Power quality improvement in multi bus system using unified power flow controller

Abstract

The possibility of controlling power flow in electric system without any rescheduling and topological changes can improve the power system performances. It has been proved that, instead of building new transmission lines, an efficient usage of the existing line to their thermal limit is possible. Flexible AC Transmission Systems (FACTS), which are power electronic based devices, can change parameters like impedance, voltage and phase angle. Therefore they have the ability to control power flow pattern and enhance the usable capacity of the existing lines. The important feature of FACTS Technology is that they can vary the parameters rapidly and continuously, which will allow a desirable control of the system operation. The Unified Power Flow Controller (UPFC) is a Flexible AC Transmission Systems device used for improving the power quality in power systems. The UPFC consists of a combination of series device and shunt device, the DC terminals of which are connected to a common DC link capacitor. The series device acts as a controllable voltage source VC, whereas the shunt device acts as a controllable current source IC. The main purpose of the shunt device is to regulate the DC link voltage by adjusting the amount of active power drawn from the transmission line. In addition, the shunt device has the capability of controlling reactive power. This work deals with power quality improvement using UPFC. UPFC is capable of controlling real and reactive powers in a transmission system. The voltage compensation capability of UPFC depends on the maximum voltage that can be injected. Voltage injection using five level Voltage Source Inverter (VSI) based UPFC is proposed to enhance the voltage stability. The simulink models and simulation results of UPFC system employing multilevel inverter are presented. Pulse Width Modulation (PWM) based inverter and Space Vector Modulation (SVM) based inverter are proposed as a block in the UPFC system.