Active support control method for power grid of offshore wind power low-frequency power transmission system

Abstract

The invention discloses a power grid active support control method for an offshore wind power low-frequency power transmission system. Real-time response of low-frequency power grid angular frequency to power-frequency power grid angular frequency is realized by using an onshore M3C converter; the active power response of the low-frequency fan to the angular frequency of the low-frequency power grid is realized by utilizing a low-frequency fan side converter and a grid side converter; through cooperative control of the M3C converter and the low-frequency fan, active support of the offshore wind power low-frequency power transmission system for frequency fluctuation of an onshore power frequency power grid can be realized, the system stability is improved, and the method has a good application prospect.

Description

Technical Field

[0001] This invention belongs to the field of new energy power generation technology, specifically relating to an active support control method for the power grid of an offshore wind power low-frequency transmission system. Background Technology

[0002] Currently, the construction of offshore wind farms is gradually shifting from short-distance, small-capacity operations to large-scale, deep-sea deployments. When offshore wind farms are located beyond a certain distance from shore, traditional power frequency AC transmission suffers from excessive reactive power consumption due to the equivalent capacitance of the submarine cable. Flexible low-frequency (LHF) transmission, with its significantly lower transmission frequency than power frequency transmission, requires less charging power for the LHF AC submarine cable, thus increasing its transmission capacity. Furthermore, compared to flexible DC transmission, LHF transmission offers advantages such as zero-crossing current interruption and ease of grid connection, allowing wind turbines to directly output low-frequency energy without the need for an offshore converter platform. Therefore, offshore wind power flexible LHF AC transmission combines the advantages of both power frequency AC and flexible DC transmission, making it advantageous for medium- to long-distance offshore wind power transmission.

[0003] The large-scale integration of offshore wind farms will bring stability challenges to the power grid, such as insufficient voltage and frequency support. The Chinese national standard "Technical Specifications for Wind Farm Integration into Power Systems Part 2: Offshore Wind Power" (GBT 19963.2-2024) explicitly requires offshore wind farms to provide frequency support to the power grid. However, in offshore wind power low-frequency transmission systems, if existing control methods are used, the onshore M3C converter isolates the onshore power grid from the offshore low-frequency grid. This prevents offshore low-frequency wind turbines from detecting frequency fluctuations in the onshore grid in real time, hindering the implementation of active support functions. Therefore, it is urgent to research active grid support control methods for offshore wind power low-frequency transmission systems. Summary of the Invention

[0004] The purpose of this invention is to solve the problem that existing control methods for offshore wind power low-frequency transmission systems cannot provide active frequency support for onshore power grids. This invention provides an active grid support control method for offshore wind power low-frequency transmission systems, which achieves active frequency support for onshore power AC grids by coordinating the control of onshore M3C converters and offshore low-frequency wind turbines.

[0005] To achieve the above-mentioned objectives, this method adopts the following technical solution:

[0006] A method for active grid support control of a low-frequency transmission system for offshore wind power is characterized by obtaining the angular frequency feedback value of the power grid on the power frequency side of the onshore M3C converter, and providing a reference value for the low-frequency grid angular frequency based on the error between the feedback value and the reference value on the low-frequency side. A phase reference value for the low-frequency grid is then obtained based on this reference value, achieving real-time response of the low-frequency grid angular frequency to the power frequency angular frequency. The grid-side converter of the low-frequency wind turbine employs a virtual synchronous machine control strategy, while the turbine-side converter adopts a constant DC bus voltage and reactive power control strategy, enabling active power response of the low-frequency wind turbine to the low-frequency grid angular frequency. This scheme achieves active support of the offshore wind power low-frequency transmission system for frequency fluctuations in the onshore power grid, improving system stability.

[0007] Furthermore, the onshore M3C converter control system for implementing the method includes: a low-frequency grid angular frequency reference value calculation module, a low-frequency grid phase reference value calculation module, a low-frequency AC voltage control module, a low-frequency current control module, a low-frequency Park inverse transformation module, a low-frequency internal circulating current 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 circulating current control module, a bridge arm voltage calculation module, and a modulation module;

[0008] The low-frequency power grid angular frequency reference value calculation module calculates the low-frequency power grid angular frequency reference value ω based on the feedback value and reference value of the power frequency power grid angular frequency. g1ref ;

[0009] The low-frequency power grid phase reference value calculation module calculates the low-frequency power grid angular frequency reference value ω. g1ref Calculate the low-frequency power grid phase reference value θ g1ref ;

[0010] The low-frequency AC voltage control module controls the low-frequency d-axis and q-axis voltages U. gdq1 The system is controlled by a low-frequency AC voltage PI controller to follow the given reference value u. gd1ref and u gq1ref The output of the low-frequency AC voltage PI controller, after being limited by a limiting circuit, is used as the reference value i for the d-axis and q-axis currents, respectively. vd1ref and i vq1ref ;

[0011] The low-frequency current control module controls the low-frequency d-axis and q-axis currents I. vdq1 A low-frequency current PI controller is used for control, so that it follows the reference value i of the d-axis and q-axis currents. vd1ref and i vq1ref The output of the low-frequency current PI controller, after passing through a limiting circuit, becomes the low-frequency output voltage U. vdq1 ;

[0012] The low-frequency Park inverse converter module outputs a low-frequency voltage U. vdq1 Performing the inverse Park transform, we obtain the low-frequency output voltage U in the stationary three-phase coordinate system. vαβ1 The low-frequency Park inverse transform uses the low-frequency power grid phase reference value θ as the angle. g1ref ;

[0013] The low-frequency internal circulating current control module will control the low-frequency internal circulating current I. cαβ1 When the control is set to 0, the output of the low-frequency internal circulating current control module serves as the low-frequency internal circulating current voltage U. cαβ1 ;

[0014] On the power frequency side of the onshore M3C converter, the power frequency grid angular frequency feedback value is obtained through a phase-locked loop (PLL). The power frequency PLL module is based on the power frequency grid voltage U. gabc2 The angular frequency feedback value ω of the power grid was calculated. g2 Phase θ of the power frequency grid g2 ;

[0015] The capacitor voltage and reactive power control module controls the average capacitor voltage U. c and power frequency reactive power Q g2 Control is achieved through a capacitor voltage and reactive power PI controller, making them follow the given reference value U. cref and Q g2ref The capacitor voltage and reactive power outputs of the PI controller, after being limited by a limiting circuit, are used as reference values ​​i for the d-axis and q-axis currents, respectively. vd2ref and i vq2ref ;

[0016] The power frequency current control module controls the power frequency d-axis and q-axis currents I. vdq2 A power frequency current PI controller is used for control, so that it follows the reference value I. vdq2ref The output of the power frequency current PI controller is used as the power frequency output voltage U. vdq2 ;

[0017] The power frequency Park inverse converter module converts the power frequency output voltage U... vdq2 Performing the inverse Park transform, we obtain the power frequency output voltage U in the stationary three-phase coordinate system. vαβ2 The angle used in the power frequency Park inverse transform is the phase θ of the power frequency grid voltage. g2 ;

[0018] The power frequency internal circulating current control module will control the power frequency internal circulating current I. cαβ2 The control is set to 0, and the output of the power frequency internal circulating current control module serves as the power frequency internal circulating current voltage U. cαβ2 ;

[0019] The bridge arm voltage calculation module utilizes the low-frequency output voltage U vαβ1 Low-frequency internal circulating voltage U cαβ1 , power frequency output voltage U vαβ2 Internal circulating current voltage U at power frequency cαβ2 The reference voltages of the nine arms of the onshore M3C converter were calculated.

[0020] The modulation module generates modulation commands based on the reference voltages of the nine arms of the onshore M3C converter, thereby controlling the onshore M3C converter.

[0021] Furthermore, the low-frequency angular frequency reference value calculation module calculates the low-frequency angular frequency reference value ω according to the following method. g1ref :

[0022] ω g1ref =ω g1n -F PI1 (s)(ω g2ref -ω g2 )

[0023]

[0024] Wherein: F PI1 (s) is the transfer function of the angular frequency PI controller, k p1 k is the proportionality coefficient. i1 ω is the integral coefficient. g2ref ω is the reference value for the angular frequency of the power grid. g2 ω is the angular frequency feedback value of the power grid. g1n This is the rated value of the angular frequency of the low-frequency power grid.

[0025] Furthermore, the low-frequency wind turbine grid-side converter control system implementing the method includes: a virtual synchronous machine mechanical module, a virtual synchronous machine excitation module, a grid-side Park conversion module, a grid-side voltage outer loop controller, a grid-side current inner loop controller, a grid-side Park inverse conversion module, and a grid-side modulation module; wherein, the virtual synchronous machine mechanical module and the virtual synchronous machine excitation module provide active power response to the low-frequency grid angular frequency by simulating the mechanical and excitation modules of a traditional synchronous motor, and the outputs of the virtual synchronous machine mechanical module and the excitation module serve as the voltage reference value for the voltage outer loop controller;

[0026] The control system for the low-frequency wind turbine machine-side converter that implements the method includes: a rotor position observer, a machine-side Park conversion module, an outer loop controller for DC bus voltage and reactive power, an inner loop controller for machine-side current, a machine-side Park inverse conversion module, and a machine-side modulation module.

[0027] The beneficial effects of this invention are:

[0028] By adopting the technical solution of this invention, the onshore M3C converter is used to realize the real-time response of the low-frequency grid angular frequency to the power frequency grid angular frequency; the low-frequency wind turbine machine-side converter and grid-side converter are used to realize the active power response of the low-frequency wind turbine to the low-frequency grid angular frequency; through the coordinated control of the M3C converter and the low-frequency wind turbine, the offshore wind power low-frequency transmission system can actively support the frequency fluctuations of the onshore power frequency grid, thereby improving system stability. Attached Figure Description

[0029] Figure 1 This is a typical topology diagram of the offshore wind power low-frequency transmission system of the present invention.

[0030] Figure 2 This is a typical topology diagram of the onshore M3C converter in this invention.

[0031] Figure 3 This is a schematic diagram of a specific example of the control system for the onshore M3C converter in this invention.

[0032] Figure 4 This is a schematic diagram of a specific example of the low-frequency wind turbine grid-side converter control system in this invention.

[0033] Figure 5 This is a schematic diagram of a specific example of the low-frequency wind turbine converter control system in this invention. Detailed Implementation

[0034] To describe the present invention in more detail, the technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0035] like Figure 1 As shown in the embodiment of the present invention, the offshore wind power low-frequency transmission system includes: a wind turbine generator 1, a low-frequency wind turbine generator-side converter 2, a low-frequency wind turbine grid-side converter 3, a low-frequency wind turbine transformer 4, a low-frequency collector submarine cable 5, an offshore low-frequency low-voltage switchgear 6, an offshore low-frequency step-up transformer 7, an offshore low-frequency high-voltage switchgear 8, a low-frequency transmission submarine cable 9, an onshore low-frequency grid-side switchgear 10, an onshore low-frequency connection transformer 11, an onshore low-frequency valve-side switchgear 12, an onshore M3C converter 13, an onshore starting resistor 14, an onshore power frequency valve-side switchgear 15, an onshore power frequency connection transformer 16, and an onshore power frequency grid-side switchgear 17.

[0036] like Figure 2As shown, in this embodiment of the invention, the onshore M3C converter 13 is composed of three frequency conversion modules, each of which includes three bridge arm branches. The entire onshore M3C converter is composed of nine bridge arm branches. Each bridge arm branch is composed of multiple cascaded full-bridge sub-modules and bridge arm reactors connected in series. The three-phase AC systems on both sides of the onshore M3C converter are connected through the bridge arm branches. Each phase of one system is connected to the three phases of the other system through three bridge arm branches.

[0037] like Figure 3 As shown, in this embodiment of the invention, the control system adopted by the onshore M3C converter includes: a low-frequency grid angular frequency reference value calculation module 101, a low-frequency grid phase reference value calculation module 102, a low-frequency AC voltage control module 103, a low-frequency current control module 104, a low-frequency Park inverse transformation module 105, a low-frequency internal circulating current control module 106, a power frequency phase-locked loop module 107, a capacitor voltage and reactive power control module 108, a power frequency current control module 109, a power frequency Park inverse transformation module 110, a power frequency internal circulating current control module 111, a bridge arm voltage calculation module 112, and a modulation module 113.

[0038] In this embodiment of the invention, the low-frequency power grid angular frequency reference value calculation module 101 calculates the low-frequency power grid angular frequency reference value ω based on the feedback value and reference value of the power frequency power grid angular frequency. g1ref The specific implementation method is as follows:

[0039] ω g1ref =ω g1n -F PI1 (s)(ω g2ref -ω g2 )

[0040]

[0041] Wherein: F PI1 (s) is the transfer function of the angular frequency PI controller, k p1 k is the proportionality coefficient. i1 ω is the integral coefficient. g2ref ω is the reference value for the angular frequency of the power grid. g2 ω is the angular frequency feedback value of the power grid. g1n This is the rated value of the angular frequency of the low-frequency power grid.

[0042] The low-frequency power grid phase reference value calculation module 102 calculates the low-frequency power grid angular frequency reference value ω. g1ref Calculate the low-frequency power grid phase reference value θ g1ref .

[0043] The low-frequency AC voltage control module 103 controls the low-frequency d-axis and q-axis voltage U. gdq1The system is controlled by a low-frequency AC voltage PI controller to follow the given reference value u. gd1ref and u gq1ref The output of the low-frequency AC voltage PI controller, after being limited by a limiting circuit, is used as the reference value i for the d-axis and q-axis currents, respectively. vd1ref and i vq1ref The specific implementation method is as follows:

[0044]

[0045] Wherein: F PI2 (s) is the transfer function of the low-frequency AC voltage PI controller, k p2 k is the proportionality coefficient. i2 Let i be the integral coefficient. vd1ref i vq1ref Corresponding to the current vector I vdq1ref The d-axis and q-axis components.

[0046] The low-frequency current control module 104 controls the low-frequency d-axis and q-axis currents I. vdq1 A low-frequency current PI controller is used for control, so that it follows the reference value i of the d-axis and q-axis currents. vd1ref and i vq1ref The output of the low-frequency current PI controller, after passing through a limiting circuit, becomes the low-frequency output voltage U. vdq1 The specific implementation method is as follows:

[0047]

[0048] Wherein: F PI3 (s) is the transfer function of the low-frequency current PI controller, k p3 k is the proportionality coefficient. i3 U is the integral coefficient, L1 is the equivalent inductance including the onshore low-frequency connection transformer and the bridge arm reactor, and u vd1 ,u vq1 Corresponding to voltage vector U vdq1 The d-axis and q-axis components.

[0049] The low-frequency Park inverse converter module 105 outputs a low-frequency voltage U. vdq1 Performing the inverse Park transform, we obtain the low-frequency output voltage U in the stationary three-phase coordinate system. vαβ1 The low-frequency Park inverse transform uses the low-frequency power grid phase reference value θ as the angle. g1ref The specific implementation method is as follows:

[0050]

[0051] Among them, u vd1 ,u vq1 Corresponding to voltage vector Uvdq1 d-axis, q-axis components, u vα1 ,u vβ1 Corresponding to voltage vector U vαβ1 The α-axis and β-axis components.

[0052] The low-frequency internal circulating current control module 106 will control the low-frequency internal circulating current I. cαβ1 As the feedback value of the controller, control is achieved in the stationary coordinate system. The output of the low-frequency internal circulating current controller serves as the low-frequency internal circulating current voltage U. cαβ1 The specific implementation method is as follows:

[0053]

[0054] Among them, u cα1 ,u cβ1 Corresponding to voltage vector U cαβ1 α-axis, β-axis components, i cα1 i cβ1 Corresponding to the current vector I cαβ1 The α-axis and β-axis components.

[0055] The power frequency phase-locked loop module 107 operates according to the power frequency grid voltage U. gabc2 The angular frequency feedback value ω of the power grid was calculated. g2 Phase θ of the power frequency grid g2 .

[0056] The capacitor voltage and reactive power control module 108 controls the average capacitor voltage U. c and power frequency reactive power Q g2 Control is achieved through a capacitor voltage and reactive power PI controller, making them follow the given reference value U. cref and Q g2ref The capacitor voltage and reactive power outputs of the PI controller, after being limited by a limiting circuit, are used as reference values ​​i for the d-axis and q-axis currents, respectively. vd2ref and i vq2ref The specific implementation method is as follows:

[0057]

[0058] Wherein: F PI4 (s) represents the transfer function of the capacitor voltage and reactive power PI controller, k p4 k is the proportionality coefficient. i4 Let i be the integral coefficient. vd2ref i vq2ref Corresponding to the current vector I vdq2ref The d-axis and q-axis components.

[0059] The power frequency current control module 109 controls the power frequency d-axis and q-axis currents I. vdq2A power frequency current PI controller is used for control, so that it follows 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 method is as follows:

[0060]

[0061] Wherein: F PI5 (s) is the transfer function of the power frequency current PI controller, k p5 k is the proportionality coefficient. i5 U is the integral coefficient, L2 is the equivalent inductance including the onshore power frequency connection transformer and the bridge arm reactor, and u vd2 ,u vq2 Corresponding to voltage vector U vdq2 The d-axis and q-axis components.

[0062] The power frequency Park inverse converter module 110 outputs power frequency voltage U vdq2 Performing the inverse Park transform, we obtain the power frequency output voltage U in the stationary three-phase coordinate system. vαβ2 The angle used in the power frequency Park inverse transform is the phase θ of the power frequency grid voltage. g2 The specific implementation method is as follows:

[0063]

[0064] Among them, u vd2 ,u vq2 Corresponding to voltage vector U vdq2 d-axis and q-axis components, u vα2 ,u vβ2 Corresponding to voltage vector U vαβ2 The α-axis and β-axis components.

[0065] The power frequency internal circulating current control module 111 will control the power frequency internal circulating current I cαβ2 As the feedback value of the controller, control is achieved in the stationary coordinate system. The output of the power frequency internal circulating current controller serves as the power frequency internal circulating current voltage U. cαβ2 The specific implementation method is as follows:

[0066]

[0067] Among them, u cα2 ,u cβ2 Corresponding to voltage vector U cαβ2 α-axis, β-axis components, i cα2 i cβ2 Corresponding to the current vector I cαβ2 The α-axis and β-axis components.

[0068] Bridge arm voltage calculation module 112 utilizes low-frequency output voltage U vαβ1 Low-frequency internal circulating voltage U cαβ1 , power frequency output voltage U vαβ2 Internal circulating current voltage U at power frequency cαβ2 The reference voltages of the nine arms of the onshore M3C converter were calculated.

[0069] The modulation module 113 generates modulation commands based on the reference voltages of the nine arms of the onshore M3C converter to control the onshore M3C converter.

[0070] like Figure 4 As shown in this embodiment of the invention, the control system adopted by the low-frequency wind turbine grid-side converter 3 includes: a virtual synchronous machine mechanical module 201, a virtual synchronous machine excitation module 202, a grid-side Park conversion module 203, a grid-side voltage outer loop controller 204, a grid-side current inner loop controller 205, a grid-side Park inverse conversion module 206, and a grid-side modulation module 207; wherein, the virtual synchronous machine mechanical module 201 and the virtual synchronous machine excitation module 202 provide active power response to the low-frequency grid angular frequency by simulating the mechanical and excitation modules of a traditional synchronous motor. The output serves as the voltage reference value for the grid-side voltage outer loop controller 204; the grid-side Park transformation module 203 transforms the three-phase voltage and current feedback values ​​to a two-phase synchronous rotating coordinate system, and the grid-side voltage outer loop controller 204 controls the voltage feedback value to follow the reference value. The output of the grid-side voltage outer loop controller 204 serves as the reference value for the grid-side current inner loop controller 205; the grid-side current inner loop controller 205 controls the current feedback value to follow the reference value, and the output of the grid-side current inner loop controller 205, after undergoing Park inverse transformation, serves as the reference value for the grid-side modulation module 207, which generates a modulation signal based on the reference value.

[0071] In the virtual synchronizer mechanical module 201, the reference phase θ is calculated according to the following method. g :

[0072] θ g (k+1)=∫ω g (k+1)dt

[0073]

[0074] Where, θ g (k+1) is the reference phase for the next sampling period, ω g (k+1) is the angular frequency of the next sampling period, ω g (k) is the angular frequency of this sampling period, ω n P is the rated angular frequency. gref P is the active power reference value. g(k) represents the active power in this sampling period, J represents the virtual rotor moment of inertia, and D... p This is the active damping coefficient.

[0075] In the virtual synchronous machine excitation module 202, the d-axis voltage reference value u is calculated according to the following method. gdref :

[0076]

[0077] Among them, u gdref (k+1) is the d-axis voltage reference value for the next sampling period, |U g (k)| represents the voltage amplitude during this sampling period, U ref Q is the reference value for voltage amplitude. gref Q is the reactive power reference value. g (k) represents the reactive power in this sampling period, K is the virtual excitation coefficient, and D q This is the reactive damping coefficient.

[0078] The implementation method of the grid-side voltage outer loop controller 204 is as follows:

[0079]

[0080] Wherein: F PI6 (s) is the transfer function of the grid-side voltage PI controller, k p6 k is the proportionality coefficient. i6 Let i be the integral coefficient. gdref i gqref Corresponding to the current vector I gdqref d-axis, q-axis components, u gdref ,u gqref Corresponding to the voltage vector reference value U gdqref d-axis, q-axis components, u gd ,u gq Corresponding to the voltage vector reference value U gdq The d-axis and q-axis components.

[0081] The implementation method of the grid-side current inner loop controller 205 is as follows:

[0082]

[0083] Wherein: F PI7 (s) is the transfer function of the grid-side current PI controller, k p7 k is the proportionality coefficient. i7 U is the integral coefficient. vdref ,u vqref Corresponding to voltage vector U vdqref d-axis, q-axis components, u gd ,u gqCorresponding to voltage vector U gdq d-axis, q-axis components, i gd i gq Corresponding to the current vector I gdq d-axis and q-axis components, ω g L is the angular frequency of the grid voltage. g This is a filter inductor.

[0084] like Figure 5 As shown in this embodiment of the invention, the control system adopted by the low-frequency wind turbine side converter 2 includes: a rotor position observer 301, a machine-side Park conversion module 302, a DC bus voltage and reactive power outer loop controller 303, a machine-side current inner loop controller 304, a machine-side Park inverse conversion module 305, and a machine-side modulation module 306; wherein, the DC bus voltage reference value is the rated DC bus voltage of the wind turbine, and the reactive power reference value is given according to the reactive power demand of the offshore AC grid; the rotor position observer 301 is for Park conversion and inverse conversion. The module provides a reference phase; the DC bus voltage and reactive power outer loop controller 303 controls the DC bus voltage and reactive power to follow the reference value, and its output serves as the reference value for the machine-side current inner loop controller 304; the machine-side Park transformation module 302 transforms the three-phase current feedback value to a two-phase synchronous rotating coordinate system, and the machine-side current inner loop controller 304 controls the current feedback value to follow the reference value. The output of the machine-side current inner loop controller 304, after being inversely transformed by Park, serves as the reference value for the machine-side modulation module 306, which generates a modulation signal based on the reference value.

[0085] The implementation method of the DC bus voltage and reactive power outer loop controller 303 is as follows:

[0086]

[0087] Wherein: F PI8 (s) represents the transfer function of the DC bus voltage and reactive power PI controller, k p8 k is the proportionality coefficient. i8 Let i be the integral coefficient. sdref i sqref Corresponding to the current vector I sdqref d-axis and q-axis components, U dcref U is the reference value for the DC bus voltage. dc Q is the DC bus voltage. sref Q is the reactive power reference value. s This refers to reactive power.

[0088] The implementation method of the machine-side current inner loop controller 304 is as follows:

[0089]

[0090] Wherein: F PI9 (s) is the transfer function of the machine-side current PI controller, k p9 k is the proportionality coefficient. i9 U is the integral coefficient. sdref ,u sqref Corresponding to voltage vector U sdqref d-axis and q-axis components, i sd i sq Corresponding to the current vector I sdq d-axis and q-axis components, ω r L is the rotor angular frequency. s Ψ represents the stator inductance of the wind turbine, and Ψ represents the rotor permanent magnet flux linkage.

[0091] The above description of the embodiments is provided to enable those skilled in the art to understand and apply the present invention. It will be apparent to those skilled in the art that various modifications can be made to the above embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made to the present invention by those skilled in the art based on the disclosure thereof should be within the scope of protection of the present invention.

Claims

1. A method for active grid support control of a low-frequency transmission system for offshore wind power, characterized in that, On the power frequency side of the onshore M3C converter, the power frequency grid angular frequency feedback value is obtained. On the low frequency side, based on the error between the feedback value and the reference value of the power frequency grid angular frequency, a low frequency grid angular frequency reference value is given, and the low frequency grid phase reference value is obtained based on the low frequency grid angular frequency reference value, realizing the real-time response of the low frequency grid angular frequency to the power frequency grid angular frequency. The grid-side converter of the low frequency wind turbine adopts a virtual synchronous machine control strategy, and the machine-side converter of the low frequency wind turbine adopts a constant DC bus voltage and reactive power control strategy, realizing the active power response of the low frequency wind turbine to the low frequency grid angular frequency.

2. The active grid support control method for offshore wind power low-frequency transmission systems according to claim 1, characterized in that: The onshore M3C converter control system for implementing the method includes: a low-frequency grid angular frequency reference value calculation module, a low-frequency grid phase reference value calculation module, a low-frequency AC voltage control module, a low-frequency current control module, a low-frequency Park inverse transformation module, a low-frequency internal circulating current 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 circulating current control module, a bridge arm voltage calculation module, and a modulation module; The low-frequency power grid angular frequency reference value calculation module calculates the low-frequency power grid angular frequency reference value ω based on the feedback value and reference value of the power frequency power grid angular frequency. g1ref ; The low-frequency power grid phase reference value calculation module calculates the low-frequency power grid angular frequency reference value ω. g1ref Calculate the low-frequency power grid phase reference value θ g1ref ; The low-frequency AC voltage control module controls the low-frequency d-axis and q-axis voltages U. gdq1 The system is controlled by a low-frequency AC voltage PI controller to follow the given reference value u. gd1ref and u gq1ref The output of the low-frequency AC voltage PI controller, after being limited by a limiting circuit, is used as the reference value i for the d-axis and q-axis currents, respectively. vd1ref and i vq1ref ; The low-frequency current control module controls the low-frequency d-axis and q-axis currents I. vdq1 A low-frequency current PI controller is used for control, so that it follows the reference value i of the d-axis and q-axis currents. vd1ref and i vq1ref The output of the low-frequency current PI controller, after passing through a limiting circuit, becomes the low-frequency output voltage U. vdq1 ; The low-frequency Park inverse transformer module outputs a low-frequency voltage U. vdq1 Performing the inverse Park transform, we obtain the low-frequency output voltage U in the stationary three-phase coordinate system. vαβ1 The low-frequency Park inverse transform uses the low-frequency power grid phase reference value θ. g1ref ; The low-frequency internal circulating current control module will control the low-frequency internal circulating current I. cαβ1 When the control is set to 0, the output of the low-frequency internal circulating current control module serves as the low-frequency internal circulating current voltage U. cαβ1 ; On the power frequency side of the onshore M3C converter, the power frequency grid angular frequency feedback value is obtained through a phase-locked loop (PLL). The power frequency PLL module is based on the power frequency grid voltage U. gabc2 The angular frequency feedback value ω of the power grid was calculated. g2 Phase θ of the power frequency grid g2 ; The capacitor voltage and reactive power control module controls the average capacitor voltage U. c and power frequency reactive power Q g2 Control is achieved through a capacitor voltage and reactive power PI controller, making them follow the given reference value U. cref and Q g2ref The capacitor voltage and reactive power outputs of the PI controller, after being limited by a limiting circuit, are used as reference values ​​i for the d-axis and q-axis currents, respectively. vd2ref and i vq2ref ; The power frequency current control module controls the power frequency d-axis and q-axis currents I. vdq2 A power frequency current PI controller is used for control, so that it follows 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 power frequency Park inverse converter module converts the power frequency output voltage U... vdq2 Performing the inverse Park transform, we obtain the power frequency output voltage U in the stationary three-phase coordinate system. vαβ2 The angle used in the power frequency Park inverse transform is the phase θ of the power frequency grid voltage. g2 ; The power frequency internal circulating current control module will control the power frequency internal circulating current I. cαβ2 The control is set to 0, and the output of the power frequency internal circulating current control module serves as the power frequency internal circulating current voltage U. cαβ2 ; The bridge arm voltage calculation module utilizes the low-frequency output voltage U vαβ1 Low-frequency internal circulating voltage U cαβ1 , power frequency output voltage U vαβ2 Internal circulating current voltage U at power frequency cαβ2 The reference voltages of the nine arms of the onshore M3C converter were calculated. The modulation module generates modulation commands based on the reference voltages of the nine arms of the onshore M3C converter, thereby controlling the onshore M3C converter.

3. The active grid support control method for offshore wind power low-frequency transmission systems according to claim 2, characterized in that: The low-frequency angular frequency reference value calculation module calculates the low-frequency angular frequency reference value ω according to the following method. g1ref : oh g1ref =ω g1n -F PI1 (s)(ω) g2ref -oh g2 ) Wherein: F PI1 (s) is the transfer function of the angular frequency PI controller, k p1 k is the proportionality coefficient. i1 ω is the integral coefficient. g2ref ω is the reference value for the angular frequency of the power grid. g2 ω is the angular frequency feedback value of the power grid. g1n This is the rated value of the angular frequency of the low-frequency power grid.

4. The active grid support control method for offshore wind power low-frequency transmission systems according to claim 1, characterized in that: The low-frequency wind turbine grid-side converter control system implementing the method includes: a virtual synchronous machine mechanical module, a virtual synchronous machine excitation module, a grid-side Park conversion module, a grid-side voltage outer loop controller, a grid-side current inner loop controller, a grid-side Park inverse conversion module, and a grid-side modulation module; wherein, the virtual synchronous machine mechanical module and the virtual synchronous machine excitation module provide active power response to the low-frequency grid angular frequency by simulating the mechanical and excitation modules of a traditional synchronous motor, and the outputs of the virtual synchronous machine mechanical module and the excitation module serve as the voltage reference value for the voltage outer loop controller; The control system for the low-frequency wind turbine machine-side converter that implements the method includes: a rotor position observer, a machine-side Park conversion module, an outer loop controller for DC bus voltage and reactive power, an inner loop controller for machine-side current, a machine-side Park inverse conversion module, and a machine-side modulation module.

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