Light gas protection only operates when gas within the transformer collection chamber reaches threshold volume. Such criterion failed to reflect transformer conditions accurately due to the lack of understanding for defect developing process and corresponding gas generation- convergence pattern, which has led to lower protection reliability and has caused several transformer explosions. To address current problem, an experimental tank with structure resembling that of a power transformer is set up, inside which the needle-plate oil-pressboard breakdown model is planted. Experimental results indicate that throughout fault development, voltage drops became more frequent until arcing occurs, so is HFCT signal; meanwhile gas generation rate increases through slight partial discharge, spark discharge and arcing stage, the collected gas volume corresponds with patterns above. The purpose of this study is to clarify the defect developing process and the gas generation-convergence pattern for oil-immersed transformer internal fault in guidance for a more reliable protection method.

transformer,fault developing pattern,gas generation-convergence pattern,light gas protection