CN112751346B - Design method of DFIG-PSS controller based on virtual impedance - Google Patents

Abstract

The invention provides a design method of a DFIG-PSS controller based on virtual impedance, which comprises the following steps: firstly, constructing a power grid system containing a double-fed induction motor model, combining a power system stabilizer and virtual impedance, connecting the power system stabilizer and the virtual impedance to a rotor side converter, and then obtaining the relation between the output voltage of the rotor side converter and the rotor current by adopting a stator flux linkage directional vector control technology; secondly, obtaining the relation between the stator current and the output power of the stator according to the relation among the stator voltage, the electronic flux linkage and the electronic current; then, obtaining a relation between a parameter of a power system stabilizer-virtual impedance and the output power of the stator according to the relation between the output voltage of the rotor side converter and the rotor current and the relation between the stator current and the output power of the stator; and finally, the output power of the stator is adjusted by changing the parameters of the power system stabilizer-virtual impedance, so that the damping characteristic of the power grid system is influenced. The invention can improve the low-frequency oscillation characteristic of the wind power system.

Description

A Design Method of DFIG-PSS Controller Based on Virtual Impedance

technical field

The invention relates to the technical field of wind power systems, in particular to a design method for a DFIG-PSS controller based on virtual impedance.

Background technique

The grid connection of large-scale wind turbines has brought new challenges to the stable operation of the power system. Wind turbines such as doubly fed induction generators (DFIG) will affect the original oscillation mode of the system and may introduce new oscillation modes. The dynamic stability of the system, especially the small disturbance stability will be adversely affected. Traditional synchronous generators use a power system stabilizer (PSS) to provide additional damping when the system is disturbed and improve the low-frequency oscillation characteristics of the system. With the increase of wind power penetration, the power grid requires wind turbines to also provide damping for the power system, and the study of additional damping control strategies similar to traditional synchronous generators has become crucial.

In recent years, a lot of research work has been carried out on the low-frequency oscillation problem caused by large-scale wind power grid-connected at home and abroad, and rich research results have been obtained. Studies have shown that the impact of large-capacity wind turbines on power systems cannot be ignored, so it is particularly important to study the impact of wind turbines, especially the widely used doubly-fed wind turbines, on the stability of small disturbances in interconnected systems. Literature[JoseLuisDominguez-Garc1, Oriol Gomis-Bellmunta, Fernando D.Bianchi, Andreas Summera. Power oscillation damping supported by wind power:Areview[J].Renewable and Sustainable Energy Reviews,2012,16(7):4982-4993.] The impact of wind power generation on small-signal stability is clarified, and control methods for wind turbines to suppress internal and power system oscillation modes are introduced. Literature [Liu Yong, Joe R. Gracia, Thomas J. King, Liu Yilu. Frequency regulation and oscillation damping contributions of variable-speed wind generators in the U.S. Eastern Interconnection (EI) [J]. IEEE Transactions on Sustainable Energy, 2015,6 (3):951-957.] presents fast active power control techniques for variable-speed wind turbines and discusses how these controls can be applied to frequency regulation and oscillation damping in the Eastern United States Interconnection System. Literature [Mohit Singh, AliciaJ.Allen, Eduard Muljadi, VahanGevorgian, Zhang Yingchen, Surya Santoso.Interareaoscillation damping controls for wind power plants.IEEE Transactions onSustainable Energy,2015,6(3):967-975.]Add the control signal to In the active power control loop, the small disturbance stability of the system is improved by adjusting the DFIG active power output. Document [Liao Kai, He Zhengyou, Xu Yan, GuoChen, Zhao Yangdong, Kit Po Wong. Asliding mode based damping control of DFIG for interarea power oscillations. IEEE Transactions on Sustainable Energy, 2017, 8(1): 258-267.] proposed A second-order sliding mode controller modulates DFIG reactive power output, document [FanLingling, YinHaiping, MiaoZhixin.On Active/Reactive Power Modulation of DFIG-Based Wind Generation for Interarea Oscillation Damping.IEEE Transactions onEnergy Conversion, 2011,26 (2):513-521] show that power modulation may cause shaft oscillation, and reactive power modulation is a better choice. Using PSS to suppress low-frequency oscillation is a common method in traditional power systems. DFIG can also adopt an additional damping control strategy similar to synchronous generators to improve the low-frequency oscillation characteristics of interconnected power systems, that is, DFIG-PSS. The literature [F.Michael Hughes, Olimpo Anaya-Lara, Nicholas Jenkins, Goran Strbac.Apowersystem stabilizer for DFIG-based wind generation.IEEE Transactions on PowerSystems,2006,21(2):763-772.] clarifies that double-fed wind power generation The addition of a properly designed power system stabilizer can significantly enhance the wind farm's contribution to grid damping and can be achieved without degrading the quality of voltage control provided. Literature[Bian Xiaoyan, Geng Yan, Yuan Fangqi, Li Xuewu, Fu Yang. Discussion on DFIG-PSS Input Signal Selection under Multiple Operation Modes[J].Journal of Electric Power System and Automation,2016,28(07):47-50 .] The influence of DFIG-PSS position and input on its damped oscillation is analyzed. Literature [Li Shenghu, Sun Qi, Shi Xuemei, Huang Jiejie. Weakly damped low-frequency oscillation mode suppression of wind power system based on regional pole configuration[J]. Power System Protection and Control, 2017,45(20):14-20.] Application of regional pole configuration Design DFIG-PSS by method. Literature [BianXiaoyan, Ding Yang, Jia Qingyu, Shi Lei, Zhang Xiaoping, L.LO Kwok. Mitigation of sub-synchronous control interaction of a power system with DFIG-based wind farm under multi-operating points [J]. IET Generation, Transmission & Distribution ,2018,12(21):5834-5842.] Based on subsynchronous control interaction probability sensitivity, select DFIG-PSS position and input, and optimize control parameters. Literature [Li Shenghu, Zhang Hao. Sensitivity Analysis of Wind Power System Oscillation Modes to DFIG-PSS Transfer Function[J]. Power System Protection and Control, 2020,48(16):11-17.] Establishing the State Equation of DFIG-PSS System , the eigenvalue increment is introduced into the incremental iterative operation of the transfer function, and the control effect of the correspondingly designed DFIG-PSS is compared with the parameter sensitivity and the transfer function sensitivity error through simulation.

When wind power is connected to the grid, the output voltage of the system has certain requirements on the output impedance characteristics, and the corresponding virtual impedance (VI) control strategy can be used to make the equivalent output impedance present the expected characteristics of the system. Literature [Zhou Peng, Zhang Xinyan, Di Qiang, Yue Jiahui, Xing Chen. Pre-synchronization grid connection strategy of doubly-fed wind turbine based on virtual synchronous machine control[J]. Automation of Electric Power Systems, 2020,44(14):71-84. ] proposed a pre-synchronization control strategy for doubly-fed wind turbines controlled by a virtual synchronous generator (VSG) without a phase-locked loop, and introduced a virtual impedance in the reactive power control loop, and replaced the reference input with a virtual current to achieve Doubly-fed wind turbines are quickly and efficiently connected to the grid smoothly. Literature [Ju Yuntao, Ma Yarong, Qi Zhinan. Research on the Influence Mechanism of Virtual Synchronous Machine on Small Disturbance Stability Based on Damping Torque Analysis[J/OL]. Chinese Journal of Electrical Engineering, 1-10[2020-11-30].] The influence mechanism of the virtual synchronous control strategy on the small-disturbance stability is clarified, and it is found that increasing the inductance of the virtual impedance link is beneficial to the small-disturbance stability of the system. Literature [Li Hui, Wang Kun, Hu Yu, Wang Xiao, Xia Guisen. Impedance Modeling and Stability Analysis of Virtual Synchronous Control of Double-fed Wind Power System[J]. Chinese Journal of Electrical Engineering, 2019, 39(12): 3434-3443 .] Based on the virtual synchronous control strategy of double-fed wind power system, a virtual synchronous control strategy in the d-q rotating coordinate system is proposed.

The traditional PSS adopts open-loop control, which is easy to generate some oscillation signals that adversely affect the system. When the grid-connected inverter operates with a nonlinear load, the system output current contains harmonics, and the output impedance presents non-resistive characteristics, and the virtual impedance self-calibration can be used to adjust the output impedance characteristics.

Contents of the invention

Aiming at the shortcomings in the above-mentioned background technology, the present invention proposes a DFIG-PSS controller design method based on virtual impedance, which solves the technical problem of low-frequency oscillation in the existing wind power system.

Technical scheme of the present invention is realized like this:

A kind of DFIG-PSS controller design method based on virtual impedance, its steps are as follows:

Step 1: Construct a power grid system with a doubly-fed induction motor model, where the doubly-fed induction motor model includes a rotor-side converter and a wind turbine, and the rotor-side converter controls the wind turbine through the stator and rotor;

Step 2: Connect the power system stabilizer and virtual impedance to the rotor-side converter, and use the stator flux linkage oriented vector control technology to obtain the relationship between the output voltage of the rotor-side converter and the rotor current;

Step 3: Obtain the relationship between the stator current and the output power of the stator according to the relationship between the stator voltage, the stator flux linkage and the stator current;

Step 4: According to the relationship between the output voltage of the rotor-side converter and the rotor current and the relationship between the stator current and the output power of the stator, the relationship between the parameters of the power system stabilizer-virtual impedance and the output power of the stator is obtained;

Step 5: Adjust the output power of the stator by changing the parameters of the power system stabilizer-virtual impedance, thereby affecting the damping characteristics of the grid system.

The relationship between the output voltage of the rotor-side converter and the rotor current is:

表示转子无功功率参考值，Q_s表示转子无功功率测量值，i_rq表示q轴转子电流，s表示微分算子，u_errs为电力系统稳定器-虚拟阻抗经自动电压调节器装置后的输入信号。Among them, K _p1 , K _i1 , K _p3 , K _i3 , K _q1 , and K _i2 are the parameters of the rotor-side converter control proportional integral controller, V _rd represents the d-axis rotor voltage, and P _s ^* represents the rotor active power reference value, P _s represents the rotor active power measurement value, i _rd represents the d-axis rotor current, V _rq represents the q-axis rotor voltage,

Indicates the reference value of rotor reactive power, Q _s indicates the measured value of rotor reactive power, i _rq indicates the q-axis rotor current, s indicates the differential operator, u _errs is the power system stabilizer-virtual impedance after the automatic voltage regulator device input signal.

The input signal of the power system stabilizer-virtual impedance after the automatic voltage regulator device is:

In the formula, T _e , K _f1 , T _c , T _b , T _f1 , K _a , T _a , T _r , K are the control parameters of the automatic voltage regulator, and △P represents the rotor active power reference value P _s ^* and The difference of the rotor active power measurement value P _s , u represents the stator voltage of the double-fed induction motor model, G ₁ (s) represents the transfer function of the power system stabilizer-virtual impedance.

The relationship between the stator current and the output power of the stator is:

In the formula: L _m is the mutual inductance of the stator and rotor coaxial equivalent windings in the dq coordinate system, L _s is the self-inductance of the stator equivalent two-phase windings in the dq coordinate system, ω _s is the rotational angular velocity of the synchronous magnetic field, and P _s ' is the active power of the stator Output power, Q _s 'is the reactive output power of the stator, _ids represents the d-axis stator current, i _qs represents the q-axis stator current, u _ds represents the d-axis stator voltage, U _s represents the stator voltage.

The stator current is:

The relationship between the parameters of the power system stabilizer-virtual impedance and the output power of the stator is:

Compared with prior art, the beneficial effect that the present invention produces is:

1) The present invention adds a virtual impedance link on the basis of the traditional PSS, adopts a closed-loop negative feedback control method to enhance the controller’s anti-interference ability to the outside world, and verifies the improvement of the controller by the virtual impedance link by performing a step response test on the controller effect.

2) Build the DFIG-PSS-VI model of the present invention in the DIgSILENT/PowerFactory simulation software, and take the 4-machine 2-area system as an example, add the built controller into the reactive power control loop of the DFIG rotor side controller, and pass the time domain The simulation verifies that the designed controller can improve the low-frequency oscillation characteristics of the system.

3) The overshoot based on the virtual impedance controller is lower than that of the traditional controller, and it has a good step response characteristic, and has a more obvious effect on improving the stability of the system.

4) The addition of PSS-based reactive power control loop on the rotor side of DFIG can improve the low-frequency oscillation characteristics of the power system including wind power, and it also has a certain effect when the power of the tie line changes.

5) DFIG-PSS-VI gain and virtual impedance inductance have a certain influence on the system damping, which has a limited effect on improving intra-regional oscillations, but it provides a new idea for suppressing inter-regional low-frequency oscillations of regional interconnection systems containing wind power.

Description of drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings that need to be used in the description of the embodiments or the prior art. Obviously, the accompanying drawings in the following description are only These are some embodiments of the present invention. Those skilled in the art can also obtain other drawings based on these drawings without creative work.

Figure 1 is a virtual impedance vector diagram.

Figure 2 is the PSS-VI step response curve.

Figure 3 is the PSS-VI unit step response curve.

Fig. 4 is a control structure diagram of DFIG-PSS-VI of the present invention.

Fig. 5 is a block diagram of the 4-machine 2-area system including DFIG of the present invention.

Fig. 6 is the influence distribution of the PSS-VI gain K ₁ of the present invention on the characteristic root of the inter-area oscillation mode of the 4-machine 2-area system containing DFIG.

Fig. 7 is the influence distribution of the PSS-VI parameter K _L of the present invention on the characteristic root of the inter-area oscillation mode of the 4-machine 2-area system containing DFIG.

Fig. 8 is the load fluctuation response curve after adding PSS-VI to the 4-machine 2-area system containing DFIG of the present invention, wherein (a) is the power angle response curve of generator _G1 , and (b) is the voltage response of generator _G1 curve.

Fig. 9 is the three-phase short-circuit response curve after adding PSS-VI to the 4-machine 2-area system containing DFIG of the present invention, wherein (a) is the power angle response curve of generator _G1 , and (b) is the voltage of generator _G1 response curve.

Detailed ways

The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only some, not all, embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without making creative efforts belong to the protection scope of the present invention.

Virtual impedance is a control method that introduces a feedback loop in the control link to simulate the actual line impedance. The implementation method of virtual impedance is more flexible. When the output impedance of the controller or the line impedance is small, the virtual impedance can be introduced to compensate. By selecting a reasonable virtual impedance Z _vi , it forms a new output impedance Z _i with the output impedance Z _oi of the PSS, and then changes the nature of the system impedance to achieve the purpose of improving system stability.

According to the stability criterion of the open-loop system, if you want to improve the system stability, you can start by increasing the system damping and reducing the system overshoot. It can be seen from Figure 1 that when Z _vi changes, the phase angle of Z _i can be changed , by changing the virtual impedance to adjust the PSS-VI output impedance, thereby ensuring a good unit step response of the controller. The use of closed-loop feedback links can ensure that the controller has higher precision, enhance the anti-interference ability of the controller, and ensure the stability of the system with small disturbances.

PSS-VI is shown in Figure 2, and its transfer function is:

In the formula, K _L and K _r are the parameters of the virtual impedance link; K ₁ is the gain of the PSS-VI controller, G ₁ (s) is the transfer function of the PSS-VI, T _W is the time constant of the filter link, T ₁ , T ₂ , T ₃ and T ₄ represent the time constant of the compensation link.

Figure 3 shows the unit step response of PSS-VI. It can be seen that although PSS-VI adopts the closed-loop negative feedback control method, there are no problems such as overshoot and oscillation that are common in closed-loop systems, and it has good step response characteristics. The parameter K _L of the virtual impedance link can adjust the unit step response of the controller.

The embodiment of the present invention provides a method for designing a DFIG-PSS controller based on virtual impedance, and the specific steps are as follows:

Step 1: Construct a power grid system with a double-fed induction motor model, where the double-fed induction motor model includes the rotor-side converter and the wind turbine, and the rotor-side converter controls the wind turbine through the stator and rotor; the double-fed generator adopts vector The control strategy realizes the decoupling control of active power and reactive power, and considers the maximum wind power and reactive power compensation, as shown in Figure 4. The controller adopts the traditional nested loop structure, that is, the outer speed/active power and reactive power control loop and the inner d-q rotor current control loop, which can effectively estimate the rotor d-q reference current from the actual reference value and reactive power reference value, The power loop controller of the nested control structure is omitted.

The DFIG with basic power and voltage control loops has a small damping effect on the electromechanical oscillation mode of the grid, but by introducing additional damping control, the damping effect can be significantly enhanced. The additional damping control signal is added to the DFIG active power control loop or reactive power control loop through an automatic voltage regulator (AVR).

It is more effective to change the electromagnetic torque of the generator through active power regulation, but the active power regulation will deteriorate the dynamic characteristics of the DFIG shaft, and the reactive power regulation may deteriorate the dynamic characteristics of the stator voltage. Sometimes, DFIG needs to sacrifice part of its own dynamic characteristics for damping control. In actual implementation, damping control and system dynamic performance need to be considered at the same time.

Step 2: Connect the power system stabilizer and virtual impedance to the rotor side converter, and use the stator flux oriented vector control technology to obtain the relationship between the output voltage of the rotor side converter and the rotor current; PSS-VI input Signal selection has a great influence on the performance of the controller. The input signal can be selected from rotor speed, voltage, current or frequency. Based on residual analysis is the most commonly used signal selection method. When the output signal is added to the quadrature component of the rotor voltage, that is, the reactive power control loop, the difference ΔP between the active power reference value P _s ^* of DFIG and the active power measurement value P _s can be selected as the input signal. In the present invention, the PSS-VI output control signal U _PSS-VI is added into the reactive power control loop, and the sum is obtained to obtain the quadrature component of the rotor voltage.

The DFIG rotor side converter adopts the stator flux oriented vector control technology, and the relationship between the rotor current and the RSC output voltage is:

After adding the PSS-VI controller, the relationship between the rotor current and the RSC output voltage can be obtained from formula (2):

Among them, K _p1 , K _i1 , K _p3 , K _i3 , K _q1 , and K _i2 are the parameters of the rotor-side converter control PI (proportional-integral) controller, V _rd represents the d-axis rotor voltage, and P _s ^* represents the rotor Active power reference value, P _s means rotor active power measurement value, i _rd means d-axis rotor current, V _rq means q-axis rotor voltage, Q _s ^* means rotor reactive power reference value, Q _s means rotor reactive power measurement value , i _rq represents the q-axis rotor current, s represents the differential operator, u _errs is the input signal of the power system stabilizer-virtual impedance after the automatic voltage regulator device.

u _errs is the input signal of the power system stabilizer-virtual impedance after passing through the AVR device.

In the formula, T _e , K _f1 , T _c , T _b , T _f1 , K _a , T _a , T _r , K are all AVR control parameters, U _PSS-VI means PSS-VI output signal, u means DFIG stator voltage .

Step 3: Obtain the relationship between the stator current and the output power of the stator according to the relationship between the stator voltage, the stator flux linkage and the stator current;

Under stator voltage orientation, the d-axis of the synchronous rotating coordinate system coincides with the stator voltage vector u _s direction, and the stator magnetic flux vector coincides with the negative direction of the q-axis after ignoring the stator resistance, lagging the stator voltage vector by 90°. From the voltage, flux linkage and current relationship on the stator side, the stator current and power can be expressed as:

In the formula: L _m is the mutual inductance of the stator and rotor coaxial equivalent windings in the dq coordinate system, L _s is the self-inductance of the stator equivalent two-phase windings in the dq coordinate system, ω _s is the rotational angular velocity of the synchronous magnetic field, and P _s ' is the active power of the stator Output power, Q _s 'is the reactive output power of the stator, _ids represents the d-axis stator current, i _qs represents the q-axis stator current, u _ds represents the d-axis stator voltage, U _s represents the stator voltage.

Step 4: According to the relationship between the output voltage of the rotor-side converter and the rotor current and the relationship between the stator current and the output power of the stator, the relationship between the parameters of the power system stabilizer-virtual impedance and the output power of the stator is obtained;

According to formulas (4)-(6), the relationship between the output power of the stator and the parameters of the PSS-VI controller can be obtained as follows:

Step 5: Adjust the output power of the stator by changing the parameters of the power system stabilizer-virtual impedance, thereby affecting the damping characteristics of the grid system. According to formula (7), when the PSS-VI parameters are changed, the output reactive power of DFIG will change accordingly, which will affect the damping characteristics of the system.

Simulation

In order to study the effectiveness of the method of the present invention in improving the low-frequency oscillation characteristics of power systems containing wind power, the DFIG-PSS-VI model is built in the DigSilent/PowerFactory simulation software, and the characteristic root analysis and Time domain simulations are analyzed. The base capacity of this system is 100MVA, the frequency is 50Hz, and the transmission power of the tie line is 400MW. Area 1 and area 2 of this system are connected by double-circuit connecting lines. G ₁ , G ₂ , G ₃ , and G ₄ are 4 thermal power units with a rated capacity of 900MVA and 20kV, among which node 3 is the reference node, and the active power of each generating set The output is 700MW. The DFIG access point is the busbar 10, and the system diagram of this system is shown in Figure 5.

In this simulation experiment, the influence of the controller gain is first studied, and the PSS-VI controller gain K ₁ is adjusted under the premise of keeping other control parameters unchanged. From formula (7) and formula (4), it can be seen that when K ₁ decreases, the DFIG output has no The work power decreases accordingly. Figure 6 shows the distribution of the characteristic roots of the system's inter-regional oscillation mode. It can be seen from Fig. 6 that as the PSS-VI controller gain decreases, the system's inter-regional oscillation mode eigenvalues tend to shift to the left. The damping ratio tends to increase.

PSS-VI controller gain K ₁ = 0.45, keep other parameters unchanged, change the virtual impedance link parameter K _L , Fig. 7 shows the distribution of the characteristic root of the oscillation mode between the system regions, it can be seen from Fig. 7 that with the PSS- As the parameter K _L of the virtual impedance link of the VI controller decreases, the eigenvalues of the system's interregional oscillation mode eigenvalues tend to shift to the left, and the damping ratio increases.

According to the previous analysis, the PSS-VI parameters are taken as: T _W =0.01, T ₁ =0.2, T ₂ =0.08, T ₃ =0.06, T ₄ =0.4, K ₁ =0.45, K _r =0.2, K _L =0.5. Table 1 shows some eigenvalues of the system before and after the PSS-VI controller is installed. Mode 1 and Mode 4 are inter-area oscillation modes, and Mode 2 and Mode 3 are intra-area oscillation modes. It can be seen that the damping ratio of interregional oscillation mode 1 is significantly increased, and the robustness of the system is improved.

Table 1 System oscillation mode after installing PSS-VI

Assuming that the active power of the system load _L1 steps 5% in 1s, the system load recovers in 1.5s, the transmission power of the tie line is 400MW, and the simulation time is 20s. Figure 8 shows that when the load fluctuation occurs, the system adds PSS-VI The relative power angle δ of generator G ₁ and the voltage change curve of bus 6 before and after the controller.

Assuming that a three-phase short circuit occurs between tie lines 6 and 7 at t=1s, the fault is cleared at t=1.1s, and the simulation time is 20s, Fig. 9 shows the relative power angle δ of generator G1 and the frequency variation curve of bus 6.

It can be seen from Figures 8 and 9 that when different types of disturbances occur in the system, the relative power angle δ of generator G ₁ and the voltage oscillation amplitude of bus 6 all decrease to varying degrees after adding the PSS-VI controller to the system, which shows that The addition of the PSS-VI controller has a certain inhibitory effect on the system oscillation and improves the damping characteristics of the wind power system.

Changes in generator output or load power will change the transmission power of the tie line, and even change the power transmission direction of the tie line. In order to further verify the established model, it is considered to change the power of the tie line by changing the output of the synchronous generator, and the improvement effect is studied under different power of the tie line.

Table 2 Oscillation modes of the system under different tie line transmission power

Adjust the tie line power to 300MW, 450MW, and 600MW respectively, and transmit power from area 1 to area 2. Table 2 gives the system characteristic values at different tie line transmission powers. It can be seen from Table 2 that after the PSS-VI controller is installed, the damping ratio change of the oscillation mode 1 between the system areas is improved under the condition of different transmission power of the tie line.

Aiming at the problem of low-frequency oscillation in the wind power grid-connected system, the present invention constructs a DFIG-PSS controller based on virtual impedance, and then analyzes the step response of the built controller to verify the promotion effect of the virtual impedance link on the controller. Build the designed DFIG-PSS-VI model in the DIgSILENT/PowerFactory simulation software, and take the 4-machine 2-area system as an example, add the built controller to the reactive power control loop of the DFIG rotor side controller, and verify it through time domain simulation The designed controller can improve the low-frequency oscillation characteristics of the system. The present invention has the following advantages:

(1) The overshoot of the virtual impedance controller is lower than that of the traditional controller, and it has a good step response characteristic, which can improve the stability of the system more obviously.

(2) The addition of PSS-based reactive power control loop on the rotor side of DFIG can improve the low-frequency oscillation characteristics of the power system including wind power, and it also has a certain effect when the power of the tie line changes.

(3) DFIG-PSS-VI gain and virtual impedance inductance have a certain influence on the system damping, which has a limited effect on improving intra-regional oscillations, but it provides a new idea for suppressing inter-regional low-frequency oscillations of regional interconnection systems containing wind power.

The above descriptions are only preferred embodiments of the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included in the scope of the present invention. within the scope of protection.

Claims (4)

1. a kind of DFIG-PSS controller design method based on virtual impedance, it is characterized in that, its steps are as follows: Step 1: Construct a power grid system with a doubly-fed induction motor model, where the doubly-fed induction motor model includes a rotor-side converter and a wind turbine, and the rotor-side converter controls the wind turbine through the stator and rotor; Step 2: Connect the power system stabilizer and virtual impedance to the rotor-side converter, and use the stator flux linkage oriented vector control technology to obtain the relationship between the output voltage of the rotor-side converter and the rotor current; The relationship between the output voltage of the rotor-side converter and the rotor current is:

Among them, K _p1 , K _i1 , K _p3 , K _i3 , K _q1 , and K _i2 are the parameters of the rotor-side converter control proportional integral controller, V _rd represents the d-axis rotor voltage, and P _s ^* represents the rotor active power reference value, P _s represents the rotor active power measurement value, i _rd represents the d-axis rotor current, V _rq represents the q-axis rotor voltage,

Indicates the reference value of rotor reactive power, Q _s indicates the measured value of rotor reactive power, i _rq indicates the q-axis rotor current, s indicates the differential operator, u _errs is the power system stabilizer-virtual impedance after the automatic voltage regulator device input signal; The input signal of the power system stabilizer-virtual impedance after the automatic voltage regulator device is:

In the formula, T _e , K _f1 , T _c , T _b , T _f1 , K _a , T _a , T _r , K are the control parameters of the automatic voltage regulator, and △P represents the rotor active power reference value P _s ^* and The difference of the rotor active power measurement value P _s , u represents the stator voltage of the doubly-fed induction motor model, G ₁ (s) represents the transfer function of the power system stabilizer-virtual impedance;

In the formula, K _L and K _r are parameters of the virtual impedance link; K ₁ is the gain of the PSS-VI controller, T _W is the time constant of the filter link, T ₁ , T ₂ , T ₃ , and T ₄ are the time constants of the compensation link; Step 3: Obtain the relationship between the stator current and the output power of the stator according to the relationship between the stator voltage, the stator flux linkage and the stator current; Step 4: According to the relationship between the output voltage of the rotor-side converter and the rotor current and the relationship between the stator current and the output power of the stator, the relationship between the parameters of the power system stabilizer-virtual impedance and the output power of the stator is obtained; Step 5: Adjust the output power of the stator by changing the parameters of the power system stabilizer-virtual impedance, thereby affecting the damping characteristics of the grid system. 2. the DFIG-PSS controller design method based on virtual impedance according to claim 1, is characterized in that, the relation of the output power of described stator current and stator is:

In the formula: L _m is the mutual inductance of the stator and rotor coaxial equivalent windings in the dq coordinate system, L _s is the self-inductance of the stator equivalent two-phase windings in the dq coordinate system, ω _s is the rotational angular velocity of the synchronous magnetic field, and P _s ' is the active power of the stator Output power, Q _s 'is the reactive output power of the stator, _ids represents the d-axis stator current, i _qs represents the q-axis stator current, u _ds represents the d-axis stator voltage, U _s represents the stator voltage. 3. the DFIG-PSS controller design method based on virtual impedance according to claim 2, is characterized in that, described stator current is:

4. the DFIG-PSS controller design method based on virtual impedance according to claim 2, is characterized in that, the relation of the parameter of described power system stabilizer-virtual impedance and the output power of stator is:

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