IMPROVEMENT OF POWER QUALITY BY FRIENDS DEVICES


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INTRODUCTION

POWER QUALITY issues are currently receiving a great deal of attention in the light of distribution generation, deregulation, liberalization and privatization of the electrical energy market. For AC transmission and distribution system, power quality broadly refers to maintaining a near sinusoidal bus voltage at rated magnitude and frequency. The power quality of a supply voltage can deteriorate due to very high-speed.


STATIC SWITCH

This is the combination of two anti-parallel thyristors as shown in Fig. 1. Assume that an alternating current flows from the left to the right of the static switch. The positive half cycle current is passed by the thyristor that is in forward biased condition. Similarly the negative half cycle current is passed by the other thyristor. Thus each thyristor conducts for half cycle and remains in the off state for another half cycle. The current flowing through static switch can be interrupted by blocking the firing signal.


STATIC CIRCUIT BREAKER

The schematic diagram for SCB is shown in Fig.3 .The main components of this equipment are a high-speed vacuum circuit breaker (VCB) and a GTO based static switch connected in parallel. The usual load current is carried out by the VCB. In case of system fault, the fault current is commutated to and interrupted by the GTO based static switch by opening the vacuum switch. This equipment does not require a large cooling system and is compact compared with conventional solid state equipment.


STATIC TRANSFER SWITCH

The thyristor based single phase STS is illustrated in the Fig. 4. The success of the device is mainly due to its low cost and fast response due to the phenomenon of fast switching, usually referred to as “make before break” switching (MBB). The name refers to the possibility of firing the thyristors of the static switch that is not conducting (alternative source), thus initiating the transfer, before the current on the corresponding switch in the preferred source has become zero.


COORDINATION
A generic distribution system is shown in Fig. 5 in which the SCL and SCB are to be placed at positions labeled with numerals. This distribution system has two incoming transformers, each connected to one main bus. These two buses are connected by a bus tie-breaker. Devices below the buses are referred to as down stream devices.