San Diego Gas & Electric (SDG&E; San Diego, California, U.S.) uses interconnected windings to save space in substations and provide a stable ground/neutral point of reference. After two of these transformers failed due to internal faults at two separate substations in October 1996 and July 1999, SDG&E decided to have the units rebuilt with new parts, which included the current coils, bushings, arresters, wiring, current transformers and load tap changer.

The two units were rebuilt and tested according to the IEEE Standard C57.12.90. After a review of the test report, some of the numbers were questioned. After several consultations with the manufacturer, SDG&E determined that the zero sequence test was valid for wye-delta, delta-wye and three winding transformers, and excluded transformers with inter-connected windings such as wye-delta-ZigZag.

After much research and analysis, SDG&E decided to evaluate these units using symmetrical components. The utility developed an accurate method of calculating Zo for use by transformer manufacturers or utility personnel.

An actual test was performed at the manufacturer site on March 26, 2002. A subsequent test was performed at SDG&E's Monserate Substation on April 12, 2002. Both of these tests provided the expected results.

SDG&E is including the developed test procedure as part of its specification for any new or refurbished wye-delta-ZigZag transformer purchases. Engineers, utilities or manufacturers now have a proven way to test interconnected winding transformers.
Circle 179 on Reader Service Card or visit freeproductinfo.net/tdw