A novel directional comparison pilot protection scheme based on Neutral-point Zero-sequence Current of UHV Auto-transformer

With the development of UHV power system, the capacity of autotransformer gets larger and zero-sequence impedance of autotransformer gets smaller accordingly. Therefore, the zero-sequence directional relay may mal-operate or misoperate due to the poor sensitive caused by the small zerosequence voltage, even the zero-sequence current threshold is reached. However, the large value of zero-sequence current of the neutral point in autotransformer is not utilized effectively. In view of this problem, this paper proposed a new method that using zero-sequence current of the neutral point in autotransformer instead of using two sides´ zero-sequence voltage of autotransformer to construct the novel zerosequence current directional protection scheme. Therefore, A Novel Directional Comparison Pilot Protection Scheme based on Neutral-point Zero-sequence Current of UHV Autotransformer is proposed in this paper. With the zero-sequence current of neutral point as reference, 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. When the polarities of both sides are the same, the internal fault can be discriminated. When the polarities of both sides are different, the external fault can be discriminated. The proposed scheme eliminates the adverse effects yielded by zero-sequence voltage effectively. The miss-operation and refuse-operation problem of the traditional zero-sequence directional relay in UHV power system could be avoided.