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Much like a doctor can diagnose internal problems of a patient by the use of blood tests, oil testing can diagnose unseen problems inside a transformer. Mineral oil, much like blood in the body, comes in contact with the internal workings of the transformer and contains valuable information regarding the condition of the transformer. Two things are critical to make a correct diagnosis of the transformer’s health: one is information about the equipment and the other is a properly drawn sample. In Part 1 of this series we will discuss the information that is needed in order to obtain a correct diagnosis from the oil test results of your electrical equipment.
One of the most basic pieces of needed information is the equipment type. For example, a transformer and an LTC (load tap changer) are not diagnosed the same way. Acetylene is common in an LTC, whereas in a transformer it is most likely a sign of a more critical issue. Further, all LTCs are not created equal. A vacuum LTC has a very different set of parameters for interpretation of oil test results than does an ordinary oil-filled unit. These are just two cases where the same test results would cause a misdiagnosis if the equipment type is listed incorrectly.
For proper diagnosis of oil test results on electrical equipment, the following information needs to be collected:
- Sample Date
- Nameplate Information – serial number, manufacturer, manufacture date, kVA rating, high voltage, low voltage, gallons, etc.
- Equipment Type – transformer, cabinet transformer, pop-top transformer, regulating transformer, GSU (generator step-up transformer), furnace transformer, precipitator transformer, rectifier, reactor, step voltage regulator, LTC, vacuum LTC, deenergized tap changer, OCB (oil circuit breaker), disconnect switch, switch reclosure, etc.
- Liquid Type – oil, silicone, R-Temp, FR3, Biotemp, Askarel, Wecosol, Perclene, etc.
- Sub Name and Unit Number – unique identification for the unit that is meaningful to the owner
- Conservator Tank – yes or no
- Breather information – free, free w/desiccant, N2 blanket, N2 system (active), or bladder
- Insulation Type (Rated Temperature Rise) – 55 oC, 65 oC, 55/65 oC, etc.
- Additional Equipment – radiators, fans, water cooled, oil pumps, etc.
- Liquid Level – very low, low, normal, or high
- Top Liquid (Gauge) Temperature
- Actual Sample Temperature – measured while the sample is being pulled
- Pressure/Vacuum Gauge Reading
- Leaks – note the location of any leaks
- Service performed on the unit – hot oil cleaning, moisture reduction, reinhibiting, degassing, rewinding, etc.
- Testing requested – Oil Screen, Karl Fischer Moisture, Dissolve Gas Analysis, Inhibitor, Power Factor, Furans, Dissolved Metals, PCB, Particle Count, Particle Count/Filming Compounds, Corrosive Sulfur, etc.
For LTCs, additional information is helpful: selector range, sweep range, and counter reading.
It is also very important to identity to which transformer the LTC is attached.
Finally, make sure you check with the laboratory you are using regarding sample volumes and proper containers that will be needed for the tests you plan on having performed.
In Part 2 of this series we will examine what is involved in drawing a representative sample.
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