Offshore wind power low-frequency networking system fault ride-through method based on hybrid topology

Abstract

The invention discloses an offshore wind power low-frequency networking system fault ride-through method based on hybrid topology, and the method comprises the steps: when a land power node power frequency AC power grid breaks down, through the cooperative control of a land voltage node M3C current converter, a land power node MMC current converter and an offshore low-frequency wind turbine generator, the fault ride-through of the offshore wind power low-frequency networking system is realized under different operation conditions; different surplus power consumption strategies are adopted, fault ride-through of the offshore wind power low-frequency networking system based on the hybrid topology is achieved, an energy consumption device does not need to be additionally built, and the method has good application prospects.

Description

Technical Field

[0001] The present invention belongs to the field of renewable energy power generation, and in particular relates to a hybrid topology-based offshore wind power low-frequency networking system fault ride-through method. 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 the medium and long-distance offshore wind power networking and transmission scenarios.

[0003] In the existing offshore wind power low-frequency networking scheme, all receiving nodes use M3C converters, resulting in high project construction costs. In fact, it is not necessary for all receiving nodes to have the ability to establish grid voltage. It is possible to consider using M3C topology for one receiving node and a hybrid topology of diodes and MMC for the other nodes, thereby reducing the overall construction cost of the project while ensuring reliable operation of the system. When a grid fault occurs in an onshore receiving node using a hybrid topology of diodes and MMC, the output power capacity of the node is limited, and surplus power will be generated in the offshore wind power low-frequency networking system, posing a serious threat to the safe and stable operation of the system. Since the system contains multiple power electronic devices such as M3C converters, DRU converters, MMC converters, and low-frequency wind turbine converters, how to achieve system fault crossing through the coordination between different devices is an urgent problem to be solved, and there is currently a lack of effective fault crossing methods. Summary of the invention

[0004] The purpose of the present invention is to solve the problem of surplus power generated by an offshore wind power low-frequency networking system based on a hybrid topology when a receiving-end power grid fails, and to provide a fault ride-through method for an offshore wind power low-frequency networking system based on a hybrid topology. Through the coordinated cooperation of an M3C converter, an MMC converter and a low-frequency wind turbine generator set, smooth fault ride-through of the offshore wind power low-frequency networking system is achieved.

[0005] In order to achieve the above-mentioned object of the invention, the present invention adopts the following technical scheme:

[0006] A hybrid topology-based offshore wind power low-frequency networking system fault ride-through method, characterized in that the hybrid topology-based offshore wind power low-frequency networking system comprises: an offshore low-frequency wind turbine, a low-frequency collection submarine cable, M offshore low-frequency booster stations, 1 onshore voltage node and N-1 onshore power nodes, the above-mentioned M+N nodes are connected through a low-frequency transmission submarine cable to form an offshore low-frequency power grid, and each node is connected to the offshore low-frequency power grid through at least two low-frequency transmission submarine cables; low-frequency circuit breakers are set at both ends of each section of the low-frequency transmission submarine cable; the onshore voltage node includes an M3C low-frequency connection transformer, an M3C converter , M3C industrial frequency connection transformer, M3C low-frequency connection transformer is connected to the onshore M3C converter, low-frequency electric energy is converted into industrial frequency electric energy through the M3C converter, and then connected to the industrial frequency AC power grid through the M3C industrial frequency connection transformer; the onshore power node includes a DRU connection transformer, a DRU converter, an MMC converter, and an MMC connection transformer, the DRU connection transformer is connected to the DRU converter, rectified into DC electric energy through the DRU converter and then connected to the MMC converter, inverted into industrial frequency AC electric energy through the MMC converter and then connected to the onshore industrial frequency AC power grid through the MMC connection transformer;

[0007] The M3C converter control system for implementing the fault ride-through method includes: a low-frequency voltage reference value calculation module, a low-frequency voltage control module, a low-frequency current control module, a low-frequency Park inverse transformation module, a low-frequency internal circulation control module, a power frequency phase-locked loop module, a capacitor voltage and reactive power control module, a power frequency current control module, a power frequency Park inverse transformation module, a power frequency internal circulation control module, a bridge arm voltage calculation module, and a modulation module;

[0008] The MMC converter control system for implementing the fault ride-through method comprises: an active power control module, a DC bus voltage and reactive power control module, a differential mode current control module, a Park inverse transformation module, a common mode current control module, a bridge arm voltage calculation module, and a modulation module;

[0009] When a fault occurs in the power frequency AC grid of a certain onshore power node, the voltage of the power frequency AC grid of the onshore power node decreases, the output current of the MMC converter reaches the limit value, the output power of the MMC converter is limited, and the power output capacity of the onshore power node decreases; the fault ride-through method performs the following fault ride-through steps:

[0010] First, determine whether the output current of the MMC converter of other onshore power nodes has reached the limit value. If the output current of the MMC converter of other onshore power nodes has not reached the limit value, increase the active power reference value of other onshore power nodes that have not reached the limit value, and consume the surplus power through other onshore power nodes that have not reached the limit value to achieve system fault ride-through; after the fault is eliminated, adjust the active power reference value of other onshore power nodes that consume the surplus power to the state before the fault occurs to achieve fault ride-through;

[0011] If the output current of the MMC converters at other onshore power nodes has reached the limit value, in order to maintain the voltage and frequency stability of the offshore low-frequency power grid, the active power flowing into the low-frequency side of the M3C converter at the onshore voltage node will increase;

[0012] Further determine whether the output current on the power frequency side of the M3C converter has reached the limit value. If the output current on the power frequency side of the M3C converter has not reached the limit value, no action is required. The power that cannot be delivered due to the power node fault is transmitted through the M3C converter, thereby achieving system fault ride-through.

[0013] If the output current on the power frequency side of the M3C converter reaches the limit value, the output power on the power frequency side of the M3C converter is limited, and the surplus power inside the M3C converter will charge the M3C submodule capacitor, causing the M3C submodule capacitor voltage to increase; using the low-frequency voltage reference value calculation module, according to the average value of the M3C submodule capacitor voltage, adjust the low-frequency voltage reference value of the M3C converter to reduce the AC voltage on the low-frequency side of the M3C converter;

[0014] After the AC voltage on the low-frequency side of the M3C converter is reduced, the offshore low-frequency wind turbine enters low-voltage ride-through operation, and the wind turbine crowbar circuit is put into operation to consume the surplus power inside the wind turbine; after the fault is eliminated, the low-frequency voltage of the M3C converter is adjusted to the rated value, the wind turbine crowbar circuit is shut down, the offshore low-frequency wind turbine resumes normal operation, and the fault ride-through process ends.

[0015] Furthermore, the low-frequency voltage reference value calculation module in the onshore M3C converter control system calculates the low-frequency voltage reference value by the following method:

[0016]

[0017] u gq1ref =0

[0018] Among them, u gd1ref and u gq1ref is the low frequency voltage reference value U gdq1ref The d and q axis components, U c is the average value of the capacitor voltage of the M3C submodule.

[0019] Furthermore, the active power control module in the onshore MMC converter control system calculates the DC bus voltage reference value by the following method:

[0020] U dc2ref =U dc2N -F PI1 (s)(P s2ref -P s2 )

[0021]

[0022] Where: P s2ref is the active power reference value, P s2 is the active power, U dc2ref is the DC bus voltage reference value, U dc2N is the rated voltage of the DC bus, F PI1 (s) is the transfer function of the active power PI controller, k p1 is the proportionality coefficient, k i1 is the integration coefficient.

[0023] Due to the adoption of the technical solution of the present invention, the present invention can realize fault riding of the offshore wind power low-frequency networking system based on hybrid topology under different operating conditions through the coordinated control of the onshore voltage node M3C converter, the onshore power node MMC converter and the offshore low-frequency wind turbine set, without the need to construct additional energy-consuming devices, and has good application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 The present invention is a flow chart of a fault ride-through method for an offshore wind power low-frequency networking system based on a hybrid topology.

[0025] Figure 2 This is a typical topology diagram of the offshore wind power low-frequency networking system based on the hybrid topology of the present invention.

[0026] Figure 3 This is a typical topological diagram of the onshore M3C converter in the present invention.

[0027] Figure 4 This is a typical topological diagram of the onshore DRU converter in the present invention.

[0028] Figure 5 This is a typical topological diagram of the onshore MMC converter in the present invention.

[0029] Figure 6 It is a specific example system schematic diagram of the onshore M3C converter control system in the present invention.

[0030] Figure 7It is a specific example system schematic diagram of the onshore MMC converter control system in the present invention. 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.

[0032] like Figure 2 As shown, in an embodiment of the present invention, an offshore wind power low-frequency networking system based on a hybrid topology includes: an offshore low-frequency wind turbine 1, a low-frequency collection submarine cable 2, M offshore low-frequency booster stations 3, 1 onshore voltage node and N-1 onshore power nodes, and the above-mentioned M+N nodes are connected through a low-frequency transmission submarine cable 5 to form an offshore low-frequency power grid, and each node is connected to the offshore low-frequency power grid through at least two low-frequency transmission submarine cables; Figure 2 The connection method given in the figure is a connection method adopted in this embodiment, and other connection methods that can meet the above requirements can also achieve the technical effect of the present invention; a low-frequency circuit breaker 4 is set at both ends of each section of low-frequency transmission submarine cable; the onshore voltage node includes an M3C low-frequency connection transformer 10, an M3C converter 11, and an M3C power frequency connection transformer 12. The M3C low-frequency connection transformer 10 is connected to the onshore M3C converter 11, and the low-frequency electric energy is converted into power frequency electric energy through the M3C converter 11, and then connected to the power frequency AC power grid through the M3C power frequency connection transformer 12; the onshore power node includes a DRU connection transformer 6, a DRU converter 7, an MMC converter 8, and an MMC connection transformer 9. The DRU connection transformer 6 is connected to the DRU converter 7, and is connected to the MMC converter 8 after being rectified into DC power through the DRU converter 7, and is connected to the onshore power frequency AC power grid through the MMC connection transformer 9 after being inverted into power frequency AC power through the MMC converter 8.

[0033] like Figure 3 As shown, in an embodiment of the present invention, the onshore M3C converter 11 is composed of three frequency conversion modules, each frequency conversion module includes three bridge arm branches, and the entire M3C converter is composed of nine bridge arm branches in total. Each bridge arm branch is composed of multiple cascaded full-bridge sub-modules and bridge arm inductors 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.

[0034] like Figure 4 As shown, in an embodiment of the present invention, the onshore DRU converter 7 adopts a twelve-pulse uncontrolled rectifier topology, which is obtained by combining two six-pulse uncontrolled rectifier circuits in series on the DC side and in parallel on the AC side. The AC side input voltage amplitudes of the two six-pulse uncontrolled rectifier circuits are equal and the phase difference is 30°.

[0035] like Figure 5 As shown, in the embodiment of the present invention, the onshore MMC converter 8 includes six bridge arms, each bridge arm includes a plurality of series-connected sub-modules and a bridge arm reactor, and the sub-modules adopt a half-bridge topology.

[0036] like Figure 1 As shown, in an embodiment of the present invention, a fault ride-through method for an offshore wind power low-frequency networking system based on a hybrid topology is as follows:

[0037] When a fault occurs in the industrial frequency AC grid of a certain onshore power node, the voltage of the industrial frequency AC grid of the onshore power node decreases, the output current of the MMC converter 8 reaches the limit value, the output power of the MMC converter 8 is limited, and the power output capacity of the onshore power node is reduced.

[0038] First, determine whether the output current of the MMC converter 8 of other onshore power nodes reaches the limit value. If the output current of the MMC converter 8 of other onshore power nodes does not reach the limit value, increase the active power reference value of other onshore power nodes that have not reached the limit value, and consume the surplus power through other onshore power nodes that have not reached the limit value to achieve system fault ride-through; after the fault is eliminated, adjust the active power reference value of other onshore power nodes that consume the surplus power to the state before the fault occurs to achieve fault ride-through.

[0039] If the output current of the MMC converter 8 of other onshore power nodes has reached the limit value, in order to maintain the voltage and frequency stability of the offshore low-frequency power grid, the active power flowing into the low-frequency side of the M3C converter 11 of the onshore voltage node will increase.

[0040] It is further determined whether the output current on the power frequency side of the M3C converter 11 reaches the limit value. If the output current on the power frequency side of the M3C converter 11 does not reach the limit value, the system does not need to take any action. The power that cannot be delivered due to the power node fault is transmitted through the M3C converter 11, thereby achieving system fault ride-through.

[0041] If the output current on the power frequency side of the M3C converter 11 reaches the limit value, the output power on the power frequency side of the M3C converter is limited, and the surplus power inside the M3C converter will charge the M3C submodule capacitor, causing the M3C submodule capacitor voltage to increase; using the low-frequency voltage reference value calculation module, according to the average value of the M3C submodule capacitor voltage, adjust the low-frequency voltage reference value of the M3C converter 11 to reduce the AC voltage on the low-frequency side of the M3C converter 11.

[0042] After the AC voltage on the low-frequency side of the M3C converter 11 is reduced, the offshore low-frequency wind turbine 1 enters low-voltage ride-through operation, and the wind turbine crowbar circuit is put into operation to consume the surplus power inside the wind turbine; after the fault is eliminated, the low-frequency voltage of the M3C converter 11 is adjusted to the rated value, the wind turbine crowbar circuit is shut down, the offshore low-frequency wind turbine 1 resumes normal operation, and the fault ride-through process ends.

[0043] like Figure 6 As shown, in an embodiment of the present invention, an onshore M3C converter control system that implements a hybrid topology-based offshore wind power low-frequency networking system fault ride-through method includes: a low-frequency voltage reference value calculation module 101, a low-frequency voltage control module 102, a low-frequency current control module 103, a low-frequency Park inverse transformation module 104, a low-frequency internal circulation control module 105, a power frequency phase-locked loop module 106, a capacitor voltage and reactive power control module 107, a power frequency current control module 108, a power frequency Park inverse transformation module 109, a power frequency internal circulation control module 110, a bridge arm voltage calculation module 111, and a modulation module 112.

[0044] In the embodiment of the present invention, the low-frequency voltage reference value calculation module 101 calculates the low-frequency voltage reference value by the following method:

[0045]

[0046] u gq1ref =0

[0047] Among them, u gd1ref and u gq1ref is the low frequency voltage reference value U gdq1ref The d and q axis components, U c is the average value of the capacitor voltage of the M3C submodule.

[0048] The low frequency voltage control module 102 controls the low frequency d and q axis voltages U gdq1 The PI controller is used to control the following reference values ​​u gd1ref and u gq1ref The output of the low-frequency voltage PI controller is passed through the limiting link and is used as the d-axis and q-axis components i of the low-frequency current reference value. vd1ref and i vq1ref The specific implementation of the low-frequency voltage control module 102 is as follows:

[0049]

[0050] Among them: F PI2 (s) is the transfer function of the low-frequency voltage PI controller, k p2 is the proportionality coefficient, k i2 is the integral coefficient, i vd1ref ,ivq1ref Corresponding to the current vector I vdq1ref The d-axis and q-axis components of .

[0051] The low-frequency current control module 103 controls the low-frequency d-axis and q-axis currents I vdq1 The PI controller is used to control it to follow the d and q axis low-frequency current reference value 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 103 is as follows:

[0052]

[0053] Among them: F PI3 (s) is the transfer function of the low-frequency current PI controller, k p3 is the proportionality coefficient, k i3 is the integral coefficient, L1 is the equivalent inductance including the low-frequency side converter transformer and bridge arm reactor, u vd1 ,u vq1 The corresponding voltage vector is U vdq1 The d-axis and q-axis components, ω g1 is the angular frequency of the low-frequency grid voltage.

[0054] The low-frequency Park inverse transformation module 104 performs Park inverse transformation on the low-frequency output voltage to obtain the low-frequency output voltage in the stationary three-phase coordinate system; the low-frequency internal circulation control module 105 suppresses the low-frequency internal circulation, and its output is the low-frequency internal circulation voltage; the power frequency phase-locked loop module 106 calculates the power frequency grid voltage phase according to the power frequency grid voltage.

[0055] The capacitor voltage and reactive power control module 107 controls the capacitor voltage average value U c , and the power frequency reactive power Q g2 The PI controller is used to control the following reference values ​​U cref and Q g2ref The outputs of capacitor voltage and reactive power PI controller are used as reference values ​​of d-axis and q-axis currents after passing through the limiting link. vd2ref and i vq2ref The specific implementation of the capacitor voltage and reactive power control module 107 is as follows:

[0056]

[0057] Among them: F PI4 (s) is the transfer function of capacitor voltage and reactive power PI controller, k p4 is the proportionality coefficient, k i4is the integral coefficient, i vd2ref ,i vq2ref Corresponding to the current vector I vdq2ref The d-axis and q-axis components of .

[0058] The power frequency current control module 108 controls the power frequency d and q axis currents I vdq2 Use 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 108 is as follows:

[0059]

[0060] Among them: F PI5 (s) is the transfer function of the power frequency current PI controller, k p5 is the proportionality coefficient, k i5 is the integral coefficient, L2 is the equivalent inductance of the power frequency side converter transformer and bridge arm reactor, u vd2 ,u vq2 The corresponding voltage vector is U vdq2 The d-axis and q-axis components, ω g2 is the angular frequency of the industrial frequency grid voltage.

[0061] The power frequency Park inverse transformation module 109 performs Park inverse transformation on the power frequency output voltage to obtain the power frequency output voltage in the stationary three-phase coordinate system; the power frequency internal circulation control module 110 realizes the suppression of the power frequency internal circulation, and its output is the power frequency internal circulation voltage; the bridge arm voltage calculation module 111 calculates the reference voltage of the nine bridge arms of the onshore M3C converter using the low-frequency output voltage, the low-frequency internal circulation voltage, the power frequency output voltage, and the power frequency internal circulation voltage; the modulation module 112 generates a modulation instruction according to the reference voltage of the nine bridge arms of the onshore M3C converter to realize the control of the onshore M3C converter.

[0062] like Figure 7 As shown, in an embodiment of the present invention, an onshore MMC converter control system that implements a hybrid topology-based offshore wind power low-frequency networking system startup method includes: an active power control module 201, a DC bus voltage and reactive power control module 202, a differential mode current control module 203, a Park inverse transformation module 204, a common mode current control module 205, a bridge arm voltage calculation module 206, and a modulation module 207.

[0063] In the embodiment of the present invention, the active power control module 201 controls the active power through a PI controller, and the output of the active power control module is a DC bus voltage reference value, which is specifically implemented as follows:

[0064] Udc2ref =U dc2N -F PI1 (s)(P s2ref -P s2 )

[0065]

[0066] Where: P s2ref is the active power reference value, P s2 is the active power, U dc2ref is the DC bus voltage reference value, U dc2N is the rated voltage of the DC bus, F PI1 (s) is the transfer function of the active power PI controller, k p1 is the proportionality coefficient, k i1 is the integration coefficient.

[0067] The DC bus voltage and reactive power control module 202 controls the DC bus voltage and reactive power through a PI controller. The outputs of the DC bus voltage and reactive power PI controllers are used as reference values ​​of the differential mode current d and q axis components respectively after passing through a limiting link. The specific implementation of the DC bus voltage and reactive power control module 202 is as follows:

[0068]

[0069] Among them: F PI6 (s) is the transfer function of the DC bus voltage and reactive power PI controller, k p6 is the proportionality coefficient, k i6 is the integral coefficient, i sd2ref ,i sq2ref Corresponding to the current vector I sdq2ref The d-axis and q-axis components, Q s2ref is the reactive power reference value, Q s2 is the reactive power, U dc2 is the DC bus voltage.

[0070] The differential mode current control module 203 controls the differential mode current through a PI controller, and the output of the differential mode current control module is a differential mode voltage reference value; the specific implementation of the differential mode current control module 203 is as follows:

[0071]

[0072] Among them: F PI7 (s) is the transfer function of the differential mode current PI controller, k p7 is the proportionality coefficient, k i7 is the integral coefficient, L s is the equivalent inductance including the connecting transformer and bridge arm reactor, udifd2 ,u difq2 The corresponding voltage vector is U difdq2 The d-axis and q-axis components, ω s2 is the grid voltage angular frequency.

[0073] The Park inverse transformation module 204 performs a Park inverse transformation on the differential mode voltage reference value to obtain the differential mode voltage reference value in the stationary three-phase coordinate system; the common mode current control module 205 controls the common mode current through a PI controller, and the output of the common mode current control module is the common mode voltage reference value; the bridge arm voltage calculation module 206 calculates the bridge arm voltage reference value according to the differential mode voltage reference value and the common mode voltage reference value; the modulation module 207 realizes the control of the onshore MMC converter according to the bridge arm voltage reference value.

[0074] 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 hybrid topology-based offshore wind power low-frequency networking system fault ride-through method, characterized in that: The offshore wind power low-frequency networking system based on hybrid topology includes: offshore low-frequency wind turbines, low-frequency collection submarine cables, M offshore low-frequency booster stations, 1 onshore voltage node and N-1 onshore power nodes. The above M+N nodes are connected through low-frequency transmission submarine cables to form an offshore low-frequency power grid. Each node is connected to the offshore low-frequency power grid through at least two low-frequency transmission submarine cables. Low-frequency circuit breakers are set at both ends of each section of low-frequency transmission submarine cables. The onshore voltage node includes an M3C low-frequency connection transformer, an M3C converter, an M3C power frequency connection transformer, and an M3C low-frequency power transmission cable. The DRU power node includes a DRU connection transformer, a DRU converter, an MMC converter, and an MMC connection transformer. The DRU connection transformer is connected to the DRU converter, rectified into DC power by the DRU converter, and then connected to the MMC converter. The DRU power node is inverted into industrial frequency AC power by the MMC converter and then connected to the onshore industrial frequency AC grid through the MMC connection transformer. The M3C converter control system for implementing the fault ride-through method includes: a low-frequency voltage reference value calculation module, a low-frequency voltage control module, a low-frequency current control module, a low-frequency Park inverse transformation module, a low-frequency internal circulation control module, a power frequency phase-locked loop module, a capacitor voltage and reactive power control module, a power frequency current control module, a power frequency Park inverse transformation module, a power frequency internal circulation control module, a bridge arm voltage calculation module, and a modulation module; The MMC converter control system for implementing the fault ride-through method includes: an active power control module, a DC bus voltage and reactive power control module, a differential mode current control module, a Park inverse transformation module, a common mode current control module, a bridge arm voltage calculation module, and a modulation module; when a power frequency AC grid of a certain onshore power node fails, the power frequency AC grid voltage of the onshore power node decreases, the output current of the MMC converter reaches the limit value, the output power of the MMC converter is limited, and the power output capacity of the onshore power node is reduced; the fault ride-through method performs the following fault ride-through steps: First, determine whether the output current of the MMC converter of other onshore power nodes has reached the limit value. If the output current of the MMC converter of other onshore power nodes has not reached the limit value, increase the active power reference value of other onshore power nodes that have not reached the limit value, and consume the surplus power through other onshore power nodes that have not reached the limit value to achieve system fault ride-through; after the fault is eliminated, adjust the active power reference value of other onshore power nodes that consume the surplus power to the state before the fault occurs to achieve fault ride-through; If the output current of the MMC converters at other onshore power nodes has reached the limit value, in order to maintain the voltage and frequency stability of the offshore low-frequency power grid, the active power flowing into the low-frequency side of the M3C converter at the onshore voltage node will increase; It is further determined whether the output current on the power frequency side of the M3C converter has reached the limit value. If the output current on the power frequency side of the M3C converter has not reached the limit value, no action is required. The power that cannot be delivered due to the power node fault is transmitted through the M3C converter, so that the system fault ride-through can be achieved. If the output current on the power frequency side of the M3C converter reaches the limit value, the output power on the power frequency side of the M3C converter is limited, and the surplus power inside the M3C converter will charge the capacitor of the M3C submodule, resulting in an increase in the capacitor voltage of the M3C submodule. The low-frequency voltage reference value calculation module is used to adjust the low-frequency voltage reference value of the M3C converter according to the average value of the capacitor voltage of the M3C submodule, thereby reducing the AC voltage on the low-frequency side of the M3C converter. After the AC voltage on the low-frequency side of the M3C converter is reduced, the offshore low-frequency wind turbine enters low-voltage ride-through operation, and the wind turbine crowbar circuit is put into operation to consume the surplus power inside the wind turbine; after the fault is eliminated, the low-frequency voltage of the M3C converter is adjusted to the rated value, the wind turbine crowbar circuit is shut down, the offshore low-frequency wind turbine resumes normal operation, and the fault ride-through process ends.

2. The hybrid topology-based offshore wind power low-frequency networking system fault ride-through method according to claim 1 is characterized in that: The low-frequency voltage reference value calculation module in the onshore M3C converter control system calculates the low-frequency voltage reference value by the following method: u gq1ref ＝0 Among them, u gd1ref and u gq1ref is the low frequency voltage reference value U gdq1ref The d and q axis components, U c is the average value of the capacitor voltage of the M3C submodule.

3. The hybrid topology-based offshore wind power low-frequency networking system fault ride-through method according to claim 1, characterized in that: The active power control module in the onshore MMC converter control system calculates the DC bus voltage reference value by the following method: U dc2ref =U dc2N -F PI1 (s)(P s2ref -P s2 ) Where: P s2ref is the active power reference value, P s2 is the active power, U dc2ref is the DC bus voltage reference value, U dc2N is the rated voltage of the DC bus, F PI1 (s) is the transfer function of the active power PI controller, k p1 is the proportionality coefficient, k i1 is the integration coefficient.

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