Use of Resistive Voltage Dividers for Transformer Tertiary Ground Detection

The Bonneville Power Administration (BPA) has a ‘tertiary ground detection’ scheme that detects line-to-ground faults by looking for an imbalance in the delta-configured transformer tertiary windings. This is a rather complex arrangement involving special 25-Hz PTs and auxiliary PTs all carefully designed to avoid Ferro-resonance. Despite these precautions, high voltage fuses feeding the PTs in this scheme still occasionally blow, resulting in unwanted outages necessary to replace them. Engineers at BPA have proposed the use of off-the-shelf resistive voltage dividers (sensors) to measure the voltage to ground on each phase. These are robust, inexpensive, and immune to Ferro-resonance. The output of the sensor is measurable by a modern digital relay, which would detect the imbalance and react accordingly. Engineers must take into account the impact of the parallel resistance of the measuring relay to resistance of the voltage divider. Another consideration is that the cable length for these sensors is limited. The relay must measure the output voltage of the sensors near the structure where they are to be installed. 61850-compliant relays have been configured for this application using merging units for the remote sensing required for this scheme. These relays and their respective merging units are be tested in the lab using real time based digital power system simulators (RTDS). Following the computer modeling and lab testing, the proposed scheme is to be demonstrated at the BPA Medium Power Lab on the Ross complex in Vancouver, WA. The Medium Power Lab is fed from a transformer bank which is already equipped with a conventional tertiary ground detection scheme. Full and partial line-to-ground faults will be applied to this feeder as needed to test the scheme. This paper will describe the existing the scheme, the proposed scheme, the modeling and lab testing of the scheme, and an analysis of the data collected from the results of the stage fault testing of the new scheme.