Real case interaction between a sympathetic inrush and a resonant system

An incident occurred in the Portuguese HV electrical distribution system, where an EHV/HV power transformer supplied by the EHV transmission system was tripped by an elevated fifth harmonic residual current on its secondary side, leading to a power outage. At the time, a second power transformer was being energized for parallel operation, in order to restore normal grid operation after reconfiguration from commissioning works. A study was conducted to identify the origin of the fifth harmonic current, and to establish possible mitigation solutions. It was found that the sympathetic inrush, caused by the inrush of the second transformer, produced a rich harmonic disturbance in the zero-sequence voltage component on the HV side, which in turn excited an existing zero-sequence resonance between the transformer and the extensive underground cable network. This paper presents the conducted analysis, along with the measures taken to prevent further unwanted trips and to lower the overall system disturbance.