Protection Research For Transformers Of The Large Generator-transformer Unit Based On Break-Variable

In order to protect transformers of the large generator-transformer unit in power systems, the paper puts forward a kind of new principle of transformer protection based on break-variable of current. Now, the generator-transformer unit always adopts quasi-synchronization operation. In such a case, the magnetizing inrush current in no-load transformer switching on and out of order in a single source don't exist. So the protection only distinguishes the internal fault, the external fault and the magnetizing inrush current in double source. It distinguishes the internal fault and the external fault by comparing sampled-value polarity of break-variable of current. And by comparing sampled-value polarity of break-variable of absolute current, it can distinguish the internal fault and the magnetizing inrush current. The paper analyzes the criterion of the transformer protection and uses PSCAD/EMTDC simulation to check the performance of the method. The protection has easy arithmetic, high sensibility and swiftness of action.