Fault power transfer method for hybrid offshore wind power low-frequency networking system

Abstract

The invention discloses a fault power transfer method for a hybrid offshore wind power low-frequency networking system, and the method comprises the steps: carrying out the cooperative control of an M3C current converter of an onshore voltage node, an MMC current converter of an onshore power node, and an offshore low-frequency wind turbine generator when a low-frequency transmission submarine cable breaks down; the method can suppress overvoltage of a non-fault phase of a system in a submarine cable fault process, can adjust a control mode of a wind turbine generator according to the operation condition of an offshore wind power plant, matches the output power of the wind power plant with the current-carrying capacity of residual submarine cables, and solves the problem of power coordination distribution among different nodes of an onshore receiving end after the operation state of the system is changed. Good application prospects are realized.

Description

Technical Field

[0001] The present invention belongs to the field of renewable energy power generation, and in particular relates to a method for transferring fault power of a hybrid offshore wind power low-frequency networking system. 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 that of 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. In the point-to-point low-frequency transmission system, the offshore grid voltage is established by the onshore M3C converter, while in the offshore wind power low-frequency networking system, due to the presence of multiple onshore receiving nodes, it is not necessary for all receiving nodes to have the ability to establish the grid voltage. In this case, it is possible to consider using an M3C topology for one receiving node and a diode and MMC hybrid topology for the remaining nodes, thereby reducing the overall construction cost of the project while ensuring reliable operation of the system.

[0003] When a fault occurs in the sending submarine cable, it is generally a permanent fault. The faulty submarine cable needs to be cut out from the system, and the output power of the wind farm needs to be sent out through the remaining non-faulty submarine cables. In the process of the above-mentioned fault power transfer, there are two problems that need to be solved urgently in the low-frequency networking system using a hybrid topology of diodes and MMC: First, when an asymmetric fault occurs in the sending submarine cable, since both ends of the sending submarine cable are power electronic converters, their overvoltage characteristics are significantly different from those of traditional power grids, and it is necessary to reduce the overvoltage of the non-fault phase during the fault process through optimized control strategies; second, when the faulty submarine cable is cut out, the current carrying capacity of the remaining sending submarine cable may not be able to meet the active power delivery requirements of the wind farm at this time. It is necessary to adjust the control method of the wind turbine in combination with the operation of the wind farm during the fault. At the same time, it is also necessary to consider the power coordination and distribution between different nodes on the land receiving end after the system operation status changes. Summary of the invention

[0004] The purpose of the present invention is to solve the problems of overvoltage and power coordination control in the process of sending submarine cable failure in the offshore wind power low-frequency networking system based on hybrid topology, and to provide a hybrid offshore wind power low-frequency networking system fault power transfer method, through the coordinated cooperation of the M3C converter of the onshore voltage node, the MMC converter of the onshore power node and the low-frequency wind turbine, to achieve the smooth transfer of the output power of the offshore wind farm to the non-fault submarine cable when the sending submarine cable fails.

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

[0006] A hybrid offshore wind power low-frequency networking system fault power transfer method, characterized in that the hybrid 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 by 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 low-frequency transmission submarine cable; the onshore voltage node includes an M3C low-frequency connection transformer, an M3C converter, and an M3C converter. C power frequency connection transformer, M3C low frequency connection transformer is connected to the onshore M3C converter, low frequency electric energy is converted into power frequency electric energy through the M3C converter, and then connected to the power frequency AC power grid through the M3C power 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 power through the DRU converter and then connected to the MMC converter, inverted into power frequency AC power through the MMC converter and then connected to the onshore power frequency AC power grid through the MMC connection transformer;

[0007] The M3C converter control system for implementing the fault power transfer method includes: a low-frequency voltage reference value calculation module, a low-frequency voltage control module, a low-frequency positive sequence current control module, a low-frequency negative sequence current control module, a low-frequency coordinate 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 power transfer 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;

[0009] When a ground fault occurs in a low-frequency transmission submarine cable, the following fault power transfer steps are performed:

[0010] (1) The M3C converter at the onshore voltage node adjusts the low-frequency voltage reference value according to the negative-sequence voltage feedback value to suppress overvoltage in the offshore low-frequency grid;

[0011] (2) The low-frequency circuit breakers at both ends of the faulty submarine cable are disconnected;

[0012] (3) If the real-time power of the offshore wind farm does not exceed the current carrying capacity of the non-faulty submarine cable at this time, the system can resume normal operation after the faulty submarine cable is cut out, and the output power of all wind farms is transferred to the non-faulty submarine cable; if the real-time power of the offshore wind farm exceeds the current carrying capacity of the non-faulty submarine cable at this time, the wind turbines are further adjusted to switch from the MPPT operation mode to the fixed power operation mode, and the power reference value of the wind turbines is reduced to ensure that the output active power of the wind farm does not exceed the current carrying capacity of the non-faulty submarine cable. Then, by reducing the active power reference value of the MMC converter at the onshore power node, the power distribution between the onshore receiving nodes is coordinated, so that the output power of all wind farms is transferred to the non-faulty submarine cable, and the process of transferring the faulty power of the sending submarine cable is completed.

[0013] Furthermore, the low-frequency voltage reference value calculation module in the M3C converter control system of the onshore voltage node calculates the low-frequency voltage reference value by the following method:

[0014]

[0015] Among them, |U g- | is the negative sequence voltage amplitude of the power grid, 1p.u. means the per-unit value is 1, that is, the rated voltage.

[0016] Furthermore, the active power control module in the MMC converter control system of the onshore power node calculates the DC bus voltage reference value by the following method:

[0017] U dc2ref =U dc2N -F PI1 (s)(P s2ref -P s2 )

[0018]

[0019] Where: P s2ref is the active power reference value. In the fault power transfer stage, by adjusting P s2ref Realize the coordinated power distribution between different land receiving nodes, 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, ki1 is the integration coefficient.

[0020] The present invention can suppress the overvoltage of the non-fault phase of the system during the failure of the sending submarine cable through the coordinated control of the M3C converter of the onshore voltage node, the MMC converter of the onshore power node and the offshore low-frequency wind turbine. It can also adjust the control mode of the wind turbine according to the operation of the offshore wind farm, match the output power of the wind farm with the current carrying capacity of the remaining submarine cable, and solve the problem of power coordination and allocation between different nodes on the onshore receiving end after the system operation status changes. The present invention has good application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 The present invention is a flow chart of a method for transferring fault power of a hybrid offshore wind power low-frequency networking system.

[0022] Figure 2 This is a typical topology diagram of the hybrid offshore wind power low-frequency networking system of the present invention.

[0023] Figure 3 This is a typical topological diagram of the onshore M3C converter in the present invention.

[0024] Figure 4 This is a typical topological diagram of the onshore DRU converter in the present invention.

[0025] Figure 5 This is a typical topological diagram of the onshore MMC converter in the present invention.

[0026] Figure 6 It is a specific example system schematic diagram of the M3C converter control system of the onshore voltage node in the present invention.

[0027] Figure 7 It is a specific example system schematic diagram of the MMC converter control system of the onshore power node in the present invention. DETAILED DESCRIPTION

[0028] 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.

[0029] like Figure 2 As shown, in the embodiment of the present invention, the hybrid offshore wind power low-frequency networking system 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 2The 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.

[0030] 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.

[0031] 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°.

[0032] 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.

[0033] like Figure 1 As shown, in an embodiment of the present invention, a method for transferring fault power of a hybrid offshore wind power low-frequency networking system is as follows:

[0034] When a ground fault occurs in the low-frequency transmission submarine cable 5, the M3C converter 11 at the onshore voltage node adjusts the low-frequency voltage reference value according to the negative sequence voltage feedback value to suppress overvoltage in the offshore low-frequency power grid;

[0035] After the protection system identifies the fault, the low-frequency circuit breakers 4 at both ends of the faulty submarine cable are disconnected. If the real-time power of the offshore wind farm does not exceed the current carrying capacity of the non-faulty submarine cable at this time, the system can resume normal operation after the faulty submarine cable is cut out, and all wind farm output power is transferred to the non-faulty submarine cable;

[0036] If the real-time power of the offshore wind farm exceeds the current carrying capacity of the non-faulty submarine cable at this time, the wind turbine set 1 is further adjusted to switch from the MPPT operation mode to the fixed power operation mode, and the power reference value of the wind turbine set is reduced to ensure that the active power output of the wind farm does not exceed the current carrying capacity of the non-faulty submarine cable. Then, by reducing the active power reference value of the MMC converter 8 of the onshore power node, the power distribution between the onshore receiving nodes is coordinated. Through the above method, the output power of all wind farms is transferred to the non-faulty submarine cable, and the process of transferring the faulty power of the sending submarine cable is completed.

[0037] like Figure 6 As shown, in an embodiment of the present invention, an onshore M3C converter control system that implements a hybrid offshore wind power low-frequency networking system fault power transfer method includes: a low-frequency voltage reference value calculation module 101, a low-frequency voltage control module 102, a low-frequency positive-sequence current control module 103, a low-frequency negative-sequence current control module 104, a low-frequency coordinate transformation module 105, a low-frequency internal circulation control module 106, a power frequency phase-locked loop module 107, a capacitor voltage and reactive power control module 108, a power frequency current control module 109, a power frequency Park inverse transformation module 110, a power frequency internal circulation control module 111, a bridge arm voltage calculation module 112, and a modulation module 113.

[0038] 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:

[0039]

[0040] Among them, |U g- | is the negative sequence voltage amplitude of the power grid, 1p.u. means the per-unit value is 1, that is, the rated voltage.

[0041] 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:

[0042]

[0043] Among them: FPI2 (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 ,i vq1ref Corresponding to the current vector I vdq1ref The d-axis and q-axis components of .

[0044] The low-frequency positive sequence current control module 103 controls the low-frequency d and q axis currents I vdq1 The positive sequence component of is controlled by a PI controller to follow the reference value i of the d and q axis currents. vd1ref and i vq1ref The output of the low-frequency positive sequence current PI controller passes through the limiting link and is used as the low-frequency output voltage positive sequence component U vdq1+ The specific implementation of the low-frequency positive sequence current control module 103 is as follows:

[0045]

[0046] Among them: F PI3 (s) is the transfer function of the low-frequency positive sequence 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 r1 is the angular frequency of the low-frequency grid voltage.

[0047] The low-frequency negative sequence current control module 104 controls the low-frequency d and q axis currents I vdq1 The negative sequence component of the resonant controller is controlled, and the negative sequence current reference value is given as 0. The output of the resonant controller passes through the limiting link as the low-frequency output voltage negative sequence component U vdq1- The specific implementation of the low-frequency negative sequence current control module 104 is as follows:

[0048]

[0049] Among them: F R100 (s) is the transfer function of the resonant controller, k g is the gain factor of the resonant controller, ω c is the cut-off frequency. In this module, the cut-off frequency ω c Select 12Hz; vd1- ,u vq1- The corresponding voltage vector is U vdq1- The d-axis and q-axis components of .

[0050] The low-frequency coordinate transformation module 105 is based on the low-frequency output voltage positive sequence component U vdq1+ And the low-frequency output voltage negative sequence component U vdq1- , calculate the low-frequency output voltage U in the stationary three-phase coordinate system vαβ1 The specific implementation of the low-frequency coordinate transformation module 105 is as follows:

[0051]

[0052] Among them, u vd1+ ,u vq1+ The corresponding voltage vector is U vdq1+ The d-axis and q-axis components, u vd1- ,u vq1- The corresponding voltage vector is U vdq1- The d-axis and q-axis components, u vα1 ,u vβ1 The corresponding voltage vector is U vαβ1 The α-axis, β-axis components, θ r1 It is the reference phase of the low-frequency AC power grid.

[0053] The low-frequency internal circulation control module 106 controls the low-frequency internal circulation I cαβ1 As the feedback value of the controller, the output of the low-frequency internal circulation control module is used as the low-frequency internal circulation voltage U cαβ1 .

[0054] The power frequency phase-locked loop module 107 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 108 controls the capacitor voltage average value U c , and the power frequency reactive power Q g2 The PI controller is used to control the cref and Q g2ref The outputs of the capacitor voltage and reactive power PI controller are used as the reference values ​​of the 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 108 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 i4 is the integral coefficient, i vd2ref ,i vq2refCorresponding to the current vector I vdq2ref The d-axis and q-axis components of .

[0058] The power frequency current control module 109 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 109 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 110 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 111 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 112 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 113 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 offshore wind power low-frequency networking system fault power transfer 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] U dc2ref =U dc2N -FPI1 (s)(P s2ref -P s2 )

[0065]

[0066] Where: P s2ref is the active power reference value. In the fault power transfer stage, by adjusting P s2ref Realize the coordinated power distribution between different land receiving nodes, 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 sis the equivalent inductance including the connecting transformer and bridge arm reactor, u difd2 ,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 offshore wind power low-frequency networking system fault power transfer method, characterized in that: The hybrid 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 M+N nodes are connected via a low-frequency transmission submarine cable to form an offshore low-frequency power grid. Each node is connected to the offshore low-frequency power grid via at least two low-frequency transmission submarine cables. Low-frequency circuit breakers are provided at both ends of each section of the low-frequency transmission submarine cable. The onshore voltage node comprises an M3C low-frequency connection transformer, an M3C converter, an M3C power frequency connection transformer, and an M3C low-frequency connection transformer. The connecting transformer is connected to the onshore M3C converter, and the 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 connecting transformer; the onshore power node includes a DRU connecting transformer, a DRU converter, an MMC converter, and an MMC connecting transformer, the DRU connecting transformer is connected to the DRU converter, and is connected to the MMC converter after being rectified into DC electric energy through the DRU converter, and is connected to the onshore industrial frequency AC power grid through the MMC connecting transformer after being inverted into industrial frequency AC electric energy through the MMC converter; The M3C converter control system for implementing the fault power transfer method includes: a low-frequency voltage reference value calculation module, a low-frequency voltage control module, a low-frequency positive sequence current control module, a low-frequency negative sequence current control module, a low-frequency coordinate 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 power transfer 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 ground fault occurs in a low-frequency transmission submarine cable, the following fault power transfer steps are performed: (1) The M3C converter at the onshore voltage node adjusts the low-frequency voltage reference value according to the negative-sequence voltage feedback value to suppress overvoltage in the offshore low-frequency grid; (2) The low-frequency circuit breakers at both ends of the faulty submarine cable are disconnected; (3) If the real-time power of the offshore wind farm does not exceed the current carrying capacity of the non-faulty submarine cable at this time, the system can resume normal operation after the faulty submarine cable is cut out, and the output power of all wind farms is transferred to the non-faulty submarine cable; if the real-time power of the offshore wind farm exceeds the current carrying capacity of the non-faulty submarine cable at this time, the wind turbines are further adjusted to switch from the MPPT operation mode to the fixed power operation mode, and the power reference value of the wind turbines is reduced to ensure that the output active power of the wind farm does not exceed the current carrying capacity of the non-faulty submarine cable. Then, by reducing the active power reference value of the MMC converter at the onshore power node, the power distribution between the onshore receiving nodes is coordinated, so that the output power of all wind farms is transferred to the non-faulty submarine cable, and the process of transferring the faulty power of the sending submarine cable is completed.

2. The hybrid offshore wind power low-frequency networking system fault power transfer method according to claim 1 is characterized in that: The low-frequency voltage reference value calculation module in the M3C converter control system of the onshore voltage node calculates the low-frequency voltage reference value by the following method: Among them, |U g- | is the negative sequence voltage amplitude of the power grid, 1p.u. means the per-unit value is 1, that is, the rated voltage.

3. The hybrid offshore wind power low-frequency networking system fault power transfer method according to claim 1, characterized in that: The active power control module in the MMC converter control system of the onshore power node 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. In the fault power transfer stage, by adjusting P s2ref Realize the coordinated power distribution between different land receiving nodes, 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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