Design and Development of Crowbar Circuit for High Voltage D. C. Application

Abstract

The crowbar circuit is useful for protection of load against over voltage and over current transients. When such conditions are detected the switching device shorts the supply and voltage is reduced to a very low level. In this way it protects the load circuit.The crowbar circuit finds applications in high voltage dc power supply fed loads, protection of the frequency converter circuit in the rotor circuit of the doubly fed generator and HV tube protection such as klystron etc. The various crowbar circuits used for different applications operate in the range of few milli seconds to few micro seconds. The method of discrimination is absolute comparison of parameters. For critical application the crowbar circuit needs to be operated very fast. Further if di/dt and dv/dt of the load circuit can be sensed and used as feedback the fault in the early stage can be identified. Such kind of discrimination features are not incorporated in most of the crowbar circuit reviewed under different articles. The objective of the project is to develop the crowbar circuit that will discriminate the fault,trigger the switching device and isolate the load. The development of project aims to achieve the total time required to accomplish the complete operation in one microsecond. This novel concept is introduced first time for the crowbar applications. Further the sensing of di/dt and dv/dt will reduce the stress on the load because fault can be detected at early stages of development. Another application of crowbar is developed for protection of excitation circuit of alternator. Normally the crowbar protection is provided when over voltage protection of field circuit fails. The features in the development of project includes the protection of field circuit against positive swing, negative swing in the input voltage to field circuit and external triggering in emergency condition. Further, the crowbar operates faster than field circuit breaker.The test is successful and the total response time of the crowbar from the instant of discrimination to turn on of crowbar thyristor varies in the range of 50 s to 150 s.