Neural Network Compensation Control for Output Power Optimization of Wind Energy Conversion System Based on Data-Driven Control

Due to the uncertainty of wind and because wind energy conversion systems (WECSs) have strong nonlinear characteristics, accurate model of the WECS is di ﬃ cult to be built. To solve this problem, data-driven control technology is selected and data-driven controller for the WECS is designed based on the Markov model. The neural networks are designed to optimize the output of the system based on the data-driven control system model. In order to improve the e ﬃ ciency of the neural network training, three di ﬀ erent learning rules are compared. Analysis results and SCADA data of the wind farm are compared, and it is shown that the method e ﬀ ectively reduces ﬂuctuations of the generator speed, the safety of the wind turbines can be enhanced, the accuracy of the WECS output is improved, and more wind energy is captured.


Introduction
In recent years, the global wind industry has grown rapidly, wind energy has become one of the most important renewable energy sources, installed capacity growth rate of wind turbines has become more than 25% annually, and wind energy has been widely applied to water irrigation [1], urban power supply [2], and many other fields.However, there are still some difficult issues in wind energy control technology: how to get maximum power output of wind turbine is one of the main concerns.Domestic and foreign scholars have done a lot of research on capturing the largest wind energy; the most common control strategy is maximum power point tracking control (MPPT) [3].The maximum wind energy is captured by controlling the output error of the wind generator speed, when the wind speed changes.PI control [4,5], LQG control [6,7], and Fuzzy sliding mode control [8] are commonly used as the control methods.However, the parameters of PI controller are almost adjusted through a large number of experiments.They will be affected by the change of load and wind speed, and therefore, PI regulator will lose the flexibility at this time.The LQG control method does not have high modeling accuracy, so the error of system output is too large.Fuzzy sliding mode control cannot reduce the chattering phenomenon; it will lead to low accuracy and poor robust performance.
Data-driven control theory has gradually become a hot research area for domestic and foreign scholars [9][10][11][12][13][14].The data-driven control theory can be used to design a controller for a complex nonlinear system whose model is unknown.The control method has general applicability and is successfully applied to many fields, such as flight control [15,16], pattern recognition [17], and robot control [18,19].
However, the shortcomings of the data-driven controller are parameter perturbation and excessive computing.The data-driven approach is improved by many well-known scholars [20][21][22][23].A new data-driven controller is designed in the literature [20]; the method uses only measured input and output data of the controlled plant and guarantees bounded input and bounded output stability.It can be known by comparing this data-driven approach with the general adaptive control approach.The method has practical  application value and can be combined with other control methods, and thus the cost of the controller is greatly reduced.
Neural networks have independence-not relying on accurate mathematical model of the system-and a strong classification ability for spatial data model.In fact, neural networks have been successfully applicated to wind power control systems [24][25][26][27][28][29][30].Literatures [24][25][26] introduce the neural network predictive control in the application of the WECS, using neural network predictive controller to compensate system output errors that are caused by uncertainty parameters in WECS; disturbance output is reduced when the wind turbine is at run time.Literatures [31,32] introduce observers applied to wind turbines; robust performance and dynamic performance of the WECS can be improved by the observers.When wind speed is larger, power output may exceed the rated value; this will cause high wind turbine torque load.Power output quality of the WECS is optimized by controlling the blade pitch angle in literatures [27,28], so the wind turbine shutdown and overload can be reduced.There exist analytical models of wind turbines as in [33][34][35][36] and models based on input output data as in [29,30,37], and in the paper, the second case is considered.Literatures [29,30,37] introduce data-driven controller designed for the WECS based on neural network, this method collects wind farms data by supervisory control and data acquisition (SCADA) system, such as wind energy utilization coefficient, wind generator speed, and wind speed.Using data-driven control to identify input and output of the controlled object, different control methods are used to regulate WECS output; the results proved that this method can be effectively applied to different wind turbines.
Data-driven controller for the WECS based on neural network compensation control is designed in this paper.Firstly, a 10 min data of wind turbine was selected by SCADA system; the data-driven control system Markov parameters can be obtained by input and output data of the WECS.Secondly, the controller gain can be obtained by closed solution of differential Riccati.
In order to improve the WECS control precision and capture more wind energy, a neural network compensator is designed to counteract dynamic disturbances caused by unkown parameters of the WECS, different neural network learning rules are selected in the training process, and another neural network is used to optimize the wind energy utilization coefficient and reduce the mechanical shocks on the WECS.Analysis results show that this way more wind energy can be captured and safety of the wind turbine can be enhanced.

Problem Description
Wind speed and wind direction are uncontrollable variables while, wind generator speed and torque are controllable variables [29,30,37,38].The SCADA system of wind turbines is shown in Figure 1; the system can monitor multiple wind farm data, for example, wind turbines, grid data, and substation data.Data of output power, wind generator speed, wind generator torque, and wind speed is selected in this paper.
Wind farm data collected is given in Table 1; 10 min data is used in this paper, the data from 2010/07/02 00:10:00 A. to 2010/07/20 02:20:00 AM. v(t − 1) is wind speed data of previous sampling time period, and P a (t − 1) is wind generator data of previous sampling time.
According to the literature [1], the relationship between wind energy utilization coefficient and tip speed ratio can be known.As in Figure 2, the tip speed ratio reaches optimum value, the maximum wind energy utilization coefficient C p max is around 0.47, tip speed ratio optimum value is around 7, and blade pitch angle is around 0 degree;  the figure has general applicability to the variable speed constant frequency wind turbine: Letting, C = 0.5πρR 3 , in (1), Γ is wind wheel torque, ρ is air density, R is radius of the wind wheel, and v is wind speed.The relationship between wind speed and wind turbine power output is shown in Figure 3, There are multiple unknown parameters and uncertainties in the WECS, the system has strong nonlinear characteristics, and its model is difficult to be established.Using the equivalent model and ignoring a number of uncertainties are often the solutions; however, the modeling accuracy is greatly reduced and seriously affects the control effect.Data-driven control only needs system input and output data.So, data-driven control model for the wind power system can be expressed as where y j is wind generator power output.Because three groups of data are selected for the experiment, j = 3, j can be selected as a different value according to actual needs.x j is system state, including wind generator speed, wind generator torque, and wind speed.u j is the system control input.Q j and R j are positive semidefinite symmetric weight matrices, M i j = C j A (i−1) j B j , where M i j are wind power control system Markov parameters:

Data-Driven Control Methodology
Lemma 1 (see [39]).The closed solution of differential Riccati equation is where Theorem 2. For given Markov parameters of the WECS, after introduction of a closed solution of the differential Riccati equation, the data-driven controller can be expressed as where G(k) is the data-driven controller gain x c (k) is the datadriven controller state where where A j , B j , and C j of the Markov parameters M i j can be obtained by the input u j and output y j , and the column vectors u j (k) and y j (k) are formed by p-Step input and output data from the beginning of the {u j , y j }: Then, matrices Y j and V j in (11) can be formed by the input data and output data.The relationship between input and output data can be expressed as (12) where P j1 , P j2 , and matrix T j p can be obtained by solving (9); when p = N + 1, the WECS Markov parameters M i j can be extracted from T j p .
The test time is from 2010/07/21 00:30:10 AM to 2010/ 07/29 00:25:30 AM, corresponding control input data and output data set are {u 1 , y 1 },{u 2 , y 2 },{u 3 , y 3 } and The original power output and the optimized power output are shown by Figures 4(a)-4(c).It can be seen that data-driven controller can effectively identify wind turbine power output and control precision achieves the desired effect.

Neural Network Optimization
As shown in Figure 2, the tip speed ratio can be adjusted by controlling wind generator speed to improve wind energy utilization coefficient C p .Wind generator speed is optimized by neural network compensator to improve wind energy capture efficiency.Neural network performance evaluation [40][41][42][43] is expressed as ( 14) where O ti is the target output of the compensator and i = 1, 2, . . ., n. O i is the actual output of the compensator, which is Δu jnn (k) in (17).Neural networks have many learning rules; different learning rules lead to different training efficiency, OUTSTAR learning rules are used to train neural network compensator in this paper, which is intended to generate an m-dimensional desired output vector.Weight vector ΔW k of OUTSTAR learning rule can be expressed as Before adding the data-driven controller, open-loop input data which constitutes the matrixY j and output data which constitutes the matrixV j are collected, row vector of matrixV j and matrixY j must be linearly independent.Inputs of the neural network compensator are e j (k − 1), y j (k − 1), and Δu j (k);Δu j (k) is the error between datadriven controller current input u(k, x j ) and previous input u(k −1, x j ).Output of the compensator is Δu jnn (k); Δu jnn (k) is the error between neural network current input u jnn (k, x j ) and previous input u jnn (k − 1, x j ) : The method has general applicability, so randomly wind turbine 1, wind turbine 2, and wind turbine 3 are selected as the study objects, and learning rules of EBPA and LMS are used to train the neural network compensator also.Equation (18) give the wind turbine output power mean absolute error (MAE), the standard deviation of mean absolute error (SD of MAE), the relative mean absolute error (RMAE), and  the standard deviation of relative mean absolute error (SD of RMAE), where y K is control output, y 0 is instance value, and n is sample number.It can be seen that the model built on the collected data is stabler with the OUTSTAR learning rules.Output error with different training rules is given in Table 2:

Analysis and Results
The original value and optimal value of wind energy utilization coefficient are compared in Figure 5, it can be known wind energy captured is increasing by adjusting tip speed ratio value, as the tip speed ratio is close to the optimal value, the value of wind energy utilization coefficient is increasing.
Wind turbine 1 is taken as study object, and a 5-17-1 neural network is selected to optimize wind energy captured by the wind wheel.Figure 6(a) gives the original and optimal generator speeds.It can be seen that the optimal generator speed is stabler than the original one.Figure 6(b) gives the original and optimal power output.It can be seen that the optimal power is more than the original one.

Conclusion
In this paper, data-driven controller for the WECS is designed based on neural network, Markov parameters of the data-driven controller can be obtained by input and output data of the WECS.To overcome the inadequacy of data-driven control, neural network compensator is used to compensate for the deviation output of wind power datadriven control system; the results show that the system stable performance is improved.10 min data of wind farm SCADA system is collected, including wind generator power output, wind generator speed, wind energy utilization coefficient, and tip speed ratio.The method first determines the WECS Markov parameters, the system output is controlled by adjusting the data-driven controller gain, and the neural network is trained by different  neural network learning rules.The results show that the generator speed fluctuations can be reduced, the security of wind turbine operation is high, more wind energy is captured, and the method is relatively simple and easy to understand.

Figure 1 :
Figure 1: SCADA system for wind farm.

P 7 Figure 2 :
Figure 2: Wind energy utilization coefficient versus tip speed ratio.

Figure 3 :
Figure 3: Wind speed versus wind generator power.
coefficient (b) Wind energy utilization coefficient

Figure 5 :
Figure 5: Wind energy utilization coefficient versus tip speed ratio.

Table 1 :
10 min sample data of SCADA system. .

Table 2 :
Output error with different training rules.