A Most Unusual Event Analysis
On 2/25/17 a transformer differential relay associated with a 115KV wind farm interconnection in northern Vermont operated during a severe lightning storm. A review of the relay event reports and DFR records indicated that a large DC current pulse flowed across the Northern VT 115KV, 46KV and 34.5KV transmission systems with the highest flow occurring on the wind farm transformer grounded wye 115KV windings with resulting misoperation of the transformer differential relay. The cause of the DC current flow was found to be a -450KV HVDC line fault to a 115KV line shield wire at the line crossing in northern Vermont. The fault self-cleared and the physical evidence indicates that this has occurred multiple times in the past. This paper provides an analysis of this event and ensuing corrective actions.