage is cut out of operation when it is lower than the second voltage threshold; the specific control logic is as follows: When the DC bus voltage of the low-frequency fan U dcGreater 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 operating state of the wind turbine crowbar circuit remains unchanged.

[0053] like Figure 6 As shown, in the embodiment of the present invention, the low-frequency wind turbine side converter control system for realizing the overvoltage suppression method of the offshore wind power low-frequency transmission system under the M3C locking condition includes: a rotor position observation module 301, a machine-side Park conversion module 302, a maximum power tracking module 303, a fault stage 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. PMSG is a permanent magnet synchronous wind generator.

[0054] 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 the three-phase stationary coordinate system to the two-phase synchronous rotating coordinate system, the machine-side power control module 305 realizes the control of the machine-side active power and reactive power through the machine-side power PI controller, when the active power signal flag is reduced to the second flag, the active power reference value is given by the maximum power tracking module 303, when the active power signal flag is reduced to 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 realizes the control of the machine-side current through the machine-side current PI controller, the machine-side Park inverse transformation module 307 transforms the machine-side output voltage reference value from the two-phase synchronous rotating coordinate system to the three-phase stationary coordinate system, and the machine-side modulation module 308 generates a switching signal according to the machine-side output voltage reference value to realize the control of the wind turbine machine-side converter.

[0055] In the 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 7This is the simulated waveform of the offshore 220kV low-frequency power grid voltage when the traditional method is used under M3C fault lockout, where 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 locked 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, and at 5.185s, the offshore wind turbine grid-side converter is locked due to overvoltage, and the overvoltage phase amplitude of the 220kV low-frequency power grid reaches 385kV. Figure 8 The simulation waveform of the offshore 220kV low-frequency grid voltage when the method of the present invention is used, wherein the red, blue and green waveforms represent the voltage waveforms of the three phases A, B and C respectively; at 5.047s, the onshore M3C converter is locked due to a fault, at 5.051s, the wind turbine crowbar circuit is put into operation, at 5.107s, the onshore power frequency valve side switchgear is disconnected, at 5.146s, the onshore low-frequency valve side switchgear is disconnected, and at 5.151s, the low-frequency wind turbine circuit breaker is disconnected. During the whole process, the overvoltage of the 220kV low-frequency grid is effectively suppressed, and the maximum phase amplitude is 335kV. Through the above simulation results, the effectiveness of the overvoltage suppression method of the offshore wind power low-frequency transmission system under the M3C locking condition proposed by the present invention can be verified.

[0056] The above description of the embodiments is to facilitate the understanding and application of the present invention by those skilled in the art. It is obvious that those skilled in the art can easily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without creative work. Therefore, the present invention is not limited to the above embodiments, and improvements and modifications made to the present invention by those skilled in the art based on the disclosure of the present invention should be 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 in that: 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 an active power reduction signal is sent to the low frequency wind turbine; 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 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 turbine crowbar circuit resume the control strategy operation before the M3C locking condition occurs; The onshore power frequency valve side switchgear, onshore low frequency valve side switchgear, and low frequency fan circuit breaker are disconnected after receiving the disconnection signal; The output power of the low-frequency fan side converter gradually drops to zero. During the power reduction process, the surplus power accumulated on the DC bus of the low-frequency fan is absorbed by the fan crowbar circuit.

2. According to the method for suppressing overvoltage of an offshore wind power low-frequency transmission system in an M3C locking condition in claim 1, it is characterized in that: The offshore wind power low-frequency transmission system includes: low-frequency wind turbines, low-frequency power 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 comprises: 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 generator 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. After the generated electricity is stepped up by the low-frequency wind turbine transformer, it is connected to the low-frequency power collection cable through the low-frequency wind turbine circuit breaker. After being collected by the low-frequency power collection cable, it is connected to the offshore low-frequency step-up transformer through the offshore low-frequency low-voltage side switchgear. After stepping up, it is connected to the low-frequency sending submarine cable through the offshore low-frequency high-voltage side switchgear. After the low-frequency sending submarine cable lands, it is connected to the low-frequency power transmission cable through the onshore low-frequency grid. The side switch device 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 switch device. After the onshore M3C converter is converted to industrial frequency, it is connected to the onshore industrial frequency connection transformer through the onshore industrial frequency valve-side switch device, and after voltage transformation, it is connected to the onshore industrial frequency bus through the onshore industrial frequency grid-side switch device; 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. According to the method for overvoltage suppression of an offshore wind power low-frequency transmission system in M3C locking condition of claim 1, it is characterized by: The onshore M3C converter adopts a constant capacitor voltage and reactive power control strategy on the industrial frequency side to maintain the capacitor voltage stability of its full-bridge submodule, and a fixed 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. According to claim 2, a method for suppressing overvoltage in an offshore wind power low-frequency transmission system under M3C locking conditions, 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 signal flag is reduced to the second flag, the active power reference value is given by the maximum power tracking module. When the active power signal flag is reduced to 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 condition according to claim 1, characterized in that: The fan crowbar circuit, when the active power reduction signal flag is the second flag, when the low-frequency fan DC bus voltage exceeds the first voltage threshold, is put into operation, and when the low-frequency fan DC bus voltage is lower than the second voltage threshold, it is cut out of operation; when the active power reduction signal flag is the first flag, the fan crowbar circuit is actively put into operation, and the specific control logic is as follows: When the DC bus voltage of the low-frequency fan 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 the active power reduction signal flag is the second flag, the fan crowbar circuit operating state remains unchanged 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 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 operating state of the wind turbine crowbar circuit remains unchanged.

6. The method for suppressing overvoltage in an offshore wind power low-frequency transmission system under M3C locking condition according to claim 1, characterized in that: The low-frequency wind turbine machine-side converter control system includes: a rotor position observation module, a machine-side Park conversion module, a maximum power tracking module, a fault stage power command module, a machine-side power control module, a machine-side current control module, a machine-side Park inverse conversion module, and a 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 flag is reduced to the second flag, the active power reference value is given by the maximum power tracking module, and when the active power signal flag is reduced to 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 according to the machine-side output voltage reference value to control the wind turbine machine-side converter.

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