If winding resistances are to be compared to factory values, resistance measurements will have to be converted to the reference temperature used at the factory usually 75deg C. Formula to use:
Ts + Tk
Rs = Rm ------------
Tm + Tk
Rs = Resistance at the factory reference temperature (found in transformer manual)
Rm = Resistance you actually measured
Ts = Factory reference temperature (usually 75deg C)
Tm = Temperature at which you took the measurements
Tk = Constant for the particular metal the winding is made from: 234.5deg C for copper, 225deg C for aluminium.
Note that it is very difficult to determine winding temperature in the field. Phase resistance comparison between each other or sister transformer of similar temperature are usually sufficient. Do ensure temperature are equalized within the windings after de-energization.
Core insulation resistance and core ground test is used if an unintentional core ground is suspected. Ethane and/or ethylene and possibly methane would flag up in DGA. These gases may also be present if there is a poor connection at the bottom of a bushing or a bad tap changer contact. This test is only necessary if winding resistance test did not pick up anything on all connections and tap changer contacts. If it is to be Megger-ed, use 1000V Megger. 1000MOhm min is expected on new transformer, service aged transformer should read greater than 100MOhm. 10-100MOhm reading is an indicative of deteriorating insulation between core and ground. 10MOhm is sufficient to cause destructive ciculating currents and not recommended to be return to service so as to Zero Ohm. "Burning off" using dc or ac current is possible but extremely risky. Do not perform without consultation from manufacturer and with others experienced in this task. DGA should be conducted before and after the burn off.
Doble testing is highly recommended to determine the condition of a transformer, because it can detect winding and bushing insulation integrity and problems in the winding and core. The Doble test set should also include the Insulation Power Factor test, results should not exceed 0.5%@20degC. From my personal experience with Doble equipments, their software do track the history provided the maintenance personnel store that history and load them up for analysis. Doble as a company in general have been very easy to dealt with from my experience.
Capacitance test should also be conducted. They are a meaqsurements between HV & LV windings, HV & ground, LV and ground. Values changes as transformer ages, events occur such as lightning strikes, through faults, indication of winding deformation and structural problems such as displaced wedging and winding support.
Excitation current test is to be carry outto detect short-circuited turns, poor electrical connections, core de-laminations, core lamination shorts, tap changer problems and other possible core and winding problems. Results are often compared between phases. Please consult Doble manual and support for result interpretation. Rule of thumb, on the two higher currents with excitation less than 50mA, differences between the two high currents should be less than 10%. Excitation of more than 50mA would close the difference down to less than 5%. In general, if there is an internal problem, these differences will be greater. Again, trend the results.
Bushing should be tested using Doble Power Factor testing on a scheduled basis. Deterioration will be evident from the trend. 90% of bushing failures may be attributed to moisture ingress, evidenced by an increasing power factor trend.
Other than that are the common stuff such as winding temperature gauge, pressure relief valve, bucholz relay, conservator bladder and breather, cooling fans, cooling oil flow, cooling oil isolation valves, cooling fan motor, oil leaks, and etc. DGA should also specify for dissolved metals and metal particle count for metals if transformer cooling runs on oil pump bearings. There is a risk of flashover inside the tank resulting catastrophic failure.
If there is an opportunity to carry out internal inspection, DP test can be carry out by sampling the center phase with a pair of tweezers. Center phase is usually the hottest most part of a transformer. New insulation DP value would be between 1000 to 1400. At 500DP, it is around 60-66% of life remaining. At 300DP, 30% of life remaining. 200DP is end of life.
Most cases, an internal inspection does more harm than good. Therefore a good reason has to be established prior to opening up a transformer. Good reasons such as listed below are justifiable:
- Extensive testing shows serious problems.
- Unexplained relay operation takes the transformer offline, and testing is inconclusive.
- Acetylene is being generated in DGA (indicating active arcing)
- Ethylene and ethane are being generated in sufficient quantities to cause grave concern. Indicating bad connection on bushing bottom or tap changer, circulating currents, additional core ground or static discharges.
- A core ground must be repaired, or an additional core ground has developed which must be removed.
- Vibration and ultrasonic analysis indicate loose windings that are generating gasses from heat caused by friction of the vibrating coils; loose wedges must be located and replaced.
- CO2/CO ratio are very low (2 to 3), indicating severe paper deterioration from overheating. Cooling must be checked carefully before opening the transformer.
- Furans are high indicating excessive aging rate; a DP test must be carry out.
- Metal particle count is above 5000 in 10ml of oil taken specifically for metal particle count.
You would noticed that these reasons are justifiable as any further delay may result to catastrophic failure. Which the maintenance personnel should ask, is there a point packaging a DP test if the issues are already that severe. The slight additional cost and work is up to individual site to justify.
Reference of this article:
- Transformers: Basics, Maintenance, and Diagnostics - Reclamation, US Department of the Interior Bureau of Reclamation, April 2005.