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Transformer Riddle No.56 - Reason for tan delts higher value of Tansformer
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?
Author : Gajjala Sreelatha - From: India- Hyderabad
 
#1
Tue, November 30th, 2010 - 18:10
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°C. When equipment is tested near freezing temperatures where a large correction factor may cause the resultant PF to be unacceptably high, then the equipment should be retested at a higher temperature before the equipment is condemned. Similarly, when high PF results are encountered at high temperature, the equipment should be retested after it has been allowed to cool down. Also, PF tests should not be performed for detection of presence of moisture in the insulation when the temperatures are much below freezing, because the ice has a resistivity of approximately 144 times that of water.
Although, the temperature correction factors have been developed for correcting the measured PF results to a common base temperature, no such factors are available for humidity effects because of other variable effects.
One of variables that affects the insulation measurement is surface leakage, which is dependent upon the moisture and the cleanliness of the surface of the specimen under test. When making PF tests, the effects of surface leakage (due to humidity, dirt, etc.) should be recognized and addressed accordingly.
The effects of surface leakage current may be minimized by cleaning and drying external surfaces to reduce the losses, or using guard collars to divert the surface leakage current from the measuring circuit, or using the combination of the two approaches. Some cases may be handled quite easily with no thought or effort as to control of surface leakage, while others may require an extra effort to produce good results. It should also be recognized that there will be times when it will be best to postpone tests until another day.
The PF test results may be converted to the reference temperature of 20°C (68°F) using the conversion factors given in the test manual. The procedure for normalizing the test results to 20°C consists of (1) determine the test specimen PF, (2) measure the test specimen temperature, (3) obtain the appropriate correction factor from the table corresponding to the specimen temperature, and (4) multiply the calculated PF value with the correction factor.

After the PF tests are completed and results obtained, each apparatus and equipment insulation should be evaluated to its serviceability. The evaluation criteria may be divided into four categories. They are
1 – Good , Insulation condition is good and suitable for continued service
2 - Deteriorated, Insulation condition is satisfactory for service but should be checked within six months to see if the condition has further degraded
3- Marginal, Insulation condition is not satisfactory for service—immediate investigation of the degraded conditions should be begun and if this is not possible then it should be begun as soon as possible
4- Bad, Remove from service and recondition to restore insulation to good condition, if not possible, then replace

The recommended practice for evaluating test results is not only to assess whether the test results fall into one of the four categories mentioned above, but also to compare the test results with previous year’s results to see how much change has occurred in the condition of the insulation since the last test.
This is to say that the year-to-year test results are compared for trending purposes to signify any changes in the health of the insulation due to normal aging, but as well as other causes. Any sudden and large changes in test results between two test intervals should be a cause of concern and should be investigated before putting equipment back in service.
Usually, the failure hazards of electrical equipment are expressed in terms of maximum allowable PF values, however changes in the normal dielectric losses (watts loss), capacitance, and AC resistance are also used for indicating problems in the insulation. Depending upon the type of equipment, many manufacturers publish factory and operating limits for PF or capacitance values for their equipment which can then be used for evaluating the test results. The normal test values for various types of equipment used in the industry, and discussed in this text have been obtained from testing similar equipment in the field and factory over many years. The abnormal or unsafe limits have been established from correlation of known test values at which equipment insulation has failed in service. Insulation in deteriorated condition may operate for a period of time without a failure depending upon its exposure to abnormal operating conditions, such as voltage and current transients, short circuits, temperature, etc. However, it should be recognized that deteriorated insulation creates a definite operating hazard and if goes uncorrected will result in service interruptions and equipment damage.
If deteriorated insulation is removed before failure, it may be reconditioned and restored to service with substantial savings in equipment cost and unnecessary service outages. 
Author : Hamid - From: Iran- Firouzabad in Fars
 
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