Performance Analysis of Unified Power Quality Controller for a Two Bus System with Different types of Loads

1. Introduction

The purpose of a power transmission line is to group generating stations and Load centres. It is generally constrained by line impedance, operating variables voltages and currents. Therefore generating stations may not be able to supply power demand. In addition now a days deformation of Power Quality which is effected by non linear loads and electronically switched devices is also a constraint. This affects the cost and reliability of the supply of the electricity. This may lead to large power flows with inadequate control, excessive reactive power in various parts of the power system, large dynamic swings between different parts of the system, thus the full potential of transmission line can not be utilized.

This need emerged new technology Flexible AC Transmission systems (FACTS). FACTS technology uses switching power electronics to control power flow. The FACTS technology is a collection of controllers, which can be applied individually with others to control one or more of the interrelated system parameters (Padiyar, 2007). FACTS controllers can enable a transmission line to carry power closer to its thermal rating. The idea of FACTS is to use power electronics for controlling power flow in a transmission line and allowing it to fully load up to its capability (Hingorani & Gyugyi, 2011).

The over view of the FACTS devices is shown in the Table 1. The thyristor based FACTS devices are Static VAR compensator (SVC), Thyristor Controlled Series Compensator (TCSC) and Dynamic Flow Controller (DFC). These devices have low losses due to their low switching frequency.

The other type of FACT devices are voltage source converters based uses mainly Insulated Gate Bipolar Transistors (IGBT) or Insulated Gate Commutated Thyristor (IGCT) as devices. The FACT devices are namely Static Synchronous Compensator (SSC or STATCOM), Static Synchronous Series Compensator (SSSC) and Unified Power Flow Controller (UPFC). These types of FACTS controllers are more advanced Technology. The use of IGBT with Pulse Width Modulation (PWM) allows high modulation frequencies to get low harmonics in the output voltage in magnitude and phase. With the increase in switching frequency, the switching losses also increase.

The shunt compensating FACT devices SVC and STATCOM are primarily for reactive power compensation, a current is injected in to the system at the point of common coupling and it’s a good way to control the voltage. The SVC provides smother and precise control in comparison with mechanically switched shunt compensation and improves stability. The STATCOM in comparison with SVC provides power quality in against even dips and flickers. Shunt compensator can not provide control of the power flow in the lines.