An Extra High Voltage Tva Event!

Following a switching procedure to return a 500-kV transmission path to service, the field reported trouble at one terminal, requesting that the line be again removed from service. The path was comprised to two 500-kV transmission lines (74 miles and 51 miles), with a 500/161-kV transformer bank located in the middle of the lines.

In the process of deenergizing the path, the transformer bank was left connected to the two open-ended 500-kV transmission lines and a very weak underlying 161-kV system. This contributed to measured high-side voltages exceeding 660-kV (1.3 per-unit) and low-side area voltages as high as 200-kV (1.24 per-unit). During this condition a nearby 161-kV transmission terminal tripped by reverse phase distance, resulting in further weakening of the 161-kV network and driving high-side voltages to 700-kV (1.4 per-unit). This situation persisted for nearly 30 seconds before the transformer was disconnected from the 500-kV transmission lines. It was noted that the station arresters were not challenged by the high voltage during the event.

This paper describes the event, including the unavailability (and ineffectiveness) of tertiary shunt reactors), the evaluation of reverse phase distance elements and their operation on extreme incoming reactive power flow, recommended switching procedures to minimize the risk of future occurrence, and the subsequent implementation of 500-kV phase overvoltage protection at this and similar substations.