A Novel Scheme for UHV Auto-Transformer Phase Comparison Longitudinal Protection Principle Based on Zero Sequence Current

The zero-sequence current differential protection for grounding fault has the advantages of high sensitivity, action current's independence with transformer tap joint, small influence by inrush current and principle simplicity. Therefore, it has been widely used in the area of auto-transformer protection. However, the zero-sequence current differential protection operate by measuring the phase sum of zero sequence current in three sides of autotransformer, which is badly influenced by the saturation of current transformer and hard to set the threshold. In the external fault, it will lead to mal-operation. Based on this, A novel scheme for UHV auto-transformer phase comparison longitudinal protection principle based on zero sequence current is proposed in this paper. The fault direction can be determined by comparing the phase shift between zero sequence current of high voltage side, medium voltage side and the reference current. So the internal fault and external fault can be discriminated. The results of dynamic simulation experiment and digital simulation shows that the proposed method can detect grounding fault with high sensitivity and good selectivity. Even under the serious saturation of current transformer, this method is not influenced by it.