Transformers are subject to the following tests in addition to the insulation testing. However, dry-type transformers would not be subject to fault-gas analysis and insulating liquid analysis due to the fact that these types of transformers usually feature a gaseous or dry compound insulating medium. Cast coil transformers, which are defined as transformers with high voltage coils cast in an epoxy resin, would also not be subject to fault-gas analysis and insulating liquid analysis due to the fact that the core and coils of these types of transformers are not immersed in an insulating liquid. Cast coil transformers are considered to be dry-type transformers.
A. Transformer turns-ratio and polarity tests.
to the number of turns in the other windings of the same phase on the transformer. The polarity test determines the vectorial relationship of the various transformer windings. The tests are applicable to all power and distribution transformers. Cast coil transformers are also subject to these tests. The turns-ratio test and the polarity test can be done on transformers of all sizes. The turns-ratio test is also commonly performed on instrument transformers. The test equipment used will ordinarily be a turns-ratio test set designed for the purpose, although, If not available, two voltmeters or two ammeters (for current transformers only) can be used. When two AC voltmeters are used, one is connected to the high-voltage winding, and the other is connected to the low-voltage winding. The high-voltage winding is excited to a voltage not exceeding the rating of the voltmeter. Both voltmeters are read simultaneously. A second set of readings should be taken with the metering instruments interchanged. The values indicated should be averaged to calculate the ratio. A meaningful ratio measurement may be made using only a few volts of excitation. The transformer should be excited from the highest voltage winding to avoid possibly unsafe high voltages. Care should be taken during the application of voltage and the measurement. It is important that simultaneous readings for both voltmeters must be made. The voltmeters used should have accuracy matching the requirements of a 0.5 %ratio calculation. When a turns-ratio test is performed, the ratio should be determined for all no–load taps. If the transformer is equipped with a load-tap changer, the ratio should be determined for each load-tap changer position.
B. Fault-gas analysis.
The analysis of the percentage of combustible gases present in the nitrogen cap of sealed, pressurized oil-filled transformers can provide information as to the likelihood of incipient faults in the transformer. Fault-gas analysis can be performed on mineral-oil-immersed transformers of all sizes. When arcing or excessive heating occurs below the top surface of the oil, some oil decomposes. Some of the products of the decomposition are combustible gases that rise to the top of the oil and mix with the nitrogen above the oil. The test set for this test is designed for the purpose. A small sample of nitrogen is removed from the transformer and analyzed.
C. Insulating-liquid analysis.
Insulating-liquid analysis can be performed in the field on oil-immersed power transformers of all sizes.