1. Field of Invention
This invention relates to high voltage power distribution systems and more particularly to apparatus and methods for monitoring for a fault condition in such systems
2. Description of Related Art
Power distribution transformers in a high voltage power distribution system are susceptible to internal and external faults. Internal faults may occur due to gradual degradation and/or sudden breakdown of insulating properties of internal structures of the transformer resulting from thermal, electrical and mechanical stresses. The loss of insulating properties of internal structures can lead to a short circuit within the transformer, creating an internal arcing fault. Such fault may occur in a winding of the transformer, for example. A short circuit of this type usually causes burning of the winding, ultimately resulting in an open circuit in the primary winding, whether the fault was initiated in the primary or secondary winding. When such a fault occurs, a fuse, normally installed in series between the high voltage line and the transformer, blows, interrupting current flow to the transformer. Attempts to re-energize the transformer by replacing the fuse can result in further arcing within the transformer and this can rapidly increase the pressure inside the transformer to the point of explosion where burning oil and shards of th transformer casing may be rapidly expelled at the risk of injuring persons and property nearby.
External faults may occur due to insulation failures in a secondary circuit connected to the secondary winding of the transformer. External faults are normally detectable by inspection of the secondary circuit. Re-energizing a transformer having a fault in the secondary circuit do s not normally cause an increase in pressure inside the transformer sufficient to create an explosive threat. Internal faults are thus normally more dangerous than external faults.
Various mechanical devices have been devised to detect internal faults, most of which rely on pressure build-up caused by arcing to activate a mechanism. Such devices are described in U.S. Pat. Nos. 5,078,078, 5,623,891, and 6,429,662, for example. Each of these devices employs an efficient but relatively elaborate mechanism to operate a signal device in response to a pressure surge in a transformer caused by an internal fault.