CN109038659A - DFIG grid side converter low voltage traversing control method based on state feedback linearization - Google Patents

Abstract

The invention discloses a low-voltage ride-through control method for a DFIG grid-side converter based on state feedback linearization, and proposes a control strategy for protecting a wind power system and its units under voltage drop of a doubly-fed wind turbine, solving the problem of input voltage After the drop, the problems of overcurrent and DC bus overvoltage appear on the rotor side; according to the simulation and experimental results, this method has the following advantages: it can effectively suppress the fault component in the transient process, and suppress the rotor side under a certain grid voltage drop. The current fluctuation can keep the DC bus voltage basically stable, and has good control performance, which ensures the stability of the wind power grid-connected system.

Description

Low voltage ride through control method of DFIG grid-side converter based on state feedback linearization

technical field

The invention relates to the technical field of nonlinear control theory, and more specifically relates to a low-voltage ride-through control method for a DFIG grid-side converter based on state feedback linearization.

Background technique

DFIG has been widely used in wind power generation systems because of its small capacity, low cost, and adjustable amplitude, phase, and frequency. However, since the stator side is directly connected to the grid, it is quite sensitive to voltage drops. By improving the control strategy to realize the LVRT operation of the unit, it has attracted widespread attention because it does not need to increase hardware circuits and has good economic performance. In the 26th issue of "Acta Electrotechnical Society" in 2011, the article "Electromagnetic Transition Process and Control Strategy of Double-fed Wind Turbine Generator under Grid Voltage Drop" introduced that when the voltage drop will cause serious overcurrent and overvoltage to the wind turbine, In severe cases, it may even cause damage to the converter of the wind turbine, causing the off-grid operation of the wind turbine. In the process of voltage drop, the low-voltage operation capability of the unit can be improved by optimizing the control strategy to ensure the uninterrupted operation of the unit. In the 32nd issue of "Proceedings of the Chinese Society for Electrical Engineering" in 2012, the article "Several hot issues in the grid-connected operation of doubly-fed asynchronous wind turbines" analyzed the passivity of the DFIG system model, introduced and improved the passivity control strategy , to improve the control performance of the rotor current to cope with the small value drop of the grid voltage.

At present, some literatures have proposed corresponding control methods for the low-voltage ride-through problem of double-fed wind turbines: "An improved vector control strategy based on DFIG under grid symmetrical faults" in "Industrial Instrumentation and Automation Devices" No. 3, 2014 "Using the existing mature vector control strategy, the article improves the calculation method of the fault current on the rotor side and improves the dynamic and steady-state characteristics of the system; in the 34th issue of "Power System Automation" in 2010, "Considering Crowbar resistance and exit time The paper considers that when the grid voltage drops sharply, wind turbines often use the Crowbar protection device on the rotor side, but if the Crowbar protection device is used for too long, the rotor winding will be short-circuited, and a large amount of power will be absorbed from the grid. Reactive power worsens the fault environment at the grid connection. Therefore, the concept of setting the correct investment time of the Crowbar protection device optimizes the dynamic response speed of the entire system, but requires additional hardware circuits and control systems, which undoubtedly increases the cost and complexity of the system; the 38th issue of 2014 "Power System The article "Accurate Calculation of Transient Performance of Double-fed Induction Generator and Optimization of Crowbar Circuit Parameters" in "Automation" considers various elements of the motor in the transient process, and calculates the voltage drop of double-fed induction motor by selecting an appropriate calculation method. The calculation formula of the stator and rotor currents in the process takes into account the influence of the resistance of the crowbar on the current. This method is accurate and easy to implement, and requires less hardware resources, but the algorithm is more complicated.

Therefore, how to provide a DFIG grid-side converter low voltage ride through control method that keeps the DC bus voltage basically stable, has good control performance and ensures the stability of the wind power grid-connected system is an urgent problem to be solved by those skilled in the art.

Contents of the invention

In view of this, the present invention provides a low-voltage ride-through control method of DFIG grid-side converter based on state feedback linearization, and establishes an affine nonlinear model of the grid-side converter of doubly-fed induction wind power generator; The objective function of the system, using the method of state feedback linearization and coordinate transformation, deduces the nonlinear state feedback expression of the system, and proposes a GSC low-voltage ride-through control strategy based on input-output feedback linearization, which realizes the nonlinear The linearization of the system has completed the design of the related controller.

In order to achieve the above object, the present invention adopts the following technical solutions:

A DFIG grid-side converter low-voltage ride-through control method based on state feedback linearization, characterized in that the control method includes:

S1. GSC mathematical model conversion coordinate system: For the GSC mathematical model transformed into the dq0 synchronous coordinate system, the d-axis of the synchronous rotating coordinate system is oriented on the grid voltage vector U _s , and the DC bus voltage U _dc and The DC bus current _idcr calculates the active power P _r output by the converter on the rotor side, namely: P _r = U _dc i _dcr

Arrange the mathematical model of GSC:

Among them, u _gd and u _gq are the d _q -axis components of the GSC grid-connected point voltage; _igd and i _gq are the d _q -axis components of the AC side current; C is the DC bus capacitance; L _g and R _g are the The inductance of the line reactor and the equivalent resistance of the line; U _dc is the DC side voltage of the converter; u _cd , u _cq are the d _q axis components of the AC side voltage; ω ₁ is the synchronous rotation angular velocity; P _g , P _r is the active power output by the grid-side and rotor-side converters, respectively;

S2. Determine the target and construct the output equation:

According to the affine type m-in-m-out system, y _i ＝h _i (x); set the functional relationship between the feedback input u and the new reference input v:

Select the state variable x＝[x ₁ x ₂ ] ^T ＝[i _gd i _gq ] ^T , the input variable u＝[u ₁ u ₂ ]T＝[u _gd u _gq ] ^T , to control the unit under low voltage failure With the target as a reference, construct the output equation:

In the formula, the state variable U is an open set on the manifold N=R ⁿ ; u is the input variable; the vector function f, g _i ∈ R ⁿ , is defined on U; the m-dimensional input vector function u ∈ J, J is the allowable control set, and the output vector Function h(x)∈Y, Y is a linear space composed of p-dimensional real-valued functions; α _i (x), β _ij (x) are scalar smooth functions defined on R ⁿ -open set; v=[ v ₁ ... v _m ] ^T is the new reference input, and let u and v have the same dimension;

S3. Coordinate transformation: carry out coordinate transformation, select:

Transform the original system into Brunovsky's canonical form:

The relationship between the new control variable v and the original control variable u can be obtained as follows:

In the formula:

S4. Control law expression: the linearized feedback u is obtained as:

In the control law expression shown in S4, only v=[v ₁ v ₂ ] ^T is an unknown variable. To make the performance index function obtain the extreme value, the optimal control law is: v=-Kz In the formula, v represents the most Optimal control quantity; K is the optimal feedback gain matrix, where K=B ^T P, P is the positive definite symmetric solution of the Ricardian matrix equation A ^T P+PA-PBP ^-1 B ^T P+Q=0; R and Q are weight factor matrix and weight matrix, respectively. A new input variable v is obtained by using the linear optimal control design method (LQR method) with quadratic performance index.

It can be seen from the above technical solutions that, compared with the prior art, the present invention provides a DFIG grid-side converter low voltage ride-through control method based on state feedback linearization. When the grid voltage drops, traditional vector control cannot Fast response, severe electromagnetic oscillation, weak active and reactive power control capability, and limited low voltage ride-through capability; while state feedback linear control can respond to grid voltage changes faster, and realize active power, reactive power and intermediate DC bus Fast adjustment, weakening electromagnetic oscillation, good dynamic performance, good stability, and stronger system robustness.

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 It is an embodiment of the present invention, and those skilled in the art can also obtain other drawings according to the provided drawings without creative work.

Accompanying drawing of Fig. 1 is the grid-side converter fundamental wave equivalent circuit diagram provided by the present invention;

Accompanying drawing of Fig. 2 is the nonlinear decoupling control block diagram of GSC provided by the present invention;

Accompanying drawing of Fig. 3 is the 1.5MWDFIG simulation structural diagram provided by the present invention;

Accompanying drawing of Fig. 4 is the power grid voltage waveform diagram under the fault provided by the present invention;

Accompanying drawing of Fig. 5 is the stator current waveform diagram under the fault that the present invention provides;

Fig. 6 accompanying drawing is the rotor current waveform diagram under the fault provided by the present invention;

Fig. 7 is the simulation waveform diagram of the DC bus voltage provided by the present invention with the addition of state feedback linearization and traditional vector control.

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.

The embodiment of the present invention discloses a low-voltage ride-through control method based on DFIG grid-side converter based on state feedback linearization. Through the GSC state feedback linearization control strategy and related controllers, the grid-side nonlinear controller is It can maintain the basic stability of the DC bus voltage under the grid voltage drop, and has good control performance and dynamic and static characteristics.

Referring to accompanying drawing 1, it is the fundamental wave equivalent circuit diagram of grid-side converter; Accompanying drawing 2 is the non-linear decoupling control block diagram of GSC provided by the present invention, the control system shown in the figure adopts double closed-loop structure, and there are two feedback control quantities : One is the reactive power; the other is the output DC voltage; the reactive power loop forces the input power to follow a given value, and the output voltage loop can stabilize the output DC voltage. It also increases the stability of the control system;

Accompanying drawing 3 is a 1.5MWDFIG simulation structure diagram; Accompanying drawing 4 is a grid voltage waveform diagram under a fault; Accompanying drawing 5 is a stator current waveform diagram under a fault; Accompanying drawing 6 is a rotor current waveform diagram under a fault; State feedback linearization and traditional vector control DC bus voltage simulation waveform diagram.

The present invention will be further described below in conjunction with the accompanying drawings.

S1. GSC mathematical model conversion: in the dq0 synchronous rotating coordinate system: the mathematical model of GSC is as follows:

In the formula, u _gd and u _gq are the d _q axis components of the GSC grid-connected point voltage; i _gd and i _gq are the d _q axis components of the AC side current; C is the DC bus capacitance; L _g and R _g are each The inductance of the phase-in line reactor and the equivalent resistance of the line; U _dc is the DC side voltage of the converter; u _cd , u _cq are the d _q axis components of the AC side voltage; ω ₁ is the synchronous rotation angular velocity; P _g , P _r is the active power output by the grid-side and rotor-side converters respectively;

It can be seen from the above formula that the DC bus voltage U _dc reflects the fluctuation of active power, and its active power can be controlled by controlling U _dc ; in order to simplify the control algorithm, the d-axis of the synchronous rotating coordinate system is oriented on the grid voltage vector U _s , It should be pointed out that due to the switching characteristics of the converter, the voltage on the rotor side contains a large number of harmonic components, which will lead to a large error in the calculation of P _r obtained from the instantaneous value. Therefore, the calculation of P _r in this embodiment adopts DC voltage U _dc and DC current _idcr with less fluctuation, that is, P _r = U _dc i _dcr and put it into the above formula to get:

State Feedback Linearization: State Variables The output equation y _i = _hi (x); assume the feedback input u is a function of the state x and the new reference input v:

In the formula: state variable U is an open set on the manifold N=R ⁿ ; the vector function f, g _i ∈ R ⁿ , is defined on U; the m-dimensional input vector function u ∈ J, J is the allowable control set, and the output vector function h(x) ∈Y, Y is a linear space composed of p-dimensional real-valued functions. α _i (x), β _ij (x) are scalar smooth functions defined on an open set of R ⁿ ; v=[v ₁ ...v _m ] ^T is the new reference input, and u and v are assumed to have the same dimension ;

(1) For all x in the field of x ⁰ and all 1≤j≤m, 1≤i≤m, 0≤k≤ρ _i -1, satisfy:

(2) At point x=x ⁰ , the following m×m matrix is non-singular

Then the relative order of the _system is _{called ρ 1} , _{ρ 2} , . If the relative order satisfies: ρ=ρ ₁ +ρ ₂ +...+ρ _m =n, where n is the dimension of the state vector x, then the system satisfies the necessary and sufficient condition for state feedback linearization and realizes feedback linearity change.

Select the set of coordinate transformation functions defined in the ^x0 field:

S2. Determine the target and construct the output equation:

According to the affine type m-in-m-out system, y _i ＝h _i (x); set the functional relationship between the feedback input u and the new reference input v:

Select the state variable x＝[x ₁ x ₂ ] ^T ＝[i _gd i _gq ] ^T , the input variable u＝[u ₁ u ₂ ] ^T ＝[u _gd u _gq ] ^T , to control With the target as a reference, construct the output equation:

Refine the affine system:

Where, f(x)=[f ₁ f ₂ ] ^T ＝[(-R _g /L _g )i _gd +ω ₁ i _gq +u _gd /L _g -(R _g /L _g )i _gq -ω ₁ i _gd ] ^T , g ₁ =[-1/L _g 0] ^T , g ₂ =[0-1/L _g ] ^T ;

Then, verify the linearization condition:

The matrix can be obtained:

Where the rank is equal to the order n of the system, the vector field: D=[g ₁ (x) g ₂ (x) ad _f g ₁ (x) ad _f g ₂ (x)] is a summation.

S3, carry out coordinate transformation, select:

Transform the original system into Brunovsky's standard form: z=v

The relationship between the new control variable v and the original control variable u can be obtained as follows:

In the formula:

S4, obtain u as:

In the control law expression shown in S4, only v=[v ₁ v ₂ ] ^T is undetermined. To make the performance index function obtain the extreme value, the optimal control law is: v=-Kz In the formula, v represents Optimal control quantity; K is the optimal feedback gain matrix, where K=B ^T P, P is the positive definite symmetric solution of the Ricardian matrix equation A ^T P+PA-PBP ^-1 B ^T P+Q=0; R and Q are weight factor matrix and weight matrix respectively. A new input variable v is obtained by using the linear optimal control design method (LQR method) with quadratic performance index.

Use Matlab/simulink simulation software to build a system simulation model, simulation parameters: Among them, the main parameters of DFIG are as follows: stator rated voltage U _N = 690V, stator rated power P _s = 1.5MW, stator voltage frequency f ₁ = 50Hz, stator and rotor resistance R = 2.1mΩ, stator leakage inductance L _IS = 55.8μH, rotor leakage inductance L _Ir = 44.65μH, stator-rotor mutual inductance L _m = 1.73mH; grid-side converter parameters are as follows: AC side inductance L _g = 5mH; AC side resistance R _g =0.02Ω; DC side capacitance C=470μF; DC side resistance R ₀ =40Ω; grid voltage peak value E _gd =50V; DC voltage U _dc =120V; switching frequency f _s =10kHz.

Attached Figure 3, Figure 4 and Figure 5 are the stator voltage, current and rotor current waveforms under a fault; when a low voltage fault occurs, set the grid voltage to drop at 1.5s, return to normal at 1.7s, last for 0.2s, drop The final voltage is 0.8Us; when a voltage drop fault occurs, it will cause a sudden drop in the stator terminal voltage, and the corresponding electromagnetic torque will also decrease according to the depth of the voltage drop, but the input mechanical torque of the wind turbine remains unchanged. That is, the wind speed is constant, so ω _r will increase due to the mismatch of electromagnetic power; for wind turbines without any protection and control strategy, the DC bus voltage rises rapidly, and the voltage close to 1600V may cause the converter to fail. Damage to power device switching devices.

The DC bus voltage waveform in Figure 6, comparing the two curves in the figure, it can be found that after adding the feedback linearization control strategy, compared with the traditional vector control strategy, during the voltage surge period, the active output decreases, and the reactive output is caused by the power grid. The required inductive reactive power, the increase of the DC bus voltage is obviously slowed down after the control strategy is added, so that the quality of the output voltage waveform is good.

To sum up, the present invention provides a DFIG grid-side converter low-voltage ride-through control method based on state feedback linearization, which solves the problem of overcurrent and overvoltage on the stator side during the voltage drop fault of the doubly-fed wind turbine. By controlling the selected objective function, using the method of state feedback linearization and coordinate transformation, the nonlinear state feedback expression of the system is derived, and a grid-side converter low-voltage ride-through based on input-output feedback linearization is proposed The control strategy realizes the linearization of the nonlinear system and completes the design of related controllers; the simulation results verify the effectiveness of the proposed method, and the grid-side nonlinear controller suppresses the rotor-side current under a certain grid voltage drop. It can maintain the basic stability of the DC bus voltage and has good control performance, which ensures the stability of the wind power grid-connected system and has high engineering practical value.

Each embodiment in this specification is described in a progressive manner, each embodiment focuses on the difference from other embodiments, and the same and similar parts of each embodiment can be referred to each other. As for the device disclosed in the embodiment, since it corresponds to the method disclosed in the embodiment, the description is relatively simple, and for the related information, please refer to the description of the method part.

The above description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the present invention will not be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims (3)

1. A DFIG grid side converter low voltage ride through control method based on state feedback linearization, characterized in that the control method comprises: S1. GSC mathematical model conversion coordinate system: For the GSC mathematical model transformed into the dq0 synchronous coordinate system, the d-axis of the synchronous rotating coordinate system is oriented on the grid voltage vector U _s , and the DC bus voltage U _dc and The DC bus current _idcr calculates the active power P _r output by the converter on the rotor side, namely: P _r = U _dc i _dcr Arrange the mathematical model of GSC: Among them, u _gd and u _gq are the dq-axis components of the GSC grid-connected point voltage; _igd and i _gq are the dq-axis components of the AC side current; C is the DC bus capacitance; L _g and R _g are the incoming lines of each phase The inductance of the reactor and the equivalent resistance of the line; U _dc is the DC side voltage of the converter; u _cd and u _cq are the dq axis components of the AC side voltage; ω ₁ is the synchronous rotation angular velocity; P _g and P _r are respectively Active power output by grid-side and rotor-side converters; S2. Determine the target and construct the output equation: According to the affine type m-in-m-out system, y _i ＝h _i (x); set the functional relationship between the feedback input u and the new reference input v: Select the state variable x＝[x ₁ x ₂ ] ^T ＝[i _gd i _gq ] ^T , the input variable u＝[u ₁ u ₂ ] ^T ＝[u _gd u _gq ] ^T , to control With the target as a reference, construct the output equation: In the formula, the state variable U is an open set on the manifold N=R ⁿ ; u is the input variable; the vector function f, g _i ∈ R ⁿ , is defined on U; the m-dimensional input vector function u ∈ J, J is the allowable control set, and the output vector Function h(x)∈Y, Y is a linear space composed of p-dimensional real-valued functions; α _i (x), β _ij (x) are scalar smooth functions defined on R ⁿ -open set; v=[ v ₁ ... v _m ] ^T is the new reference input, and let u and v have the same dimension; S3. Coordinate transformation: a set of coordinate transformation functions defined in the field of ^x0 : choose, Transform the original system into Brunovsky's canonical form: The relationship between the new control variable v and the original control variable u is obtained as follows: In the formula: in, is the set of coordinate transformation functions; S4. Control law expression: the linearized feedback u is obtained as: 2. The DFIG grid-side converter low-voltage ride-through control method based on state feedback linearization according to claim 1, wherein the affine system is refined: Where, f(x)=[f ₁ f ₂ ] ^T ＝[(-R _g /L _g )i _gd +ω ₁ i _gq +u _gd /L _g -(R _g /L _g )i _gq -ω ₁ i _gd ] ^T , g ₁ =[-1/L _g 0] ^T , g ₂ =[0 -1/L _g ] ^T ; Then, verify the linearization condition: The matrix can be obtained: Wherein, the rank is equal to the order n of the system, and the vector field D=[g ₁ (x) g ₂ (x) ad _f g ₁ (x) ad _f g ₂ (x)] is a summation. 3. The DFIG grid-side converter low-voltage ride-through control method based on state feedback linearization according to claim 1, wherein v=[v ₁ v ₂ ] ^T is an unknown variable in the step S4, and it is necessary to make The performance index function obtains the extreme value, then the optimal control law is: v=-Kz In the formula, v represents the optimal control quantity; K is the optimal feedback gain matrix, where K=B ^T P, P is the positive definite of the Ricardian matrix equation A ^T P+PA-PBR ^-1 B ^T P+Q=0 Symmetric solution; R and Q are weight factor matrix and weight matrix respectively; A new input variable v is obtained by using the linear optimal control design method with quadratic performance index.