A Simulation study on Distributed Generation Systems with PEM Fuel Cell and PV Cell for Power Quality Enrichment

Abstract

The main focus of this thesis is fundamental investigations of control techniques of inverter-based microgrids. It aims to develop new and improved control techniques to enhance performance and reliability so as to improve the overall power quality. It focuses on analysis of power quality disturbances occurring in the microgrid. Conventional current controllers are only effective when the grid voltage is ideally balanced and sinusoidal. Due to the popular use of nonlinear loads, the grid voltage at the point of common coupling (PCC) is typically not pure sinusoidal, but instead can be unbalanced or distorted. These abnormal grid voltage conditions can strongly deteriorate the performance of the regulating grid current. The control methods, MPC and Droop control however have been emerged as potential control power to achieve high quality grid current and to improve the system dynamic response, eliminate steady state error and to prevent the use of the feedforward. A current predictive based MPC method is proposed in our research work, which has demand to define cost functions and can further simplify the calculation process. MPC has been proposed for the current control, which is the preeminent powerful alternative to conventional method for the conventional current control. This control scheme predicts the future load current behaviour for each valid switching state of the converter, in terms of the measured load current and predicted load voltage. The predictions are evaluated with a cost function that minimizes the error between the predicted currents and their references at the end of each sampling time. A nonlinear control technique has been developed for three-phase voltage-source converters. The converter switching states are selected from a switching table. This algorithm selects the appropriate voltage vectors and calculates duty cycles in every sampling period to minimize the errors of the instantaneous active and reactive power. Simulation studies are performed to verify the performance of the MPC control method and its strategy. In inverter-based microgrids, the paralleled inverters need to work in both grid-connected mode and Islanded mode and should be able to transfer seamlessly between the two modes. In grid-connected mode, the inverters control the amount of power injected into the grid. In Islanded mode, however, the inverters control the island voltage while the output power is dictated by the load. This can be achieved using droop control. This thesis proposes a simple and effective control technique for interconnection of DG resources to the power grid via interfacing converters based on Phase locked loop (PLL) and Droop control. The behaviour of a Microgrid (MG) system during the transition from islanded mode to gridconnected mode of operation has been studied. A dynamic phase shifted PLL technique is locally designed for generating phase reference of each inverter. Droop relations are established between the voltage and current in dq synchronous reference frame (SRF). To provide better dynamics and higher stability, the SRF voltage and current are decoupled by introducing a current vector, which is aligned with the voltage vector. The phase angle between filter capacitor voltage vector and d-axis is dynamically adjusted with the change in q-axis inverter current to generate the phase reference of each inverter. During fluctuations in load capacity, the grid-connected system must be able to supply balanced power from the utility grid side and microgrid side. Therefore, droop control is implemented to maintain a balanced power sharing. An adjusted droop control method for equivalent load sharing of parallel connected Inverters, without any communication between individual inverters, has been presented. The control loops are tested with aid of MATLAB Simulink tool during several operating conditions.