Starting method of network-forming type offshore wind power delivery system with different offshore distances

Abstract

The invention discloses a method for starting a network-building type offshore wind power sending-out system at different offshore distances, and the method achieves the smooth starting of the network-building type offshore wind power sending-out system at different offshore distances through the cooperation of a land MMC current converter, a medium-frequency and low-frequency network-building type fan network side current converter, a machine side current converter and a DC / DC current converter. Compared with a traditional scheme of independently sending out by respectively adopting an MMC flexible direct current converter valve and an M3C low-frequency converter valve, the method has the advantages that the engineering construction cost can be obviously reduced under the condition of ensuring reliable starting of the system, and the method has a good application prospect.

Description

Technical Field

[0001] The present invention belongs to the technical field of renewable energy power generation, and in particular relates to a method for starting a grid-type offshore wind power transmission system at different offshore distances. Background Art

[0002] At present, offshore wind power is developing towards the deep sea. Transmission schemes such as industrial frequency AC transmission, flexible DC transmission, and low-frequency AC transmission have been applied in the offshore wind power field. The above transmission schemes are suitable for offshore wind power transmission at different offshore distances.

[0003] In existing projects, transmission schemes for different offshore wind farm sites are designed independently, resulting in high overall construction costs for offshore wind power transmission projects. Furthermore, in offshore wind power flexible direct current transmission systems, offshore converter stations using the MMC topology are large and heavy, resulting in high construction and transportation costs. In offshore wind power low-frequency transmission systems, converter valves using the M3C topology also face high equipment costs, hindering the affordable access of offshore wind power to the grid. To address these issues, a grid-type wind turbine and diode converter valve solution can be used to replace traditional MMC flexible direct current converter valves and M3C low-frequency converter valves. Furthermore, by comprehensively considering the transmission requirements of offshore wind farms at different offshore distances, a holistic transmission system solution can be designed to enable the sharing of key electrical equipment such as converter valves and transformers across different transmission schemes, reducing the amount of equipment and, consequently, the construction costs of offshore wind power transmission projects.

[0004] However, the startup process of the above-mentioned offshore wind power transmission systems with different offshore distances is relatively complicated, involving the startup and grid connection of multiple systems and key equipment such as medium-frequency and low-frequency grid-type wind turbines, MMC converters, DRU converters, etc., and there is currently a lack of effective startup solutions. Summary of the Invention

[0005] The present invention addresses the complex startup process for grid-type offshore wind power transmission systems at varying offshore distances, a problem currently lacking an effective solution. By providing a startup method for grid-type offshore wind power transmission systems at varying offshore distances, the present invention achieves smooth startup of grid-type offshore wind power transmission systems at varying offshore distances through the coordinated cooperation of onshore MMC converters, medium- and low-frequency grid-type wind turbine grid-side converters, turbine-side converters, and DC / DC converters.

[0006] In order to achieve the above-mentioned purpose of the invention, the present method adopts the following technical solutions:

[0007] A method for starting a grid-type offshore wind power transmission system with different offshore distances, characterized in that the grid-type offshore wind power transmission system with different offshore distances comprises: a medium-frequency grid-type wind turbine generator set, a medium-frequency power collection submarine cable, a medium-frequency converter transformer, a medium-frequency diode converter, a DC power transmission submarine cable, a low-frequency grid-type wind turbine generator set, a low-frequency power collection submarine cable, a low-frequency step-up transformer, a low-frequency power transmission submarine cable, a low-frequency converter transformer, a low-frequency diode converter, an onshore MMC converter, a starting resistor, a power frequency connection transformer, a power frequency wind turbine generator set, a power frequency power collection submarine cable, a power frequency step-up transformer, and a power frequency power transmission submarine cable;

[0008] The electricity generated by the medium-frequency grid-type wind turbines is collected through the medium-frequency collecting submarine cable, boosted by the medium-frequency converter transformer, and then rectified into DC by the medium-frequency diode converter, and transmitted to the land by the DC transmission submarine cable, and connected to the onshore DC bus; the electricity generated by the low-frequency grid-type wind turbines is collected through the low-frequency collecting submarine cable, boosted by the low-frequency booster transformer, and transmitted to the land by the low-frequency transmission submarine cable, and then connected to the low-frequency diode converter by the low-frequency converter transformer, rectified into DC by the low-frequency diode converter, and connected to the onshore DC bus, and then inverted into industrial frequency AC by the onshore MMC converter, and then connected to the receiving-end AC power grid through the starting resistor and the industrial frequency connecting transformer; the electricity generated by the industrial frequency wind turbines is collected through the industrial frequency collecting submarine cable, boosted by the industrial frequency booster transformer, and transmitted to the land by the industrial frequency transmission submarine cable, and then connected to the receiving-end AC power grid;

[0009] The medium-frequency grid-type wind turbine generator set includes a wind turbine generator, a medium-frequency wind turbine machine-side converter, a medium-frequency wind turbine grid-side converter, a medium-frequency wind turbine step-up transformer, a medium-frequency wind turbine energy storage battery, and a medium-frequency wind turbine DC / DC converter; the low-frequency grid-type wind turbine generator set includes a wind turbine generator, a low-frequency wind turbine machine-side converter, a low-frequency wind turbine grid-side converter, a low-frequency wind turbine step-up transformer, a low-frequency wind turbine energy storage battery, and a low-frequency wind turbine DC / DC converter; the industrial frequency wind turbine generator set includes a wind turbine generator, an industrial frequency wind turbine machine-side converter, an industrial frequency wind turbine grid-side converter, and an industrial frequency wind turbine step-up transformer;

[0010] The specific system startup process is as follows:

[0011] charging the onshore MMC converter via a starting resistor, unlocking the onshore MMC converter after charging is complete, and adjusting the onshore DC bus voltage to a rated value;

[0012] Start the medium-frequency wind turbine energy storage battery and the medium-frequency wind turbine DC / DC converter, and adjust the medium-frequency wind turbine DC bus voltage to the rated value;

[0013] Start the grid-side converter of the medium-frequency wind turbine and establish the offshore medium-frequency AC grid voltage through the grid-type control strategy; gradually increase the offshore medium-frequency AC grid voltage to the rated value through the grid-side converter of the medium-frequency wind turbine, so that the medium-frequency diode converter is ready for conduction;

[0014] Start the medium-frequency wind turbine-side converter and wind turbine generator, and gradually increase the output active power of the medium-frequency wind turbine to the rated value;

[0015] Other medium-frequency wind turbines are connected to the grid one by one;

[0016] Start the low-frequency fan energy storage battery and low-frequency fan DC / DC converter, and adjust the low-frequency fan DC bus voltage to the rated value;

[0017] Start the grid-side converter of the low-frequency wind turbine and establish the offshore low-frequency AC grid voltage through the grid-type control strategy; gradually increase the offshore low-frequency AC grid voltage to the rated value through the grid-side converter of the low-frequency wind turbine, so that the low-frequency diode converter is ready for conduction;

[0018] Start the low-frequency wind turbine side converter and wind turbine generator, and gradually increase the output active power of the low-frequency wind turbine to the rated value;

[0019] Other low-frequency wind turbines are connected to the grid one by one;

[0020] The industrial frequency wind turbines are connected to the grid one by one, and the system startup process is completed.

[0021] The offshore distance of medium-frequency grid-type wind turbines is farther than that of low-frequency grid-type wind turbines, and the offshore distance of low-frequency grid-type wind turbines is farther than that of industrial-frequency wind turbines.

[0022] Furthermore, the grid-side converter of the medium-frequency wind turbine and the grid-side converter of the low-frequency wind turbine adopt a grid-type control strategy. During the startup process, they are responsible for establishing the voltage of the medium-frequency AC grid and the low-frequency AC grid, adjusting the grid voltage and frequency, and adjusting the active power and reactive power output by the grid-side converter. The grid-side rated frequency of the grid-side converter of the medium-frequency wind turbine is preferably 150 Hz, and the grid-side rated frequency of the grid-side converter of the low-frequency wind turbine is preferably 20 Hz.

[0023] The medium-frequency wind turbine side converter and the low-frequency wind turbine side converter adopt a constant DC bus voltage and reactive power control strategy to maintain the stability of the wind turbine DC bus voltage;

[0024] The medium-frequency fan DC / DC converter and the low-frequency fan DC / DC converter adopt a fixed DC bus voltage control strategy and are responsible for establishing the medium-frequency fan DC bus voltage and the low-frequency fan DC bus voltage during the startup phase;

[0025] The grid-side converter of the power-frequency wind turbine adopts a constant DC bus voltage and reactive power control strategy to maintain the stability of the DC bus voltage of the power-frequency wind turbine. The grid-side voltage and frequency of the grid-side converter of the power-frequency wind turbine follow the receiving AC grid, and the grid-side rated frequency is 50Hz.

[0026] The power frequency wind turbine side converter adopts a fixed active power and reactive power control strategy and a maximum power tracking operation mode;

[0027] The onshore MMC converter adopts a constant DC bus voltage and reactive power control strategy, and is responsible for establishing the onshore DC bus voltage and maintaining the onshore DC bus voltage stable during startup.

[0028] The beneficial effects of the present invention are:

[0029] The technical solution of this invention utilizes medium-frequency and low-frequency grid-type wind turbines to establish offshore medium-frequency AC power grids and mid-sea low-frequency AC power grids, respectively. Through the coordinated cooperation of onshore MMC converters, grid-side converters for medium-frequency and low-frequency grid-type wind turbines, turbine-side converters, and DC / DC converters, this system enables the smooth startup of grid-type offshore wind power transmission systems at varying offshore distances. Compared to traditional solutions that use separate MMC flexible DC converter valves and M3C low-frequency converter valves for independent transmission, this system significantly reduces construction costs while ensuring reliable system startup, demonstrating promising application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is a flow chart of the method for starting a grid-type offshore wind power transmission system at different offshore distances according to the present invention.

[0031] Figure 2 This is a typical topological diagram of the grid-type offshore wind power transmission system of the present invention at different offshore distances.

[0032] Figure 3 This is a specific example system schematic diagram of the control method of the medium-frequency wind turbine side converter and the low-frequency wind turbine side converter of the present invention.

[0033] Figure 4 This is a specific example system schematic diagram of the control method of the medium-frequency wind turbine grid-side converter and the low-frequency wind turbine grid-side converter of the present invention.

[0034] Figure 5 This is a system schematic diagram of a specific example of the onshore MMC converter control method of the present invention. DETAILED DESCRIPTION

[0035] 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.

[0036] In the embodiment of the present invention, the offshore wind power transmission system with different offshore distances is as follows: Figure 2 As shown, it includes a medium-frequency grid-type wind turbine 1, a medium-frequency power collection cable 2, a medium-frequency converter transformer 3, a medium-frequency diode converter 4, a DC transmission cable 5, a low-frequency grid-type wind turbine 6, a low-frequency power collection cable 7, a low-frequency step-up transformer 8, a low-frequency transmission cable 9, a low-frequency converter transformer 10, a low-frequency diode converter 11, an onshore MMC converter 12, a starting resistor 13, a power frequency connection transformer 14, a power frequency wind turbine 15, a power frequency power collection cable 16, a power frequency step-up transformer 17, and a power frequency transmission cable 18.

[0037] In an embodiment of the present invention, an offshore wind farm using a medium-frequency grid-type wind turbine 1 is located in the far sea area more than 150 kilometers offshore, an offshore wind farm using a low-frequency grid-type wind turbine 6 is located in the medium-to-far sea area 80-150 kilometers offshore, and an offshore wind farm using an industrial frequency wind turbine 15 is located in the near sea area within 80 kilometers offshore.

[0038] The electricity generated by the medium-frequency grid-type wind turbine 1 is collected in the open sea through the medium-frequency power collection cable 2, boosted by the medium-frequency converter transformer 3, rectified into DC by the medium-frequency diode converter 4, and transmitted to the land via the DC transmission cable 5, and connected to the onshore DC bus; the electricity generated by the low-frequency grid-type wind turbine 6 is collected in the open sea through the low-frequency power collection cable 7, boosted by the low-frequency boost transformer 8, transmitted to the land via the low-frequency transmission cable 9, and then connected to the land via the low-frequency converter transformer 10. The power frequency wind turbine 15 is connected to the onshore DC bus after being rectified into power frequency AC power by the low-frequency diode converter 11, and is then connected to the onshore DC bus. After being inverted into power frequency AC power by the onshore MMC converter 12, it is connected to the receiving-end AC power grid through the starting resistor 13 and the power frequency connection transformer 14; the power frequency wind turbine 15 is collected offshore through the power frequency collection cable 16, stepped up by the power frequency step-up transformer 17, and transmitted to the land through the power frequency transmission cable 18, and is then connected to the receiving-end AC power grid.

[0039] The medium-frequency grid-type wind turbine group 1 includes a wind turbine, a medium-frequency wind turbine machine-side converter, a medium-frequency wind turbine grid-side converter, a medium-frequency wind turbine step-up transformer, a medium-frequency wind turbine energy storage battery, and a medium-frequency wind turbine DC / DC converter; the low-frequency grid-type wind turbine group 6 includes a wind turbine, a low-frequency wind turbine machine-side converter, a low-frequency wind turbine grid-side converter, a low-frequency wind turbine step-up transformer, a low-frequency wind turbine energy storage battery, and a low-frequency wind turbine DC / DC converter; the industrial frequency wind turbine group 15 includes a wind turbine, an industrial frequency wind turbine machine-side converter, an industrial frequency wind turbine grid-side converter, and an industrial frequency wind turbine step-up transformer.

[0040] like Figure 1As shown, in the embodiment of the present invention, the specific startup process of the offshore wind power transmission system with different offshore distances is as follows:

[0041] The onshore MMC converter 12 is charged through the starting resistor 13 . After charging is completed, the onshore MMC converter 12 is unlocked and the DC bus voltage is adjusted to a rated value.

[0042] In the embodiment of the present invention, the onshore MMC converter 12 adopts a constant DC bus voltage and reactive power control strategy. During the startup process, it is responsible for establishing the onshore DC bus voltage and maintaining the onshore DC bus voltage stable. The schematic diagram of its control system is shown in FIG. Figure 5 As shown, it includes: a DC bus voltage and reactive power control module 301, a differential mode current control module 302, a Park inverse transformation module 303, an internal circulating current control module 304, a bridge arm voltage calculation module 305, and a modulation module 306.

[0043] The DC bus voltage and reactive power control module 301 controls the DC bus voltage U dc2 The eighth PI controller is used to control the reference value U dc2ref The output of the controller is limited by the amplitude limiter and used as the reference value of the d-axis current i gdref2 ; For reactive power Q g2 The ninth PI controller is used to control the Q g2ref The output of the controller passes through the limiting link and serves as the reference value of the q-axis current i gqref2 .

[0044] The differential mode current control module 302 controls the d and q axis currents i of the onshore MMC converter. gd2 and i gq2 The tenth PI controller is used to control the reference value i gdref2 and i gqref2 , the output of the controller is used as the reference differential mode voltage U difdq2 .

[0045] The Park inverse transformation module 303 is used to convert the onshore MMC converter reference differential mode voltage U difdq2 Perform Park inverse transform to obtain the reference differential mode voltage U in the stationary three-phase coordinate system difabc2 The angle used in the Park inverse transformation is the phase of the receiving AC grid θ s .

[0046] Internal circulation control module 304 controls the internal circulation I of the onshore MMC converter. cabc2 A resonant controller is used for control, and the output of the internal circulating current control module is used as the reference common mode voltage U comabc2 .

[0047] The bridge arm voltage calculation module 305 calculates the bridge arm voltage according to the reference differential mode voltage U difabc2 and the reference common mode voltage U comabc2 , the reference voltage U of the upper and lower bridge arms is obtained through calculation prefabc2 with U nrefabc2 and generates a switching signal through the modulation module 306 to control the onshore MMC converter.

[0048] Start the medium-frequency wind turbine energy storage battery and DC / DC converter, and adjust the medium-frequency wind turbine DC bus voltage to the rated value.

[0049] In an embodiment of the present invention, the medium-frequency wind turbine DC / DC converter and the low-frequency wind turbine DC / DC converter adopt a fixed DC bus voltage control strategy and are respectively responsible for establishing the DC bus voltage of the medium-frequency wind turbine and the low-frequency wind turbine during the startup phase.

[0050] Start the medium-frequency wind turbine grid-side converter, and establish the offshore medium-frequency AC grid voltage through the grid-type control strategy; gradually increase the offshore medium-frequency AC grid voltage to the rated value through the medium-frequency wind turbine grid-side converter, so that the medium-frequency diode converter 4 has the conduction conditions.

[0051] In the embodiment of the present invention, the grid-side converter of the medium-frequency wind turbine and the grid-side converter of the low-frequency wind turbine adopt a grid-type control strategy. During the startup process, they are respectively responsible for establishing the voltage of the medium-frequency AC grid and the low-frequency AC grid, adjusting the grid voltage and frequency, and adjusting the active power and reactive power output by the grid-side converter. The grid-side rated frequency of the grid-side converter of the medium-frequency wind turbine is preferably 150Hz, and the grid-side rated frequency of the grid-side converter of the low-frequency wind turbine is preferably 20Hz. The schematic diagram of the control system of the grid-side converter of the medium-frequency wind turbine and the grid-side converter of the low-frequency wind turbine is shown in FIG. Figure 4 As shown, it includes: a grid-side active power control module 201, a grid-side reactive power control module 202, a grid-side Park conversion module 203, a voltage outer loop control module 204, a grid-side current inner loop control module 205, a grid-side Park inverse conversion module 206, and a grid-side modulation module 207.

[0052] The grid-side active power control module 201 calculates the grid-side voltage reference value according to the following method:

[0053]

[0054] Among them: F PI1 (s) is the transfer function of the first PI controller, k p1 is the proportionality coefficient, k i1 is the integral coefficient, u gdref ,u gqref Corresponding to the voltage reference value vector U gdqref The d-axis and q-axis components, U refis the voltage amplitude reference value, P gref is the grid side active power reference value, P g is the grid-side active power.

[0055] The grid-side reactive power control module 202 calculates the grid-side phase reference value according to the following method:

[0056] θ g =∫ω g dt

[0057] ω g =F PI2 (s)(Q gref -Q g )+ω n

[0058]

[0059] Among them: F PI2 (s) is the transfer function of the second PI controller, k p2 is the proportionality coefficient, k i2 is the integral coefficient, θ g is the grid side phase reference value, ω g is the grid side angular frequency reference value, ω n is the rated angular frequency, Q gref is the grid-side reactive power reference value, Q g is the grid-side reactive power.

[0060] The grid-side Park conversion module 203 is used to convert the grid-side voltage U gabc , grid-side current I gabc Perform Park transformation to obtain the grid-side voltage U in the synchronous rotating coordinate system gdq , grid-side current I gdq The angle used by Park transformation is the grid-side phase reference value θ g .

[0061] The grid-side voltage outer loop control module 204 is implemented as follows:

[0062]

[0063] Among them: F PI3 (s) is the transfer function of the third PI controller, k p3 is the proportionality coefficient, k i3 is the integral coefficient, i gdref ,i gqref Corresponding to the current vector I gdqref The d-axis and q-axis components, u gdref ,u gqref Corresponding to the voltage reference value vector U gdqrefThe d-axis and q-axis components, u gd ,u gq Corresponding to the voltage vector U gdq The d-axis and q-axis components of .

[0064] The grid-side current inner loop control module 205 is implemented as follows:

[0065]

[0066] Among them: F PI4 (s) is the transfer function of the fourth PI controller, k p4 is the proportionality coefficient, k i4 is the integral coefficient, u vdref ,u vqref Corresponding to the voltage vector U vdqref The d-axis and q-axis components, u gd ,u gq Corresponding to the voltage vector U gdq The d-axis and q-axis components, i gd ,i gq Corresponding to the current vector I gdq The d-axis and q-axis components, ω g is the grid voltage angular frequency, L g For the filter inductor.

[0067] The grid-side Park inverse conversion module 206 is used to convert the grid-side reference voltage U vdqref Perform Park inverse transformation to obtain the grid-side reference voltage U in the stationary three-phase coordinate system vabcref The angle used in the Park inverse transform is the grid-side phase reference value θ g .

[0068] The grid side modulation module 207 is based on the grid side reference voltage U vabcref , generate switching signals to control the grid-side converter.

[0069] Start the medium-frequency wind turbine side converter and wind turbine, and gradually increase the output active power of the medium-frequency wind turbine to the rated value.

[0070] In the embodiment of the present invention, the medium-frequency wind turbine side converter and the low-frequency wind turbine side converter adopt a constant DC bus voltage and reactive power control strategy to maintain the stability of the wind turbine DC bus voltage. The control system schematic diagram of the medium-frequency wind turbine side converter and the low-frequency wind turbine side converter is shown in FIG. Figure 3 As shown, it includes: a rotor position observation module 101, a machine-side Park transformation module 102, a machine-side reactive power control module 103, a DC bus voltage control module 104, a machine-side current inner loop control module 105, a machine-side Park inverse transformation module 106, and a machine-side modulation module 107.

[0071] The rotor position observation module 101 obtains the rotor phase θ by observation and calculation r .

[0072] The generator side Park conversion module 102 converts the generator side current I sabc Perform Park transformation to obtain the machine-side current I in the synchronous rotating coordinate system sdq , the angle used by Park transformation is the rotor phase θ r .

[0073] The implementation of the machine-side reactive power control module 103 is as follows:

[0074] i sdref =F PI5 (s)(Q sref -Q s )

[0075]

[0076] Among them: F PI5 (s) is the transfer function of the fifth PI controller, k p5 is the proportionality coefficient, k i5 is the integral coefficient, i sdref is the current vector I sdqref The d-axis component, Q sref is the reactive power reference value, Q s is the reactive power.

[0077] The DC bus voltage control module 104 is implemented as follows:

[0078] i sqref =F PI6 (s)(U dcref -U dc )

[0079]

[0080] Among them: F PI6 (s) is the transfer function of the sixth PI controller, k p6 is the proportionality coefficient, k i6 is the integral coefficient, i sqref is the current vector I sdqref The q-axis component, U dcref is the DC bus voltage reference value of the fan, U dc is the DC bus voltage of the fan.

[0081] The implementation of the generator-side current inner loop control module 105 is as follows:

[0082]

[0083] Among them: F PI7 (s) is the transfer function of the seventh PI controller, k p7 is the proportionality coefficient, k i7 is the integral coefficient, u sdref ,u sqref Corresponding to the voltage vector U sdqref The d-axis and q-axis components, i sd ,i sq Corresponding to the current vector I sdq The d-axis and q-axis components, ω r is the rotor angular frequency, L s is the stator inductance of the wind turbine, and Ψ is the magnetic flux of the rotor permanent magnet.

[0084] The Park inverse conversion module 106 on the machine side is used to convert the machine side reference voltage U sdqref Perform Park inverse transformation to obtain the machine-side reference voltage U in the stationary three-phase coordinate system sabcref , the angle used by the Park inverse transform is the rotor phase θ r .

[0085] The machine side modulation module 107 is based on the machine side reference voltage U sabcref , generate switching signals to control the machine-side converter.

[0086] Other medium-frequency wind turbines in offshore wind farms are connected to the grid one by one using the grid-connected method of conventional grid-following wind turbines.

[0087] Start the low-frequency wind turbine energy storage battery and DC / DC converter, and adjust the low-frequency wind turbine DC bus voltage to the rated value.

[0088] Start the low-frequency wind turbine grid-side converter, and establish the offshore low-frequency AC grid voltage through the grid-type control strategy; gradually increase the offshore low-frequency AC grid voltage to the rated value through the low-frequency wind turbine grid-side converter, so that the low-frequency diode converter 11 has the conduction condition.

[0089] Start the low-frequency wind turbine side converter and wind turbine, and gradually increase the output active power of the low-frequency wind turbine to the rated value.

[0090] Other low-frequency wind turbines in COSCO Wind Farm are connected to the grid one by one using the grid-connected method of conventional grid-following wind turbines.

[0091] The industrial frequency wind turbines in the offshore wind farm are connected to the grid one by one, and the system startup process is completed.

[0092] In this embodiment of the present invention, the grid-side converter of the power-frequency wind turbine uses a constant DC bus voltage and reactive power control strategy to maintain a stable DC bus voltage. The grid-side voltage and frequency of the grid-side converter follow the receiving AC grid, with a rated grid-side frequency of 50 Hz. The machine-side converter of the power-frequency wind turbine uses a constant active power and reactive power control strategy and operates in maximum power point tracking mode.

[0093] The above description of the embodiments is intended to facilitate understanding and application of the present invention by those skilled in the art. It will be apparent that those skilled in the art can readily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without requiring inventive effort. Therefore, the present invention is not limited to the above embodiments, and improvements and modifications made by those skilled in the art based on the disclosure of the present invention should fall within the scope of protection of the present invention.

Claims

1. A method for starting a grid-type offshore wind power transmission system at different offshore distances, characterized in that: The offshore wind power transmission system with different offshore distances includes: a medium-frequency grid-type wind turbine, a medium-frequency power collection cable, a medium-frequency converter transformer, a medium-frequency diode converter, a DC transmission cable, a low-frequency grid-type wind turbine, a low-frequency power collection cable, a low-frequency step-up transformer, a low-frequency transmission cable, a low-frequency converter transformer, a low-frequency diode converter, an onshore MMC converter, a starting resistor, a power frequency connection transformer, a power frequency wind turbine, a power frequency power collection cable, a power frequency step-up transformer, and a power frequency transmission cable. The electricity generated by the medium-frequency grid-type wind turbines is collected through the medium-frequency collecting submarine cable, boosted by the medium-frequency converter transformer, and then rectified into DC by the medium-frequency diode converter, and transmitted to the land by the DC transmission submarine cable, and connected to the onshore DC bus; the electricity generated by the low-frequency grid-type wind turbines is collected through the low-frequency collecting submarine cable, boosted by the low-frequency booster transformer, and transmitted to the land by the low-frequency transmission submarine cable, and then connected to the low-frequency diode converter by the low-frequency converter transformer, rectified into DC by the low-frequency diode converter, and connected to the onshore DC bus, and then inverted into industrial frequency AC by the onshore MMC converter, and then connected to the receiving-end AC power grid through the starting resistor and the industrial frequency connecting transformer; the electricity generated by the industrial frequency wind turbines is collected through the industrial frequency collecting submarine cable, boosted by the industrial frequency booster transformer, and transmitted to the land by the industrial frequency transmission submarine cable, and then connected to the receiving-end AC power grid; The medium-frequency grid-type wind turbine generator set includes a wind turbine generator, a medium-frequency wind turbine machine-side converter, a medium-frequency wind turbine grid-side converter, a medium-frequency wind turbine step-up transformer, a medium-frequency wind turbine energy storage battery, and a medium-frequency wind turbine DC / DC converter; the low-frequency grid-type wind turbine generator set includes a wind turbine generator, a low-frequency wind turbine machine-side converter, a low-frequency wind turbine grid-side converter, a low-frequency wind turbine step-up transformer, a low-frequency wind turbine energy storage battery, and a low-frequency wind turbine DC / DC converter; the industrial frequency wind turbine generator set includes a wind turbine generator, an industrial frequency wind turbine machine-side converter, an industrial frequency wind turbine grid-side converter, and an industrial frequency wind turbine step-up transformer; The specific system startup process is as follows: charging the onshore MMC converter via a starting resistor, unlocking the onshore MMC converter after charging is complete, and adjusting the onshore DC bus voltage to a rated value; Start the medium-frequency wind turbine energy storage battery and the medium-frequency wind turbine DC / DC converter, and adjust the medium-frequency wind turbine DC bus voltage to the rated value; Start the grid-side converter of the medium-frequency wind turbine and establish the offshore medium-frequency AC grid voltage through the grid-type control strategy; gradually increase the offshore medium-frequency AC grid voltage to the rated value through the grid-side converter of the medium-frequency wind turbine, so that the medium-frequency diode converter is ready for conduction; Start the medium-frequency wind turbine-side converter and wind turbine generator, and gradually increase the output active power of the medium-frequency wind turbine to the rated value; Other medium-frequency wind turbines are connected to the grid one by one; Start the low-frequency fan energy storage battery and low-frequency fan DC / DC converter, and adjust the low-frequency fan DC bus voltage to the rated value; Start the grid-side converter of the low-frequency wind turbine and establish the offshore low-frequency AC grid voltage through the grid-type control strategy; gradually increase the offshore low-frequency AC grid voltage to the rated value through the grid-side converter of the low-frequency wind turbine, so that the low-frequency diode converter is ready for conduction; Start the low-frequency wind turbine side converter and wind turbine generator, and gradually increase the output active power of the low-frequency wind turbine to the rated value; Other low-frequency wind turbines are connected to the grid one by one; The industrial frequency wind turbines are connected to the grid one by one, and the system startup process is completed.

2. The method for starting a grid-type offshore wind power transmission system at different offshore distances according to claim 1, characterized in that: The grid-side converter of the medium-frequency wind turbine and the grid-side converter of the low-frequency wind turbine adopt a grid-type control strategy. During the startup process, they are responsible for establishing the voltage of the medium-frequency AC grid and the low-frequency AC grid, regulating the grid voltage and frequency, and regulating the active power and reactive power output by the grid-side converter. The grid-side rated frequency of the grid-side converter of the medium-frequency wind turbine is preferably 150 Hz, and the grid-side rated frequency of the grid-side converter of the low-frequency wind turbine is preferably 20 Hz. The medium-frequency wind turbine side converter and the low-frequency wind turbine side converter adopt a constant DC bus voltage and reactive power control strategy to maintain the stability of the wind turbine DC bus voltage; The medium-frequency fan DC / DC converter and the low-frequency fan DC / DC converter adopt a fixed DC bus voltage control strategy and are responsible for establishing the medium-frequency fan DC bus voltage and the low-frequency fan DC bus voltage during the startup phase; The grid-side converter of the power-frequency wind turbine adopts a constant DC bus voltage and reactive power control strategy to maintain the stability of the DC bus voltage of the power-frequency wind turbine. The grid-side voltage and frequency of the grid-side converter of the power-frequency wind turbine follow the receiving AC grid, and the grid-side rated frequency is 50Hz. The power frequency wind turbine side converter adopts a fixed active power and reactive power control strategy and a maximum power tracking operation mode; The onshore MMC converter adopts a constant DC bus voltage and reactive power control strategy, and is responsible for establishing the onshore DC bus voltage and maintaining the onshore DC bus voltage stable during startup.

3. The method for starting a grid-type offshore wind power transmission system at different offshore distances according to claim 1, characterized in that: The offshore distance of medium-frequency grid-type wind turbines is farther than that of low-frequency grid-type wind turbines, and the offshore distance of low-frequency grid-type wind turbines is farther than that of industrial-frequency wind turbines.

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