CONVERTER UNIT
This usually consists of two three phase converters bridges connected in series to form a 12 pulse converter unit. The total number of valves in such a unit are twelve. The valves can be packaged as signal valve, double valve or quadrivalve arrangements. Each valve is used to switch in a segment of an AC voltage waveform. The converter is fed by converter transformers connected in star/star and star/delta arrangements.
The valves are cooled by air,oil,water or freon.Liquid cooling using deionized water is more efficient and results in the reduction of station losses. The ratings of a valve group are limited more by the permissible short circuit currents than steady state load requirements.The design of valves is based on the modular concept where each module contains a limited number of series connected thyristor levels.
Valve firing signals are generated in the converter control at ground potential and are transmitted to each thyristor in the valve through a fiber optic light guide systems. The light signal received at the thyristor level is converted to an electrical signal using gate drive amplifiers with pulse transformers.
The valves are protected using snubber circuits, protective firing and gapless surge arresters. Some of the details of control and protection of thyristor valves are given.
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