DGA and power factor testing on a 500 MVA GSU with rated voltage 13.5 kV to 240kV indicated an internal problem. Kinectrics was approached to perform diagnostics on this unit—which is located 3000 ft. underground. The project objective was to determine the cause of the high power factor and if the transformer was ready to be placed back in service.
The transformer had suffered from a design issue involving shipping pins that had caused internal arcing between the core and upper tank at the top of the transformer. The problem was discovered due to an increase in gassing.
The transformer oil was removed and the design issue was fixed by introducing galvanic contact of the shipping pins to the tank in October 2008. The oil was degassed, and reused and the transformer was ready to be placed back into service. But a power factor test for CL and CH showed a considerable increase (100%) from a previous measurement. Based on experience with similar situations, Kinectrics proposed a DFR (Dielectric Frequency Response) test.
DFR is the measure of dielectric loss or power factor measured at a wide range of frequencies, usually in the range of 0.001 Hz to a 1kHz. This is a relatively advanced technique for insulation diagnostics. This technique has been proven accurate enough for moisture content evaluation, as well as assessment of other insulation defects and contaminations.
The recorded curve of power factor for CL did not match with any moisture model curve by post-processing software. This indicated that the transformer had experienced a problem other than moisture.
Since the whole CL curve was shifted up compared with CHL, it was concluded that this winding was covered with a layer of contamination—possibly carbon particles that had been generated by shipping pin arcing. This internal arcing had been active for a long time, enough to cover the insulation with black carbon deposits, which had increased the power factors.
Based on the DFR test, it was recommended that the transformer be repaired and not energized, and be replaced by a spare transformer.
Before being shipped for repair, the transformer was inspected in the substation by removing the cover and partial draining. Inspection in the repair shop further confirmed the CL windings were covered by arcing by-products.