There is a 35kV series reactor short-circuit fault on the low-voltage side of one main transformer in a 500kV substation.The preliminary analysis concludes that there is an inter-turn short circuit occurred in A-phase of the series reactor.Although there is no fault recorder installed on the low-voltage side of all the main transformers

the fault developing process is indirectly analyzed and deduced by the fault recorder on the mid-voltage side by symmetrical component method.The conclusion of the paper is that there is not only an inter-turn short circuit occurred

but also an inter-phase short circuit occurred in the series reactor

which eventually leads to the breaker trip triggered by the protection.The suggestion for protecting equipment is also proposed in the paper.