CN109039180A - The fractional order control method of double fed induction generators and network process - Google Patents

Abstract

The invention discloses the fractional order control methods of double fed induction generators and network process, comprising: construction motor control model constructs fractional order operator；For dual feedback wind power generation system, rationalization realization is carried out to fractional order operator using continuous transfer function method, i.e., by a Rational Transfer, makes it can effective simulation fractional order operator on amplitude-frequency characteristic and phase frequency feature；Establish fractional order PI^λController: controlling motor control model, controls double fed induction generators idle grid connection process.Fractional order PI^λStartup stage active power and reactive power can be effectively reduced in controller, and system is made smoothly to enter steady-state process, avoid power grid by large impact；Under short trouble state, using fractional order PI^λControl method also can be to the concussion of the system of reduction.

Description

Fractional-order control method for doubly-fed induction generator grid-connected process

technical field

The invention relates to the technical field of control, in particular to a fractional-order control method for a doubly-fed induction generator grid-connected process.

Background technique

Doubly-fed induction generator (DFIG) is widely used in variable-speed constant-frequency wind power generation systems because of its flexible frequency control, good regulation performance, good dynamic and steady-state characteristics, and decoupling control of active and reactive power. and ship power systems. The control of the doubly-fed induction generator is realized through the control of the rotor AC excitation converter. The stator and rotor system of the doubly-fed induction motor is a strongly coupled system. In order to achieve decoupling control, the vector control method of stator flux orientation is often used to decompose the AC into active power and reactive power, and a double closed-loop PI regulator is used to separate Take control of it.

PI control is still the most commonly used control method in doubly-fed power generation control systems. It has the advantages of simple structure, convenient implementation, and wide adaptability. However, this control method is based on an accurate model, and the working conditions of the controlled object change After that, its control performance will decrease accordingly. Considering the complex dynamic part, the double-fed power generation system actually becomes a multivariable nonlinear system. At this time, the traditional PI control system has disadvantages such as poor dynamic performance, large overshoot and weak anti-interference ability. protrude.

In view of the shortcomings of traditional PI control, the improvement schemes involved in domestic and foreign literature mainly include:

1. Use the RBF neural network to adjust the PID controller parameters online to deal with the influence of system parameter uncertainty and external disturbance on the control performance, but it requires more historical data and the training process is longer.

2. A neural network-based integral sliding mode control strategy is proposed. It is verified that the method has strong robustness to disturbances and has good grid-connected performance. However, the chattering phenomenon of the sliding mode control strategy is not can be solved.

3. Propose a new excitation control strategy with variable parameter PI and neural network coordinated control. Its control effect does not depend on system parameters and has good dynamic adjustment and online decoupling capabilities. Lag and other disadvantages.

4. The speed sensorless doubly-fed induction generator control technology based on model reference self-adaptation is studied, but this control idea lacks a systematic design method and the control accuracy is relatively low.

In the field of motors, some scholars have also applied fractional-order proportional-integral controllers to maximum power tracking in permanent magnet synchronous power generation systems, and verified that fractional-order PI controllers have faster response speed and higher power output performance. Or design a fractional order PID controller for the excitation system of permanent magnet synchronous generator, and optimize the parameters with particle swarm algorithm. However, the improvement of PI control for doubly-fed induction generators is seldom reported.

Contents of the invention

In order to solve the deficiencies of the prior art, the present invention provides a fractional-order control method for the doubly-fed induction generator grid-connected process. The present invention introduces an adjustable integral order λ to increase its control dimension. It has certain development potential and application value to effectively weaken the motor power oscillation under grid-connected transient process and fault disturbance.

Fractional-order control method for doubly-fed induction generator grid-connected process, including:

A motor control model is constructed, the model is established based on a synchronous rotating coordinate system, and is composed of a voltage equation, a flux linkage equation and an electromagnetic torque equation;

Construct a fractional operator: Based on the Grunwald-Letnikov fractional calculus definition as a theoretical basis, determine the fractional differential calculation algorithm, find the approximate value of the numerical differential of the function for the fractional differential calculation algorithm and prove its calculation accuracy, according to the Grunwald-Letnikov fraction First-order calculus is defined to calculate its fractional derivative;

For the doubly-fed wind power generation system, the continuous transfer function method is used to rationalize the fractional order operator, that is, through a rational transfer function, it can effectively simulate the fractional order operator in terms of amplitude-frequency characteristics and phase-frequency characteristics;

Establish a fractional order PI ^λ controller: control the motor control model, and control the no-load grid connection process of the doubly-fed induction generator.

In a further preferred technical solution, the fractional-order integral term I ^λ in the fractional-order PI ^λ controller is obtained by an improved Oustaloup filtering algorithm.

A further preferred technical solution is to use the improved Oustaloup filter algorithm to construct fractional operators.

A further preferred technical solution, the doubly-fed induction generator is based on the mathematical model of the synchronous rotating coordinate system:

Voltage equation:

Among them, ω _s is slip angular velocity; ω _s ＝ω ₁ -ω _r , u _ds : stator voltage d-axis component, u _qs : stator voltage q-axis component, u _dr : rotor voltage d-axis component, u _qr : rotor voltage q-axis component, i _ds : stator current d-axis component, i _qs : stator current q-axis component, i _dr : rotor current d-axis component, i _qr : rotor current q-axis component, R _s : stator side resistance, R _r : Rotor side resistance, Ψ _qs : stator flux q-axis component, Ψ _ds : stator flux d-axis component, Ψ _qr : rotor flux q-axis component, Ψ _dr : rotor flux d-axis component, p: differential sign.

Flux linkage equation:

Electromagnetic torque equation:

T _e =n _p L _m (i _ds i _qr −i _qs i _dr ) (3).

As a further preferred technical solution, in the start-up phase of the doubly-fed wind turbine, it is necessary to perform no-load grid connection control to make the motor smoothly cut into the grid, and the amplitude of each electrical component is required to be as small as possible. When the generator is no-load, the stator current components are all 0, that is, i _ds =i _qs =0. The no-load mathematical model of the doubly-fed induction generator is as follows:

T _e =0 (6)

In the formula, T ₀ —generator no-load torque.

In a further preferred technical solution, the simplified equation of the motor model ignoring the motor stator resistance is:

Further preferred technical solution, Grunwald-Letnikov fractional calculus definition:

A further preferred technical solution, the fractional order differential calculation algorithm is:

In the formula, is the polynomial coefficient of the function, assuming that the step size h is small enough, the approximate value of the numerical differentiation of the function can be directly obtained according to (12), and it can be proved that the calculation accuracy is o(h), when the function expression of f(t) is determined , the fractional derivative can be calculated directly from formula (11).

Further preferred technical scheme, PI ^λ controller function is:

Among them, λ is the integration order.

In a further preferred technical solution, the control steps of the PI ^λ controller are as follows: first detect the voltage and current of the stator and rotor of the motor, and then perform coordinate transformation to calculate the stator flux linkage Ψ ₁ , stator active power P and reactive power Q; In fact, it is necessary to set the stator active power command P* and reactive power command Q*, and compare them. The difference obtained is obtained by the fractional order PI ^λ regulator to obtain the active component command i _qs * and the reactive component command i _ds * of the stator current. Then enter the current inner loop control.

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

(1) The use of fractional order PI ^λ controller can make the control system of double-fed wind power generation system flexible to deal with nonlinear control objects, and can enhance the robustness in the dynamic process.

(2) The fractional-order PI ^λ controller can effectively reduce the active power and reactive power in the start-up phase, so that the system can enter the steady-state stage smoothly and avoid excessive impact on the power grid; in the short-circuit fault state, the fractional-order PI ^λ control method is adopted It can also reduce system shock.

Description of drawings

The accompanying drawings constituting a part of the present application are used to provide further understanding of the present application, and the schematic embodiments and descriptions of the present application are used to explain the present application, and do not constitute improper limitations to the present application.

Figure 1 is a dual PWM converter;

Figure 2 is a block diagram of the power outer loop PIλ control;

Figure 3 is a doubly-fed induction motor RSC control structure model;

Figure 4 Active power under integer order and fractional order;

Figure 5 Reactive power at integer and fractional orders;

Figure 6 shows the active power oscillation in the state of three-phase short-circuit fault.

Detailed ways

It should be pointed out that the following detailed description is exemplary and intended to provide further explanation to the present application. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs.

It should be noted that the terminology used here is only for describing specific implementations, and is not intended to limit the exemplary implementations according to the present application. As used herein, unless the context clearly dictates otherwise, the singular is intended to include the plural, and it should also be understood that when the terms "comprising" and/or "comprising" are used in this specification, they mean There are features, steps, operations, means, components and/or combinations thereof.

In a typical implementation of the present application, the steps of the present invention are:

(1) Construct the motor control model

Double-fed induction generators use dual PWM converters for AC excitation, which are called grid-side PWM converters (GSC) and rotor-side PWM converters (RSC), respectively. As shown in Figure 1, from the circuit structure, GSC and RSC are decoupled through the DC bus and independent of each other, so the mathematical models and control strategies of the grid side and the rotor side can be studied separately.

GSC has the following functions: it can ensure that the waveform of the input current is close to sinusoidal, and the use of GSC can ensure low harmonic content and meet the requirements of the power factor, and provide a system power factor control method; control the input current to ensure the stability of the DC bus voltage.

The control goal of RSC is to provide excitation current for the motor rotor to adjust the reactive power output by the stator side, and to control the motor speed or the active power output by the motor by controlling the DFIG rotor current and torque current, so as to realize the variable speed of maximum wind energy tracking Constant frequency operation.

In this scheme, the fractional-order PI ^λ controller is applied to the rotor-side power control outer loop in the doubly-fed asynchronous induction motor control strategy, which makes the grid-connected dynamic process more robust. Therefore, this modeling focuses on RSC and no-load Grid control.

Since the active power and reactive power output by the motor are closely related to the d and q axis current components of the rotor, the d and q components of the rotor current are the main control targets.

The mathematical model of the double-fed induction generator based on the synchronous rotating coordinate system is as follows:

Voltage equation:

Among them, ω _s is slip angular velocity; ω _s ＝ω ₁ -ω _r , u _ds : stator voltage d-axis component, u _qs : stator voltage q-axis component, u _dr : rotor voltage d-axis component, u _qr : rotor voltage q-axis component, i _ds : stator current d-axis component, i _qs : stator current q-axis component, i _dr : rotor current d-axis component, i _qr : rotor current q-axis component, R _s : stator side resistance, R _r : Rotor side resistance, Ψ _qs : stator flux q-axis component, Ψ _ds : stator flux d-axis component, Ψ _qr : rotor flux q-axis component, Ψ _dr : rotor flux d-axis component, p: differential sign.

Flux linkage equation:

L _s : stator self-inductance, L _r : rotor self-inductance, L _m : mutual inductance.

Electromagnetic torque equation:

T _e ＝n _p L _m (i _ds i _qr -i _qs i _dr ) (3)

T _e : electromagnetic torque, n _p : number of pole pairs.

In the start-up phase of the doubly-fed wind turbine, no-load grid connection control is required to make the motor smoothly cut into the grid, and the amplitude of each electrical component is required to be as small as possible. When the generator is no-load, the stator current components are all 0, that is, i _ds =i _qs =0, and the no-load mathematical model of DFIG is as follows:

T _e =0 (6)

In the formula, T ₀ —generator no-load torque.

Since this vector control is stator flux orientation, and the stator resistance voltage drop is much lower than the reactance voltage drop and motor back electromotive force at power frequency, the motor stator resistance can be ignored in the calculation.

From this, the simplified equation can be obtained as follows:

From (8) ~ (10) can determine the basic principle of DFIG no-load grid-connected control. After the double-fed generator is integrated into the grid, the motor mainly adjusts the active power and reactive power.

(2) Fractional operator and rationalization realization

To be precise, fractional calculus should be called "non-integer order" integration. The order of fractional calculus can be in fractional form, and theoretically it can be expanded to the order of complex numbers or even irrational numbers. However, there are very few related studies on complex number and irrational number order calculus, and engineering applications have not yet been carried out. Generally, it is only in the category of rational numbers. Discuss the application of fractional order.

This implementation example takes the Grunwald-Letnikov fractional calculus definition as the theoretical basis:

Based on this definition, fractional order rationalization is carried out, and according to formula (11), the calculation algorithm of fractional order differential is determined as:

In the formula, is the polynomial coefficient of the function. Assuming that the step size h is small enough, the approximate value of the numerical differentiation of the function can be obtained directly according to (12), and its calculation accuracy can be proved to be o(h). When the function expression of f(t) is determined, its fractional derivative can be calculated directly from formula (11).

Since the double-fed wind power generation system is a strongly coupled and nonlinear system, it is difficult to obtain its accurate function expression. For the doubly-fed wind power generation system, the continuous transfer function method is used to rationalize the fractional operator, that is, through a rational transfer function, it can effectively simulate the fractional operator in terms of amplitude-frequency characteristics and phase-frequency characteristics.

This implementation example uses the improved Oustaloup filtering algorithm to construct a fractional operator:

The above-mentioned fractional operators are prior art, and the theoretical basis is taken from a series of papers by Oustaloup.

By convention, the weighting parameters take values b=10 and d=9. The zero-pole and gain of the above fractional differential operator can be calculated by the following formula:

In order to ensure the stability of the algorithm, the cut-off frequency and parameter N are generally selected as [0.01, 100] and N=4, respectively.

(3) Fractional-order PI ^λ controller (use this fractional-order controller to control the wind turbine grid-connected model)

The fractional order PID controller can be written as PI ^λ D ^μ . After introducing the differential and integral orders μ and λ in the fractional order controller, the adjustable parameters are expanded to make the setting range larger and the controlled object can be controlled more flexibly. The transfer function of PI ^λ D ^μ is:

Since the differential term of PI ^λ D ^μ control mainly performs lead and lag correction, this correction is not needed in grid-connected control, so the PI ^λ controller is simplified, and its control block diagram is shown in Figure 2. The fractional integral term I ^λ is realized by the improved Oustaloup filtering algorithm in step (2).

The PI ^lambda controller function is:

Among them, λ is the integration order.

The whole system adopts double closed-loop control structure - power outer loop and current inner loop. First detect the stator and rotor voltage and current of the motor, and then perform coordinate transformation to calculate the stator flux linkage Ψ ₁ , stator active power P and reactive power Q; set the stator active power command P* and reactive power according to actual needs The command Q* is compared, and the difference obtained is passed through the fractional PI ^λ regulator to obtain the active component command i _qs * and the reactive component command i _ds * of the stator current, and then enter the current inner loop control.

Construct a detailed RSC control model, Simulink block diagram, as shown in Figure 3, apply the fractional order control theory to the no-load grid connection process of the doubly-fed induction generator, and enhance its robustness.

In the PI controller of the rotor side power control outer loop in the doubly-fed asynchronous induction motor control strategy, an adjustable integral term order λ is introduced to increase its control dimension, and it is designed as a fractional order controller. It makes the control system more flexible to deal with nonlinear control objects, and effectively weakens the motor power oscillation under grid-connected transient process and fault disturbance.

Simulation

This application builds a doubly-fed induction motor grid-connected model under the MATLAB/simulink platform, and its RSC control structure model is shown in Figure 3. After the simulation experiment test, the results are shown in Table 1:

Table 1 Grid connection under different orders

After comprehensive comparison, it is finally determined that the fractional order control is selected as the order of 0.8, and the grid connection process of the generator system is simulated. The active power and reactive power are shown in Figure 4 and Figure 5.

Figure 4 is the graph of active power in the grid-connected process. It can be seen from the figure that using the fractional-order PI ^λ control, although the time to completely enter the steady state is slightly longer than the integer-order PI ^λ control, the fractional-order control starts to tend to 0.17s. It is stable, while the integer-order PI control ends the large oscillation at 0.46s and begins to enter the steady state. The peak interval of the oscillation under the integer order PI ^λ control is [-4.47×106, 6.25×106], while the peak interval under the fractional order PI ^λ control is much lower, only [-3.11×106, 2.43×106]. Therefore, the system can enter the normal working state more smoothly, and the impact on the power grid when connected to the grid is reduced.

Figure 5 is the graph of reactive power during grid connection. The state of reactive power and active power are consistent under the control of integer order and fractional order reactive power PI ^λ . When using fractional-order PI ^λ control, the time to fully enter the steady state is slightly longer than using integer-order PI ^λ control, but the peak value of the oscillation is significantly reduced, from [-4.77×106, 8.41×106] to [-1.018×104, 7.55×106].

Figure 6 shows the active power oscillation under the action of two controllers when a three-phase short-circuit fault occurs during the steady-state operation of the system.

As shown in Figure 6, the active power oscillation interval under the integer order PI ^λ control is [-1.349×106, 3.352×106], and under the fractional order PI ^λ control, the oscillation interval becomes [-1.349×106, 3.352×106] . It is proved that the use of fractional order control can effectively suppress the oscillation.

This application is aimed at the rotor side of the traditional doubly-fed induction generator, which usually adopts double-closed-loop PQ decoupling control. The PI controller used in the power outer loop has shortcomings such as rough control and large overshoot, which affects the dynamic performance of the doubly-fed generator. In this paper, the fractional order theory and PI control technology are combined, the adjustable order of the integral term is introduced, the control dimension is increased, the control performance of the doubly-fed induction motor is improved, and the grid-connected and short-circuit fault simulation research is carried out. The simulation results show that the use of fractional PI control can effectively reduce the oscillation of active and reactive power in the start-up phase, and make the system enter the steady state smoothly; in the short-circuit fault state, the use of fractional PI control method can also reduce the system oscillation.

The above descriptions are only preferred embodiments of the present application, and are not intended to limit the present application. For those skilled in the art, there may be various modifications and changes in the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of this application shall be included within the protection scope of this application.

Claims (10)

1. the fractional order control method of double fed induction generators and network process, characterized in that include:

Motor control model is constructed, the model is established based on synchronous rotating frame, by voltage equation, flux linkage equations and electromagnetism Torque equation composition；

Building fractional order operator: theoretical basis is defined as with Grunwald-Letnikov fractional calculus, determines fractional order Differential calculation algorithm finds out the approximation of function value differential for fractional order differential computational algorithm and proves its computational accuracy, It, which is calculated, according to the definition of Grunwald-Letnikov fractional calculus goes out Fractional Derivative；

For dual feedback wind power generation system, rationalization realization is carried out to fractional order operator using continuous transfer function method, i.e., it is logical A Rational Transfer is crossed, makes it can effective simulation fractional order operator on amplitude-frequency characteristic and phase frequency feature；

Establish fractional order PI^λController: controlling motor control model, controls double fed induction generators idle grid connection process.

2. the fractional order control method of double fed induction generators as described in claim 1 and network process, characterized in that fractional order PI^λFractional order integration item I in controller^λIt is obtained by improving Oustaloup filtering algorithm.

3. the fractional order control method of double fed induction generators as described in claim 1 and network process, characterized in that use and change Fractional order operator is constructed into Oustaloup filtering algorithm.

4. the fractional order control method of double fed induction generators as described in claim 1 and network process, characterized in that double-fed sense Answer mathematical model of the generator based on synchronous rotating frame:

Voltage equation:

Wherein, ω_sFor slip angular velocity；ω_s=ω₁-ω_r, u_ds: stator voltage d axis component, u_qs: stator voltage q axis component, u_dr: rotor voltage d axis component, u_qr: rotor voltage q axis component, i_ds: stator current d axis component, i_qs: stator current q axis component, i_dr: rotor current d axis component, i_qr: rotor current q axis component, R_s: stator side resistance, R_r: rotor side resistance, Ψ_qs: stator magnet Chain q axis component, Ψ_ds: stator magnetic linkage d axis component, Ψ_qr: rotor flux q axis component, Ψ_dr: rotor flux d axis component, p: differential Symbol.

5. the fractional order control method of double fed induction generators as claimed in claim 4 and network process, characterized in that magnetic linkage side Journey:

6. the fractional order control method of double fed induction generators as claimed in claim 4 and network process, characterized in that electromagnetism turns Moment equation:

T_e=n_pL_m(i_dsi_qr-i_qsi_dr) (3)。

7. the fractional order control method of double fed induction generators as claimed in claim 4 and network process, characterized in that double-fed wind The startup stage of power generator group needs to carry out idle grid connection control, motor is made steadily to cut power grid, it is desirable that each electrical component vibration Width is small as far as possible, and stator current components are 0 when generator zero load, i.e. i_ds=i_qs=0, the zero load of double fed induction generators Mathematical model is as follows:

T_e=0 (6)

In formula, T₀- generator no-load torque.

8. the fractional order control method of double fed induction generators as claimed in claim 6 and network process, characterized in that ignore electricity The motor model reduced equation of machine stator resistance are as follows:

9. the fractional order control method of double fed induction generators as described in claim 1 and network process, characterized in that The definition of Grunwald-Letnikov fractional calculus:

Fractional order differential computational algorithm are as follows:

In formula,For the multinomial coefficient of function, it is assumed that step-length h is sufficiently small, and it is micro- directly to find out function value according to (12) The approximation divided, and can prove that its computational accuracy is that o (h) can be directly by formula when the function expression of f (t) determines (11) it calculates it and goes out Fractional Derivative.

10. the fractional order control method of double fed induction generators as described in claim 1 and network process, characterized in that PI^λControl Device function processed are as follows:

Wherein, λ is integral order；

PI^λController controlling step are as follows: detect the stator and rotor voltage and current of motor first, then be coordinately transformed, calculated Stator magnetic linkage Ψ out₁, stator active-power P and reactive power Q；P* and nothing are instructed by actual needs setting stator active power Function power instruction Q*, is compared, and gained difference passes through fractional order PI^λAdjuster obtains the active component instruction of stator current i_qs* i is instructed with reactive component_ds*, current inner loop control is subsequently entered.