Safety of distribution transformers against internal failure

Tests have been carried out by LABORELEC and PAUWELS, on ELECTRABEL's initiative, in order to verify the efficiency of current limiting fuses and overpressure detecting devices to avoid any external effect in case of transformer internal failure. The test programme was set up with the following objectives: to know the pressure/volume relationship for a transformer tank of the type used on the network; to know the highest pressure in service, including overloading conditions; to know the pressure withstand limit; to know the time and current limits for the tank withstand against internal failure; to investigate the behaviour of arcs in oil; to make full scale tests with simulation of different types of faults; and to check the working of two types of pressure detecting devices. The tests have been done on hermetically sealed transformers which therefore have a specific design of the tank and cooling ribs. For the considered transformers, the tests have confirmed the efficiency of the fuse protection which could prevent any external effect during the tests. Despite those results and of the good service performance, one can not completely exclude that a fault in the secondary windings causes a pressure rise which goes beyond the tank withstand limit, but those can be controlled by a complementary protection using pressure detection.