GRID-FORMING WIND TURBINE CONTROL METHOD FOR DIODE RECTIFIER UNIT-BASED OFFSHORE WIND POWER TRANSMISSION SYSTEM

Abstract

A grid-forming wind turbine control method for a diode rectifier unit-based offshore wind power transmission system. A control system for controlling a grid-side converter has a three-layered structure, where a first layer is a combination of an active power controller and a reactive power controller; a second layer is a voltage controller; and a third layer is a current controller. The actual reactive power is represented by a per-unit value of a capacity of a corresponding wind turbine unit. The wind turbine units have the same reactive-power reference value, which is constant and does not change with time. The reactive power controllers of all wind turbine units have the same structure and parameters.

Claims

1. A grid-forming wind turbine control method for a diode rectifier unit (DRU)-based offshore wind power transmission system, the DRU-based offshore wind power transmission system comprising an offshore wind farm; the offshore wind farm comprising a plurality of wind turbine units; the grid-forming wind turbine control method being used to control a grid side converter of each of the plurality of wind turbine units; and the grid-forming wind turbine control method comprising: (1) converting, by an active power controller, a difference between P.sub.wt* and P.sub.wt into a voltage amplitude reference value U.sub.wt*; and converting, by a reactive power controller, a difference between Q.sub.wt and Q.sub.wt* into a frequency f of each of the plurality of wind turbine units; wherein P.sub.wt* is an active power reference value of each of the plurality of wind turbine units; Q.sub.wt* is a reactive power reference value of each of the plurality of wind turbine units; P.sub.wt is an actual active power output of each of the plurality of wind turbine units; and Q.sub.wt is an actual reactive power output of each of the plurality of wind turbine units; (2) taking U.sub.wt* as a d-axis voltage reference value u.sub.fd*; letting a q-axis voltage reference value u.sub.fq* be equal to 0; converting u.sub.fd* into a d-axis modulating voltage reference value u.sub.vd* through modulation by using a voltage controller and a current controller in sequence; and converting u.sub.fq* into a q-axis modulating voltage reference value u.sub.vq* through modulation by using the voltage controller and the current controller in sequence; and (3) subjecting the frequency f of each of the plurality of wind turbine units to integral transformation to obtain a reference phase θ of each of the plurality of wind turbine units; subjecting the u.sub.vd* and u.sub.vq* to coordinate transformation to obtain three-phase modulating voltage reference values u.sub.va, u.sub.vb and u.sub.vc in an abc coordinate system by using the reference phase θ; and subjecting the u.sub.va, u.sub.vb and u.sub.vc to pulse width modulation (PWM) to control power switching devices in the grid-side converter of each of the plurality of wind turbine units.

2. The grid-forming wind turbine control method of claim 1, wherein the actual reactive power output Q.sub.wt of each of the plurality of wind turbine units is represented based on a per-unit value of a capacity of a corresponding wind turbine unit.

3. The grid-forming wind turbine control method of claim 1, wherein the plurality of wind turbine units are the same in the reactive power reference value Q.sub.wt*; and the reactive power reference value Q.sub.wt* is a constant value and does not change with time.

4. The grid-forming wind turbine control method of claim 1, wherein reactive power controllers respectively used for controlling grid-side converters of the plurality of wind turbine units have the same structure and parameters.

5. The grid-forming wind turbine control method of claim 1, wherein the active power controller adopts a lead-lag link in series with an integral link, and a transfer function of the lead-lag link in series with the integral link is expressed as $K_1\frac{T_{P1}s+1}{T_{P2}{s}^2+s};$ wherein K.sub.1 is a proportional coefficient; T.sub.P1 and T.sub.P2 are time constants; and s is a Laplace operator.

6. The grid-forming wind turbine control method of claim 1, wherein the reactive power controller adopts a lead-lag link, and a transfer function of the lead-lag link is expressed as $K_2\frac{T_{Q1}s+1}{T_{Q2}s+1};$ wherein an output of the reactive power controller is a frequency deviation Δf, the Δf and a rated frequency f.sub.0 of each of the plurality of wind turbine units are added to obtain the frequency f of a corresponding wind turbine unit; wherein K.sub.2 is a proportional coefficient; T.sub.Q1 and T.sub.Q2 are time constants; and s is a Laplace operator.

7. The grid-forming wind turbine control method of claim 1, wherein the reference phase θ is an integral result of the frequency f of each of the plurality of wind turbine units with respect to time.

8. The grid-forming wind turbine control method of claim 1, wherein the DRU-based offshore wind power transmission system is a low-frequency offshore wind power alternating-current (AC) collection and transmission system, a power-frequency offshore wind power AC collection and direct-current (DC) transmission system or a medium-frequency offshore wind power AC collection and DC transmission system.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] FIG. 1 is a structural diagram of a diode rectifier unit (DRU)-based offshore wind power transmission system according to an embodiment of this application;

[0029] FIG. 2 schematically shows an operation principle of a control system for a grid-side converter of a wind turbine according to an embodiment of this application;

[0030] FIG. 3a is a structural diagram of an active-power controller according to an embodiment of this application;

[0031] FIG. 3b is a structural diagram of a reactive-power controller according to an embodiment of this application;

[0032] FIG. 4a schematically illustrates simulation waveforms of active power and reactive power output by the wind turbine according to an embodiment of this application;

[0033] FIG. 4b schematically shows a simulation waveform of an alternating current frequency of the wind turbine according to an embodiment of this application;

[0034] FIG. 5a schematically depicts a simulation waveform of an effective alternating voltage of a DRU-based converter station according to an embodiment of this application;

[0035] FIG. 5b schematically shows simulation waveforms of active power absorbed by a DRU-based converter and reactive power absorbed by the DRU converter according to an embodiment of this application;

[0036] FIG. 6a schematically illustrates a simulation waveform of a direct current voltage of the DRU-based offshore wind power transmission system according to an embodiment of this application; and

[0037] FIG. 6b schematically illustrates a simulation waveform of a direct current of the DRU-based offshore wind power transmission system according to an embodiment of this application.

DETAILED DESCRIPTION OF EMBODIMENTS

[0038] This application will be described in detail below with reference to the accompanying drawings and embodiments.

[0039] In a grid-forming wind turbine control method provided herein for a diode rectifier unit (DRU)-based offshore wind power transmission system, the control system for controlling a grid-side converter includes three layers, where a first layer is an active-power controller and a reactive power controller; a second layer is a voltage controller; and a third layer is a current controller. The controllers of the second layer and the third layer adopt the conventional passive controllers of modular multilevel converter (MMC) (Xu Zheng et al. “Voltage source converter based high-voltage direct current (VSC-HVDC) transmission system” (2nd edition), [M]. Beijing: China Machine Press, 2017).

[0040] In this embodiment, an active-power controller is a lead-lag link in series with an integral link, in which a numerator of the lead-lag link is expressed as K.sub.1(T.sub.P1+1); and a denominator of the lead-lag link is expressed as (T.sub.P2s+1). An input of the active-power controller is a value obtained by subtracting an actual active power output P.sub.wt of each of the plurality of wind turbine units from an active power controller reference value P.sub.wt* of each of the plurality of wind turbine units. An output of the active-power controller is a voltage amplitude reference value U.sub.wt*.

[0041] In this embodiment, a reactive power controller is a lead-lag link, in which a numerator of the lead-lag link is expressed as K.sub.2(T.sub.Q1s+1); and a denominator of the lead-lag link is expressed as (T.sub.Q2s+1). An input of the reactive-power controller is a value obtained by subtracting a reactive-power reference value Q.sub.wt* from an actual reactive power output Q.sub.wt* of each of the plurality of wind turbine units, and an output of the reactive-power controller is a wind turbine frequency f. The actual reactive power output Q.sub.wt of each of the plurality of wind turbine units is represented based on a per-unit value of a capacity of a corresponding wind turbine unit. A reference phase θ of the control system is an integral result of a frequency f of each of the plurality of wind turbine units with respect to time.

[0042] The voltage amplitude reference value U.sub.wt* output by the active power controller is a d-axis modulating voltage reference value u.sub.fd* of the voltage controller. A q-axis voltage reference value u.sub.fq* of the voltage controller is set to be zero.

[0043] The plurality of wind turbine units are the same in the reactive power reference value Q.sub.wt*. The reactive power reference value Q.sub.wt* is a constant value and does not change with time. The reactive power controllers used by all the wind turbines connected to the system have the same structure and parameters.

[0044] Referring to an embodiment shown in FIG. 1, the DRU-based offshore wind power transmission system is composed of an offshore wind farm, a medium-frequency alternating-current (AC) submarine cable, a rectifier station, a high-voltage direct current submarine cable, an inverter station and an onshore power grid, all of which are connected in sequence. The offshore wind farm includes two wind turbine units, and each of the two wind turbine units is composed of a wind turbine, a machine-side converter, a grid-side converter, and a step-up transformer, all of which are connected in sequence. The control system of the grid-side inverter is shown in FIG. 2, and specifically implemented through the following steps. [0045] (1) The active-power controller converts a difference between P.sub.wt* and P.sub.wtinto a voltage amplitude reference value U.sub.wt*. The reactive-power controller converts a difference between Q.sub.wt and Q.sub.wt* into a frequency f of each of the two wind turbine units. P.sub.wt* is an active power reference value of each of the two wind turbine units; Q.sub.wt* is a reactive power reference value of each of the two wind turbine units; P.sub.wt is an actual active power output of each of the two wind turbine units; and Q.sub.wt is an actual reactive power output of each of the two wind turbine units.

[0046] As shown in FIG. 3a, the active power controller adopts a lead-lag link in series with an integral link. A transfer function of the lead-lag link in series with the integral link is expressed as

[00003]$K_1\frac{T_{P1}s+1}{T_{P2}{s}^2+s},$

where K.sub.1 is a proportional coefficient; T.sub.P1 and T.sub.P2 are time constants; and s is a Laplace operator.

[0047] As shown in FIG. 3b, the reactive power controller adopts a lead-lag link, and a transfer function of the lead-lag link is expressed as

[00004]$K_2\frac{T_{Q1}s+1}{T_{Q2}s+1},$

where an output of the reactive power controller is a frequency deviation Δf, Δf and a rated frequency f.sub.0 of each of the two wind turbine units are added to obtain a frequency f of a corresponding wind turbine unit; K.sub.2 is a proportional coefficient; T.sub.Q1 and T.sub.Q2 are time constants; and s is a Laplace operator. [0048] (2) U.sub.wt* is taken as a d-axis voltage reference value u.sub.fd*. A q-axis voltage reference value u.sub.fq* is set to be 0. u.sub.fd* is converted into a d-axis modulating voltage reference value u.sub.vd* through modulation by using a voltage controller and a current controller in sequence; and u.sub.fq* is converted into a q-axis modulating voltage reference value u.sub.vq* through modulation by using the voltage controller and the current controller in sequence. [0049] (3) The frequency f of each of the two wind turbine units is subjected to integral transformation to obtain a reference phase θ of each of the two wind turbine units. u.sub.vd* and u.sub.vq* are subjected to coordinate transformation to obtain three-phase modulating voltage reference values u.sub.va, u.sub.vb and u.sub.vc in an abc coordinate system by using the reference phase θ. The u.sub.va, u.sub.vb and u.sub.vc are subjected to pulse width modulation (PWM) to control the power switching devices in the grid-side converter of each of the wind-turbine units.

[0050] In this embodiment, the parameters of the system provided herein are shown in the following Table 1.

TABLE-US-00001 TABLE 1 Parameters of DRU-based offshore wind power transmission system Items Scale Equivalent wind turbine units Rated power 150/150 MW Rated fundamental frequency of 100 Hz wind-turbine grid-side converter Alternating-current submarine cable Rated voltage 66 kV Length 5 km(#1), 10 km(#2) Rectifier station Converter transformer capacity 2 × 165 MVA Converter transformer ratio 66 kV/89 kV Converter transformer leakage 0.15 p.u. reactance Direct-current submarine cable Rated direct current voltage ±110 kV Length 120 km Inverter Converter transformer capacity 330 MVA Converter transformer ratio 220 kV/110 kV Converter transformer leakage 0.15 p.u. reactance Rated direct current voltage ±110 kV

[0051] A corresponding simulation platform was built in the electromagnetic transient simulation software (Power System Computer Aided Design) PSCAD/EMTDC to simulate the fluctuation of the wind speed of the wind turbine WT.sub.1. Before t=2.0 s, the two wind turbines have been running stably at a rated wind speed of 12 m/s. Assuming that the wind speed of the wind turbine WT.sub.1 drops from 12 m/s to 11 m/s stepwise at t=2.0 s, FIGS. 4a and 4b show the simulation results of the key electrical quantities of the grid-forming wind turbine, FIGS. 5a and 5b show the simulation results of the key electrical quantities of the DRU rectifier station, FIGS. 6a and 6b show the simulation results of the DC voltage and the DC current. The simulation results illustrated in the figures prove the effectiveness of the control method provided herein.

[0052] Described above are merely illustrative of this application, and are intended to facilitate the understanding and implementation of this application. It should be understood that various modifications, improvements and replacements made by those skilled in the art without departing from the spirit and scope of this application shall fall within the scope of this application defined by the appended claims.