Zero-mode Inrush Current Characteristics and Zero-sequence Protection Countermeasure under Sympathetic Interaction

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. From the on-site waveform record, the ZMIC attenuates very slowly and leads to the zero-sequence zone Ⅲ backup protection malfunction of the upstream transmission line of the transformer. The malfunction expands outages and has an adverse effect on the equipment safety and system stability. To solve this problem, an ZMIC analytical model under the sympathetic interaction considering residual flux, closing angles, transformer types, and other factors is constructed to analyze the characteristics of the ZMIC. Subsequently, a protection countermeasure based on the third harmonic component of the ZMIC to prevent the zero-sequence protection malfunction under the sympathetic interaction is proposed. The countermeasure is simple in principle and only uses the information of its own substation. However, it can deal with various energization and complex failure situations even the situation that the transformer is energized to the minor internal fault effectively. Besides, it is less affected by transition resistance and CT saturation. The effectiveness of countermeasure has been verified by the digital simulations, dynamic physical simulation tests and field wave recording tests.