A Control Technique for Grid-Connected PV System under Unbalanced Conditions

Abstract

This thesis deals with the unbalance problem as one important issue related to the power quality aspects. The bad effects of the unbalance appear on the power system components as heat, losses, and reducing the system efficiency. The unbalance reasons essentially come from three sources; asymmetric supply, asymmetric network, and finally from the scattered distributed loads. Also, the increase in the expanding use of renewable energy sources adds a burden to the problems of the network in terms of the unbalance. The grid-connected PV systems can be attached in a single-phase in a scattered manner. Also, the profile of the phases varies all the time. Therefore, these reasons can contribute to the unbalance issue. So power capturing from PVs or Distributed Generators (DGs) to solve the unbalance problem is considered as one objective from the thesis even for the local loads. There are many topologies used to solve the unbalance problem. The concept for unbalance mitigation is based on controlling each phase independently. Creating a neutral point connection is a key to construct four-wire systems using either mid-capacitor-point inverter, four-leg inverter, or three single-phase H-bridges. In this thesis, the mid-capacitor-point is selected as a compensator. The Three Dimensional Space Vector Pulse Width Modulation (3D-SVPWM) is considered as an effective tool for dealing with the unbalance. While the Two Dimensional Space Vector Pulse Width Modulation (2D-SVPWM) is restricted with a balanced load. This thesis introduces two SVPWM techniques. The first one is the conventional 3D-SVPWM which is based on several and more calculation procedures. The second PWM is based on the simplified 3D-SVPWM as a contribution. The main objective of the simplified method is to reduce the calculation time to avoid the computational burden of conventional methods. The main idea of the simplified method 3D-SVPWM is based on the curve fitting method. The technique is tested as an open-loop controller and based on a pure sine wave reference signal.