Sympathetic interaction between three-winding transformers and impact in ground relaying in a giant oil and gas field environment

This paper will describe an analysis of a well-documented sympathetic transformer inrush event that occurred at an onshore oil and gas production and treatment processing facility. The facility is powered by a large power system that includes 518 megawatts (MW) of onsite power generation capacity. During an energization attempt of a three-winding transformer rated 80/40/40 MVA, the 35 kV distribution system experienced a very prolonged and atypical transformer inrush current that resulted in non-selective operation of residually connected non-directional (51N) and directional (67N), ground overcurrent protection schemes. The event analysis disclosed that the sympathetic interaction between two large three-winding transformers connected in series during the energization had an influence in the magnitude and duration of the inrush current. This event was well documented by several fault records from different digital fault recorders (DFRs) and microprocessor based relays at different locations. In addition, the paper will also discuss the actual non-selective operation of the 35 kV system ground overcurrent relaying scheme during the sympathetic inrush phenomenon and the laboratory testing conducted on the microprocessor based relay to assess the behavior of directional controlling algorithm of the directional ground overcurrent 67N and 67G functions when the polarization source is absent. The results also corroborated that the phenomenon of sympathetic interaction during energization of series connected transformers follows the same phenomenon behavior of sympathetic inrush in parallel transformer energization events.