A large zero sequence voltage was found to have been induced onto a 138 kV line from adjacent 500 kV lines where these share the same transmission right-of-way. This zero sequence voltage distorted the 2.5-element revenue metering schemes used for two large industrial customers supplied directly from the affected 138 kV line. As a result, these two customers were overcharged, on average, approximately 3.5% for 15 years. The work done to trace the origins of the zero sequence voltage and quantify the metering error is described. Customer refunds which, in the end, totalled $4 million are calculated.
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