A Reconstruction of the WAMS-Detected Transformer Sympathetic Inrush Phenomenon

The transformer-energization inrush phenomenon is to be expected in cases when a single transformer is connected to an ac voltage source. The most commonly employed mitigation technique is to use mechanisms that enable per-phase controlled switching based on a residual magnetic-flux estimation in the transformer's ferromagnetic iron core. However, the time scale of the phenomenon is relatively short and imposes no extremely critical issues to wide-scale power-system operation. On the other hand, the sympathetic inrush phenomenon that might occur among several parallel transformers exhibits much longer time constants (several seconds and even tens of seconds). The resulting system impact becomes longer lasting. Recently, national wide area monitoring system (WAMS) measurements indicated that a sympathetic interaction occurred between two parallel 400/110-kV transformers in the Slovenian power system. The cause was the magnetization of a second transformer in parallel to the already operational and fully loaded one in the substation in electrical vicinity to a large power plant. Apart from the WAMS, the event was also captured by the transformer's over-current protection relay. The specifics of the event are described in this paper together with a dynamic-simulation-based reconstruction that is very important for the operator's understanding of on-line, WAMS-provided, and system measurements.