Study on zero-mode inrush current characteristics and its influence on protection under sympathetic interaction

Abstract

When the transformer is energized, the three-phase inrush current with the zero-mode component named zero-mode inrush current (ZMIC) will be generated due to the inconsistent saturation of the three-phase iron core. The on-site waveform record demonstrates that the ZMIC with a large amplitude and very slow attenuation is generated, and it leads to the malfunction of the line zero-sequence backup protection. To explain the mechanism of the uncommon phenomenon and prevent the protection malfunction, this paper investigates the ZMIC characteristics under sympathetic interaction and its impaction on the protection of the upstream line. First, a zero-mode equivalent circuit that can reflect the transformer winding arrangement structure and sympathetic interaction is proposed for mechanism analysis. It explores that the “complementary effect” between the zero-mode potential causes the slow attenuation of the ZMIC of the line. Then, the simulation in the case of the on-site protection malfunction is implemented to discover that the sympathetic interaction is the main reason for the protection malfunction on-site. Moreover, the further simulation analysis also reveals sympathetic interaction may lead to the long-delay zero-sequence backup protection malfunction of the upstream line of the operating transformer. Furthermore, the inrush current restraint principle based on the second and third harmonics of ZMIC is discussed to improve the anti-malfunction performance of protection.