Robust Active and Reactive Power Controllers for a Grid Connected Wind Energy Conversion System

Abstract

Permanent Magnet Synchronous Generator (PMSG) is widely used in a Wind energy Conversion System (WECS) due to its several advantages, such as gear less construction, high power density, less noise, high torque and ease of maintenance. The control objective for a PMSG based WECS is to regulate the active power extraction from the wind and at the same time to maintain the reactive power at zero value in order to achieve unity power factor operation of WECS. A number of control algorithms have been proposed in the past to control the active power and reactive power of WECS. However, a WECS is subjected to a large number of parametric uncertainties and external disturbances. Thus, it is necessary to negate the effect of parametric uncertainties and external disturbances appearing in the WECS dynamics in order to achieve satisfactory performance by designing suitable robust control algorithms.

A lot of research works have been directed to control the active power and reactive power of WECS. But parametric uncertainties greatly influence the active power and reactive power control performance in a WECS. Thus, challenges in designing suitable controllers for
regulating active power and reactive power of the WECS in face of parametric uncertainties and disturbances.

The thesis focuses on development of different robust control algorithms for a PMSG based grid connected WECS to control both reactive power in face of parametric uncertainties and disturbances. Since both the active power and reactive power of the PMSG are coupled with each other, hence it is necessary to design controllers such that the active power and reactive power can be controlled independently by appropriately decoupling the active power and reactive power control loops.

The thesis begins with development of a robust H∞ controller to regulate the active power and reactive power of PMSG based WECS delivered to the grid. The detailed design of the inner current control loop and the outer speed control loop are also presented. The H∞ based
robust controller for the PMSG is synthesized using MATLAB/Simulink. The performance of the proposed controller is first verified at nominal conditions of PMSG and then in the presence of the parametric uncertainties. To investigate the robustness of the proposed H∞ controller, stator resistance and stator inductance of the PMSG are varied and efficiency of the proposed H∞ controller in rejecting the effect of the external disturbance is also evaluated. The peak overshoot and the settling times of the active power response obtained by applying the proposed H∞
controller with the variation in these aforesaid parameters are compared. From the comparison, it is observed that the proposed controller is efficient in regulating the active power and reactive power of PMSG in face of parametric uncertainty and also able to reject the adverse effect of external disturbance in set point tracking.The efficiency of the proposed H∞ controller is also evaluated experimentally. Comparing the simulation and experimentation results it is concluded that the proposed H∞ controller effectively handles the parametric variation for setpoint tracking of active power and reactive power of WECS.

Although the proposed H∞ controller exhibits robust performance in regulating both the active power and reactive power but, the controller has some drawback i.e. the nominal performance of the WECS may slightly degrade. The design of this controller involves complex
mathematical calculations. Further, the designed H∞ controller is of high order and thus necessitates reduction of the order. Even though the proposed H∞ controller is able to reject the disturbance but the active power tracking performance is poor when disturbance is present. Moreover, the effect of the external disturbance needs to be attenuated faster than obtained with the proposed H∞ controller to minimize its adverse effect. In view of this,a Multi loop Active Disturbance Rejection Controller (MADRC) is designed to resolve the above limitations of the proposed H∞ controller. The basic idea behind designing the Active Disturbance Rejection Controller (ADRC) is to consider the internal dynamics, parametric uncertainties, the coupling term and the external disturbances as a lumped disturbance which can be estimated by designing an Extended State Observe (ESO). Subsequently, these lumped disturbances can be rejected by designing a suitable control law. The proposed MADRC is designed for speed control loop, d-axis current control loop and q-axis current control loop. The performance of the proposed MADRC is verified at nominal condition of PMSG from which it is observed that the peak overshoot and the settling time yielded by applying MADRC are less compared to the corresponding values obtained with the proposed H∞ controller. Also, the efficacy of the proposed controller is evaluated for variation of stator resistance and stator inductance of PMSG. For this, a step and variable wind speeds are applied to the WECS. From the obtained results it is observed that the MADRC regulates both the active power and reactive power under the aforesaid conditions effectively by generating suitable control actions. The performance of the proposed MADRC in active power and reactive power tracking in face of parametric uncertainties is also compared with the Single loop Active Disturbance rejection Controller (SADRC) by applying a step change in wind speed. On comparing the results, it is found that in the case of MADRC, the peak overshoot and settling time are less than that of SADRC in q-axis current and in active power responses of the PMSG. Further, the effectiveness of the proposed MADRC to reject the effect of the disturbance is verified by applying a step change in wind speed. From the result analysis, it is observed that the proposed MADRC is efficient to reject the external disturbance faster as compared to the H∞ controller. The adverse effect of the disturbance is less visible in terms of lower values peak overshoot yielded by MADRC as compared to the H∞ controller.

Although, the performance MADRC is satisfactory in active and reactive power tracking of WECS in the presence of parametric variation and external disturbances, there lie some drawbacks in this control scheme are as well. The exact estimation of the lumped disturbance by the ESO is difficult. Moreover, if the total disturbance is not constant, the estimation error in ESO may not converge to zero. Hence, to resolve this problem a Two Degree of Freedom Internal Model Controller based Active disturbance Rejection Controller (TDFIMC-ADRC) is designed. In TDFIMC-ADRC, the exact mathematical model of WECS is not required when designing a controller for WECS. In TDFIMC-ADRC, the ESO is omitted and a set point and a disturbance rejection filters are incorporated in this control scheme. Hence, the setpoint tracking and disturbance rejection are achieved in two individual decoupled control loop. The design of setpoint tracking and disturbance rejection filters for all the control loops (speed, d-axis current and q-axis current) is presented. With these filters, the proposed TDFIMC-ADRC is designed and then the performance of the proposed TDFIMC-ADRC is evaluated for active and reactive power tracking in a PMSG based WECS. The performance of the controller is verified at nominal condition of the PMSG and subsequently by varying Rs and Ls of PMSG. From the results analysis, it is observe that the proposed TDFIMC-ADRC exhibits superior performance i.e. yielded less peak overshoot and less settling time as compared to the corresponding values obtained with MADRC with a step change in wind speed and a variable change in wind speed. The efficacy of the proposed TDFIMC-ADRC controller to attenuate the adverse effect of the external disturbance is also studied. It is observed that the proposed TDFIMC-ADRC rejects the disturbance faster compared to the both H∞ controller and MADRC.

From the simulation and experimental results obtained when by applying all the above three proposed controllers, it is observed that these controllers exhibit robust performance for set point tracking of active power and reactive power of the PMSG based WECS. The performance indices such as peak overshoot and settling time in the active power and reactive
power responses obtained by applying the proposed three robust controllers are compared in the presence of the parametric uncertainty. From the comparative assessment, it is envisaged that the TDFIMC-ADRC exhibits superior performance amongst the three in face of parametric
uncertainties. The effect of the disturbance in the active power and reactive power tracking performance for all the proposed controller are compared. From this comparison, it is also observed that the TDFIMC-ADRC rejects the disturbance accurately and quickly compared
to both H∞ and MADRC. Moreover, the proposed TDFIMC-ADRC has simple structure and its implementation in both simulation and experiment becomes less complex as compared to both H∞ controller and MADRC for a PMSG based WECS in face of parametric uncertainties and external disturbances.Thus it is concluded for achieving robust active power and reactive power control performances when parametric uncertainties and external disturbances exist, the TDFIMC-ADRC is the best choice amongst all the proposed controllers described in the thesis.