A control Technique for Four Leg Inverter to Diminish Electrical Grid Unbalance

Abstract

Power quality problems are increasing rapidly, especially current harmonics, reactive power, and neutral current. Current harmonics lead to various issues in distribution systems and consumer products such as overheating machines, unsafe operation of safety equipment, overheating transformers, etc. Further neutral current leads to overheating and also to neutral line damage. High reactive current/power also increases network losses and reduces their reliability. The unbalance reasons essentially come from three sources; asymmetric supply, asymmetric network, and finally from the scattered distributed loads. In order to protect equipment from power interruption, various forms of power improvement devices have progressed over the years. This thesis decreases the problems of unbalance by compensating the source current harmonics and the reactive power utilizing a shunt active power filter (SAPF). In literature, many topologies are proposed to solve power quality problems. The concept for solving the problem is based on controlling each phase independently. Therefore, creating a neutral point connection is a key to construct the four-wire such as Mid Capacitor Point, Four-Leg Inverter, and three single-phase of H-bridge. In this thesis, the Four-Leg Inverter (4LI) is selected as a compensator. The performance of SAPF depends on inverter parameters, control schemes, and reference current detection control techniques. Increasing nonlinear load leads to growing the SAPF rating also increases the accumulating system capacity and cost. This thesis tries to enhance the SAPF performance by using a suitable control scheme. The control system of SAPF consists of three parts: harmonic extraction (including reference current generation), DC-link voltage control, and current control. This dissertation presents an investigation and evaluation of the performance of the SAPF based on 4LI utilizing different DC-voltage and current controllers: the HCC, conventional PI, FLCT-1, and FLCT-2 controllers. These controllers are compared with the proposed controller which is the simplified universal intelligent (SUI-PI) controller. In addition, this thesis presents the three-dimensional space vector modulation (3DSVM) which is considered an effective switching technique for the 4LI. The proposed system is simulated on the Matlab /Simulink software to study the dynamic performance under three different unbalanced and/or nonlinear loads. The THD values are compared before and after the compensation process for the each of the proposed controllers under different study cases.