Analysis of and reflection on the short-circuit fault of a large hydraulic generator

As a large hydro-generator carries a heavy load for the power grid, any fault in the generator would have extremely negative consequences for the power system, the power plant as well as for the local or even national economy. To minimise the risk of failure, the design, manufacture and operation of any large hydro-generator necessitates meticulous care and caution. Taking the real case of one power plant in China, this study conducts a detailed analysis of the cause of a failure at its large hydro-generator terminal because of a short-circuit fault. Based on a series of simplified calculations, the analysis finds that the main causes of the failure are 2-fold. On the one hand, instantaneous over-voltage by the short circuit generated strong over current and released huge electro-dynamic forces; on the other hand, the long period of de-excitation inevitably prolonged the short-circuit current, resulting in damage to the generator´s windings as well as to other parts. The analysis uses the notion of instantaneous symmetrical components to explain the instantaneous over-voltage and strong over-current generated by the conversion fault in this particular case. It further emphasises the importance of rapid de-excitation in similar circumstances.