Generator circuit breakers : studies to support the IEC/IEEE standardisation and novel application aspects

The present PhD thesis is specifically focused on a comprehensive analysis of the major aspects concerning the application of generator circuit-breakers (GenCBs). The main motivation comes from the recently started project within IEC and IEEE to jointly develop an international standard for GenCBs. This project represents an important step in standards development, especially because it will be the first IEC standard for GenCBs (the scope of IEC 62271-100 specifically excludes GenCB). The question whether the requirements laid down in IEEE Std C37.013-1997 (R2008) and in IEEE Std C37.013a-2007 are still adequate for the application of GenCBs in modern power stations is considered in the present work. In order to quantify the requirements for the application of GenCBs, a comprehensive survey of 185 power stations has been performed. Generators ranging from 16 to 2002 MVA have been taken into consideration. The current and the transient recovery voltage (TRV) associated to its interruption have been analysed for different fault conditions by means of simulations: Electromagnetic Transients Program (EMTP) has been used for this purpose. The system-source and the generator-source short-circuit currents as well as the fault currents resulting from synchronising under different out-of-phase angles have been analysed. The delayed current zeros (DCZ) phenomenon has been thoroughly studied and fault conditions which can lead to DCZ have been identified. In order to set the requirements for the application of GenCBs in case of DCZs, a mathematical model of the arc of the GenCB has been developed and validated by means of tests carried out in power testing stations. A comparison between different GenCB arc-extinguishing technologies is also provided. Based on the results of this analysis, a new type test procedure has been developed to reproduce the stress imposed on GenCBs due to the occurrence of DCZs. A detailed analysis of the TRVs has also been performed. A method based on genetic algorithms has been developed to investigate the adequacy of the TRV requirements laid down in IEEE Std C37.013-1997 (R2008) and in IEEE Std C37.013a-2007. Additional investigations have been carried out on different aspects that concern the application of GenCBs with respect to overvoltages either generated by the GenCB’s operation or by its sole presence in the open position. Furthermore, a comprehensive study on the impact of GenCBs on the availability of power station layouts has been performed. A survey on major failures occurred to GenCBs has been carried out. Failure data obtained from the operating history of air-blast and SF6 GenCBs operating in 108 countries from January 1970 to June 2011 have been analysed. A sample of 52696 GenCB-years installed in different types of power stations has been surveyed. This sample is by far the largest one ever analysed for GenCBs reliability assessment according to international published literature. The analysis provides reliability parameters which have been used to develop an empirical model of air-blast and SF6 GenCBs. An algorithm based on sequential Monte Carlo method has been developed to quantify the availability of power station layouts. The proposed method is specifically intended to quantify the impact of a GenCB in addition to a circuit-breaker installed at the higher voltage side of the unit auxiliary transformer on the availability of a power station layout taking into consideration the simplified operational procedures and the higher protection brought about by the use of these circuit-breakers. The results show that the presence of a GenCB, in conjunction with a circuit-breaker installed at the higher voltage side of the unit auxiliary transformer, increases the throughput power of the generator thus making the investment for these circuit-breakers cost effective. It can be concluded that the installation of SF6 GenCBs can thus free the choice of alternative power station layouts not commonly considered till today.