THE VALUE OF POWER FACTOR TESTING
Nov 1, 2005 12:00 PM
by John Bleyer and Phillip Prout, National Grid
NATIONAL GRID DISCOVERED A HIGH POWER FACTOR DURING ACCEPTANCE TESTING of a new 40-MVA transformer in 2004. The unit was then returned to the factory for repairs, burdening both the utility and manufacturer with additional costs and delays. This article reviews the information gathered and lessons learned from this event, including testing and root-cause determination results from the teardown/repair process.
National Grid's electricity delivery companies in the United States serve 3.3 million electricity customers in New England and upstate New York through more than 1200 substations. As part of a substation expansion project, a 24/32/40-MVA, 115-kVD/13.2-kVY transformer was purchased and installed at a National Grid substation in New England.
The transformer was delivered to the substation. The radiators and bushings were installed. The unit was vacuum processed, filled under vacuum and tested by the transformer manufacturer's personnel. The following tests were performed at the substation after the unit was oil filled:
Transformer turns ratio on all taps
Insulation power factor of transformer and all bushings (C1 and C2)
10-kV excitation on all taps
Core ground and winding resistance
Oil quality and dissolved gas analysis (DGA).
The power factor of the high-voltage winding was elevated. The measured value of 0.58% did not meet industry-standard acceptable values or National Grid's required values. All other tests results were acceptable.
POWER FACTOR TESTING
Insulation power factor tests are used to measure dielectric losses, which relate the wetness, dryness or deterioration of transformer insulation. Both factory and field testing are performed to verify the insulation integrity of substation transformers. Power factor testing a two-winding transformer is conducted by energizing the winding at a known ac voltage (typically 10 kV for windings rated greater than 10 kV) with the common winding bushings shorted together.
The results of overall power factor tests on power transformers reflect the insulation condition of the windings, barriers, tap changers, bushings and oil. Modern oil-filled power transformers should have power factors of 0.5% or less, corrected to 20°C (68°F), for individual windings to ground (CH and CL) and interwinding insulations (CHL). The National Grid transformer specification states that the power factor of the insulation system shall not exceed 0.5% at 20°C.
As part of the investigation into the high power factor, the transformer manufacturer retested the power factor with similar results. The high-voltage bushings were replaced and the unit was retested. The results did not change.
The unit was drained and an internal inspection was performed; nothing was found. The vendor performed a 24-hour hot oil and vacuum process to rule out the possibility of moisture in the insulation. The power factor was again retested and still had high CH results. Both the seller and purchaser agreed that the unit should be returned to the factory for further investigation.