Offshore wind power low-frequency networking system based on hybrid topology and control method

Abstract

The invention discloses an offshore wind power low-frequency networking system based on hybrid topology and a control method, in N onshore receiving end nodes of the offshore wind power low-frequency networking system, one node adopts an M3C current converter, N-1 nodes adopt DRU and MMC current converters, an offshore low-frequency power grid voltage is established through the M3C current converter, and the M3C current converter is connected with the M3C current converter; the power flow control of other power control nodes is realized through the DRU and the MMC, and by adopting the technical scheme of the invention, the engineering construction cost can be obviously reduced under the condition of ensuring the reliable operation of the system, and the method has a good application prospect.

Description

Technical Field

[0001] The present invention belongs to the field of renewable energy power generation, and in particular relates to an offshore wind power low-frequency networking system and a control method based on a hybrid topology. 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 point-to-point low-frequency transmission system, the offshore grid voltage is established by the onshore M3C converter. However, in the offshore wind power low-frequency networking system, due to the existence of multiple onshore receiving nodes, all receiving nodes in the existing offshore wind power low-frequency networking solutions use M3C converters, resulting in high project construction costs. Therefore, it is urgent to propose a new offshore wind power low-frequency networking system topology with lower overall cost, so as to reduce the overall project construction cost while ensuring the reliable operation of the system. Summary of the invention

[0004] The purpose of the present invention is to reduce the construction cost of an offshore wind power low-frequency networking system and to provide an offshore wind power low-frequency networking system and a control method based on a hybrid topology.

[0005] To achieve the above-mentioned object of the invention, according to the first aspect of the present invention, the present invention adopts the following technical solution:

[0006] An offshore wind power low-frequency networking system based on hybrid topology, characterized in that the offshore wind power low-frequency networking system based on hybrid topology includes: an offshore low-frequency wind turbine, a low-frequency collection submarine cable, M offshore low-frequency booster stations, a low-frequency circuit breaker, a low-frequency transmission submarine cable, N-1 onshore DRU connection transformers, N-1 onshore DRU converters, N-1 onshore MMC converters, N-1 onshore MMC connection transformers, 1 onshore M3C low-frequency connection transformer, 1 onshore M3C converter, and 1 onshore M3C power frequency connection transformer;

[0007] The electricity generated by the offshore low-frequency wind turbines is collected by low-frequency collecting submarine cables and connected to M offshore low-frequency booster stations. After being boosted at the offshore low-frequency booster stations, it is transmitted to the land through low-frequency transmission submarine cables. The low-frequency transmission submarine cables connect the M offshore low-frequency booster stations and 1 onshore M3C low-frequency connection transformer and N-1 onshore DRU connection transformers to form an offshore low-frequency power grid. Each offshore low-frequency booster station, onshore M3C low-frequency connection transformer and onshore DRU connection transformer are connected to the offshore low-frequency power grid through at least two low-frequency transmission submarine cables. Low-frequency circuit breakers are installed at both ends of each section of low-frequency transmission submarine cables for use in the low-frequency power grid. When a submarine cable fails, the faulty line is cut off; the onshore M3C low-frequency connection transformer is connected to the onshore M3C converter, and the low-frequency electric energy is converted into industrial frequency electric energy through the onshore M3C converter, and then connected to the onshore industrial frequency AC power grid through the onshore M3C industrial frequency connection transformer; the N-1 onshore DRU connection transformers are connected to the N-1 onshore DRU converters, rectified into DC electric energy through the onshore DRU converter, and then connected to the N-1 onshore MMC converters, and then inverted into industrial frequency AC electric energy through the MMC converter, and then connected to the onshore industrial frequency AC power grid through the N-1 onshore MMC connection transformers.

[0008] To achieve the above-mentioned object of the invention, according to the second aspect of the present invention, the present invention adopts the following technical solution:

[0009] A control method for an offshore wind power low-frequency networking system based on a hybrid topology, characterized in that an onshore M3C converter adopts a fixed low-frequency voltage control strategy to maintain the voltage stability of an offshore low-frequency AC power grid; N-1 onshore MMC converters adopt a fixed power control strategy to control the active power and reactive power flowing into the corresponding onshore MMC converter; the active power and reactive power reference values ​​of the N-1 onshore MMC converters adopting the fixed power control strategy are given by an offshore wind power low-frequency power grid centralized control system according to a power grid dispatching instruction;

[0010] The onshore M3C converter control system for implementing the control method includes: 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;

[0011] The low-frequency voltage control module controls the d- and q-axis low-frequency voltages through a low-frequency voltage PI controller, and the output of the low-frequency AC voltage control module is a low-frequency current reference value; the low-frequency current control module controls the d- and q-axis low-frequency currents using a low-frequency current PI controller, and the output of the low-frequency current control module is a low-frequency output voltage; the low-frequency Park inverse transformation module performs a Park inverse transformation on the low-frequency output voltage to obtain a low-frequency output voltage in a stationary three-phase coordinate system; the low-frequency internal circulation control module suppresses the low-frequency internal circulation, and its output is a low-frequency internal circulation voltage; the power frequency phase-locked loop module calculates the power frequency grid voltage phase according to the power frequency grid voltage; the capacitor voltage and reactive power control module controls the capacitor voltage average value and the power frequency reactive power through the capacitor voltage and reactive power PI controller The output of the capacitor voltage and reactive power control module is the power frequency current reference value; the power frequency current control module uses the power frequency current PI controller to control the d and q axis power frequency currents, and the output of the power frequency current control module is the power frequency output voltage; the power frequency Park inverse transformation module performs Park inverse transformation on the power frequency output voltage to obtain the power frequency output voltage in the static three-phase coordinate system; the power frequency internal circulation control module 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 uses the low-frequency output voltage, the low-frequency internal circulation voltage, the power frequency output voltage, and the power frequency internal circulation voltage to calculate the reference voltage of the nine bridge arms of the onshore M3C converter; the modulation module 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;

[0012] The onshore MMC converter control system for implementing the control 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;

[0013] The active power control module controls the active power through an active power PI controller, and the output of the active power control module is a DC bus voltage reference value; the DC bus voltage and reactive power control module controls the DC bus voltage and reactive power through a DC bus voltage and reactive power PI controller, and the output of the DC bus voltage and reactive power control module is a differential mode current reference value; the differential mode current control module controls the differential mode current through a differential mode current PI controller, and the output of the differential mode current control module is a differential mode voltage reference value; the common mode current control module controls the common mode current through a common mode current PI controller, and the output of the common mode current control module is a common mode voltage reference value; the bridge arm voltage calculation module 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 realizes control of the onshore MMC converter according to the bridge arm voltage reference value.

[0014] Furthermore, the active power control module in the onshore MMC converter control system calculates the DC bus voltage reference value by the following method:

[0015] U dc2ref =U dc2N -F PI1 (s)(P s2ref -P s2 )

[0016]

[0017] 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.

[0018] Due to the adoption of the technical solution of the present invention, among the N onshore receiving nodes of the offshore wind power low-frequency networking system, one node adopts the M3C converter, and N-1 nodes adopt the DRU and MMC converters. The offshore low-frequency grid voltage is established through the M3C converter, and the flow control of other power control nodes is realized through the DRU and MMC converters. This can significantly reduce the engineering construction cost while ensuring the reliable operation of the system, and has good application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is a typical topology diagram of the offshore wind power low-frequency networking system based on the hybrid topology of the present invention.

[0020] Figure 2 This is a typical topological diagram of the onshore M3C converter in the present invention.

[0021] Figure 3 This is a typical topological diagram of the onshore DRU converter in the present invention.

[0022] Figure 4 This is a typical topological diagram of the onshore MMC converter in the present invention.

[0023] Figure 5 It is a specific example system schematic diagram of the onshore M3C converter control system in the present invention.

[0024] Figure 6 It is a specific example system schematic diagram of the onshore MMC converter control system in the present invention. DETAILED DESCRIPTION

[0025] 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.

[0026] like Figure 1 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, a low-frequency circuit breaker 4, a low-frequency transmission submarine cable 5, N-1 onshore DRU connection transformers 6, N-1 onshore DRU converters 7, N-1 onshore MMC converters 8, N-1 onshore MMC connection transformers 9, 1 onshore M3C low-frequency connection transformer 10, 1 onshore M3C converter 11, and 1 onshore M3C industrial frequency connection transformer 12.

[0027] The offshore low-frequency wind turbine set 1 is connected to the low-frequency collection submarine cable 2. The electricity generated by the offshore low-frequency wind turbine set 1 is collected at sea through the low-frequency collection submarine cable 2, and then connected to M offshore low-frequency booster stations 3. After boosting at the offshore low-frequency booster station 3, it is transmitted to land through the low-frequency transmission submarine cable 5. The low-frequency transmission submarine cable 5 connects the M offshore low-frequency booster stations 3 and 1 onshore M3C low-frequency connection transformer 10 and N-1 onshore DRU connection transformers 6. Each offshore low-frequency booster station, onshore M3C low-frequency connection transformer and onshore DRU connection transformer are connected to the offshore low-frequency power grid through at least two low-frequency transmission submarine cables. Figure 1The connection method given in the figure is a connection method adopted in this embodiment. Other connection methods that can meet the above requirements can also achieve the technical effects of the present invention. At both ends of each section of low-frequency transmission submarine cable, a low-frequency circuit breaker 4 is set to cut out the faulty line when a fault occurs in the submarine cable to ensure the normal operation of other parts of the offshore low-frequency power grid. The onshore M3C low-frequency connection transformer 10 is connected to the onshore M3C converter 11, and the low-frequency electric energy is converted into industrial frequency electric energy through the onshore M3C converter 11, and then the low-frequency electric energy is converted into industrial frequency electric energy through the onshore M3C converter 11. It is connected to the onshore industrial frequency AC power grid through the onshore M3C industrial frequency connection transformer 12; the N-1 onshore DRU connection transformers 6 are connected one-to-one with the N-1 onshore DRU converters 7, and are rectified into DC power by the onshore DRU converter 7 and then connected one-to-one with the N-1 onshore MMC converters 8, and are inverted into industrial frequency AC power by the MMC converter 8 and then connected to the onshore industrial frequency AC power grid through N-1 one-to-one corresponding onshore MMC connection transformers 9, and are connected to the onshore industrial frequency AC power grid together.

[0028] like Figure 2 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.

[0029] like Figure 3 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°.

[0030] like Figure 4 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.

[0031] like Figure 5As 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 control method includes: a low-frequency 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.

[0032] In the embodiment of the present invention, the low-frequency voltage control module 101 controls the d-axis and q-axis low-frequency 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 101 is as follows:

[0033]

[0034] 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 ,i vq1ref Corresponding to the current vector I vdq1ref The d-axis and q-axis components of .

[0035] The low-frequency current control module 102 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 102 is as follows:

[0036]

[0037] 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 vdq1The d-axis and q-axis components, ω g1 is the angular frequency of the low-frequency grid voltage.

[0038] The low-frequency Park inverse transformation module 103 is based on the low-frequency output voltage U vdq1 The low-frequency output voltage U in the stationary three-phase coordinate system is calculated vαβ1 The specific implementation of the low-frequency Park inverse transform module 103 is as follows:

[0039]

[0040] Among them, 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 g1 It is the reference phase of the low-frequency AC power grid.

[0041] The low-frequency internal circulation control module 104 controls the low-frequency internal circulation I cαβ1 As the feedback value of the 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:

[0042]

[0043] Among them, u cα1 ,u cβ1 The corresponding voltage vector is U cαβ1 The α-axis and β-axis components, i cα1 ,i cβ1 Corresponding to the current vector I cαβ1 The α-axis and β-axis components.

[0044] 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 .

[0045] The capacitor voltage and reactive power control module 106 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 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 vq2refThe specific implementation of the capacitor voltage and reactive power control module 106 is as follows:

[0046]

[0047] 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 vq2ref Corresponding to the current vector I vdq2ref The d-axis and q-axis components of .

[0048] The power frequency current control module 107 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 107 is as follows:

[0049]

[0050] 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.

[0051] The power frequency Park inverse transformation module 108 outputs the power frequency voltage U vdq2 Perform Park inverse transformation to obtain the power frequency output voltage U in the stationary three-phase coordinate system vαβ2 The angle used by the power frequency Park inverse transformation is the power frequency grid voltage phase θ g2 The specific implementation of the power frequency Park inverse transformation module 108 is as follows:

[0052]

[0053] Among them, u vd2 ,u vq2 The corresponding voltage vector is U vdq2 The d-axis and q-axis components, u vα2 ,u vβ2 The corresponding voltage vector is U vαβ2 The α-axis and β-axis components.

[0054] The power frequency internal circulation control module 109 converts the power frequency internal circulation I cαβ2 As the feedback value of the controller, it can be controlled in a stationary coordinate system without rotating the coordinates. The output of the power frequency internal circulating current controller is used as the power frequency internal circulating current voltage U cαβ2 The specific implementation of the power frequency internal circulation control module 109 is as follows:

[0055]

[0056] Among them, u cα2 ,u cβ2 The corresponding voltage vector is U cαβ2 The α-axis and β-axis components, i cα2 ,i cβ2 Corresponding to the current vector I cαβ2 The α-axis and β-axis components.

[0057] The bridge arm voltage calculation module 110 uses the low-frequency output voltage U vdq1 , low frequency internal circulating current voltage U cαβ1 , power frequency output voltage U vdq2 , power frequency internal circulating current voltage U cαβ2 , the reference voltage of the nine bridge arms of the M3C frequency-changing valve is calculated.

[0058] 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.

[0059] like Figure 6 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 control 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.

[0060] 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; the specific implementation of the active power control module 201 is as follows:

[0061] U dc2ref =U dc2N -F PI1 (s)(P s2ref -P s2 )

[0062]

[0063] 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.

[0064] 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:

[0065]

[0066] 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.

[0067] 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:

[0068]

[0069] 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, 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.

[0070] The Park inverse transformation module 204 performs a Park inverse transformation on the differential mode voltage reference value to obtain a differential mode voltage reference value in a stationary three-phase coordinate system.

[0071] The common mode current control module 205 suppresses the common mode current.

[0072] The bridge arm voltage calculation module 206 calculates a bridge arm voltage reference value according to the differential mode voltage reference value and the common mode voltage reference value.

[0073] The modulation module 207 controls 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. An offshore wind power low-frequency networking system based on hybrid topology, 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, low-frequency circuit breakers, low-frequency transmission submarine cables, N-1 onshore DRU connection transformers, N-1 onshore DRU converters, N-1 onshore MMC converters, N-1 onshore MMC connection transformers, 1 onshore M3C low-frequency connection transformer, 1 onshore M3C converter, and 1 onshore M3C power frequency connection transformer; The electricity generated by the offshore low-frequency wind turbines is collected through low-frequency collection submarine cables and connected to M offshore low-frequency booster stations. After being boosted at the offshore low-frequency booster stations, it is transmitted to the land through low-frequency transmission submarine cables. The low-frequency transmission submarine cables connect the M offshore low-frequency booster stations and 1 onshore M3C low-frequency connection transformer and N-1 onshore DRU connection transformers to form an offshore low-frequency power grid. Each offshore low-frequency booster station, onshore M3C low-frequency connection transformer and onshore DRU connection transformer are connected to the offshore low-frequency power grid through at least two low-frequency transmission submarine cables. Low-frequency circuit breakers are installed at both ends of each section of the low-frequency transmission submarine cable for the When a submarine cable fails, the faulty line is cut off; the onshore M3C low-frequency connection transformer is connected to the onshore M3C converter, and the low-frequency electric energy is converted into industrial frequency electric energy through the onshore M3C converter, and then connected to the onshore industrial frequency AC power grid through the onshore M3C industrial frequency connection transformer; the N-1 onshore DRU connection transformers are connected to the N-1 onshore DRU converters, rectified into DC electric energy through the onshore DRU converter, and then connected to the N-1 onshore MMC converters, and then inverted into industrial frequency AC electric energy through the MMC converter, and then connected to the onshore industrial frequency AC power grid through the N-1 onshore MMC connection transformers.

2. A control method for a hybrid topology-based offshore wind power low-frequency networking system according to claim 1, characterized in that: The onshore M3C converter adopts a fixed low-frequency voltage control strategy to maintain the voltage stability of the offshore low-frequency AC power grid; the N-1 onshore MMC converters adopt a fixed power control strategy to control the active power and reactive power flowing into the corresponding onshore MMC converters; the active power and reactive power reference values ​​of the N-1 onshore MMC converters adopting the fixed power control strategy are given by the offshore wind power low-frequency power grid centralized control system according to the power grid dispatching instructions; The onshore M3C converter control system for implementing the control method includes: 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 low-frequency voltage control module controls the d- and q-axis low-frequency voltages through a low-frequency voltage PI controller, and the output of the low-frequency AC voltage control module is a low-frequency current reference value; the low-frequency current control module controls the d- and q-axis low-frequency currents using a low-frequency current PI controller, and the output of the low-frequency current control module is a low-frequency output voltage; the low-frequency Park inverse transformation module performs a Park inverse transformation on the low-frequency output voltage to obtain a low-frequency output voltage in a stationary three-phase coordinate system; the low-frequency internal circulation control module suppresses the low-frequency internal circulation, and its output is a low-frequency internal circulation voltage; the power frequency phase-locked loop module calculates the power frequency grid voltage phase according to the power frequency grid voltage; the capacitor voltage and reactive power control module controls the capacitor voltage average value and the power frequency reactive power through the capacitor voltage and reactive power PI controller The output of the capacitor voltage and reactive power control module is the power frequency current reference value; the power frequency current control module uses the power frequency current PI controller to control the d and q axis power frequency currents, and the output of the power frequency current control module is the power frequency output voltage; the power frequency Park inverse transformation module performs Park inverse transformation on the power frequency output voltage to obtain the power frequency output voltage in the static three-phase coordinate system; the power frequency internal circulation control module 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 uses the low-frequency output voltage, the low-frequency internal circulation voltage, the power frequency output voltage, and the power frequency internal circulation voltage to calculate the reference voltage of the nine bridge arms of the onshore M3C converter; the modulation module 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; The onshore MMC converter control system for implementing the control 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; The active power control module controls the active power through an active power PI controller, and the output of the active power control module is a DC bus voltage reference value; the DC bus voltage and reactive power control module controls the DC bus voltage and reactive power through a DC bus voltage and reactive power PI controller, and the output of the DC bus voltage and reactive power control module is a differential mode current reference value; the differential mode current control module controls the differential mode current through a differential mode current PI controller, and the output of the differential mode current control module is a differential mode voltage reference value; the Park inverse transformation module performs a Park inverse transformation on the differential mode voltage reference value to obtain a differential mode voltage reference value in a stationary three-phase coordinate system; the common mode current control module controls the common mode current through a common mode current PI controller, and the output of the common mode current control module is a common mode voltage reference value; the bridge arm voltage calculation module 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 realizes control of the onshore MMC converter according to the bridge arm voltage reference value.

3. The offshore wind power low-frequency networking system control method based on hybrid topology according to claim 2 is 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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