[Gajjala Sreelatha]

In one of Transformers, sometimes megger value is good, Polarisation index is of the order of 2 and the dryout of insulation is as per established VPD process, still the tandelts is high. What could be the reason?
In one of Transformers, sometimes megger value is good, Polarisation index is of the order of 2 and the dryout of insulation is as per established VPD process, still the tandelts is high. What could be the reason?

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[Hamid]

A high PF of the oil is an indication of the presence of contamination, such as moisture, carbon, acids, polar contaminants, etc. Of course you must be sure about standard test conditions.The PF measurement is a searching diagnostic tool for evaluating insulation condition. It is a fundamental concept that changes in insulation quality result in measurable changes in some of the basic electrical characteristics of the insulation, such as capacitance, dielectric loss, or PF. Therefore, by measuring these electrical characteristics over time, changes in the integrity of the insulation can be assessed. Unfortunately, PF tests cannot always be conducted under the desired or same conditions because the equipment may be located outdoors or the environment may be different from test to test. There are two environmental variables which cannot be controlled easily; they are temperature and humidity. Also, depending on the cleanliness of the insulation and relative humidity, surface leakage current can also have an effect on the PF measurements. The electrical characteristics of most insulation materials vary with temperature. In order to compare the results of routine PF tests measurements taken at different temperatures for the same equipment, it is necessary to normalize the results to a common base temperature. It is a recommended practice to convert the measured PF values to a common base temperature of 20

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