The problem of control of parallel-connected photovoltaic (PV) based distributed generation (DG) units in an AC distribution system is studied in this work. Firstly, a new robust back-stepping sliding mode based maximum power point tracking (MPPT) strategy is designed for each PV system. Then, a new control scheme is presented for power management of parallel-connected inverters of PV systems. The proposed control technique is an improvement to the instantaneous average current sharing control scheme (IACS) and uses adaptive fuzzy controllers to adjust the gains of the controllers of each inverter module. The fuzzy inference system is optimized using the H$\infty$ optimal theory to reduce the error in the fuzzy system. This adaptive fuzzy gain scheduling is then introduced to the IACS controller to improve the current and power sharing for a condition in which the system parameters change. Simulation results are presented to show the effectiveness of the proposed control strategy in improving the load sharing among parallel-connected PV systems.

Offshore wind generators are connected to the ac power grid through high-voltage direct current (HVDC) transmission systems. However, when a short-circuit fault occurs in a transmission line, which is connected to the point of common coupling (PCC) of the HVDC system, the performance of distance relays on the ac grid is affected. This is due to rapid response of reactive power control of the grid-side voltage source converter (GVSC) of the HVDC system to compensate the voltage drop of the PCC bus. In other words, during the fault, the GVSC injects the reactive power to the ac grid to restore the PCC bus voltage. This in turn causes to interfere with the distance protection scheme by affecting the viewed impedance of distance relays. To overcome this problem, based on robust control theory, the present paper introduces a robust control approach for voltage source converter of HVDC system using μ synthesis analysis. The proposed strategy controls the reactive power injected to the PCC bus by trying to keep the defined current ratio $\alpha $ constant. The effectiveness of the proposed approach was validated through simulation using MATLAB/Simulink.

In this paper, a new scheme based on state feedback control concept is presented for robust control of bidirectional interlink power converters (BILPCs) in hybrid microgrids (HMGs). The studied grid-connected HMG contains one dc and one ac microgrid, interconnected by BILPCs. To consider uncertainties, the nonlinear uncertain model of the BILPC is derived and linearized based on Lie derivative concept. As a new point of view for dynamic modeling of BILPC, it is indicated that each nonlinear uncertain BILPC model can be represented by two decoupled subsystems. Then, a virtual robust state feedback controller is designed to independently control each of these subsystems with disturbance rejection capability. The gains of the state feedback controllers are obtained using Ackerman pole placement method. To approximate the unknown inputs/unmodeled dynamics, which are needed in the design process of the state feedback controllers, a high-gain proportional-integral (PI) observer is designed. The high gains of this PI observer are determined using linear quadratic regulation method. Simulation results and comparative studies support the efficacy of the proposed control approach.

This work illustrates the application of an observer-based robust control of unified interphase power controller (UIPC) for power flow control between interconnected microgrids in hybrid microgrids. Here, the challenges of UIPC control in heterogeneous structure of hybrid microgrid are addressed, and a new control scheme for UIPC is proposed. In the proposed control scheme, the states of the UIPC dc link nonlinear dynamics are estimated using a Quasi-Luenberger observer. Then, the estimated states are implemented in a robust sliding mode controller that alleviates the effect of disturbances on the dc link dynamics and effectively controls the dc side power converter of the UIPC. The simulation results and comparison studies are provided to validate the efficacy of the proposed control scheme for hybrid microgrid.