阅读说明：本技术 一种dfig多目标优化控制目标函数的设计方法 (Design method of DFIG multi-objective optimization control objective function ) 是由 李岚 杨旭 侯传羽 杨琦 赵楠 于 2021-04-08 设计创作，主要内容包括：本发明属于双馈风力发电机的有限集模型预测控制技术领域,具体为一种DFIG多目标优化控制目标函数的设计方法,解决了背景技术中的技术问题,其包括选择两个被控对象；基于有限集模型预测控制,给出传统分层控制目标函数；将传统分层控制目标函数中的非线性项进行更新,重新构造包括两个被控对象在内的目标函数；根据电机动态反馈的效果,系统动态切换被控对象的权重系数,同时根据被控对象参考值的变化情况调整控制力度,使两个被控对象的控制精度保持稳定。本发明在传统分层控制的基础上,通过进一步改写目标函数中的非线性项,在目标函数中引入相对误差控制思想,可在被控对象参考值变化较大的情况下,实现各被控对象控制精度基本不变的目的。(The invention belongs to the technical field of finite set model predictive control of a doubly-fed wind generator, in particular to a design method of a DFIG multi-objective optimization control objective function, which solves the technical problem in the background technology and comprises the steps of selecting two controlled objects; based on finite set model predictive control, providing a traditional hierarchical control objective function; updating a nonlinear item in a traditional hierarchical control objective function, and reconstructing an objective function including two controlled objects; according to the effect of the dynamic feedback of the motor, the system dynamically switches the weight coefficients of the controlled objects, and simultaneously adjusts the control strength according to the change condition of the reference value of the controlled objects, so that the control precision of the two controlled objects is kept stable. On the basis of traditional hierarchical control, the invention can realize the purpose that the control precision of each controlled object is basically unchanged under the condition that the reference value of the controlled object has larger change by further rewriting the nonlinear item in the objective function and introducing the idea of relative error control into the objective function.)

1. A design method of a DFIG multi-objective optimization control objective function is characterized by comprising the following steps:

s1, selecting rotor current, wherein the active power of the stator is a controlled object;

s2, based on finite set model predictive control, giving a traditional hierarchical control objective function G as formula (1):

in the formula (1), the reaction mixture is,representing a reference value of a rotor current vector under a two-phase static alpha beta coordinate system at the moment k + 1;the predicted value of the rotor current vector at the moment k +1 under the two-phase static alpha beta coordinate system is shown;is a function of the non-linearity of the signal,expressed as formula (2):

in the formula (2), Δ p means that the active power k +1 of the stator is allowed to fluctuate Δ p up and down relative to the reference value,is a reference value of the moment of the active power k +1 of the stator,the predicted value of the stator active power k +1 moment is obtained;

s3, controlling the non-linear function in the target function by the traditional hierarchyThe improvement is that Represented by formula (3):

in the formula (3), δ is a prediction error range of the stator active power k +1 relative to a reference value thereof;the predicted value of the stator active power k +1 moment is obtained;the reference value is the moment of active power k +1 of the stator; according toReconstructing an objective function g comprising rotor current and stator active power, wherein the reconstructed objective function g is expressed by an equation (4):

in the formula (4), the reaction mixture is,representing a reference value of a rotor current vector under a two-phase static alpha beta coordinate system at the moment k + 1;the predicted value of the rotor current vector at the moment k +1 under the two-phase static alpha beta coordinate system is shown;

s4, according to the dynamic feedback effect of the motor, the control system dynamically switches the weight coefficient of the stator active power in the objective function according to the prediction error ranges of 7 stator active power predicted values corresponding to 7 switch states of the inverter; meanwhile, the control force is adjusted according to the change condition of the active power reference value of the stator, so that the control precision of the rotor current and the active power of the stator is kept stable.

2. The design method of the DFIG multi-objective optimization control objective function according to claim 1, wherein in step S4, the prediction error range allowed by the stator active power predicted value is expressed as formula (5):

in equation (5), δ is the prediction error range of the stator active power k +1 from its reference value.

3. The design method of the DFIG multi-objective optimization control objective function according to claim 1, wherein in step S4, the method for dynamically switching the weight coefficient of the stator active power in the objective function is as follows: when the predicted values of the active power of the 7 stators are all within the prediction error range, the system only controls the rotor current, and the weight coefficient of the active power of the stators is 0 at the moment; when the predicted values of the active power of the 7 stators exceed the prediction error range, the system only controls the active power of the stators, and the weight coefficient of the active power of the stators is positive infinite at the moment; when part of the 7 stator active power predicted values exceed the prediction error range and part of the 7 stator active power predicted values are within the prediction error range, the system selects a switch state which minimizes the total error of the rotor current and the stator active power from switch states which do not exceed the prediction error range, and the weight coefficient of the stator active power is 0, but the stator active power is preferentially controlled.

4. The design method of the DFIG multi-objective optimization control objective function according to claim 1, wherein in step S4, the method for the system to adjust the control force according to the variation of the stator active power reference value comprises: when the reference value of the active power of the stator is reduced, the control strength of the active power of the stator is increased; when the active power reference value of the stator rises, the control strength of the active power of the stator is reduced.

5. The design method of the DFIG multi-objective optimization control objective function according to any one of claims 1 to 4, wherein the controlled object can be replaced by stator reactive power and stator current.

Technical Field

The invention belongs to the technical field of control of a double-fed wind driven generator under unbalanced grid voltage, relates to a finite set model prediction control technology of the double-fed wind driven generator, and particularly relates to a design method of a DFIG multi-objective optimization control objective function.

Background

In recent years, with the occurrence of environmental problems such as air pollution and greenhouse effect, people have come to be aware of the importance of replacing fossil energy with clean energy. Among various new energy sources, wind energy is an important energy form, and in the present electrical age, people hope to convert wind energy into electric energy to serve human production and life. The doubly-fed wind generator is a mainstream machine type of wind power generation due to the advantages of wide rotating speed operation range, small capacity of a required excitation converter and the like.

The probability of asymmetric faults occurring in an actual power grid is high, and in order to enhance the capability of stable operation of a system and guarantee the safety of the power grid, the doubly-fed wind driven generator needs to have certain asymmetric fault ride-through capability. Therefore, the control of the operation capacity of the doubly-fed wind generator under the condition of asymmetric grid voltage becomes one of the main trends of the international wind power technology development.

The model predictive control can intensively optimize all control targets and constraint conditions through a uniform objective function, so that the control targets with different physical dimensions can be conveniently controlled without greatly changing a control structure. However, model predictive control needs to design a reasonable weight coefficient for each control target, and if the coefficient is not reasonable, each target cannot realize coordinated control. At present, researchers are mostly based on experience, and obtain better weight coefficient combination through multiple attempts, and the time cost is higher and is not necessarily the optimal combination. For the design problem of the weight coefficient, the existing literature and research mainly focus on optimizing the algorithm or changing the structure of the objective function to avoid the design of the weight coefficient. However, these methods also expose a number of disadvantages, and the design method for the optimization algorithm is time consuming to implement and can significantly increase the computational burden. The design method for changing the structure of the objective function may complicate the objective function, cannot be applied to other control targets, and lacks universality.

In summary, the existing design scheme for the weight coefficient cannot be explicitly selected according to the user-defined criteria, so that the problem that the design scheme for the weight coefficient is not flexible enough and the fundamental problem is not solved is avoided.

Disclosure of Invention

The invention aims to solve the technical problem that the control effect of a certain controlled object is deteriorated due to the serious deviation of control side key points caused by the change of the reference value of the controlled object in the traditional layered control strategy, and provides a design method of a DFIG multi-objective optimization control objective function. According to the method, under the condition of unbalanced grid voltage, coordination control over multiple targets of the doubly-fed wind generator can be achieved, the control strength can be automatically adjusted under the condition that the reference value of a controlled object is large in change, and the control precision of each target is kept stable.

The technical means for solving the technical problems of the invention is as follows: a design method of a DFIG multi-objective optimization control objective function comprises the following steps:

s1, selecting rotor current, wherein the active power of the stator is a controlled object;

s2, based on finite set model predictive control, giving a traditional hierarchical control objective function G as formula (1):

in the formula (1), the reaction mixture is,representing a reference value of a rotor current vector under a two-phase static alpha beta coordinate system at the moment k + 1;represents the rotor current vector at k +1 under a two-phase stationary alpha beta coordinate systemA predicted value of the time;is a non-linear function, expressed as formula (2):

in the formula (2), Δ p means that the active power k +1 of the stator is allowed to fluctuate Δ p up and down relative to the reference value,is a reference value of the moment of the active power k +1 of the stator,the predicted value of the stator active power k +1 moment is obtained;

s3, controlling the non-linear function in the target function by the traditional hierarchyThe improvement is thatRepresented by formula (3):

in the formula (3), δ is a prediction error range of the stator active power k +1 relative to a reference value thereof;the predicted value of the stator active power k +1 moment is obtained;the reference value is the moment of active power k +1 of the stator; according to

Reconstructing an objective function g comprising rotor current and stator active power, wherein the reconstructed objective function g is expressed by an equation (4):

in the formula (4), the reaction mixture is,representing a reference value of a rotor current vector under a two-phase static alpha beta coordinate system at the moment k + 1;the predicted value of the rotor current vector at the moment k +1 under the two-phase static alpha beta coordinate system is shown;

s4, according to the dynamic feedback effect of the motor, the control system dynamically switches the weight coefficient of the stator active power in the objective function according to the prediction error ranges of 7 stator active power predicted values corresponding to 7 switch states of the inverter; meanwhile, the control force is adjusted according to the change condition of the active power reference value of the stator, so that the control precision of the rotor current and the active power of the stator is kept stable.

The method can realize the coordination control of a plurality of targets of the doubly-fed wind generator under the condition of unbalanced grid voltage, can automatically adjust the control strength under the condition of large change of the reference value of the controlled object, keeps the control precision of each target stable, can perform definite selection according to the user-defined criterion, and has flexible design of the weight coefficient.

Preferably, in step S4, the prediction error range allowed by the predicted stator active power value is expressed by equation (5):

in equation (5), δ is the prediction error range of the stator active power k +1 from its reference value. The relative error of active power of the stator is used as a control standard, and the control force can be automatically adjusted according to the change condition of the reference value of the active power of the stator, so that the control precision of the rotor current and the active power of the stator is kept stable.

Preferably, in step S4, the method for dynamically switching the weighting coefficients of the stator active power in the objective function is as follows: when the predicted values of the active power of the 7 stators are all within the prediction error range, the system only controls the rotor current, and the weight coefficient of the active power of the stators is 0 at the moment; when the predicted values of the active power of the 7 stators exceed the prediction error range, the system only controls the active power of the stators, and the weight coefficient of the active power of the stators is positive infinite at the moment; when part of the 7 stator active power predicted values exceed the prediction error range and part of the 7 stator active power predicted values are within the prediction error range, the system selects a switch state which minimizes the total error of the rotor current and the stator active power from switch states which do not exceed the prediction error range, and the weight coefficient of the stator active power is 0, but the stator active power is preferentially controlled. Therefore, the user can define the control precision by himself according to the importance degree of the controlled object.

Preferably, in step S4, the method for the system to adjust the control strength according to the variation of the stator active power reference value includes: when the reference value of the active power of the stator is reduced, the control strength of the active power of the stator is increased so as to prevent the control effect of the active power of the stator from deteriorating; when the active power reference value of the stator rises, the control strength of the active power of the stator is reduced, so that the control effect of the rotor current is prevented from deteriorating. Therefore, the overall control effect of the control system is more coordinated, the control precision of each controlled object is kept stable, and the phenomenon of unbalanced control effect can not occur.

The invention has the beneficial effects that: by adopting an improved hierarchical control idea, when a reference value changes, a control system automatically adjusts control strength to ensure that the control precision is not changed, so that the control precision of a controlled object can be within an error range defined by a user; the invention comprehensively considers two aspects of optimizing control algorithm and improving coordination control capability; a system model is established in a two-phase static alpha beta coordinate system, so that a control structure is simplified; the error range of the controlled object is set by user self-definition, so that the defect that the control effect of a control target with higher importance degree is poor due to unreasonable weight coefficient design is avoided; the design method of the target function can realize multi-target coordination control, and the capability of the double-fed wind driven generator in traversing asymmetric faults is enhanced.

Drawings

FIG. 1 is a flow chart of the design of the improved hierarchical control strategy nonlinear term of the present invention.

FIG. 2 is a flow chart of the control logic for the improved objective function for multi-objective control in accordance with the present invention.

Fig. 3 is a simulation waveform of the active power of the stator (the reference value of the active power is reduced) under the traditional hierarchical control strategy.

Fig. 4 is a simulation waveform of the active power of the stator (active power reference value is reduced) under the improved hierarchical control strategy.

Fig. 5 is a simulation waveform of rotor a-phase current (active power reference value is reduced) under a traditional hierarchical control strategy.

Fig. 6 shows simulation waveforms of rotor a-phase current (active power reference value reduction) under the improved hierarchical control strategy.

Fig. 7 is a simulation waveform of the active power of the stator (the reference value of the active power is increased) under the traditional hierarchical control strategy.

Fig. 8 is a simulation waveform of the active power of the stator (the reference value of the active power is increased) under the improved hierarchical control strategy.

Fig. 9 is a simulation waveform of rotor a-phase current (active power reference value is increased) under a traditional hierarchical control strategy.

Fig. 10 shows simulation waveforms of rotor a-phase current (active power reference value is increased) under the improved hierarchical control strategy.

Detailed Description

With reference to fig. 1 to 10, a detailed description will be given of a design method of a DFIG multi-objective optimization control objective function according to the present invention.

A design method of a DFIG multi-objective optimization control objective function is shown in FIG. 1 and FIG. 2, and comprises the following steps:

s1, selecting the rotor current and the stator active power as controlled objects;

s2, based on finite set model predictive control, giving a traditional hierarchical control objective function G as formula (1):

in the formula (1), the reaction mixture is,representing a reference value of a rotor current vector under a two-phase static alpha beta coordinate system at the moment k + 1;the predicted value of the rotor current vector at the moment k +1 under the two-phase static alpha beta coordinate system is shown;is a non-linear function, expressed as formula (2):

in the formula (2), Δ p is 90, which means that the active power predicted value of the stator is allowed to fluctuate by 90W from the reference value at the time k +1,the reference value of the active power of the stator at the moment k +1 is obtained,the predicted value of the active power of the stator at the moment k +1 is obtained;

the control error of the active power of the stator is required to fluctuate within 7 percent of the reference value;

s3, nonlinear function in traditional hierarchical control objective functionThe improvement is thatRepresented by formula (3):

in the formula (3), δ is 7%, which is a prediction error range allowing the stator active power predicted value to be relative to the reference value thereof at the time k +1,the predicted value of the active power of the stator at the moment k +1 is obtained,the reference value of the active power of the stator at the moment k +1 is obtained; according toReconstructing an objective function g comprising rotor current and stator active power, wherein the reconstructed objective function g is expressed by an equation (4):

in the formula (4), the reaction mixture is,representing the reference value of the rotor current vector at the time k +1 in a two-phase stationary alpha beta coordinate system,the predicted value of the rotor current vector at the moment k +1 under the two-phase static alpha beta coordinate system is shown;

s4, as shown in fig. 2, according to the dynamic feedback effect of the motor, the control system dynamically switches the weighting coefficients of the active power of the stator in the objective function according to the prediction error ranges of the active power predicted values of the stator corresponding to the 7 switching states of the inverter, where the prediction error range allowed by the active power predicted values of the stator is expressed as equation (5):

in the formula (5), δ is an error range allowing the predicted value of the active power of the stator to be relative to the reference value thereof at the time k +1, in this embodiment, δ is taken to be 7%, the relative error relative to the reference value of the active power of the stator is taken as a control standard, and the control strength can be automatically adjusted according to the change condition of the reference value of the active power of the stator, so as to ensure that the control precision of the rotor current and the active power of the stator is kept stable; specifically, the method for dynamically switching the weight coefficient of the active power of the stator in the objective function comprises the following steps: when the predicted values of the active power of the 7 stators are within 7% of the prediction error range, the system only controls the rotor current, and the weight coefficient of the active power of the stators is 0; when the predicted values of the active power of the 7 stators exceed the prediction error range by 7%, the system only controls the active power of the stators, and the weight coefficient of the active power of the stators is positive infinite; when part of the 7 stator active power predicted values exceeds the prediction error range by 7 percent and part of the 7 stator active power predicted values is within the prediction error range by 7 percent, the system selects a switch state which minimizes the overall control error of the rotor current and the stator active power from switch states which do not exceed the prediction error range, and the weight coefficient of the stator active power is 0 at the moment, but the stator active power is preferentially controlled; meanwhile, the control force is adjusted according to the change condition of the active power reference value of the stator, so that the control precision of the rotor current and the active power of the stator is kept stable; the method for adjusting the control force by the system according to the change situation of the active power reference value comprises the following steps: when the reference value of the active power of the stator is reduced, the control strength of the active power of the stator is increased so as to prevent the control effect of the active power of the stator from deteriorating; when the active power reference value of the stator rises, the control strength of the active power of the stator is reduced, so that the control effect of the rotor current is prevented from deteriorating.

Further, as a specific implementation manner of the design method of the DFIG multi-objective optimization control objective function, the controlled object can be replaced by the stator reactive power and the stator current.

Example 1: the a-phase grid voltage alone decreases to 0.8pu at t 0.35 s.

As shown in fig. 3 and 4, when t is 0s to 0.55s, the reference value of the active power of the stator is 1147W, the active power fluctuation range of the stator under the two control strategies is 1015W to 1210W, and the control precision of the active power of the stator is 8.5%. As shown in fig. 5 and 6, the rotor current THD under the two control strategies is 2.37% and 2.31%, respectively, and the control accuracies of the two control strategies are substantially the same.

When t is 0.55s, the active power reference value of the stator is suddenly changed to 734W. As shown in fig. 3, the active power fluctuation range of the stator under the traditional hierarchical control strategy is 570W-800W, and the active power control precision of the stator is 15.67%. As shown in fig. 4, the active power fluctuation range of the stator under the improved hierarchical control strategy is 600W-750W, the active power control precision of the stator is 10.22%, as shown in fig. 5 and 6, and the rotor current THD under the two control strategies is 0.91% and 2.62%, respectively. It can be seen that when the active power reference value of the stator is reduced, the control accuracy of the active power of the stator and the current of the rotor under the improved hierarchical control strategy is basically unchanged, while the control side emphasis is seriously deviated under the traditional hierarchical control strategy.

As can be seen from fig. 2 to 6, after the active power reference value of the stator is reduced, the rotor current under the conventional hierarchical control strategy is almost undistorted, resulting in too low control accuracy of the active power of the stator and too high control accuracy of the rotor current. And the improved hierarchical control strategy has no great change in the control precision of the stator active power and the rotor current before and after the power reference value is reduced, and has better coordination control capability.

Example 2: the a-phase grid voltage alone decreases to 0.8pu at t 0.25 s.

As shown in fig. 7 and 8, when t is 0s to 0.4s, the active power reference value of the stator is 734W, the active power fluctuation range of the stator under the two control strategies is 623W to 788W, and the active power control accuracy of the stator is 11.24%. As shown in fig. 9 and 10, the rotor current THD under the two control strategies was 2.69% and 2.82%, respectively.

When t is 0.4s, the active power reference value of the stator is suddenly changed to 1473W. As shown in fig. 7, the active power fluctuation range of the stator under the traditional hierarchical control strategy is 1300W-1520W, and the active power control precision of the stator is 7.45%. As shown in fig. 8, the active power fluctuation range of the stator under the improved hierarchical control strategy is 1270W-1590W, and the active power control accuracy of the stator is 10.86%. As shown in fig. 9 and 10, the rotor current THD under both control strategies was 4.38% and 2.68%, respectively. It can be seen that when the active power reference value of the stator is increased, the control accuracy of the active power of the stator and the current of the rotor under the improved hierarchical control strategy is basically unchanged, while the control side emphasis is seriously deviated under the traditional hierarchical control strategy.

As can be seen from fig. 7 to 10, after the active power reference value is increased, the control accuracy of the active power is too high and the control accuracy of the rotor current is too low under the conventional hierarchical control strategy. And the improved hierarchical control strategy has no excessive change in the control precision of the active power of the stator and the current of the rotor before and after the power reference value is increased, and has better coordination control capability

While the preferred embodiments of the present invention have been illustrated and described, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.