中文摘要：

由于结构设计的原因，综合性能较好的一些保护方案如横差保护和不完全纵差保护等将不能应用于大型超临界汽轮发电机。文中重点讨论了纵向零序电压保护方案的改进及其在应用中的问题。研究表明，发电机发生匝间短路故障时3次谐波电势减小，而机组外部故障时发电机3次谐波电势增大。因此，为了提高纵向零序电压保护方案的性能，可以根据故障时3次谐波电压增量的特性作为选取制动量的依据，从而提高保护性能。亦即：当3次谐波电压表现为负增量时，制动量取为0或接近0；而当3次谐波电压表现为正增量时，以该增量值作为制动量。此外，对匝间保护纵向零序电压元件增加一个小延时是可行的，它可以提高保护躲暂态3次谐波影响的能力。仿真计算表明，比例制动式纵向零序电压保护能可靠区分内外部故障，符合大型超临界汽轮发电机的保护要求。

英文摘要：

The protection schemes for the large ultra-supercritical turbo-generator will suffer the disadvantage of low sensitivity as it has the high reactance and low resistance. This paper investigates the changing laws of the longitudinal zero-sequence voltage which reflects the internal faults of generators. And the third harmonic voltage of the faulted generator is also analyzed. Analysis results show that when an inter-turn fault happens, the third harmonic voltage will decrease. However, it will increase if the fault locates out of the generator. According to that, the third harmonic voltage is introduced as the restricted component to improve the performance of the longitudinal zero-sequence voltage protection. Simulation results show that the new scheme is successful in detecting the generator inter-turn faults.