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

Abstract

The invention discloses an offshore wind power low-frequency networking system starting method based on hybrid topology, and the method comprises the steps: building an offshore low-frequency power grid voltage through an M3C converter in an offshore wind power low-frequency networking system, achieving the smooth conduction of a DRU converter through the cooperative control of an MMC converter and the M3C converter, and achieving the starting of an offshore wind power low-frequency networking system. Therefore, smooth starting of the offshore wind power low-frequency networking system based on the hybrid topology is realized. And compared with a scheme in which all M3C converters are adopted, under the condition that reliable starting of the system is guaranteed, the engineering construction cost can be remarkably reduced, 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 a method for starting an offshore wind power low-frequency networking system 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 existing offshore wind power low-frequency networking solutions, all receiving nodes use M3C converters, resulting in high 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 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. The startup process of the above-mentioned offshore wind power low-frequency networking system based on hybrid topology is relatively complicated, involving the charging and startup process of multiple systems and key equipment such as M3C converters, DRU converters, and MMC converters. There is currently a lack of effective startup solutions. Summary of the invention

[0004] The purpose of the present invention is to overcome the problem of complex startup process of an offshore wind power low-frequency networking system based on a hybrid topology, and to provide a startup method for an offshore wind power low-frequency networking system based on a hybrid topology, so as to achieve smooth startup of the offshore wind power low-frequency networking system through the coordinated cooperation of an M3C converter, a DRU converter, an MMC converter and a low-frequency wind turbine.

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

[0006] A method for starting an offshore wind power low-frequency networking system based on a hybrid topology, characterized in that the offshore wind power low-frequency networking system based on a hybrid topology comprises: 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, N starting resistors, and 1 onshore M3C power frequency connection transformer;

[0007] 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, the electricity 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 set at both ends of each section of low-frequency transmission submarine cable to cut out the fault line when a fault occurs in the submarine cable. 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 starting resistor and the onshore M3C industrial frequency connection transformer; the N-1 onshore DRU connection transformers are connected to the N-1 onshore DRU converters, and are rectified into DC electric energy through the onshore DRU converters and then connected to the N-1 onshore MMC converters, and then inverted into industrial frequency AC electric energy through the MMC converters and then connected to the onshore industrial frequency AC power grid through the N-1 starting resistors and the onshore MMC connection transformers;

[0008] The onshore M3C converter control system for implementing the startup 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;

[0009] The onshore MMC converter control system for implementing the startup 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;

[0010] The hybrid topology-based offshore wind power low-frequency networking system startup method comprises the following steps:

[0011] The startup process of the hybrid topology-based offshore wind power low-frequency networking system begins by first charging the onshore M3C converter and onshore MMC converter through the startup resistor;

[0012] Unlock the onshore M3C converter. The onshore M3C converter adopts a constant capacitor voltage and reactive power control strategy on the power frequency side to adjust the M3C capacitor voltage to the rated value.

[0013] Unlock the onshore MMC converter. The onshore MMC converter adopts a fixed DC bus voltage and reactive power control strategy to adjust the MMC DC bus voltage to the rated value.

[0014] The low-frequency side of the onshore M3C converter adopts a fixed low-frequency AC voltage control strategy to adjust the low-frequency AC voltage reference value to zero;

[0015] The onshore M3C converter is used to gradually increase the offshore low-frequency AC voltage to the rated value, so that the onshore DRU converter is ready for conduction;

[0016] The onshore MMC converter control strategy is adjusted to a constant power control strategy, and the output of the active power control module is used as the DC bus voltage reference value of the MMC converter;

[0017] Start the offshore low-frequency wind turbines one by one, and gradually increase the output active power of the offshore low-frequency wind turbines to the rated value;

[0018] According to the startup status of the offshore low-frequency wind turbines, the active power reference value of the onshore MMC converter is adjusted through the offshore wind power low-frequency grid control system.

[0019] Furthermore, the DC bus voltage and reactive power control module in the onshore MMC converter control system, when the onshore MMC converter adopts a fixed DC bus voltage and reactive power control strategy, its DC bus voltage reference value is given as the DC bus voltage rated value; when the onshore MMC converter adopts a fixed power control strategy, its DC bus voltage reference value is given by the active power control module;

[0020] In the active power control module, the DC bus voltage reference value is calculated by the following method:

[0021] U dc2ref =U dc2N -F PI1 (s)(P s2ref -P s2 )

[0022]

[0023] Where: P s2ref is the active power reference value, P s2 is the active power, Udc2ref 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;

[0024] In the active power control module, the active power reference value P s2ref It is given by the offshore wind power low-frequency grid centralized control system according to the start-up status of the wind farm wind turbines.

[0025] The technical solution of the present invention is adopted. The present invention establishes the offshore low-frequency grid voltage through the M3C converter, realizes the smooth conduction of the DRU converter through the coordinated control of the MMC converter and the M3C converter, thereby realizing the smooth start-up of the offshore wind power low-frequency networking system based on the hybrid topology. Compared with the solution of using all M3C converters, it can significantly reduce the engineering construction cost under the condition of ensuring the reliable start-up of the system, and has good application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 The present invention is a flow chart of a method for starting a hybrid topology-based offshore wind power low-frequency networking system.

[0027] 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.

[0028] Figure 3 This is a typical topological diagram of the onshore M3C converter in the present invention.

[0029] Figure 4 This is a typical topological diagram of the onshore DRU converter in the present invention.

[0030] Figure 5 This is a typical topological diagram of the onshore MMC converter in the present invention.

[0031] Figure 6 It is a specific example system schematic diagram of the onshore M3C converter control system in the present invention.

[0032] Figure 7 It is a specific example system schematic diagram of the onshore MMC converter control system in the present invention. DETAILED DESCRIPTION

[0033] 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.

[0034] like Figure 2As 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, N starting resistors 12, and 1 onshore M3C industrial frequency connection transformer 13.

[0035] In the embodiment of the present invention, the electricity generated by the offshore low-frequency wind turbine 1 is collected at sea through the low-frequency collection submarine cable 2, connected to M offshore low-frequency booster stations 3, and 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 2 The 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. The onshore industrial frequency AC power grid is connected through a starting resistor 12 and an onshore M3C industrial frequency connection transformer 13; 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 converters 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 converters 8 and then connected to the onshore industrial frequency AC power grid through N-1 one-to-one corresponding starting resistors 12 and onshore MMC connection transformers 9.

[0036] 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.

[0037] like Figure 4As 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°.

[0038] 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.

[0039] like Figure 1 As shown, in an embodiment of the present invention, a method for starting an offshore wind power low-frequency networking system based on a hybrid topology is as follows:

[0040] The system startup process begins, firstly charging the onshore M3C converter 11 and the onshore MMC converter 8 through the startup resistor 12 .

[0041] The onshore M3C converter 11 is unlocked. The onshore M3C converter 11 adopts a constant capacitor voltage and reactive power control strategy on the power frequency side to adjust the M3C capacitor voltage to the rated value.

[0042] 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 startup 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.

[0043] In the embodiment of the present invention, the low-frequency voltage control module 101 controls the d-axis and q-axis low-frequency voltages through a PI controller, and the output of the low-frequency AC voltage control module 101 is a low-frequency current reference value; the low-frequency current control module 102 uses a PI controller to control the low-frequency d-axis and q-axis currents, and the output of the low-frequency current control module 102 is a low-frequency output voltage; the low-frequency Park inverse transformation module 103 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 104 suppresses the low-frequency internal circulation, and its output is a low-frequency internal circulation voltage; the power frequency phase-locked loop module 105 calculates the power frequency grid voltage phase according to the power frequency grid voltage; the capacitor voltage and reactive power control module 106 controls the capacitor voltage average value and the power frequency reactive power through a PI controller, The output of the capacitor voltage and reactive power control module 106 is the power frequency current reference value; the power frequency current control module 107 uses a PI controller to control the d and q axis power frequency currents, and the output of the power frequency current control module 107 is the power frequency output voltage; the power frequency Park inverse transformation module 108 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 109 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 110 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 111 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;

[0044] Unlock the onshore MMC converter 8, which adopts a fixed DC bus voltage and reactive power control strategy to adjust the MMC DC bus voltage to a rated value;

[0045] 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.

[0046] 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:

[0047] U dc2ref =U dc2N -FPI1 (s)(P s2ref -P s2 )

[0048]

[0049] 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 integral coefficient; in the active power control module, the active power reference value P s2ref It is given by the offshore wind power low-frequency grid centralized control system according to the start-up status of the wind farm wind turbines.

[0050] The DC bus voltage and reactive power control module 202 controls the DC bus voltage and reactive power through a PI controller, and the output of the DC bus voltage and reactive power control module is a differential mode current reference value; when the onshore MMC converter adopts a fixed DC bus voltage and reactive power control strategy, its DC bus voltage reference value is given as the DC bus voltage rated value; when the onshore MMC converter adopts a fixed power control strategy, its DC bus voltage reference value is given by the active power control module.

[0051] 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 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; 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 a 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 control of the onshore MMC converter according to the bridge arm voltage reference value.

[0052] The low-frequency side of the onshore M3C converter 11 adopts a fixed low-frequency AC voltage control strategy to adjust the low-frequency AC voltage reference value to zero.

[0053] The onshore M3C converter 11 gradually increases the offshore low-frequency AC voltage to a rated value, so that the onshore DRU converter 7 meets the conduction conditions.

[0054] The control strategy of the onshore MMC converter 8 is adjusted to a constant power control strategy, and the output of the active power control module is used as a reference value for the DC bus voltage of the MMC converter.

[0055] The offshore low-frequency wind turbines 1 are started one by one, and the output active power of the offshore low-frequency wind turbines is gradually increased to the rated value.

[0056] According to the startup status of the offshore low-frequency wind turbine generator set, the active power reference value of the onshore MMC converter 8 is adjusted through the offshore wind power low-frequency grid control system.

[0057] The above description of the embodiments is to facilitate the understanding and application of the present invention by those skilled in the art. It is obvious that those skilled in the art can easily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without creative work. Therefore, the present invention is not limited to the above embodiments, and improvements and modifications made to the present invention by those skilled in the art based on the disclosure of the present invention should be within the scope of protection of the present invention.

Claims

1. A method for starting an offshore wind power low-frequency networking system based on a 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, N starting resistors, 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, the electricity 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 set at both ends of each section of low-frequency transmission submarine cable to cut out the fault line when a fault occurs in the submarine cable. 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 starting resistor and the onshore M3C industrial frequency connection transformer; the N-1 onshore DRU connection transformers are connected to the N-1 onshore DRU converters, and are rectified into DC electric energy through the onshore DRU converters and then connected to the N-1 onshore MMC converters, and then inverted into industrial frequency AC electric energy through the MMC converters and then connected to the onshore industrial frequency AC power grid through the N-1 starting resistors and the onshore MMC connection transformers; The onshore M3C converter control system for implementing the startup 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 onshore MMC converter control system for implementing the startup 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; The hybrid topology-based offshore wind power low-frequency networking system startup method comprises the following steps: The startup process of the offshore wind power low-frequency networking system based on the hybrid topology begins by first charging the onshore M3C converter and the onshore MMC converter through the starting resistor; Unlock the onshore M3C converter. The onshore M3C converter adopts a constant capacitor voltage and reactive power control strategy on the power frequency side to adjust the M3C capacitor voltage to the rated value. Unlock the onshore MMC converter. The onshore MMC converter adopts a fixed DC bus voltage and reactive power control strategy to adjust the MMC DC bus voltage to the rated value. The low-frequency side of the onshore M3C converter adopts a fixed low-frequency AC voltage control strategy to adjust the low-frequency AC voltage reference value to zero; The onshore M3C converter is used to gradually increase the offshore low-frequency AC voltage to the rated value, so that the onshore DRU converter is ready for conduction; The onshore MMC converter control strategy is adjusted to a constant power control strategy, and the output of the active power control module is used as the DC bus voltage reference value of the MMC converter; Start the offshore low-frequency wind turbines one by one, and gradually increase the output active power of the offshore low-frequency wind turbines to the rated value; According to the startup status of the offshore low-frequency wind turbines, the active power reference value of the onshore MMC converter is adjusted through the offshore wind power low-frequency grid control system.

2. The method for starting an offshore wind power low-frequency networking system based on hybrid topology according to claim 1, characterized in that: The DC bus voltage and reactive power control module in the onshore MMC converter control system, when the onshore MMC converter adopts a fixed DC bus voltage and reactive power control strategy, its DC bus voltage reference value is given as the DC bus voltage rated value; when the onshore MMC converter adopts a fixed power control strategy, its DC bus voltage reference value is given by the active power control module; In the active power control module, the DC bus voltage reference value is calculated 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; In the active power control module, the active power reference value P s2ref It is given by the offshore wind power low-frequency grid centralized control system according to the start-up status of the wind farm wind turbines.

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