Abstract:

Operation mode and structural changes of power systems have little influence on distance protection whose protection range is relatively stable. It is widely used in the relay protection of power transmission lines.The influence of the ungrounded neutral system on the distance protection is mainly described.Firstly, based on the basic principle of the distance protection of the ungrounded neutral system, it is found out that the discrimination of positive sequence component directly affects the action of distance protection, and then the failure of the ungrounded system.The characteristics of ungrounded system faults are analyzed, and combined with the quadrilateral characteristics of the distance protection relay, an improved distance protection scheme for blocking the discrimination of positivesequence voltage direction component is proposed. Finally, the correctness of the scheme is verified by the simulation test  of the realtime digital simulator(RTDS).