Grid Ancillary Services From Doubly Fed Induction Generator-Based Wind Energy Conversion System: A Review <s> B. REACTIVE POWER CONTROL <s> From the Publisher: ::: This is a book on practical feedback control and not on system theory in general. Feedback is used in control systems to change the dynamics of the system and to reduce the sensitivity of the system to both signal and model uncertainty. The book presents a rigorous, yet easily readable, introduction to the analysis and design of robust multivariable control systems. It provides the reader with insights into the opportunities and limitations of feedback control. Its objective is to enable the engineer to design real control systems. Important topics are: extensions and classical frequency-domain methods to multivariable systems, analysis of directions using the singular value decomposition, performance limitations and input-output controllability analysis, model uncertainty and robustness including the structured singular value, control structure design, and methods for controller synthesis and model reduction. Numerous worked examples, exercises and case studies, which make frequent use of MATLAB, are included. MATLAB files for examples and figures, solutions to selected exercises, extra problems and linear state-space models for the case studies are available on the Internet. <s> BIB001 </s> Grid Ancillary Services From Doubly Fed Induction Generator-Based Wind Energy Conversion System: A Review <s> B. REACTIVE POWER CONTROL <s> This paper presents the simulation results of a grid-connected wind driven doubly fed induction machine (DFIM) together with some real machine performance results. The modeling of the machine considers operating conditions below and above synchronous speed, which are actually achieved by means of a double-sided PWM converter joining the machine rotor to the grid. In order to decouple the active and reactive powers generated by the machine, stator-flux-oriented vector control is applied. The wind generator mathematical model developed in this paper is used to show how such a control strategy offers the possibility of controlling the power factor of the energy to be generated. <s> BIB002 </s> Grid Ancillary Services From Doubly Fed Induction Generator-Based Wind Energy Conversion System: A Review <s> B. REACTIVE POWER CONTROL <s> In this paper, a robust state-feedback control law is designed for rotor side converter of doubly fed induction generator to enhance both transient and steady-state performances of grid connected variable speed wind turbine. More explicitly, a disturbance observer-based control (DOBC) is designed to compensate for the offset caused by model uncertainties and unknown external disturbances which is not considered in the DFIG modeling. Moreover, the fast dynamic response is inherited from the state-feedback control law. The composite controller consisting of the state-feedback controller and the disturbance observer allows achieving a stable and accurate control of the stator active and reactive powers. The proposed control strategy is verified by simulations and experimental testbed. Various tests are conducted to demonstrate the effectiveness of the proposed control strategy, and satisfactory results are obtained. <s> BIB003 </s> Grid Ancillary Services From Doubly Fed Induction Generator-Based Wind Energy Conversion System: A Review <s> B. REACTIVE POWER CONTROL <s> In this paper, feedback linearization is used to control the speed of a surface mount Permanent Magnet Synchronous Motor (PMSM). An extended high-gain observer is also used to estimate both the speed of the motor and the disturbance present in the system to recover the performance of feedback linearization. The disturbance is assumed to be unknown, and time-varying but bounded. Stability of the output feedback system is shown. Finally, simulation and experimental results confirm stability, robustness, and performance of the system. <s> BIB004 </s> Grid Ancillary Services From Doubly Fed Induction Generator-Based Wind Energy Conversion System: A Review <s> B. REACTIVE POWER CONTROL <s> Abstract A disturbance observer based control method for a grid-connected doubly fed induction generator is presented in this study. The proposed control method consists of a state-feedback controller and a disturbance observer (DO). The DO is used to compensate for model uncertainties with the aim of removing the steady-state error. The control objective consists of regulating the stator currents instead of the rotor currents in order to achieve direct control of the stator active and reactive powers. Such a control scheme removes the need for an exact knowledge of the machine parameters to achieve accurate control of the stator active and reactive powers. The main advantage of this control method is ensuring a good transient performance as per the controller design specifications, while guaranteeing zero steady-state error. Moreover, the proposed control method was experimentally validated on a small scale DFIG setup. <s> BIB005 </s> Grid Ancillary Services From Doubly Fed Induction Generator-Based Wind Energy Conversion System: A Review <s> B. REACTIVE POWER CONTROL <s> This paper designs a novel robust sliding-mode control using nonlinear perturbation observers for wind energy conversion systems (WECS), in which a doubly-fed induction generator (DFIG) is employed to achieve an optimal power extraction with an improved fault ride-through (FRT) capability. The strong nonlinearities originated from the aerodynamics of the wind turbine, together with the generator parameter uncertainties and wind speed randomness, are aggregated into a perturbation that is estimated online by a sliding-mode state and perturbation observer (SMSPO). Then, the perturbation estimate is fully compensated by a robust sliding-mode controller so as to provide a considerable robustness against various modelling uncertainties and to achieve a consistent control performance under stochastic wind speed variations. Moreover, the proposed approach has an integrated structure thus only the measurement of rotor speed and reactive power is required, while the classical auxiliary dq-axis current regulation loops can be completely eliminated. Four case studies are carried out which verify that a more optimal wind power extraction and an enhanced FRT capability can be realized in comparison with that of conventional vector control (VC), feedback linearization control (FLC), and sliding-mode control (SMC). <s> BIB006
As discussed previously, the DFIG system is able to provide both active and reactive powers. In the following test, a step change in the DFIG stator reactive power is conducted. Five control methods, proportional integral (PI) control BIB002 , H ∞ (Hinf) control BIB001 , state-feedback with disturbance observer (SFDO) control BIB003 , BIB005 , state-feedback with high gain observer (SFHGO) control BIB004 , and state-feedback with sliding mode perturbation observer (SFSMPO) control BIB006 , are selected as examples and investigated in terms of transient VOLUME 7, 2019 and steady-state performances. Figure 19 shows simulation results of the DFIG reactive power step change test. The five control methods seem to have similar response in steady-state region. However, the transient performance can be further analyzed as shown in Figure 20 . The PI control method requires more time to reach the reference reactive power value. The PI controller parameters are chosen using trial and error. The H ∞ control reaches to the reactive power reference faster than the PI controller. The H ∞ control has tradeoff between performance and robustness to uncertainties and disturbances. The controller weighting functions are choose to obtain a balanced performance in this test. The SFDO and SFSMPO have the best transient performance. The SFHGO control performance is similar to SFDO; however, during transients, the high gain observer needs time to force the estimated states to track their actual states. This will negatively affect the controller overall transient performance since the actual and estimated states do not match each others during transients. As a result, the state-feedback controller will see mismatched signal since it uses the state estimate as input. Table 4 summarizes the control methods performance in this DFIG reactive power test. There are a lot of other novel control methods, and each method has its own advantages and disadvantages. However, conducting a fair comparison between them is a challenging task. Saying that one control method is better than the other is not necessarily correct. The controller effectiveness depends on the control objectives, requirements, and also on the system itself. In the end, all methods are used for one supreme purpose which is improving the DFIG base WECS services.