Research on Testing of Over-exciting to Evaluate the Tolerated DC Currents of Transformers

During monopolar operation of HVDC systems with ground return, large currents returned from earth may enter into neutrals of transformers, which causes DC bias of transformers and influence transformers running. The reason of DC bias of transformers and the factor of tolerated DC bias are analyzed in this paper. Great capacity transformers constructed in the form of three-phase groups or three-phase five-legged are more prone to DC bias saturation because the air gaps between laminations are small. According to simulation of a single phase two-winding transformers model the exciting currents of transformers under AC over exciting and DC bias analyzed. More odd components appear in the symmetrical exciting current under over-exciting. Some even components appear in the asymmetrical exciting current under DC bias. Saturation effect caused by over-exciting and DC bias is similar in the influence of transformers. Exciting current can be as the indicating depicting the change of magnetic field within transformer. The testing of over-exciting in transformers can be used to evaluate the tolerated DC currents of transformers. This method of evaluation based over-exciting is simple and available