An Interesting Overreach Mis-Operation and The Process of Solving Mis-Operations

Plan to go over the details of a Zone 1 mis-operation for a fault at the remote bus, where the apparent impedance was significantly shorter than expected (46% vs 100% of line impedance). Will review the variety of possible causes that were reviewed and dismissed over the next six weeks. (Will also briefly discuss the issue of modeling regular static wire instead of OPGW static wire, which can cause 13% over estimation of line impedance.)

Will then detour to discuss the process in general of finding mis-operations’ causes. The process is similar to the scientific method: Observation, question, (research), hypothesis, experiment/test, conclusion; all in repetition until the answer is found. The most likely and/or easiest to test hypotheses should be tested first. If the answer remains elusive, additional research and/or input from colleagues can provide additional hypotheses to test. (Will provide a longer list of common mis-operation causes to consider.)

While continuing the investigation, it was finally realized that the unexplained overreach was caused by self-mutual impedance that was not known and not modeled, caused by the line traveling to and from a “tapped” station on the same poles for 54% of the total line length. This self-mutual impedance caused the apparent impedance to be 25.4% shorter, explaining the rest of the un-explained overreach.