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Modern electrical grids face serious challenges to maintain healthy operational conditions. The nominal voltage and frequency, for instance, are affected by huge load changes and renewable energy integration. Renewable energy conversion systems are traditionally installed to provide active power. Fortunately, advanced renewable energy conversion systems have the ability to provide other facilities such as reactive power control and harmonic mitigation. In fact, recent grid codes obligate renewable energy systems to serve other duties such as generating reactive power rather than just simply supply active power, participate in fault ride through, for instance. This paper presents a comprehensive study on the doubly fed induction generator (DFIG)-based wind energy conversion system (WECS), which provides ancillary services to the grid along with performing its regular duties. These services include reactive power control, voltage ride through, power quality improvement, frequency control, and power oscillation damping. The literature survey and comparative studies show that the DFIG-based WECS has the ability to support electrical grid during normal and transient conditions, and it can also help in the large-scale renewable energy penetration to the power grid. The effect of different industrial and nonlinear controllers on DFIG performance in extracting grid ancillary services is highlighted. INDEX TERMS Doubly fed induction generator (DFIG), frequency control, grid ancillary services, power oscillation damping, power quality improvement, reactive power control, voltage ride through, wind energy conversion system (WECS).

Contribution of Photovoltaic (PV) systems is rapidly growing and great attention is given to the design of PV controllers to enhance both the performance of PV systems and the low voltage ride through (LVRT) capability during abnormal operational conditions. This article presents a novel application of the salp swarm algorithm (SSA) in order to optimally tune the PV controllers to enhance the LVRT of grid-connected PV systems. Enhancement of LVRT is indicated in percentage undershoots or overshoots, settling time and steady-state error of voltage response. A control strategy is applied to the DC-DC converter to obtain a maximum power point tracking operation through a proportional-integral (PI)-based open fractional voltage control. The grid side inverter controls both the point of common coupling voltage and the DC-link voltage through PI-based cascaded-voltage control. To get PI controller parameters that guarantee the optimum design of the controllers, the fitness function is optimized by using the SSA. The proposed optimal control scheme is tested under various fault scenarios and compared with other conventional optimization-based PI controllers to examine its validity under PSCAD environment. The effectiveness of the optimal control scheme is verified by comparing the simulation results with the practical results of the PV system.

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.

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