Assessment of the requirements for the application of tee-off circuit-breakers in modern power plant layouts

Nowadays there is a continuously increasing trend to equip power plant layouts with a circuit-breaker installed at the higher voltage side of the unit auxiliary transformer. A major concern in this respect is that the design, testing and application requirements for this circuit-breaker are not standardised. Due to its installation location, this circuit-breaker has to handle currents of very high magnitude which can also exhibit delayed zero crossings. Moreover, these currents are associated to very steep transient recovery voltages which often exceed the tested capability of conventional distribution circuit-breakers. This paper is specifically concerned with a global survey of different power plants to identify the requirements for application of the circuit-breakers installed at the higher voltage side of the unit auxiliary transformers. The fault transients simulations have been performed by means of the Electromagnetic Transients Program (EMTP). A methodology to properly select the adequate circuit-breaker is finally proposed.