Improved Performance of ZSI based UPFC for Enhanced Power Flow in Interconnected System

Abstract

In today’s highly complex and interconnected power systems, there is a great need to improve power utilization while still maintaining reliability and security. Reducing the effective reactance of lines by series compensation is a direct approach to increase transmission capability. However, a power transfer capability of long transmission line is limited by stability consideration. Oscillation of generator angle or line angle are generally associated with the transmission system disturbances and can occur due to step changes in load, sudden change of generator output, transmission line switching and short circuit. This low frequency is important to damp as quickly as possible because they cause mechanical wear in power plants and cause power quality problem. If the electromechanical oscillations are not properly controlled in the electric power system operation, it may lead to a partial or total system outage. Instability problems in power systems that can lead to partial or full blackout can be broadly classified into three main categories, namely voltage, phase angle and frequency related problems. In early age this signal instability problem was solved by amortisseurs implemented in generator rotors, later with the application of fast excitation system this was solved by development & utilization of Power System Stabilizer (PSS) and however in modern power system due to the connection of power grids in vast area, for inter area oscillation damping due to the ability of controlling line impedence, power flow and bus voltage, Flexible AC transmission Systems (FACTS) devices implementation offers an alternative solution. The ZSI employs an impedance network to connect the inverter main circuit to the power circuit thus providing unique features that overcomes the conceptual and theoretical barriers limitations of VSI and CSI. This inverter has unique features in terms of voltage (both buck & boost). The focus of this research work is on a FACTS device known as Unified Power Flow Controller (UPFC) implemented with Z-Source Inverter (ZSI), which can provide simultaneous control of basic power system parameters like voltage, active power flow, reactive power flow, impedance and phase angle. Employing ZSI in UPFC further improves the performance of UPFC in transmission network. In this research work, simulation models for interconnected power systems are carried out without UPFC, with VSI based UPFC and With ZSI based UPFC. Simulation models have been incorporated into MATLAB based Power System Toolbox (PST). These models were analyzed for active power flow and reactive power flow with and without UPFC. Performance analysis of VSI based UPFC and ZSI based UPFC has been studied under faulty conditions also. The results of the power system without UPFC and with VSI based UPFC and ZSI based UPFC are compared and the conclusion is given at the end under normal and abnormal conditions.