The problem of operating power stations with the highest possible availability has become more and more important in recent years, the layout of a power station obviously has a decisive influence in this respect. The present paper is specifically concerned with the reliability assessment of large thermal power stations. It provides significant insights in the optimization of power stations layouts. The application of generator circuit-breakers for the switching of generators at their terminal voltage offers many advantages when compared with the unit connection such as lower first costs, simplified operational procedures and better fault protection. Modern SF6 generator circuit-breakers make it possible to interrupt all types of fault currents within four cycles. This rapid clearance of fault currents helps to avoid expensive secondary damage of power station equipment and consequently long down times for repair. Although they have a low probability of occurrence such outages have a substantial effect on the availability of a generating unit. It is obvious that long unavailability periods as e.g. in the case of disruptive faults on step-up transformers may affect the rate of return of investments related with power stations. With the recent successful certification of a generator circuit-breaker with a rated short-circuit breaking current of 200 kA SF6 generator circuit-breakers are now available for generating units up to 2000 MVA. Another recent development has been the integration of all the associated items of switchgear within the generator circuit-breaker enclosure as an option to their separate installation. This greatly improved functionality of generator switchgear also contributes to the realisation of simpler and more economic layouts of power stations. Beside a substantial reduction of the first costs this new solution, being fully factory assembled and tested, also makes possible considerable saving in time and expenditures for erection and commissioning.

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