STRESSES ON THYRISTOR VALVES IN SVC APPLICATION AND THEIR DESIGN REQUIREMENTS

State of art designs of Static Var Compensators (SVC) using Thyristor Switched Capacitors (TSC) and Thyristor Controlled Reactors (TCR) to perform various duties in the system as per the needs of the system. To achieve the functions of SVC, it is required that absorption and supply of reactive power be performed as fast as possible, typically within a few milliseconds. This can be achieved by use of fast acting switching and control devices like thyristors. Thyristor valves find their application in SVCs mainly to 'switch in' and 'switch out' the capacitor banks and corntrol of current in reactors. The combination of TSC - TCR type compensator is based on the capability of thyristor valves to switch capacitors and control reactive current through reactors at precise time instants to meet the system VAR demand with minimum transient disturbance and harmonic generation. This combination also ensures the control of reactive VAR demand in the network smoothly through the entire range of operation. The thyristor valves in SVC applications encounter various types of stresses especially switching transients, over voltage and over current stresses. The valves for SVC duty are designed after a detailed analysis of reverse recovery transients, system over voltages and misfiring of thyristor valves under unwanted situations. This paper discusses various aspects regarding design of SVC thyristor valves, its protection requirements and type testing requirements which were evolved after detailed design studies for thyristor valves employed for the SW installed by POWERGRID at their Kanpur Substation.