Internal faults in oil-filled distribution transformers. Fault mechanisms and choice of protection.

摘要

The aim of this work has been to study internal faults in mineral-oil-filled distribution transformers. In this work, main emphasis is placed on experimental work, both on full scale and model transformers. The service experience and fault statistics of distribution transformers situated in cable networks show that the failure rate of these transformers is less than one fault per thousand transformer years. Because of the low failure rate, the main argument for the overcurrent and fault-current protection of distribution transformers situated in cable networks is to ensure the safety of the general public and operating personnel by protecting against explosions or violent transformer tank rupture. Tests with internal short circuits between turns carried out in this work, and tests described in the literature have both shown that the line currents may reach values which sometimes may be detected by overcurrent relays or correctly dimensioned current limiting fuses. Consequently, adequate overcurrent protection against the most common type of internal faults in distribution transformers can not be achieved, neither with the implementation with primary circuit breaker and commercial overcurrent relays, nor with the fuse-loadbreaker combination. Tests with internal short circuits between turns have shown that gas-actuated Buchholz relays operate very satisfactorily and fast both for transformers with conservator and for hermetically-sealed transformers. Used together with a primary circuit breaker and a correct dimensioned overcurrent relay or a fuse-loadbreaker combination, the gas actuated relay seems to be the best safety arrangement for the most common type of fault in distribution transformers, namely short circuits between neighbouring turns or layers in the medium voltage winding.