A method for suppressing low-frequency oscillation of a weak power grid with direct-drive wind power

Abstract

A method for suppressing low-frequency oscillations in a weak power grid with direct-drive wind power, the main steps of which are: A. Real-time measurement and recording of current and voltage parameters of the direct-drive wind turbine grid connection point in the weak power grid system, and the capacitor voltage V of the direct-drive wind turbine DC link. Z , the output phase angle θ of the PLL controller P ; The steady-state value and variation of the corresponding parameters are obtained through low-pass filtering and high-pass filtering. B. Calculate the d-axis voltage V at the grid connection point of the direct-drive wind turbine d The related transfer function G p (s) and the q-axis voltage V at the grid connection point of the direct-drive wind turbine q and the output phase angle θ of the PLL controller P The related transfer function G Z (s), and finally, the current compensation command for the direct-drive wind turbine is obtained; C. The current at the direct-drive wind turbine's grid connection point is compensated and controlled based on the current compensation command. This method can effectively suppress low-frequency oscillations in weak grids within a wide range of short-circuit capacity variations. Its low-frequency oscillation suppression is effective and robust, ensuring the safe and stable operation of weak grids with direct-drive wind power.

Description

Technical Field

[0001] The present invention relates to a method for suppressing low-frequency oscillation when direct-drive wind power is integrated into a weak power grid. Background Art

[0002] After wind power is integrated into the grid, the long transmission lines and transformer leakage reactance between wind turbines and the grid's synchronous generators—the line impedance and transformer leakage reactance are non-negligible. This weakens the grid's relative strength and reduces its short-circuit capacity. The grid can no longer be considered a strong, ideal power source, but rather a relatively weak grid with significant weak grid effects, characterized by weak disturbance immunity and large voltage fluctuations. The larger the scale and power of the wind power connected, the stronger the weak grid effect. Under weak grid conditions, the unique dynamic characteristics of direct-drive wind turbine controllers cause the output impedance of direct-drive wind turbines to exhibit a negative resistance and capacitive impedance characteristic at low frequencies. When the capacitive reactance of the direct-drive wind turbine resonates with the inductive reactance of the weak grid in the low-frequency (less than 10Hz) band, the direct-drive wind turbine will experience low-frequency (less than 10Hz) power oscillations (in this case, the direct-drive wind turbine exhibits negative resistance in this low-frequency band). This low-frequency oscillation directly causes turbine output power fluctuations, resulting in resonant overvoltages in the system, dynamic instability, component damage, and even system collapse, seriously impacting the safe and stable operation of the grid. Therefore, it is necessary to suppress the low-frequency oscillations of the weak power grid in conjunction with wind power. The existing method for suppressing low-frequency oscillations of the weak power grid is to control the amplitude and phase of the wind power injection current according to a pre-set short-circuit capacity (the inverse of the system reactance, mainly composed of the inductance of the long-distance transmission line and the leakage reactance of the transformer), reduce the negative resistance characteristics of the output impedance of the direct-drive wind turbine in this low-frequency band, and achieve the suppression of the low-frequency oscillations of the direct-drive wind turbine in conjunction with the weak power grid. This type of method is more effective for weak power grid systems whose short-circuit capacity is known in advance and whose changes during operation are small. However, in fact, the operation and distribution conditions of synchronous generators and wind turbines in weak power grid systems change at any time, causing the short-circuit capacity of the weak power grid system to change at any time and the changes are large and difficult to predict. As a result, the existing method for suppressing low-frequency oscillations of the weak power grid system has a poor suppression effect on the low-frequency oscillations of the weak power grid system and cannot well guarantee the safe and stable operation of the power grid. Summary of the Invention

[0003] The purpose of the present invention is to propose a method for suppressing low-frequency oscillations of a weak power grid with direct-driven wind power. The method can effectively suppress the low-frequency oscillations of the weak power grid within a large short-circuit capacity variation range of the weak power grid system. The method has a good suppression effect on the low-frequency oscillations of the weak power grid system and strong robustness, and can better ensure the safe and stable operation of the weak power grid with direct-driven wind power.

[0004] The technical solution adopted by the present invention to achieve its invention object is a method for suppressing low-frequency oscillations of a weak power grid with direct-drive wind power, comprising the following steps:

[0005] A. Real-time measurement and recording of the d-axis voltage V in the dq coordinate system at the direct-drive wind turbine connection point in the weak power grid system d , q-axis voltage V q , d-axis output current I d , q-axis output current I q and the capacitor voltage V of the DC link of the direct-drive fan Z , and measure the output phase angle θ of the direct-drive wind turbine's phase-locked loop controller in real time P ;

[0006] For the d-axis voltage V d , d-axis output current I d , q-axis output current I q and the capacitor voltage V of the DC link of the direct-drive fan Z Perform low-pass filtering respectively to obtain the steady-state value of the d-axis voltage V at the direct-drive wind turbine grid connection point. d0 , d-axis current steady-state value I d0 , q-axis output current steady-state value I q0 And the steady-state value of the capacitor voltage of the DC link of the direct-drive wind turbine V Z0 , and then calculate the output power steady-state value P0 of the direct-drive wind turbine grid connection point, P0=V d0 ×I d0 ;

[0007] For the d-axis voltage V d , d-axis output current I d , q-axis voltage V q , q-axis output current I q and the output phase angle θ of the PLL controller P Perform high-pass filtering to obtain the d-axis voltage change ΔV at the direct-drive wind turbine grid connection point. d , q-axis voltage change ΔV q , q-axis output current change ΔI q , d-axis output current change ΔI d and the output phase angle change Δθ of the phase-locked loop controller P ;

[0008] B. Calculate the current compensation instruction of the direct drive fan

[0009] B1, the d-axis voltage steady-state value V at the direct-drive wind turbine grid connection point d0 The output power steady-state value P0 of the direct-drive wind turbine grid-connected point is calculated to obtain the d-axis voltage V d The related transfer function G p (s),

[0010]

[0011] Among them, k Ppand k Pi are the proportional coefficient and integral coefficient of the phase-locked loop controller respectively, and s represents the Laplace operator;

[0012] B2, the d-axis voltage steady-state value V at the direct-drive wind turbine grid connection point d0 , the steady-state value of the capacitor voltage of the DC link of the direct-drive wind turbine V Z0 The output power steady-state value P0 of the direct-drive wind turbine grid-connected point is calculated to obtain the q-axis voltage V q and the output phase angle θ of the PLL controller P The related transfer function G Z (s),

[0013]

[0014] Among them, k Zp and k Zi are the proportional coefficient and integral coefficient of the DC voltage controller of the grid-side voltage source converter of the direct-drive wind turbine, C Z is the capacitance value of the DC link of the direct-drive fan;

[0015] B3, the d-axis voltage V at the grid connection point with the direct-drive wind turbine d The related transfer function G p (s), calculate the d-axis current compensation instruction δI of the DC voltage controller of the direct-drive fan d ,

[0016]

[0017] The q-axis voltage V at the direct-drive wind turbine grid connection point q and the output phase angle θ of the PLL controller P The related transfer function G Z (s), calculate the q-axis current compensation instruction δI of the AC voltage controller of the direct-drive wind turbine q ,

[0018]

[0019] C. Set the d-axis current compensation instruction δI d The d-axis current command I of the DC voltage controller dr The d-axis current instruction after compensation is obtained by superposition; at the same time, the q-axis current compensation instruction δI q The q-axis current command I of the AC voltage controller qrThe compensated d-axis current command is superimposed to obtain the compensated q-axis current command; finally, the compensated d-axis current command is fed into the d-axis current controller to control the d-axis current of the direct-drive wind turbine grid-connected point; the compensated q-axis current command is fed into the q-axis current controller to control the q-axis current of the direct-drive wind turbine grid-connected point; thereby reducing the capacitive reactance characteristics of the direct-drive wind turbine output impedance and suppressing low-frequency oscillation.

[0020] The cutoff frequencies of the low-pass filter and the high-pass filter in step A are the same, and the range of the cutoff frequencies is 2 to 5 Hz.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] 1. The existing suppression method is to suppress low-frequency oscillations by reducing the negative resistance of the fixed direct-drive wind turbine in the low-frequency band under the preset system short-circuit capacity. The suppression method has poor suppression effect and weak robustness when the system short-circuit capacity changes. The present invention provides a new suppression method for the low-frequency oscillations of direct-drive wind turbines and weak power grids. By measuring the voltage and current and other related variables output by the direct-drive wind turbine in real time, the dynamic characteristics of the current in the low-frequency band are controlled accordingly to achieve decoupling of positive and negative sequence impedances, thereby reducing the capacitive impedance presented by the direct-drive wind turbine under the weak power grid, destroying the resonance point between the weak power grid and the direct-drive wind turbine, and achieving suppression of low-frequency oscillations of the weak power grid; because the decoupling of positive and negative sequence impedances (reduction of capacitive impedance) is independent of the system short-circuit capacity, the present invention can effectively suppress the low-frequency oscillations of the weak power grid with direct-drive wind turbines within a large short-circuit capacity variation range.

[0023] Second, the only parameters that require tuning in this invention are the cutoff frequencies of the low-pass and high-pass filters. All other control parameters are calculated in real time by the direct-drive wind turbine grid-connected system and require no tuning. This makes the method simple to debug, easy to implement, and readily available for widespread adoption.

[0024] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is an admittance characteristic diagram of a direct-drive wind turbine in a weak power grid with direct-drive wind power but without using the method of the present invention.

[0026] Figure 2 The figure is an admittance characteristic diagram of a direct-drive wind turbine in a weak power grid using the method of the present invention and having direct-drive wind power.

[0027] Figure 3 The figure shows the result of the open-loop transfer function matrix of the direct-drive wind turbine in a weak power grid with direct-drive wind power but without using the method of the present invention.

[0028] Figure 4The following is the result of the Grignard band analysis of the open-loop transfer function matrix of the direct-drive wind turbine in a weak power grid with direct-drive wind power using the method of the present invention.

[0029] Figure 5 The waveform diagram of the DC voltage in the weak power grid with direct-driven wind power using the method of the present invention after t=2.5s.

[0030] Figure 6 The waveform diagram of the DC power in the weak power grid with direct-driven wind power using the method of the present invention after t=2.5s.

[0031] Figure 7 The figure is a comparison diagram of the dynamic response of the DC voltage of the weak power grid with direct-drive wind power after the direct-drive wind turbine is disturbed, with and without the method of the present invention.

[0032] Figure 8 The figure is a comparison diagram of the dynamic response of the q-axis current of the weak power grid with direct-drive wind power after the direct-drive wind turbine is disturbed, with and without the method of the present invention. DETAILED DESCRIPTION

[0033] Example

[0034] A method for suppressing low-frequency oscillations in a weak power grid with direct-drive wind power comprises the following steps:

[0035] A. Real-time measurement and recording of the d-axis voltage V in the dq coordinate system at the direct-drive wind turbine connection point in the weak power grid system d , q-axis voltage V q , d-axis output current I d , q-axis output current I q and the capacitor voltage V of the DC link of the direct-drive fan Z , and measure the output phase angle θ of the direct-drive wind turbine's phase-locked loop controller in real time P ;

[0036] For the d-axis voltage V d , d-axis output current I d , q-axis output current I q and the capacitor voltage V of the DC link of the direct-drive fan Z Perform low-pass filtering respectively to obtain the steady-state value of the d-axis voltage V at the direct-drive wind turbine grid connection point. d0 , d-axis current steady-state value I d0 , q-axis output current steady-state value I q0 And the steady-state value of the capacitor voltage of the DC link of the direct-drive wind turbine V Z0 , and then calculate the output power steady-state value P0 of the direct-drive wind turbine grid connection point, P0=V d0 ×I d0 ;

[0037] For the d-axis voltage Vd , d-axis output current I d , q-axis voltage V q , q-axis output current I q and the output phase angle θ of the PLL controller P Perform high-pass filtering to obtain the d-axis voltage change ΔV at the direct-drive wind turbine grid connection point. d , q-axis voltage change ΔV q , q-axis output current change ΔI q , d-axis output current change ΔI d and the output phase angle change Δθ of the phase-locked loop controller P ;

[0038] B. Calculate the current compensation instruction of the direct drive fan

[0039] B1, the d-axis voltage steady-state value V at the direct-drive wind turbine grid connection point d0 The output power steady-state value P0 of the direct-drive wind turbine grid-connected point is calculated to obtain the d-axis voltage V d The related transfer function G p (s),

[0040]

[0041] Among them, k Pp and k Pi are the proportional coefficient and integral coefficient of the phase-locked loop controller respectively, and s represents the Laplace operator;

[0042] B2, the d-axis voltage steady-state value V at the direct-drive wind turbine grid connection point d0 , the steady-state value of the capacitor voltage of the DC link of the direct-drive wind turbine V Z0 The output power steady-state value P0 of the direct-drive wind turbine grid-connected point is calculated to obtain the q-axis voltage V q and the output phase angle θ of the PLL controller P The related transfer function G Z (s),

[0043]

[0044] Among them, k Zp and k Zi are the proportional coefficient and integral coefficient of the DC voltage controller of the grid-side voltage source converter of the direct-drive wind turbine, C Z is the capacitance value of the DC link of the direct-drive fan;

[0045] B3, the d-axis voltage V at the grid connection point with the direct-drive wind turbine d The related transfer function G p (s), calculate the d-axis current compensation instruction δI of the DC voltage controller of the direct-drive fand ,

[0046]

[0047] The q-axis voltage V at the direct-drive wind turbine grid connection point q and the output phase angle θ of the PLL controller P The related transfer function G Z (s), calculate the q-axis current compensation instruction δI of the AC voltage controller of the direct-drive wind turbine q ,

[0048]

[0049] C. Set the d-axis current compensation instruction δI d The d-axis current command I of the DC voltage controller dr The d-axis current instruction after compensation is obtained by superposition; at the same time, the q-axis current compensation instruction δI q The q-axis current command I of the AC voltage controller qr The compensated d-axis current command is superimposed to obtain the compensated q-axis current command; finally, the compensated d-axis current command is fed into the d-axis current controller to control the d-axis current of the direct-drive wind turbine grid-connected point; the compensated q-axis current command is fed into the q-axis current controller to control the q-axis current of the direct-drive wind turbine grid-connected point; thereby reducing the capacitive reactance characteristics of the direct-drive wind turbine output impedance and suppressing low-frequency oscillation.

[0050] The cutoff frequencies of the low-pass filter and the high-pass filter in step A of this example are the same, and the range of the cutoff frequencies is 2 to 5 Hz.

[0051] Simulation experiment

[0052] In order to verify the effect of the present invention, a simulation experiment was carried out, and the main parameters of the simulation experiment are as follows:

[0053] The d-axis voltage steady-state value V at the direct-drive wind turbine grid connection point in the dq coordinate system d0 =564V, d-axis current steady-state value I d0 =2100A, q-axis output current steady-state value I q0 =0A; the steady-state value of the capacitor voltage of the DC link of the direct-drive fan V Z0 =1200V, the output power steady-state value P0 of the direct-drive wind turbine grid connection point is 1.8MW, and the proportional coefficient k of the phase-locked loop controller is Pp =0.07, the integral coefficient k of the phase-locked loop controller Pi =26.89, DC voltage controller proportional coefficient k Zp =101.03, the capacitance value of the DC link of the direct-drive fan C Z =100mF, the cutoff frequency of low-pass filter and high-pass filter is the same, which is 2.5Hz.

[0054] Figure 1 The output admittance (inverse of impedance) characteristic diagram of the direct-drive wind turbine of a weak power grid with direct-drive wind power but without using the method of the present invention, wherein the system short-circuit capacity (SCR) of the sub-graphs (a), (b), (c), and (d) are 1.54, 1.3, 1.26, and 1.24, respectively. Figure 1 The four sub-graphs show that for a weak grid with direct-driven wind power and without using the method of the present invention, as the short-circuit capacity (SCR) decreases, the grid strength becomes weaker and the capacitance (B vsc ) gradually increases and is combined with the inductive reactance of the weak grid (B lg ) intersects; this indicates that the capacitive impedance presented by the direct-drive wind turbine resonates with the inductive reactance on the grid side. In particular, when the SCR is reduced to below 1.26 (c, d), the capacitance (B vsc ) and the inductive reactance of the weak grid (B lg ) falls into the low frequency range (G vsc <0); indicating that low-frequency oscillation has occurred in the weak power grid system with direct-driven wind power.

[0055] Figure 2 The output admittance (inverse of impedance) characteristic diagram of a direct-drive wind turbine in a weak power grid using the method of the present invention and having direct-drive wind power is shown in FIG. 1 , wherein the system short-circuit capacity (SCR) of sub-graphs (a), (b), (c), and (d) are 1.54, 1.3, 1.26, and 1.24, respectively. Figure 2 The four sub-graphs show that after using the method of the present invention, under four different short-circuit capacity (SCR) conditions, the capacitive impedance characteristics of the direct-drive wind turbine are greatly weakened, and no longer conflict with the inductive reactance (B lg ) intersects, indicating that the low-frequency oscillation of the direct-drive wind turbine and weak grid system is successfully suppressed.

[0056] Figure 3 This is a diagram showing the results of the open-loop transfer function matrix of a direct-drive wind turbine in a weak power grid with direct-drive wind power but without using the method of the present invention. The system short-circuit capacity (SCR) of sub-graphs (a), (b), (c), (d), and (e) are 3, 1.54, 1.3, 1.26, and 1.24, respectively.

[0057] Figure 4 This is a diagram showing the results of the open-loop transfer function matrix of a direct-drive wind turbine in a weak power grid using the method of the present invention and with direct-drive wind power. The system short-circuit capacities (SCRs) of sub-graphs (a), (b), (c), (d), and (e) are 3, 1.54, 1.3, 1.26, and 1.24, respectively.

[0058] contrast Figure 3 and Figure 4, Figure 4 The Grignard band no longer surrounds the point (-1, j0), indicating that the method of the present invention improves the stability of the weak power grid with direct-driven wind power.

[0059] Figure 5 The waveform diagram of the DC voltage in the weak power grid with direct-driven wind power using the method of the present invention after t=2.5s. Figure 6 The waveform diagram of the DC power in the weak power grid with direct-driven wind power using the method of the present invention after t=2.5s.

[0060] Figure 5 、 Figure 6 Comparison of the waveform before t=2.5s and the waveform after t=2.5s shows that the method of the present invention can effectively suppress oscillation under four different short-circuit capacities.

[0061] Figure 7 The figure is a comparison diagram of the dynamic response of the DC voltage of the weak power grid with direct-drive wind power after the direct-drive wind turbine is disturbed, with and without the method of the present invention. Figure 8 The figure is a comparison diagram of the dynamic response of the q-axis current of a weak power grid with direct-drive wind power after the direct-drive wind turbine is disturbed, which is different from the one using the method of the present invention.

[0062] Figure 7 and Figure 8 It shows that the method of the present invention has little impact on the dynamic performance of the weak power grid of direct-driven wind power.

[0063] The above simulation results prove that the method of the present invention can effectively control the dynamic characteristics of the current in the low-frequency band within the large short-circuit capacity variation range, realize the decoupling of the positive and negative sequence impedances, and thus reduce the capacitive impedance presented by the direct-drive wind turbine under the weak power grid, destroy the resonance point between the weak power grid and the direct-drive wind turbine, and achieve the suppression of the low-frequency oscillation of the weak power grid.

Claims

1. A method for suppressing low-frequency oscillations in a weak power grid with direct-drive wind power, comprising the following steps: A. Real-time measurement and recording of the d-axis voltage V in the dq coordinate system at the direct-drive wind turbine connection point in the weak power grid system d , q-axis voltage V q , d-axis output current I d , q-axis output current I q and the capacitor voltage V of the DC link of the direct-drive fan Z , and measure the output phase angle θ of the direct-drive wind turbine's phase-locked loop controller in real time P ; For the d-axis voltage V d , d-axis output current I d , q-axis output current I q and the capacitor voltage V of the DC link of the direct-drive fan Z Perform low-pass filtering respectively to obtain the steady-state value of the d-axis voltage V at the direct-drive wind turbine grid connection point. d0 , d-axis current steady-state value I d0 , q-axis output current steady-state value I q0 And the steady-state value of the capacitor voltage of the DC link of the direct-drive wind turbine V Z0 , and then calculate the output power steady-state value P0 of the direct-drive wind turbine grid connection point, P0=V d0 ×I d0 ; For the d-axis voltage V d , d-axis output current I d , q-axis voltage V q , q-axis output current I q and the output phase angle θ of the PLL controller P Perform high-pass filtering to obtain the d-axis voltage change ΔV at the direct-drive wind turbine grid connection point. d , q-axis voltage change ΔV q , q-axis output current change ΔI q , d-axis output current change ΔI d and the output phase angle change Δθ of the phase-locked loop controller P ; B. Calculate the current compensation instruction of the direct drive fan B1, the d-axis voltage steady-state value V at the direct-drive wind turbine grid connection point d0 The output power steady-state value P0 of the direct-drive wind turbine grid-connected point is calculated to obtain the d-axis voltage V d The related transfer function G p (s), Among them, k Pp and k Pi are the proportional coefficient and integral coefficient of the phase-locked loop controller respectively, and s represents the Laplace operator; B2, the d-axis voltage steady-state value V at the direct-drive wind turbine grid connection point d0 , the steady-state value of the capacitor voltage of the DC link of the direct-drive wind turbine V Z0 The output power steady-state value P0 of the direct-drive wind turbine grid-connected point is calculated to obtain the q-axis voltage V q and the output phase angle θ of the PLL controller P The related transfer function G Z (s), Among them, k Zp and k Zi are the proportional coefficient and integral coefficient of the DC voltage controller of the grid-side voltage source converter of the direct-drive wind turbine, C Z is the capacitance value of the DC link of the direct-drive fan; B3, the d-axis voltage V at the grid connection point with the direct-drive wind turbine d The related transfer function G p (s), calculate the d-axis current compensation instruction δI of the DC voltage controller of the direct-drive fan d , The q-axis voltage V at the direct-drive wind turbine grid connection point q and the output phase angle θ of the PLL controller P The related transfer function G Z (s), calculate the q-axis current compensation instruction δI of the AC voltage controller of the direct-drive wind turbine q , C. Set the d-axis current compensation instruction δI d The d-axis current command I of the DC voltage controller dr The d-axis current instruction after compensation is obtained by superposition; at the same time, the q-axis current compensation instruction δI q The q-axis current command I of the AC voltage controller qr The compensated d-axis current command is superimposed to obtain the compensated q-axis current command; finally, the compensated d-axis current command is fed into the d-axis current controller to control the d-axis current of the direct-drive wind turbine grid-connected point; the compensated q-axis current command is fed into the q-axis current controller to control the q-axis current of the direct-drive wind turbine grid-connected point; thereby reducing the capacitive reactance characteristics of the direct-drive wind turbine output impedance and suppressing low-frequency oscillation.

2. The method for suppressing low-frequency oscillation of a weak power grid with direct-drive wind power according to claim 1, characterized in that: The cutoff frequencies of the low-pass filter and the high-pass filter in step A are the same, and the range of the cutoff frequencies is 2 to 5 Hz.

Images