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This paper describes the monitoring devices that were developed to integrate electronic systems for control and monitoring of the main functions of circuit breakers with respect to three main objectives:First, they must improve the overall reliability, availability and dependability of high voltage substations, transmission system and electrical distribution networks where the circuit breakers with monitoring are installed, as for example in a strategic location in a network. Second, the monitoring and control system must offer a minimum set of connections and simplicity of operability and self-maintenance to the user, and insure interoperability with other type of equipment installed in the substation. Third, the monitoring equipment has to be designed to be flexible enough to reply to the galaxy of user requirements of the world.The result is to make the use of a high voltage circuit breaker less expensive during its life by less regular maintenance, in a "New World" of electrical power deregulation, and competition.

The /imitation of switching overvoltages on very high voltage transmission lines is traditionally carried out with the aid of circuit breakers-fitted with closing resistors. This widely used system has repeatedly shown itself to be highly efficient. Nevertheless, it is possible to envisage other methods, in particular, those giving control of the circuit breaker instant of closing. This paper examines the possibilities offered by controlled switching on UHV transmission lines and compares existing solutions. It takes into account the various network characteristics that may be encountered. It details the results in terms of overvoltage levels which could be attained, due to the performance of the circuit breaker. The paper also highlights the critical points relative to the application.

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