阅读说明：本技术 一种弱电网下永磁直驱风电机组振荡抑制方法 (Permanent magnet direct-drive wind turbine generator oscillation suppression method under weak grid ) 是由 白正宇 姜文 张瑞春 田旭 李震 于 2021-02-09 设计创作，主要内容包括：本发明公开了一种弱电网下永磁直驱风电机组振荡抑制方法,在网侧变流器中加入下垂控制,建立系统阻抗模型,并以阻抗分析法抑制系统振荡；通过下垂控制方法改变系统阻抗模型,并快速获取该阻抗模型,通过此阻抗模型优化系统参数,从而有效抑制系统振荡。(The invention discloses a method for suppressing oscillation of a permanent magnet direct-drive wind turbine generator under a weak grid, wherein droop control is added into a grid-side converter, a system impedance model is established, and system oscillation is suppressed by an impedance analysis method; the system impedance model is changed through a droop control method, the impedance model is rapidly obtained, and system parameters are optimized through the impedance model, so that system oscillation is effectively restrained.)

1. A permanent magnet direct-drive wind turbine generator oscillation suppression method under a weak grid is characterized by comprising the following steps:

step 0, adding a droop control module before the voltage and current double closed loop for providing the voltage amplitude and the angular velocity of the voltage and current double closed loop;

step 1, inputting current system parameters, including the following electrical parameters: DC voltage V_dcFilter inductor L_fFilter inductor resistor R_fFilter capacitor C_fImpedance of weak grid Z_g(ii) a Controlling parameters: sag factor K_p、K_qVoltage loop PI control parameter K_vp、K_viControl parameter K of current loop PI_ip、K_iiCoefficient of decoupling of current loop K_d(ii) a Steady state value parameter: fundamental frequency voltage amplitude V₁Amplitude of fundamental current I₁Voltage and current amplitude phase difference with fundamental frequencyObtaining grid-connected voltage and current through the three parameters;

step 2, calculating a droop control impedance model of the permanent magnet direct-drive wind turbine converter, considering a machine-network coupling effect, and calculating a uniform equivalent sequence impedance model, specifically:

the sequence impedance model is:

the coupling impedance model is:

wherein:

the effect of the machine-network coupling effect on the sequence impedance is equivalent to an impedance Z connected in parallel with the positive sequence impedance_q(s)：

Equivalent sequence impedance model Z considering machine-network coupling effect under weak current network_pq(s)：

Step 3, obtaining a system Nyquist curve according to the equivalent sequence impedance model obtained in the step 2, judging whether the system Nyquist curve is stable or not, if not, namely, the system oscillates, and adjusting a control parameter K_p，K_qThen, the equivalent sequence impedance model is adjusted, and the parameter K is continuously adjusted_p，K_qUntil the system Nyquist curve is stabilized.

2. The method for suppressing the oscillation of the permanent magnet direct-drive wind turbine generator set under the weak grid as claimed in claim 1, wherein in the step 0, the droop control module controls an expression as follows:

wherein, P₀、Q₀Rated active power and reactive power, K, of the direct drive system respectively_pIs the active sag factor, K_qIs a reactive sag factor, V₀For the rated voltage amplitude, omega, of the network₀For rating the frequency, V, of the grid_ref、ω_refRespectively, given voltage amplitude and angular velocity of the voltage-current double closed loop.

3. The method for suppressing the oscillation of the permanent magnet direct-drive wind turbine generator under the weak grid as claimed in claim 1, wherein in the step 1, the specific expression forms of grid-connected voltage and current are as follows:

V₁＝V₁/2

wherein f is_pRepresents a harmonic frequency; v_pAnd I_pRespectively representing positive sequence voltage and current of a PCC grid-connected point; the weak grid impedance is:s ═ j · 2 π f, f denotes frequency; SCR represents a short-circuit ratio, P represents active power at a grid-connected point, f₁Representing the fundamental frequency of the grid.

4. The method for suppressing the oscillation of the permanent magnet direct-drive wind turbine generator under the weak grid as claimed in claim 1, wherein the equivalent effect of the machine-grid coupling effect on the sequence impedance is that the impedance connected in parallel with the positive sequence impedance is as follows:

5. the method for suppressing the oscillation of the permanent magnet direct-drive wind turbine generator under the weak grid as claimed in claim 1, wherein the equivalent sequence impedance model considering the machine-grid coupling effect under the weak grid is as follows:

Technical Field

The invention belongs to the technical field of analysis and control of a grid-connected system, and particularly relates to a method for suppressing oscillation of a permanent magnet direct-drive wind turbine generator under a weak grid.

Background

The renewable energy power generation is important for the sustainable development of human society and environment because fossil fuels can generate a large amount of pollutants in the process of producing electric energy, so that the problem of secondary pollution of the environment is caused, and the health of people is harmed. The wind power generation technology is one of the mature new energy power generation technologies with wide development prospect, and large-scale development and application are realized in the global scope. The permanent-magnet direct-drive fan is widely applied due to the characteristics of high efficiency, long service life, small volume and low maintenance cost. The increase in the proportion of renewable energy power generation in the power grid changes the characteristics of the power system dominated by the synchronous machines, causing a number of oscillation problems.

The permanent magnet direct-drive generator is incorporated into a power grid through the machine side converter and the grid side converter, the machine side converter outputs direct-current voltage which is stable due to the existence of large capacitance and can be regarded as a direct-current voltage source, and therefore control and analysis of the grid side converter are very important. The converter control modes comprise PQ control, V/F control and droop control. The PQ control needs to acquire the voltage and frequency of a large power grid in real time, keeps synchronization with the voltage and frequency, and cannot keep a stable state. The voltage and frequency output under the V/F control are constant, but faults such as current overcurrent and the like easily occur in grid-connected operation. Compared with the former, the voltage frequency output under the droop control is output by the control of the droop control, and the output of the system is more stable during grid connection. The droop control of the grid-connected inverter can effectively adjust the power balance among all distributed power supplies, and further improve the stability of the system.

In the existing stage, a method for controlling the alternating current port of the direct-drive fan grid-side converter through droop still has a lot of stability problems, so when the electric control structure is used, the problem to be solved is urgently solved by carrying out oscillation suppression. The impedance analysis method is a stability analysis method which is widely concerned in recent years, and has the advantages of convenient analysis and simple operation structure. Therefore, the invention aims to solve the problem that an impedance model of an alternating current port of a droop control direct-drive wind turbine side converter is established to suppress oscillation of a system electrical structure.

Disclosure of Invention

In view of the above, the invention aims to provide a method for suppressing oscillation of a permanent magnet direct-drive wind turbine generator under a weak grid.

A permanent magnet direct-drive wind turbine generator oscillation suppression method under a weak grid comprises the following steps:

step 0, adding a droop control module before the voltage and current double closed loop for providing the voltage amplitude and the angular velocity of the voltage and current double closed loop, wherein the droop control module has a control expression as follows:

wherein, P₀、Q₀Rated active power and reactive power, K, of the direct drive system respectively_pIs the active sag factor, K_qIs a reactive sag factor, V₀For the rated voltage amplitude, omega, of the network₀For rating the frequency, V, of the grid_ref、ω_refRespectively setting voltage amplitude and angular velocity of the voltage-current double closed loop;

step 1, inputting current system parameters, including the following electrical parameters: DC voltage V_dcFilter inductor L_fFilter inductor resistor R_fFilter capacitor C_fImpedance of weak grid Z_g(ii) a Controlling parameters: sag factor K_p、K_qVoltage loop PI control parameter K_vp、K_viControl parameter K of current loop PI_ip、K_iiCoefficient of decoupling of current loop K_d(ii) a Steady state value parameter: fundamental frequency voltage amplitude V₁Amplitude of fundamental current I₁Voltage and current amplitude phase difference with fundamental frequencyThe three parameters are used for obtaining the concrete expression forms of grid-connected voltage and current:

wherein f is_pRepresents a harmonic frequency; v_pAnd I_pRespectively representing positive sequence voltage and current of a PCC grid-connected point; the weak grid impedance is:s ═ j · 2 π f, f denotes frequency; SCR represents a short-circuit ratio, P represents active power at a grid-connected point, f₁Representing the fundamental frequency of the power grid;

step 2, calculating a droop control impedance model of the permanent magnet direct-drive wind turbine converter, considering a machine-network coupling effect, and calculating a uniform equivalent sequence impedance model, specifically:

the sequence impedance model is:

the coupling impedance model is:

wherein:

the effect of the machine-to-net coupling effect on the sequence impedance is equivalent to an impedance in parallel with the positive sequence impedance:

the equivalent sequence impedance model considering the machine-network coupling effect under the weak grid is as follows:

step 3, obtaining a system Nyquist curve according to the equivalent sequence impedance model obtained in the step 2, judging whether the system Nyquist curve is stable or not, if not, namely, the system oscillates, and adjusting a control parameter K_p，K_qThen, the equivalent sequence impedance model is adjusted, and the parameter K is continuously adjusted_p，K_qUntil the system Nyquist curve is stabilized.

The invention has the following beneficial effects:

according to the method for suppressing the oscillation of the permanent magnet direct-drive wind turbine generator under the weak grid, the system impedance model is changed through the droop control method, the impedance model is rapidly obtained, and the system parameters are optimized through the impedance model, so that the system oscillation is effectively suppressed.

Drawings

FIG. 1 is a schematic structural diagram of a grid-side converter of a permanent magnet direct-drive wind turbine generator under a weak grid in the embodiment of the invention;

fig. 2 is a schematic view of a droop control structure of a grid-side converter according to an embodiment of the present invention;

fig. 3 is a schematic diagram of a dual closed-loop structure of a grid-side converter according to an embodiment of the present invention;

FIG. 4 is a schematic structural diagram of an oscillation suppression device of a permanent magnet direct-drive wind turbine generator based on impedance modeling under a weak power grid according to an embodiment of the invention;

FIG. 5 is a flow chart of a method for suppressing oscillation of a converter control parameter optimization of a permanent magnet direct-drive wind turbine generator based on impedance modeling;

FIG. 6 is a Nyquist curve diagram of the output impedance of the grid-side converter of the permanent magnet direct-drive wind turbine generator and the impedance ratio of the weak grid;

FIG. 7 is a graph of a grid-connected experiment result of the permanent magnet direct-drive wind turbine generator set according to the embodiment of the invention;

FIG. 8 is a Nyquist curve of the output impedance of the grid-side converter of the permanent magnet direct-drive wind turbine generator and the impedance ratio of the weak grid after the control parameters are optimized in the embodiment of the invention;

fig. 9 is a graph of a grid-connected experimental result of the permanent magnet direct-drive wind turbine generator set after control parameters are optimized according to the embodiment of the invention.

Detailed Description

The invention is described in detail below by way of example with reference to the accompanying drawings.

A permanent magnet direct-drive wind turbine generator oscillation suppression method under a weak grid comprises the following steps:

step 0, adding a droop control module before the voltage and current double closed loop, as shown in fig. 1 and 2, for providing the voltage amplitude and the angular velocity of the voltage and current double closed loop, wherein the droop control module has a control expression:

wherein, P₀、Q₀Rated active power and reactive power, K, of the direct drive system respectively_pIs the active sag factor, K_qIs a reactive sag factor, V₀For the rated voltage amplitude, omega, of the network₀For rating the frequency, V, of the grid_ref、ω_refRespectively, given voltage amplitude and angular velocity of the voltage-current double closed loop.

Fig. 3 is a schematic diagram of a dual closed-loop structure of a grid-side converter according to an embodiment of the invention. Wherein the voltage regulator and the current regulator are both PI controllers. H_v(s) and H_i(s) are transfer functions of the voltage regulator and the current regulator, respectively, expressed as follows:

wherein, K_vp、K_viRespectively representing a voltage loop proportion coefficient and an integral coefficient; k_ip、K_iiRespectively, a current loop proportionality coefficient and an integral coefficient, K_dIs a current loop decoupling coefficient.

The double closed loop transfer function is:

wherein, V_dcIs the voltage value of the direct current end of the grid-side converter, I_dAnd I_qAnd respectively representing the frequency domains of the three-phase current of the PCC points in a three-phase coordinate system.

Step 1, inputting current system parameters, including the following electrical parameters: DC voltage V_dcFilter inductor L_fFilter inductor resistor R_fFilter capacitor C_fImpedance of weak grid Z_g(ii) a Controlling parameters: sag factor K_p、K_qVoltage loop PI control parameter K_vp、K_viControl parameter K of current loop PI_ip、K_iiCoefficient of decoupling of current loop K_d(ii) a Steady state value parameter: fundamental frequency voltage amplitude V₁Amplitude of fundamental current I₁Voltage and current amplitude phase difference with fundamental frequencyThe three parameters can obtain the concrete expression forms of grid-connected voltage and current:

wherein f is_pRepresents a harmonic frequency; v_pAnd I_pRespectively representing the positive sequence voltage and current of the PCC grid-connected point.

And setting a parameter adjusting range and a parameter stepping value. Weak grid impedance Z_g(s) calculating a specific value by the short circuit ratio SCR. Wherein the content of the first and second substances,s ═ j · 2 π f, f denotes frequency; SCR denotes the short-circuit ratio (given by the grid), P denotes the active power at the grid tie-in point, f₁Representing the fundamental frequency of the power grid;

step 2, calculating a droop control impedance model of the permanent magnet direct-drive wind turbine converter, considering a machine-network coupling effect, and calculating a uniform equivalent sequence impedance model, specifically:

sequence impedance model and coupling impedance model:

wherein, I_p2Representing negative sequence current, corresponding to a frequency of + - (f)_p-2f₁)；

The effect of the machine-to-net coupling effect on the sequence impedance can be equated with an impedance in parallel with the positive sequence impedance as follows:

the equivalent sequence impedance model considering the machine-network coupling effect under the weak grid is as follows:

step 3, obtaining a system Nyquist curve according to the equivalent sequence impedance model obtained in the step 2, judging whether the system Nyquist curve is stable or not, if the system Nyquist curve is unstable, namely the system oscillates, and adjusting the control parameter K according to parameter steps in the parameter range of the droop control module at the moment_p，K_qAnd adjusting the equivalent sequence impedance model at the moment, and continuously adjusting parameters until the Nyquist curve of the system is judged to be stable.

The invention provides an oscillation suppression device of a permanent magnet direct-drive wind turbine generator based on impedance modeling under a weak power grid, which comprises the following modules as shown in figure 4:

1) and the control input module 401 is used for writing a droop control program of the grid-side converter of the permanent magnet direct-drive wind turbine generator into the control chip and running the droop control program.

2) The parameter obtaining module 402 is configured to obtain parameters of a grid-side converter of the permanent magnet direct-drive wind turbine generator, where the parameters include: DC voltage V_dcFilter inductor L_fFilter inductor resistor R_fFilter capacitor C_fImpedance of weak grid Z_g(ii) a Droop controller control parameters: sag factor K_p、K_q(ii) a Voltage current loop control parameters: voltage loop PI control parameter K_vp、K_viControl parameter K of current loop PI_ip、K_iiCoefficient of decoupling of current loop K_d(ii) a Steady state value parameter: fundamental frequency voltage amplitude V₁Amplitude of fundamental current I₁Voltage and current amplitude phase difference with fundamental frequency

3) An impedance model calculation module 403. For calculating the system sequence impedance model Z through the parameters obtained above_p(s) and a coupled impedance model Z_c(s); further, considering the machine-network coupling effect under the weak power grid, the effect is equivalent to a parallel impedance form of the sequence impedance, and a uniform sequence impedance equivalent model Z is obtained_pq(s)。

4) A parameter adjustment module 404. And the method is used for judging the stability of the system through the impedance model, optimizing the control parameters of the droop controller of the system if the system oscillates at the moment, and adjusting the impedance model of the converter of the system to ensure that the Nyquist curve judgment of the system is stable.

Fig. 6 is a Nyquist curve diagram of the output impedance of the grid-side converter of the permanent magnet direct-drive wind turbine generator and the impedance ratio of the weak grid according to the embodiment of the invention. The red dotted line represents the impedance ratio Z_g/Z_pqThe curve includes a point (-1,0) and thus the system is unstable.

Fig. 7 is a graph of a grid-connected experimental result of the permanent magnet direct-drive wind turbine generator according to the embodiment of the invention, and it can be seen from the graph that the three-phase voltage and current waveforms have obvious oscillation phenomena.

It can be seen that the Nyquist decision of the system is unstable at this time, and the system oscillates after running.

Fig. 8 is a Nyquist curve of the output impedance of the grid-side converter of the permanent magnet direct-drive wind turbine generator and the impedance ratio of the weak grid after control parameter optimization according to the embodiment of the invention. The green dotted line represents the impedance ratio Z_g/Z_pqThe curve does not contain a point (-1,0) and the system is therefore stable.

Fig. 9 is a graph of a grid-connected experimental result of the permanent magnet direct-drive wind turbine generator set after the control parameters are optimized according to the embodiment of the invention, and it can be seen from the graph that three-phase voltage and current waveforms operate stably without oscillation.

Therefore, the Nyquist judgment of the system after the control parameters are optimized is stable, and the system stably operates.

According to the oscillation suppression device of the permanent magnet direct-drive wind turbine generator based on impedance modeling under the weak grid, which is provided by the embodiment of the invention, the system impedance model can be effectively optimized, the system oscillation is suppressed, and the stable operation of the system is ensured.

In summary, the above description is only a preferred embodiment of the present invention, and is not intended to limit the scope of the present invention. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.