METHOD FOR ANALYZING STABILITY OF PERMANENT MAGNET SYNCHRONOUS GENERATOR-BASED WIND TURBINES CONNECTED TO WEAK POWER GRID CONSIDERING INFLUENCE OF POWER CONTROL

Abstract

Provided is a method for analyzing the stability of a PMSG-WT connected to a weak power grid considering the influence of power control. New energy power generation mostly uses a perturbation and observation (P&O) method for maximum power point tracking, and nonlinear discontinuous links therein make stability analysis difficult. The present application analyzes the stability of the PMSG-WT connected to the weak grid system based on a describing function method, and fully considers the nonlinear discontinuous links in the power loop, thus making the analysis result more accurate. At the same time, the describing function method is a method that can quantitatively calculate the frequency and amplitude of oscillation. The analysis method of the present application can provide a powerful and good reference for oscillation suppression and controller design.

Description

TECHNICAL FIELD

The present application relates to a method for analyzing the stability of a permanent magnet synchronous generator-based wind turbine connected to a weak power grid considering the influence of power control, and belongs to the field of stability analysis of a new energy power generation system in an electrical power system.

BACKGROUND

The proportion of renewable energy power generation continues to increase, the installed capacity of wind power generation continues to grow, and the stable operation of a grid-connected wind power system has become an important issue. A permanent magnet synchronous generator (PMSG) based wind turbines (WTs) has the advantages of high energy conversion efficiency, good operational reliability and strong controllability, and the proportion of installed capacity continues to increase. In order to make better use of wind energy, a suitable maximum power point tracking algorithm must be adopted. A perturbation and observation (P&O) method does not need to measure the wind speed, and thus has a better practical value. However, the P&O method implies a nonlinear discontinuous link, which makes the traditional small-signal stability analysis method difficult to apply. The conventional stability analysis method generally ignores a power loop based on the P&O method. A describing function method can well complete the modeling of a nonlinear link and the stability analysis of the system, the basic idea is that when the system meets certain assumed conditions, under the action of sinusoidal input, the output of the nonlinear link of the system can be approximately expressed by its first harmonic component, so as to obtain the approximate equivalent frequency characteristics of the nonlinear link. Therefore, based on the describing function method, the present application proposes a method for analyzing the stability of a permanent magnet synchronous generator-based wind turbines (PMSG-WT) connected to a weak power grid considering the influence of power control, which fully considers the nonlinear link in the power loop, and improves the accuracy of stability analysis.

SUMMARY

The present application aims to provide a method for analyzing the stability of a PMSG-WT connected to a weak power grid considering the influence of power control. The PMSG-WT includes a wind turbine, a generator, a machine-side converter (MSC), a DC capacitor, a grid-side converter (GSC), a filter, an MSC controller, and a GSC controller. The MSC controller includes a power loop, a rotating speed loop, and a machine-side current loop; and the GSC controller includes a voltage loop and a grid-side current loop.

The present application adopts the following technical solutions:

Step 1: acquiring main parameters of the PMSG-WT, establishing mathematical models of the wind turbine, the generator and the MSC, the MSC controller, the DC capacitor, the GSC and the filter, and the GSC controller, respectively, performing linearization in a dq coordinate system, and calculating steady-state operation parameters to obtain small-signal models of the wind turbine, the generator and the MSC, the MSC controller, the DC capacitor, the GSC and the filter, and the GSC controller.

Step 2: modeling the power loop in the MSC controller based on a describing function method, wherein its mathematical expression is:

${\omega }_g^{\mathrm{ref}}=\frac{\epsilon }{T_{{\rm P}}}\int \mathrm{sgn}\left(P^{\mathrm{ref}}-P_n\right)\mathrm{sgn}\left(P_n-P_{n-1}\right)\mathrm{sgn}\left({\omega }_{g,n}-{\omega }_{g,n-1}\right)\mathrm{dt}$

where ε represents a perturbation step length of the power loop, T_p represents a control period of the power loop, P^ref represents a reference value of the output power of the PMSG-WT, P represents an output power of the PMSG-WT, P_n represents an output power of the PMSG-WT at the current sampling moment n, P_n−1 represents an output power of the PMSG-WT at the previous sampling moment, ω_g represents a rotating speed of the generator, ω_g^ref represents a reference value of the rotating speed of the generator, ω_g,n represents a rotating speed of the generator at the current sampling moment, and ω_g,n−1 represents a rotating speed of the generator at the previous sampling moment. sgn(x) is a sign function, when x≥0, sgn(x)=1, and when x<0, sgn(x)=−1. Considering an actual power-rotating speed curve of the PMSG-WT, it can be deemed that:

$\mathrm{sgn}\left(P_n-P_{n-1}\right)\mathrm{sgn}\left({\omega }_{g,n}-{\omega }_{g,n-1}\right)=\{\begin{array}{cc}1& \left({\omega }_g\le {\omega }_{\mathrm{mpp}}\right)\\ -1& \left({\omega }_g>{\omega }_{\mathrm{mpp}}\right)\end{array}$

where ω_mpp represents the rotating speed of the generator at the maximum power point. Then, the power loon model can be simplified as:

${\omega }_g^{\mathrm{ref}}=\{\begin{array}{c}\frac{\epsilon }{T_{{\rm P}}}\int \mathrm{sgn}\left(P^{\mathrm{ref}}-P_n\right)\mathrm{dt},\left({\omega }_g\le {\omega }_{\mathrm{mpp}}\right)\\ -\frac{\epsilon }{T_{{\rm P}}}\int \mathrm{sgn}\left(P^{\mathrm{ref}}-P_n\right)\mathrm{dt},\left({\omega }_g>{\omega }_{\mathrm{mpp}}\right)\end{array}$

the sign function in the formula can be modeled by a describing function, and the describing function is:

$N\left(A\right)=\frac{4}{\pi A}$

in the formula, A represents an amplitude of an input signal.

Step 3: considering the influence of the weak AC power grid, combining the linear parts of the small-signal models of the weak power grid and the power loop with the small-signal model of the PMSG-WT established in step 1, and deriving a transfer function G(s) of the linear part of the system.

Step 4: drawing G(s) and −1/N(A) curves in a complex plane, and analyzing the stability of the system based on the describing function method, wherein the method is specifically that if G(s) contains a right-half-plane (RHP) pole, the system must be unstable; if the G(s) does not contain any RHP pole, then judging the stability of the system through the relationship between the G(s) curve and the −1/N (A) curve:

a. if the G(s) curve does not surround the −1/N(A) curve, the system is stable and does not oscillate;

b. if the G(s) curve and the −1/N(A) curve intersect, the system is critically stable, at this time, the system oscillates at a constant amplitude and a constant frequency, and the frequency and amplitude of the oscillation can be calculated by the following formula:

$\{\begin{array}{c}{G}_{Im}\left({\omega }_0\right)=0\\ N\left(A_0\right)=-1/G_{\mathrm{Re}}\left({\omega }_0\right)\end{array}$

where G(jω)=G_Re(ω)+jG_Im(ω), G_Re means to find the real part of a complex number, G_Im means to find the imaginary part of the complex number, ω₀ represents an angular frequency of the oscillation, and A₀ represents the amplitude of the oscillation; and

c. if the G(s) curve surrounds the −1/N(A) curve, the system is unstable.

Further, in step 1, the small-signal models of the wind turbine, the generator and the MSC, and the MSC controller are established as follows:

the mathematical model of the wind turbine is established as:

sJω _g =T _m −T _e −Bω _g

in the formula, J represents rotational inertia of an equivalent concentrated mass block of the wind turbine and the generator, T_m represents a mechanical torque of the generator, T_e represents an electromagnetic torque of the generator, B represents a self-damping coefficient, it is deemed that B=0 herein, and s represents a parameter introduced by the Laplace transform. Linearization is performed on the model to obtain:

sJΔω _g =ΔT _m −ΔT _e

the electromagnetic torque of the generator is:

T _e= 3/2n_pψ_fi_qr

n_p represents the number of pole pairs of the generator, i_qr represents stator current of the q-axis generator, and ψ_f represents a permanent magnet flux linkage of the generator. This formula is linearized to obtain:

ΔT _e= 3/2n_pψ_fΔi_qr

the mechanical torque of the generator is:

T _m =B _tω_g

in the formula,

$B_t=\frac{{\mathrm{dT}}_m}{d{\omega }_g}{|}_{{\omega }_g={\omega }_g^{*}}$

represents a linearization constant of the mechanical torque of the wind turbine, and ω_g* represents a steady-state value of the rotating speed of the generator, which is a specific constant at an equilibrium point. This formula is linearized to obtain:

ΔT _m =B _tΔω_g

from this, the small-signal model of the wind turbine can be obtained,

${\mathrm{\Delta \omega }}_g=\frac{3n_p\psi f}{2\left(B_t-\mathrm{sJ}\right)}\Delta i_{\mathrm{qr}}=\left[\begin{array}{cc}0& \frac{3n_p\psi f}{2\left(B_t-\mathrm{sJ}\right)}\end{array}\right]·\Delta i_{\mathrm{dqr}}$

setting

$G_{\mathrm{iq}\omega }=[\begin{array}{cc}0& \frac{3n_p\psi f}{2\left(B_t-\mathrm{sJ}\right)}],\end{array}$

then

Δω_g=G_iqω·Δi_dqr, Δω_e=n_pG_iqω·Δi_dqr.

The mathematical model of the generator and the MSC is:

$\left[\begin{array}{cc}{\mathrm{sL}}_s+R_s& -{\omega }_e{L}_s\\ {\omega }_e{L}_s& {\mathrm{sL}}_s+R_s\end{array}\right]·\left[\begin{array}{c}{i}_{\mathrm{dr}}\\ i_{qr}\end{array}\right]=\left[\begin{array}{c}0\\ {\omega }_e{\psi }_f\end{array}\right]-\left[\begin{array}{c}{d}_{dr}\\ d_{qr}\end{array}\right]·u_{dc}$

in the formula, R_s and L_s represent rotor resistance and armature inductance of the generator, respectively, ω_e represents an electrical angular speed of the rotor, and ω_e=n_pω_g. i_dr and i_qr represent the stator current of the generator in the dq coordinate system, d_dr and d_qr represent output duty ratios of the MSC controller in the dq coordinate system, u_dc represents a DC voltage. By linearizing the model, the small-signal model of the generator and MSC can be obtained as:

Z _dqr ·Δi _dqr =−U* _dc ·Δd _dqr −D* _dqr ·Δu _dc +G _ωe·Δω_e

where

$Z_{dqr}=\left[\begin{array}{cc}{\mathrm{sL}}_s+R_s& -{\omega }_e{L}_s\\ {\omega }_e{L}_s& {\mathrm{sL}}_s+R_s\end{array}\right],\Delta i_{dqr}=\left[\begin{array}{c}\Delta i_{dr}\\ \Delta i_{qr}\end{array}\right],\Delta d_{dqr}=\left[\begin{array}{c}\Delta d_{dr}\\ \Delta d_{qr}\end{array}\right],$ $D_{dqr}^{*}=\left[\begin{array}{c}{D}_{dr}^{*}\\ D_{qr}^{*}\end{array}\right],G_{\omega e}=\left[\begin{array}{c}{L}_s{I}_{qr}^{*}\\ {\psi }_f-L_s{I}_{dr}^{*}\end{array}\right];$

and capital letters and superscripts * represent steady-state components of corresponding lowercase variables, and Δ represents a small-signal component of the corresponding variable.

The mathematical model of the MSC controller is established as:

$\{\begin{array}{c}{d}_{\mathrm{dr}}^c{U}_{dc}^{*}=-H_{\mathrm{cr}}\left(0-i_{\mathrm{dr}}^c\right)+{\omega }_e{L}_s\\ d_{\mathrm{qr}}^c{U}_{dc}^{*}=-H_{\mathrm{cr}}\left(-H_{\omega }\left({\omega }_g^{\mathrm{ref}}-{\omega }_g\right)-i_{\mathrm{qr}}^c\right)-{\omega }_e{L}_s{i}_{\mathrm{dr}}^c+{\omega }_e{\psi }_f\end{array}$

where

$H_{\mathrm{cr}}=K_{\mathrm{cpr}}+\frac{K_{\mathrm{cir}}}{s},$

K_cpr and K_cir represent a proportional parameter and an integral parameter of machine-side current loop PI control, respectively,

$H_{\omega }=K_{\omega p}+\frac{K_{\omega i}}{s},$

and K_ωp and K_ωi represent the proportional parameter and the integral parameter of rotating speed loop PI control, respectively. Superscript c represents a dq coordinate system of the MSC controller. ω_g^ref represents the reference value of the rotating speed of the generator. By linearizing the model, the small-signal model of the MSC controller can be obtained as:

$\Delta d_{\mathrm{dqr}}^c=\frac{1}{U_{dc}^{*}}\left(G_{\mathrm{cr}}·\Delta i_{\mathrm{dqr}}^c+G_{\omega g}\left({\omega }_g^{\mathrm{ref}}-{\omega }_g\right)+G_{\omega e}·\Delta {\omega }_e\right)\mathrm{where}$ $G_{\mathrm{cr}}=\left[\begin{array}{cc}{H}_{\mathrm{cr}}& {\omega }_e^{*}{L}_s\\ -{\omega }_e^{*}{L}_s& H_{\mathrm{cr}}\end{array}\right],G_{\omega g}=\left[\begin{array}{c}0\\ H_{\mathrm{cr}}{H}_{\omega }\end{array}\right].$

ω_e* represents the steady-state value of the electrical angular speed of the rotor. Affected by the disturbance of the rotating speed, the dq coordinate system of the MSC controller will have a phase angle difference with the dq coordinate system of the generator during the dynamic process. The electrical angle of the generator is:

θ_e=n_pω_g/s

the formula is linearized to obtain:

$\Delta {\theta }_e=\frac{n_p\Delta {\omega }_g}{s}=\frac{3n_p^2{\psi }_f}{2s\left(B_t-\mathrm{sJ}\right)}\Delta i_{\mathrm{qr}}=H_{\theta e}\Delta i_{\mathrm{qr}}$

in the formula,

$H_{\theta e}=\frac{3n_p^2{\psi }_f}{2s\left(B_t-sJ\right)}.$

Therefore, the variable conversion relationship between the dq coordinate system of the MSC controller and the dq coordinate system of the generator is:

$\{\begin{array}{c}\Delta i_{\mathrm{dqr}}^c=G_{\theta e}^i·\Delta i_{\mathrm{dqr}}\\ \Delta d_{\mathrm{dqr}}^c=G_{\theta e}^d·\Delta i_{\mathrm{dqr}}+\Delta d_{\mathrm{dqr}}\end{array}\mathrm{where}$ $G_{\theta e}^i=\left[\begin{array}{cc}1& H_{\theta e}{I}_{\mathrm{qr}}^{*}\\ 0& 1-H_{\theta e}{I}_{\mathrm{dr}}^{*}\end{array}\right],G_{\theta e}^d=\left[\begin{array}{cc}0& H_{\theta e}{D}_{\mathrm{qr}}^{*}\\ 0& -H_{\theta e}{D}_{\mathrm{dr}}^{*}\end{array}\right].$

Then, the small-signal model of the MSC controller is:

$G_{\theta e}^d·\Delta i_{\mathrm{dqr}}+\Delta d_{\mathrm{dqr}}=\frac{1}{U_{dc}^{*}}\left(G_{\mathrm{cr}}·G_{\theta e}^i·\Delta i_{\mathrm{dqr}}+G_{\omega g}\left({\omega }_g^{\mathrm{ref}}-{\omega }_g\right)+G_{\omega e}·{\mathrm{\Delta \omega }}_e\right).$

Further, in step 1, the establishment process of the small-signal model is as follows:

the mathematical model of the DC capacitor is established as:

sC _dc u _dc =i _dc2 −i _dc1=1.5(d_dri_dr+d_qri_qr)−1.5(d_dgi_dg+d_qgi_qg)

in the formula, C_dc represents the DC capacitor, i_dc1 represents grid-side DC current, i_dc2 represents machine-side DC current, i_dg and i_qg represent d-axis current and q-axis current of the AC port of the GSC, respectively, and d_dg and d_qg represent the output duty ratios of the GSC controller in the dq coordinate system, respectively. u_dc represents the DC voltage, i_dr and i_qr represent the stator current of the generator in the dq coordinate system, respectively, d_dr and d_qr represent the output duty ratios of the MSC controller in the dq coordinate system, respectively, and by linearizing the model, the small-signal model of the DC capacitor can be obtained as:

sC _dc Δu _dc=1.5(D*_dqr^T·Δi_dqr+I*_dqr^T·Δd_dqr)−1.5(D*_dqg^T·Δi_dqg+I*_dqg^T·Δd_dqg)

in the formula,

$\Delta i_{\mathrm{dqg}}=\left[\begin{array}{c}\Delta i_{dg}\\ \Delta i_{\mathrm{qg}}\end{array}\right],\Delta d_{\mathrm{dqg}}=\left[\begin{array}{c}\Delta d_{dg}\\ \Delta d_{\mathrm{qg}}\end{array}\right],D_{\mathrm{dqg}}^{*}=\left[\begin{array}{c}{D}_{dg}^{*}\\ D_{\mathrm{qg}}^{*}\end{array}\right],I_{\mathrm{dqg}}^{*}=\left[\begin{array}{c}{I}_{dg}^{*}\\ I_{\mathrm{qg}}^{*}\end{array}\right].$

The capital letters and the superscripts * represent the steady-state components of corresponding lowercase variables, and Δ represents the small-signal component of the corresponding variable.

Further, in step 1, the establishment process of the small-signal models of the GSC and the filter, and the GSC controller is as follows:

the mathematical model of the GSC and the filter is established as:

$\left[\begin{array}{cc}{\mathrm{sL}}_f& -\omega L_f\\ \omega L_f& {\mathrm{sL}}_f\end{array}\right]·\left[\begin{array}{c}{i}_{dg}\\ i_{\mathrm{qg}}\end{array}\right]=\left[\begin{array}{c}{d}_{dg}\\ d_{\mathrm{qg}}\end{array}\right]·u_{dc}-\left[\begin{array}{c}{u}_{dg}\\ u_{\mathrm{qg}}\end{array}\right]$

in the formula, L_f represents a filtering inductance, ω represents a power frequency angular frequency, ω=100π rad/s, i_dg and i_qg represent the d-axis current and the q-axis current of the AC port of the GSC, respectively, d_dg and d_qg represent the output duty ratios of the GSC controller in the dq coordinate system, respectively, u_dc represents the DC voltage, and u_dg and u_qg represent a d-axis voltage and a q-axis voltage of a grid-connected point, respectively. By linearizing the model, the small-signal model of the GSC and the filter can be obtained as:

Z _f ·Δi _dqg =U* _dc ·Δd _dqg +D* _dqg ·Δu _dc −Δu _dqg

where

$Z_f=\left[\begin{array}{cc}{\mathrm{sL}}_f& -\omega L_f\\ \omega L_f& {\mathrm{sL}}_f\end{array}\right],\Delta i_{\mathrm{dqg}}=\left[\begin{array}{c}\Delta i_{dg}\\ \Delta i_{\mathrm{qg}}\end{array}\right],$ $\Delta d_{\mathrm{dqg}}=\left[\begin{array}{c}\Delta d_{dg}\\ \Delta d_{\mathrm{qg}}\end{array}\right],D_{\mathrm{dqg}}^{*}=\left[\begin{array}{c}{D}_{dg}^{*}\\ D_{\mathrm{qg}}^{*}\end{array}\right],$

The capital letters and the superscripts * represent the steady-state components of corresponding lowercase variables, and Δ represents the small-signal component of the corresponding variable.

The mathematical model of the GSC controller is established as:

$\{\begin{array}{c}{d}_{\mathrm{dr}}^c{U}_{dc}^{*}=u_{dg}^c+H_{\mathrm{cg}}\left(-H_v\left(U_{\mathrm{dcref}}-u_{dc}\right)-i_{dg}^c\right)-\omega L_f{i}_{\mathrm{qg}}^c\\ d_{\mathrm{qg}}^c{U}_{dc}^{*}=u_{dg}^c+H_{\mathrm{cg}}\left(0-i_{\mathrm{qg}}^c\right)+\omega L_f{i}_{dg}^c\end{array}$

where

$H_{cg}=K_{cpg}+\frac{K_{cig}}{s},$

K_cpg and K_cig represent a proportional parameter and an integral parameter of grid-side current loop PI control, respectively,

$H_v=K_{\mathrm{vp}}+\frac{K_{\mathrm{vi}}}{s},$

and K_vp and K_vi represent the proportional parameter and the integral parameter of grid-side voltage loop PI control, respectively. U_dcref represents the reference value of the DC voltage. In the GSC controller, a phase-locked loop is used to keep the PMSG-WT synchronous with the power grid. The superscript c represents the dq coordinate system of the GSC controller. By linearizing the model, the small-signal model of the GSC controller can be obtained as:

Δd _dqg ^c ·U* _dc =Δu _dqg ^c +G _cg Δi _dqg ^c +G _udc Δu _dc

where

$G_{\mathrm{cg}}=\left[\begin{array}{cc}-H_{\mathrm{cg}}& -\omega L_f\\ \omega L_f& -H_{\mathrm{cg}}\end{array}\right],G_{\mathrm{udc}}=\left[\begin{array}{c}{H}_{\mathrm{cg}}{H}_v\\ 0\end{array}\right].$

In addition, the dynamics of the phase-locked loop should also be considered in the GSC, and its mathematical model is:

$\theta =H_{\mathrm{pll}}·\frac{1}{s}·u_{\mathrm{qg}}^c$

where

$H_{\mathrm{pll}}=K_{\mathrm{ppll}}+\frac{K_{\mathrm{ipll}}}{s},$

K_pll and K_ipll represent the proportional parameter and the integral parameter of a phase-locked loop PI controller, respectively, and u_qg^c represents the q-axis voltage of the grid-connected point in the dq coordinate system of the GSC controller. The model is linearized to obtain:

$\Delta \theta =H_{\mathrm{pll}}·\frac{1}{s}·\Delta u_{\mathrm{qg}}^c$

wherein there is a certain deviation between the dq coordinate system of the system and the dq coordinate system of the controller, and the two coordinate systems can be converted to each other through the following equation:

$\{\begin{array}{c}\Delta x_d^c=\Delta x_d+\mathrm{\Delta \theta }·X_q^{*}\\ \Delta x_q^c=\Delta x_q+\mathrm{\Delta \theta }·X_d^{*}\end{array}$

in the formula, the variables Δx_d and Δx_q may represent the output current Δi_dg and Δi_qg of the GSC, the voltages Δu_dg and Δu_qg of the grid-connected point, or the output duty ratios Δd_dg and Δd_qg of the grid-side controller, and X*_q and X*_d represent steady-state components I*_qg, I*_dg, U*_qg, U*_dg, D*_qg, D*_dg.

From this, the small-signal model of the phase-locked loop can be derived, namely:

Δθ=G_pll·Δu_qg

where

$G_{\mathrm{pll}}=\frac{H_{\mathrm{pll}}}{s+U_{\mathrm{dg}}^{*}{H}_{\mathrm{pll}}},$

and then the relationship between the dq coordinate system of the controller and the dq coordinate system of the system can be obtained as:

$\{\begin{array}{c}\Delta u_{\mathrm{dqg}}^c=G_{\mathrm{pll}}^u·\Delta u_{\mathrm{dqg}}\\ \Delta i_{\mathrm{dqg}}^c=G_{\mathrm{pll}}^i·\Delta u_{\mathrm{dqg}}+\Delta i_{\mathrm{dqg}}\\ \Delta d_{\mathrm{dqg}}^c=G_{\mathrm{pll}}^d·\Delta u_{\mathrm{dqg}}+\Delta d_{\mathrm{dqg}}\end{array}$ $\mathrm{where}$ $G_{\mathrm{pll}}^u=\left[\begin{array}{cc}1& G_{\mathrm{pll}}{U}_{\mathrm{qg}}^{*}\\ 0& 1-G_{\mathrm{pll}}{U}_{\mathrm{dg}}^{*}\end{array}\right],G_{\mathrm{pll}}^i=\left[\begin{array}{cc}0& G_{\mathrm{pll}}{I}_{\mathrm{qg}}^{*}\\ 0& -G_{\mathrm{pll}}{I}_{\mathrm{dg}}^{*}\end{array}\right],G_{\mathrm{pll}}^d=\left[\begin{array}{cc}0& G_{\mathrm{pll}}{D}_{\mathrm{qg}}^{*}\\ 0& -G_{\mathrm{pll}}{D}_{\mathrm{dg}}^{*}\end{array}\right].$

Then, the small-signal model of the GSC controller is converted into:

(G_pll^d·Δu_dqg+Δd_dqg)·U*_dc=G_pll^u·Δu_dqg+G_cg(G_pllⁱ·Δu_dqg+Δi_dqg)+G_udcΔu_dc

Further, the step 3 is specifically:

the AC weak power grid is expressed by series equivalent inductance of an ideal voltage source, and its mathematical model is established as:

$\left[\begin{array}{cc}{\mathrm{sL}}_g& -\omega L_g\\ \omega L_g& {\mathrm{sL}}_g\end{array}\right]·\left[\begin{array}{c}{i}_{\mathrm{dg}}\\ i_{\mathrm{qg}}\end{array}\right]=\left[\begin{array}{c}{u}_{\mathrm{dg}}\\ u_{\mathrm{qg}}\end{array}\right]-\left[\begin{array}{c}{u}_{\mathrm{ds}}\\ u_{\mathrm{qs}}\end{array}\right]$

in the formula, L_g represents the equivalent inductance of the weak grid, u_ds and u_qs represent ideal voltage source voltages of the d axis and the q axis, respectively, and i_dg and i_qg represent the d axis current and the q axis current of the AC port of the GSC, respectively. The formula is linearized to obtain:

Z _g ·Δi _dqg =Δu _dqg

in the formula,

$Z_g=\left[\begin{array}{cc}{\mathrm{sL}}_g& -\omega L_g\\ \omega L_g& {\mathrm{sL}}_g\end{array}\right].$

The output power of the PMSG-WT is:

P=1.5(i_dgu_dg+i_qgu_qg)

The formula is linearized to obtain:

ΔP=1.5(I*_dqg^T·Z_g+U*_dqg^T)·Δi_dqg

in the formula,

$U_{\mathrm{dqg}}^{*}=\left[\begin{array}{c}{U}_{\mathrm{dg}}^{*}\\ U_{\mathrm{qg}}^{*}\end{array}\right].$

By combining the linear parts of various part models of the aforementioned PMSG-WT grid-connected system, the transfer function G(s) of the linear part of the system can be obtained as:

$\begin{array}{c}G\left(s\right)=-\frac{1.5\epsilon }{T_{p^S}}\left({I_{\mathrm{dqg}}^{*}}^T{Z}_g+{U_{\mathrm{dqg}}^{*}}^T\right)M_5^{-1}\left(D_{\mathrm{dqg}}^{*}+G_{\mathrm{udc}}\right)M_7^{-1}{M}_8·\frac{1}{1+T_{f^S}}·\frac{1}{1+1.5T_{p^S}}\\ M_8=-{D_{\mathrm{dqr}}^{*}}^T·M_1^{-1}·G_{\omega g}+{I_{\mathrm{dqr}}^{*}}^T·M_3\\ M_7=\left(\frac{2{\mathrm{sC}}_{\mathrm{dc}}}{3}+{I_{\mathrm{dqg}}^{*}}^T{M}_6-{I_{\mathrm{dqr}}^{*}}^T{M}_4+{D_{\mathrm{dqg}}^{*}}^T{M}_5^{-1}\left(D_{\mathrm{dqg}}^{*}+G_{\mathrm{udc}}\right)+{D_{\mathrm{dqr}}^{*}}^T{M}_1^{-1}{D}_{\mathrm{dqr}}^{*}\right)\\ M_6=\left(\left(\left(G_{\mathrm{cg}}{G}_{\mathrm{pll}}^i-G_{\mathrm{pll}}^d{U}_{\mathrm{dc}}^{*}+G_{\mathrm{pll}}^u\right)Z_g+G_{\mathrm{cg}}\right)M_5^{-1}\left(D_{\mathrm{dqg}}^{*}+G_{\mathrm{udc}}\right)+G_{\mathrm{udc}}\right)/U_{\mathrm{dc}}^{*}\\ M_5=-G_{\mathrm{cg}}+Z_f-(\left(G_{\mathrm{cg}}{G}_{\mathrm{pll}}^i-G_{\mathrm{pll}}^d{U}_{\mathrm{dc}}^{*}+G_{\mathrm{pll}}^u\right)Z_g+Z_g\\ M_4=-M_2{M}_1^{-1}{D}_{\mathrm{dqr}}^{*}/U_{\mathrm{dc}}^{*}\\ M_3=\left(-M_2{M}_1^{-1}{G}_{\omega g}+G_{\omega g}\right)/U_{\mathrm{dc}}^{*}\\ M_2=G_{\omega e}{G}_{\mathrm{iq}\omega }{n}_p+G_{\mathrm{cr}}{G}_{\theta e}^i-G_{\omega g}{G}_{\mathrm{iq}\omega }-U_{\mathrm{dc}}^{*}{G}_{\theta e}^d\\ M_1=G_{\mathrm{cr}}{G}_{\theta e}^i-G_{\omega g}{G}_{\mathrm{iq}\omega }-U_{\mathrm{dc}}^{*}{G}_{\theta e}^d+Z_{\mathrm{dqr}}\end{array}$

in the formula, T_f represents the period of a power sampling filter, 1/(1+T_fs) represents the delay of the power sampling filter, and 1/(1+1.5T_ps) represents the delay of the controller and the PWM.

Compared with the prior art, the present application has the advantages that:

(1) the present application applies the describing function method to the stability analysis of the grid-connected PMSG-WT system, fully considers the nonlinear link in the power loop based on the P&O method, and overcomes the problem that the traditional small-signal analysis method cannot be applied to discontinuous nonlinear links.

(2) When the system is in a critically stable state, oscillation occurs at constant amplitude and constant frequency. The describing function method can quantitatively calculate the frequency and amplitude of the oscillation, which provides an important basis for the prevention and suppression of the oscillation.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a topological structure (A) of a PMSG-WT and its controller structure (B and D represent coordinate conversion, C represents a phase-locked loop, E represents an MSC controller, and F represents a GSC controller);

FIG. 2 shows a distribution diagram of G(s) poles;

FIG. 3 shows G(s) and −1/N(A) curves when an L_g value is changed;

FIG. 4a shows a simulation verification waveform (a) when Lg=0.1 mH;

FIG. 4b shows FFT spectrum analysis (b) when Lg=0.1 mH;

FIG. 5a shows the simulation verification waveform (a) when Lg=0.4 mH; and

FIG. 5b shows FFT spectrum analysis (b) when Lg=0.4 mH.

DESCRIPTION OF EMBODIMENTS

The topological structure of the PMSG-WT and its controller according to the present application are shown in FIG. 1. The PMSG-WT includes a wind turbine, a generator, an MSC, a DC capacitor, a GSC, and a filter. The wind turbine captures wind energy and converts the wind energy into mechanical energy, both the MSC and the GSC are two-level voltage source converters, wherein the MSC converts the alternating current output by the permanent magnet synchronous generator into direct current, and the GSC inverts the direct current into power frequency alternating current and integrates the power frequency alternating current into the power grid. Both the MSC and the GSC adopt a vector control method in a dq coordinate system, and convert three-phase voltage and current under the abc frame into the voltage and current under d-axis and q-axis through dq transformation. The MSC controller includes a power loop (P&O), a rotating speed loop (H_ω), and a machine-side current loop (H_cr); and the GSC controller includes a voltage loop (H_v) and a grid-side current loop (H_cg). In addition, the PMSG-WT also includes a phase-locked loop (PLL) and coordinate conversion links (abc/dq and dq/abc). Hereinafter, the present application will be further described in combination with specific embodiments:

In one embodiment of the present application, main parameters of the system are shown in Table 1.

TABLE 1
Main parameters of the system
	Parameter	Value
	Equivalent inductance of a weak	0.1	mH
	power grid Lg
	Voltage amplitude of an ideal	690{square root over (2)}/{square root over (3)}
	voltage source of the weak
	power grid Us
	Filtering inductance Lf	1.2	mH
	DC capacitor Cdc	2	mF
	Armature inductance of the	1.5028	mH
	generator Ls
	Rotor resistance of generator Rs	0.0224	Ω
	Permanent magnet flux	5.3445	Wb
	linkage ψf
	Power frequency angular	100π	rad/s
	frequency ω
	The number of pole pairs of the	48
	generator np
	Equivalent rotational inertia J	1417769.4
	Perturbation step length of a	0.0001	rad/s
	power loop ε
	Control period of the power	0.1	ms
	loop Tp
	Reference value of output power	0.688	MW
	of the PMSG-WT Pref
	Proportional parameter of a	10902
	rotating speed loop Kωp
	Integral parameter of the rotating	17875
	speed loop Kωi
	Proportional parameter of a	3.13
	machine-side current Kcpr
	Integral parameter of the	109.8
	machine-side current loop Kcir
	Proportional parameter of a	19.05
	grid-side current loop Kcpg
	Integral parameter of the	176
	grid-side current loop Kcig
	Proportional parameter of a	1.6
	grid-side voltage loop Kvp
	Integral parameter of the	28
	grid-side voltage loop Kvi
	Proportional parameter of the	2.1
	phase-locked loop Kppll
	Integral parameter of the	36.6
	phase-locked loop Kipll
	Reference value of DC voltage	1400	V
	Udcref
	Steady-state value of rotating	3	rad/s
	speed of the generator ωg*
	Linearization constant of a	−2.742 × 105
	mechanical torque of the wind
	turbine Bt
	Period of a power sampling	0.01
	filter Tf

In the embodiment of the present application, step 1, acquiring main parameters of the PMSG-WT, wherein the main parameters as shown in Table 1, establishing mathematical models of the wind turbine, the generator and the MSC, the MSC controller, the DC capacitor, the GSC and the filter, and the GSC controller, respectively, performing linearization in a dq coordinate system, and calculating steady-state operation parameters to obtain small-signal models of the wind turbine, the generator and the MSC, the MSC controller, the DC capacitor, the GSC and the filter, and the GSC controller:

$\begin{array}{c}\mathrm{sJ}{\mathrm{\Delta \omega }}_g=\Delta T_m-\Delta T_e\\ \Delta T_e=-\frac{3}{2}{n}_p{\psi }_f\Delta i_{\mathrm{qr}}\\ \Delta T_m=B_t{\mathrm{\Delta \omega }}_g\\ Z_{\mathrm{dqr}}·\Delta i_{\mathrm{dqr}}=-U_{\mathrm{dc}}^{*}·\Delta d_{\mathrm{dqr}}-D_{\mathrm{dqr}}^{*}·\Delta u_{\mathrm{dc}}+G_{\omega e}·{\mathrm{\Delta \omega }}_e\\ \Delta d_{\mathrm{dqr}}^c=\frac{1}{U_{\mathrm{dc}}^{*}}\left(G_{\mathrm{cr}}·\Delta i_{\mathrm{dqr}}^c+G_{\omega g}\left({\omega }_g^{\mathrm{ref}}-{\omega }_g\right)+G_{\omega e}·{\mathrm{\Delta \omega }}_e\right)\end{array}$ $\{\begin{array}{c}\Delta i_{\mathrm{dqr}}^c=G_{\theta e}^i·\Delta i_{\mathrm{dqr}}\\ \Delta d_{\mathrm{dqg}}^c=G_{\theta e}^d·\Delta i_{\mathrm{dqr}}+\Delta d_{\mathrm{dqg}}\end{array}$ ${\mathrm{sC}}_{\mathrm{dc}}\Delta u_{\mathrm{dc}}=1.5\left({D_{\mathrm{dqr}}^{*}}^T·\Delta i_{\mathrm{dqr}}+{I_{\mathrm{dqr}}^{*}}^T·\Delta d_{\mathrm{dqr}}\right)-1.5\left({D_{\mathrm{dqg}}^{*}}^T·\Delta i_{\mathrm{dqg}}+{I_{\mathrm{dqg}}^{*}}^T·\Delta d_{\mathrm{dqg}}\right)$ $\begin{array}{c}{Z}_f·\Delta i_{\mathrm{dqg}}=U_{\mathrm{dc}}^{*}·\Delta d_{\mathrm{dqg}}+D_{\mathrm{dqg}}^{*}·\Delta u_{\mathrm{dc}}-\Delta u_{\mathrm{dqg}}\\ \Delta d_{\mathrm{dqg}}^c·U_{\mathrm{dc}}^{*}=\Delta u_{\mathrm{dqg}}^c+G_{\mathrm{cg}}\Delta i_{\mathrm{dqg}}^c+G_{\mathrm{udc}}\Delta u_{\mathrm{dc}}\end{array}$ $\{\begin{array}{c}\Delta u_{\mathrm{dqg}}^c=G_{\mathrm{pll}}^u·\Delta u_{\mathrm{dqg}}\\ \Delta i_{\mathrm{dqg}}^c=G_{\mathrm{pll}}^i·\Delta u_{\mathrm{dqg}}+\Delta i_{\mathrm{dqg}}\\ \Delta d_{\mathrm{dqg}}^c=G_{\mathrm{pll}}^d·\Delta u_{\mathrm{dqg}}+\Delta d_{\mathrm{dqg}}\end{array}$

s represents a parameter introduced by the Laplace transform, J represents rotational inertia of an equivalent concentrated mass block of the wind turbine and the generator, ω_g represents a rotating speed of the generator, T_m represents a mechanical torque of the generator, and T_e represents an electromagnetic torque of the generator. n_p represents the number of pole pairs of the generator, ψ_f represents a permanent magnet flux linkage of the generator, and B_t represents a linearization constant of the mechanical torque of the wind turbine.

$Z_{\mathrm{dqr}}=\left[\begin{array}{cc}{\mathrm{sL}}_s+R_s& -{\omega }_e^{*}{L}_s\\ {\omega }_e^{*}{L}_s& {\mathrm{sL}}_s+R_s\end{array}\right],\Delta i_{\mathrm{dqr}}=\left[\begin{array}{c}\Delta i_{\mathrm{dr}}\\ \Delta i_{\mathrm{qr}}\end{array}\right],\Delta d_{\mathrm{dqr}}=\left[\begin{array}{c}\Delta d_{\mathrm{dr}}\\ \Delta d_{\mathrm{qr}}\end{array}\right],D_{\mathrm{dqr}}^{*}=\left[\begin{array}{c}{D}_{\mathrm{dr}}^{*}\\ D_{\mathrm{qr}}^{*}\end{array}\right],G_{\omega e}=\left[\begin{array}{c}{L}_s{I}_{\mathrm{qr}}^{*}\\ {\psi }_f-L_s{I}_{\mathrm{dr}}^{*}\end{array}\right],$

R_s and L_s represent rotor resistance and armature inductance of the generator, respectively, ω_e represents an electrical angular speed of the rotor, and ω_e=n_pω_g. i_dr and i_qr represent the stator current of the generator in the dq coordinate system, d_dr and d_qr represent output duty ratios of the MSC controller in the dq coordinate system, and u_dc represents a DC voltage.

$G_{\mathrm{cr}}=\left[\begin{array}{cc}{H}_{\mathrm{cr}}& {\omega }_e^{*}{L}_s\\ -{\omega }_e^{*}{L}_s& H_{\mathrm{cr}}\end{array}\right],G_{\omega g}=\left[\begin{array}{c}0\\ H_{\mathrm{cr}}{H}_{\omega }\end{array}\right],H_{\mathrm{cr}}=K_{\mathrm{cpr}}+\frac{K_{\mathrm{cir}}}{s},$

K_cpr and K_cir represent a proportional parameter and an integral parameter of machine-side current loop PI control, respectively,

$H_{\omega }=K_{\omega p}+\frac{K_{\omega i}}{s},$

K_ωp and K_ωi represent the proportional parameter and the integral parameter of rotating speed loop PI control, respectively, ω_g^ref represents the reference value of the rotating speed of the generator, ω*_e represents the steady-state value of the electrical angular speed of the rotor, and the superscript c represents a dq coordinate system of the MSC controller.

$G_{\theta e}^i=\left[\begin{array}{cc}1& H_{\theta e}{I}_{\mathrm{qr}}^{*}\\ 0& 1-H_{\theta e}{I}_{\mathrm{dr}}^{*}\end{array}\right],G_{\theta e}^d=\left[\begin{array}{cc}0& H_{\theta e}{D}_{\mathrm{qr}}^{*}\\ 0& -H_{\theta e}{D}_{\mathrm{dr}}^{*}\end{array}\right],H_{\theta e}=\frac{3n_p^2{\psi }_f}{2s\left(B_t-\mathrm{sJ}\right)}.I_{\mathrm{dqr}}^{*}\left[\begin{array}{c}{I}_{\mathrm{dr}}^{*}\\ I_{\mathrm{qr}}^{*}\end{array}\right],\Delta i_{\mathrm{dqg}}=\left[\begin{array}{c}\Delta i_{\mathrm{dg}}\\ \Delta i_{\mathrm{qg}}\end{array}\right],\Delta d_{\mathrm{dqg}}=\left[\begin{array}{c}\Delta d_{\mathrm{dg}}\\ \Delta d_{\mathrm{qg}}\end{array}\right],D_{\mathrm{dqg}}^{*}=\left[\begin{array}{c}{D}_{\mathrm{dg}}^{*}\\ D_{\mathrm{qg}}^{*}\end{array}\right],I_{\mathrm{dqg}}^{*}=\left[\begin{array}{c}{I}_{\mathrm{dg}}^{*}\\ I_{\mathrm{qg}}^{*}\end{array}\right],$

C_dc represents the DC capacitor, i_dg and i_qg represent d-axis current and q-axis current of an AC port of the GSC, respectively, and d_dg and d_qg represent the output duty ratios of the GSC controller in the dq coordinate system, respectively.

$Z_f=\left[\begin{array}{cc}{\mathrm{sL}}_f& -\omega L_f\\ \omega L_f& {\mathrm{sL}}_f\end{array}\right],\Delta i_{\mathrm{dqg}}=\left[\begin{array}{c}\Delta i_{\mathrm{dg}}\\ \Delta i_{\mathrm{qg}}\end{array}\right],\Delta d_{\mathrm{dqg}}=\left[\begin{array}{c}\Delta d_{\mathrm{dg}}\\ \Delta d_{\mathrm{qg}}\end{array}\right],D_{\mathrm{dqg}}^{*}=\left[\begin{array}{c}{D}_{\mathrm{dg}}^{*}\\ D_{\mathrm{qg}}^{*}\end{array}\right],\Delta u_{\mathrm{dqg}}=\left[\begin{array}{c}\Delta u_{\mathrm{dg}}\\ \Delta u_{\mathrm{qg}}\end{array}\right],$

L_f represents filtering inductance, ω represents power frequency angular frequency, ω=100π rad/s, i_dg and i_qg represent the d-axis current and the q-axis current of the AC port of the GSC, respectively, d_dg and d_qg represent the output duty ratios of the GSC controller in the dq coordinate system, respectively, and u_dg and u_qg represent a d-axis voltage and a q-axis voltage of a grid-connected point, respectively.

$G_{\mathrm{cg}}=\left[\begin{array}{cc}-H_{\mathrm{cg}}& -\omega L_f\\ \omega L_f& -H_{\mathrm{cg}}\end{array}\right],G_{\mathrm{udc}}=\left[\begin{array}{c}{H}_{\mathrm{cg}}{H}_v\\ 0\end{array}\right],H_{\mathrm{cg}}=K_{\mathrm{cpg}}+\frac{K_{\mathrm{cig}}}{s},$

K_cpg and K_cig represent the proportional parameter and the integral parameter of grid-side current loop PI control, respectively,

$H_v=K_{\mathrm{vp}}+\frac{K_{\mathrm{vi}}}{s},$

and K_vp and K_vi represent the proportional parameter and the integral parameter of grid-side voltage loop PI control, respectively.

$G_{\mathrm{pll}}^u=\left[\begin{array}{cc}1& G_{\mathrm{pll}}{U}_{\mathrm{qg}}^{*}\\ 0& 1-G_{\mathrm{pll}}{U}_{\mathrm{dg}}^{*}\end{array}\right],G_{\mathrm{pll}}^i=\left[\begin{array}{cc}0& G_{\mathrm{pll}}{I}_{\mathrm{qg}}^{*}\\ 0& -G_{\mathrm{pll}}{I}_{\mathrm{dg}}^{*}\end{array}\right],G_{\mathrm{pll}}^d=\left[\begin{array}{cc}0& G_{\mathrm{pll}}{D}_{\mathrm{qg}}^{*}\\ 0& -G_{\mathrm{pll}}{D}_{\mathrm{dg}}^{*}\end{array}\right],G_{\mathrm{pll}}=\frac{H_{\mathrm{pll}}}{s+U_{\mathrm{dg}}^{*}{H}_{\mathrm{pll}}},H_{\mathrm{pll}}=K_{\mathrm{ppll}}+\frac{K_{\mathrm{ipll}}}{s},$

K_ppll and K_ipll represent the proportional parameter and the integral parameter of a phase-locked loop PI controller, respectively.

Capital letters and superscripts * represent steady-state components of corresponding lowercase variables, and the specific calculation method is as follows:

$\{\begin{array}{c}{I}_{\mathrm{qr}}^{*}=3P^{\mathrm{ref}}/n_p{\Psi }_f{\omega }_g^{*}\\ I_{\mathrm{dr}}^{*}=0\\ U_{\mathrm{dc}}^{*}=U_{\mathrm{dcref}}\\ D_{\mathrm{dr}}^{*}=\left({\omega }_e{I}_{\mathrm{qr}}^{*}{L}_s-R_s{I}_{\mathrm{dr}}^{*}\right)/U_{\mathrm{dc}}^{*}\\ D_{\mathrm{qr}}^{*}=\left(-{\omega }_e{L}_s{I}_{\mathrm{dr}}^{*}+{\omega }_e{\Psi }_f-R_s{I}_{\mathrm{qr}}^{*}\right)/U_{\mathrm{dc}}^{*}\\ U_{\mathrm{dg}}^{*}=\sqrt{\left({\left(1.5U_s\right)}^2+\sqrt{{\left(1.5U_s\right)}^4-{\left(3P^{\mathrm{ref}}\omega L_g\right)}^2}\right)/4.5}\\ U_{\mathrm{qg}}^{*}=0\\ I_{\mathrm{dg}}^{*}=P^{\mathrm{ref}}/\left(1.5U_{\mathrm{dg}}^{*}\right)\\ I_{\mathrm{qg}}^{*}=0\\ D_{\mathrm{dg}}^{*}=\left(-I_{\mathrm{qg}}^{*}\omega L_f+U_{\mathrm{dg}}^{*}\right)/U_{\mathrm{dc}}^{*}\\ D_{\mathrm{dg}}^{*}=\left(I_{\mathrm{dg}}^{*}\omega L_f+U_{\mathrm{dg}}^{*}\right)/U_{\mathrm{dc}}^{*}\end{array}$

Step 2: modeling the power loop in the MSC controller based on a describing function method, wherein its mathematical expression is:

${\omega }_g^{\mathrm{ref}}=\{\begin{array}{cc}\frac{\epsilon }{s{T}_P}\mathrm{sgn}\left(P^{\mathrm{ref}}-P_n\right),& \left({\omega }_g\le {\omega }_{\mathrm{mpp}}\right)\\ -\frac{\epsilon }{s{T}_P}\mathrm{sgn}\left(P^{\mathrm{ref}}-P_n\right),& \left({\omega }_g>{\omega }_{\mathrm{mpp}}\right)\end{array}$

the sign function in the formula can be modeled by using a describing function, and the describing function is:

$N\left(A\right)=\frac{4}{\pi A}$

Step 3: considering the influence of the weak AC power grid, the weak AC power grid model is:

Z _g ·Δi _dqg =Δu _dqg

in the formula

$,Z_g=\left[\begin{array}{cc}{\mathrm{sL}}_g& -\omega L_g\\ \omega L_g& {\mathrm{sL}}_g\end{array}\right].$

The small-signal model of the output power of the PMSG-WT is:

ΔP=1.5(I*_dqg^T·Z_g+U*_dqg^T)·Δi_dqg

combing the linear parts of the weak power grid model and the power loop model with the small-signal model of the PMSG-WT in step 1, and deriving a transfer function G(s) of the linear part of the system:

$G\left(s\right)=-\frac{1.5\epsilon }{T_{P}s}\left(I_{\mathrm{dqg}}^{*T}{Z}_g+U_{\mathrm{dqg}}^{*T}\right)M_5^{-1}\left(D_{\mathrm{dqg}}^{*}+G_{\mathrm{udc}}\right)M_7^{-1}{M}_8·\frac{1}{1+T_{f}s}·\frac{1}{1+T_{P}s}$ $\mathrm{where}$ $M_8=-D_{\mathrm{dqr}}^{*T}·M_1^{-1}·G_{\omega g}+I_{\mathrm{dqr}}^{*T}·M_3$ $M_7=\left(\frac{2s{C}_{\mathrm{dc}}}{3}+I_{\mathrm{dqg}}^{*T}{M}_6-I_{\mathrm{dqr}}^{*T}{M}_4+D_{\mathrm{dqg}}^{*T}{M}_5^{-1}\left(D_{\mathrm{dqg}}^{*}+G_{\mathrm{udc}}\right)+D_{\mathrm{dqr}}^{*T}{M}_1^{-1}{D}_{\mathrm{dqr}}^{*}\right)$ $M_6=\left(\left(\left(G_{\mathrm{cg}}{G}_{\mathrm{plli}}-G_{\mathrm{plld}}{U}_{\mathrm{dc}}^{*}+G_{\mathrm{pllu}}\right)Z_g+G_{\mathrm{cg}}\right)M_5^{-1}\left(D_{\mathrm{dqg}}^{*}+G_{\mathrm{udc}}\right)+G_{\mathrm{udc}}\right)/U_{\mathrm{dc}}^{*}$ $M_5=-G_{\mathrm{cg}}+Z_f-\left(G_{\mathrm{cg}}{G}_{\mathrm{plli}}-G_{\mathrm{plld}}{U}_{\mathrm{dc}}^{*}+G_{\mathrm{pllu}}\right)Z_g+Z_g$ $M_4=-M_2{M}_1^{-1}{D}_{\mathrm{dqr}}^{*}/U_{\mathrm{dc}}^{*}$ $M_3=\left(-M_2{M}_1^{-1}{G}_{\omega g}+G_{\omega g}\right)/U_{\mathrm{dc}}^{*}$ $M_2=G_{\omega e}{G}_{iq\omega }{n}_p+G_{\mathrm{cr}}{G}_{\theta e}^i-G_{\omega g}{G}_{iq\omega }-U_{\mathrm{dc}}^{*}{G}_{\theta e}^d$ $M_1=G_{\mathrm{cr}}{G}_{\theta e}^i-G_{\omega g}{G}_{iq\omega }-U_{\mathrm{dc}}^{*}{G}_{\theta e}^d+z_{\mathrm{dqr}}$

Step 4: analyzing the stability of the system. Firstly, a G(s) pole diagram is drawn, as shown in FIG. 2, it can be seen that G(s) does not contain the pole of the right half plane (the real part is greater than 0), so the first condition for the stability of the system is satisfied. G(s) and −1/N(A) images are drawn in a complex plane, as shown in FIG. 3, G(s) and −1/N(A) intersect, indicating that the system is in a critically stable state at this time. It can be seen from calculation that, when Lg=0.1 mH, the oscillation frequency of the system is about 129 rad/s (20.5 Hz), and the oscillation amplitude is about 61 kW; and when Lg=0.4 m, the oscillation frequency of the system is about 131 rad/s (20.9 Hz), the oscillation amplitude is about 56 kW. When the power grid strength decreases (Lg increases), the oscillation amplitude of the system decreases, indicating that under certain conditions, the increase of the equivalent impedance of the power grid is conducive to maintaining the stability of the system.

FIGS. 4a, 4b, 5a and 5b show system simulation and FFT spectrum analysis results when Lg=0.1 mH and Lg=0.4 mH, respectively. In FIGS. 4a and 4b, the DC component is 0.6889 MW, the corresponding oscillation frequency is 19.5 Hz, and the amplitude is 58 kW, which are basically consistent with the theoretical analysis results; and in FIGS. 5a and 5b, the DC component is 0.6889 MW, the corresponding oscillation frequency is 19.5 Hz, and the amplitude is 53 kW, which are basically consistent with the theoretical analysis results. The simulation results verify the effectiveness and accuracy of the analysis method.

Claims

1. A method for analyzing stability of a PMSG-WT connected to a weak power grid considering influence of power control, wherein the PMSG-WT comprises a wind turbine, a generator, an MSC, a DC capacitor, a GSC, a filter, an MSC controller, and a GSC controller; the MSC controller comprises a power loop, a rotating speed loop, and a machine-side current loop; the GSC controller comprises a voltage loop and a grid-side current loop; and the method comprises the following steps: step 1: acquiring main parameters of the PMSG-WT, establishing mathematical models of the wind turbine, the generator and the MSC, the MSC controller, the DC capacitor, the GSC and the filter, and the GSC controller, respectively, performing linearization in a dq coordinate system, and calculating steady-state operation parameters to obtain small-signal models of the wind turbine, the generator and the MSC, the MSC controller, the DC capacitor, the GSC and the filter, and the GSC controller;step 2: modeling the power loop in the MSC controller based on a describing function method, wherein its mathematical expression is:

2. The method according to claim 1, wherein in step 1, the small-signal models of the wind turbine, the generator and the MSC, and the MSC controller are established as follows: the mathematical model of the wind turbine is established as: sJωg=TM−Te−Bωg in the formula, J represents rotational inertia of an equivalent concentrated mass block of the wind turbine and the generator, Tm represents a mechanical torque of the generator, Te represents an electromagnetic torque of the generator, B represents a self-damping coefficient, it is deemed that B=0 herein, and s represents a parameter introduced by the Laplace transform; linearization is performed on the model to obtain: sJΔωg=ΔTm−ΔTe the electromagnetic torque of the generator is: Te= 3/2npψfiqr np represents the number of the pole pairs of the generator, iqr represents stator current of the q-axis generator, and ψf represents a permanent magnet flux linkage of the generator, and this formula is linearized to obtain: ΔTe= 3/2npψfΔiqr the mechanical torque of the generator is: Tm=Btωg in the formula,

3. The method according to claim 2, wherein in step 1, the establishment process of the small-signal model of the DC capacitor is as follows: the mathematical model of the DC capacitor is established as: sCdcudc=idc2−idc1=1.5(ddridr+dqriqr)−1.5(ddgidg+dqgiqg)in the formula, Cdc represents the DC capacitor, idc1 represents grid-side DC current, idc2 represents machine-side DC current, and by linearizing the model, the small-signal model of the DC capacitor can be obtained as: sCdcΔudc=1.5(D*dqrT·Δidqr+I*dqrT·Δddqr)−1.5(D*dqgT·Δidqg+I*dqgT·Δddqg)in the formula,

4. The method according to claim 3, wherein in step 1, the establishment process of the small-signal models of the GSC and the filter, and the GSC controller is as follows: the mathematical model of the GSC and the filter is established as:

5. The method according to claim 4, wherein the step 3 specifically comprises: the AC weak power grid is expressed by series equivalent inductance of an ideal voltage source, and its mathematical model is established as: