Power Factor Test of Transformer

The power factor test and dissipation factor test are considered to be synonymous because they both refer to the AC dielectric loss test. PF and DF are but two of the several measurable characteristics that can be obtained from an AC dielectric loss test used for evaluating the condition of the insulation system. 

Both of these tests are effective in locating weaknesses in the electrical insulation and hazard in the power apparatus before impending failure. PF and DF tests can measure dielectric loss, capacitance, and AC resistance of the insulation of the electrical apparatus. 

These tests can measure the presence of bad insulation even when there may be a layer of good insulation in series with the bad insulation. These tests provide information on the overall condition of the insulation in terms of a ratio (i.e., PF or DF value of insulation) that is independent of the volume of the insulation being tested. 

Moreover, they provide an assessment of the insulation under normal frequency (60 Hz or 50 Hz) operating conditions which is not time-dependent. The PF and DF tests do not overstress the insulation and can determine if the insulation is slowly degrading by comparison with the previous year’s test results, or with test results of similar equipment.

Principles of Power Factor Test

The PF/DF tests measure insulation capacitance, AC dielectric losses, and the ratio of the measured quantities. 

When insulation is energized with an AC voltage, the insulation draws a charging current. This charging current comprises two components called capacitive current and resistive current. 

The capacitive current leads the applied test voltage by 90°, whereas the resistive current is in phase with the voltage as shown in Figure.

The capacitive current is directly proportional to the dielectric constant, area, and voltage and inversely proportional to the thickness of the insulation under test. The capacitive current may be calculated by the following formula:

Icap = E/Xc = EωC = 2πfCE

Where, E = test voltage, f =frequency of test voltage, C = capacitannce of insulaton.

Changes in the capacitive current indicate degradation in the insulation, such as wetness or shorted layers, or a change in the geometry of the insulation. The resistive current supplies the energy lost due to dielectric losses such as carbon tracking, volumetric leakage, surface conduction, and corona. 

Dielectric losses due to water contamination or carbon tracking or other forms of deterioration increase by the square of the voltage, whereas dielectric losses due to corona increase exponentially as the voltage increases. PF/DF testing is sensitive enough to detect a deteriorated moisture problem in the insulation compared to an insulation resistance test. 

Normally, the power factor values are in the range of 5 to 10 percent. As the insulation deteriorates, more current will leak through the insulation and the power factor, therefore, becomes increasingly greater, (approaching 100 percent). Power-factor readings are taken periodically, as part of maintenance programs. These readings are compared, not with readings of other parts, but with previous readings on the same part. 

Power Factor test Equipment

Although there are several manufacturers of PF/DF test equipment, this text describes PF test methods and procedures based on the PF test equipment of Megger Instruments. The PF methods, theory, and principles discussed in this text are also applicable to the PF test equipment of other manufactures.

It should be noted that in the electrical industry, especially in the utility transmission and distribution (T&D) arena, the PF test may be referred to as the Doble test because of the use of the Doble test sets when performing PF tests. The terms PF test and Doble test are the same.

A PF test set generally consists of a standard reference capacitor (CS) and the insulation under test (CX). A special multi-winding transformer is the characteristic feature of the circuit. A voltage is applied to both CS and CX. The ratio arms, NS, and NX are adjusted to balance capacitive current, and the variable resistor (RS) is adjusted to balance resistive current. The null indicator is used to determine when the bridge circuit is balanced. The values of NS and NX are used to determine capacitance and the value of RS correlates to the power (dissipation) factor of the test insulation.

power factor test of transformer
Simplified circuit diagram of Megger DF test set (transformer ratio-arm bridge.)

Capacitance, Power Factor, and Dissipation Factor are terms that are not affected by changes in test voltage. They can be measured without having to set the test voltage precisely. The ‘power loss’ or ‘watts loss’ values depends on the test voltage (proportional to the square of the test voltage) and therefore need to be corrected to a ‘reference voltage’.

10 kV has become the preferred ‘reference voltage’ when testing high voltage electric power equipment. A ‘reference voltage’ of 2.5 kV is often used when testing equipment rated at distribution or lower voltages.

The test voltage used for PF testing should be sufficient to detect any latent weaknesses in the insulation, but since the test is intended to be nondestructive, the voltage should not exceed the normal line-to-neutral or line-to-ground operating voltage of the apparatus under test. 

Basic Test Connections (Test Modes) for PF Testing

Grounded-Specimen Test Mode: In grounded-specimen test (GST) mode, all current between the AC source and ground (through CX) is measured by the bridge. GST is used when one terminal of the insulation to be measured is permanently connected to the ground, such as a bushing flange, transformer tank, or grounded apparatus frame. GST mode also connects the LV lead(s) directly to the ground. This enables the lead(s) to be used to ground a specimen terminal that is not normally grounded.

GST Mode with Guard (GST-G): In Doble test sets this connection is referred to as guard-specimen test mode. In this mode, all current between the AC source and ground (through CX) is measured by the bridge. The LV lead(s) may be connected to the test circuit guard. Any current present on the LV lead(s) during the test is bypassed directly to the AC source return and is eliminated from the measurement. GST-G mode is used to isolate an individual section of insulation and test it without measuring other connected insulation.

Ungrounded-Specimen Test Mode: In the ungrounded-specimen test (UST) Mode, the only current between the AC source and the LV lead (through CX) is measured. Any current flowing to a grounded terminal is bypassed directly to the AC source return and is eliminated from the measurement. UST mode is only used to measure insulation between two ungrounded terminals of the apparatus. In UST mode, the ground is considered a guard since grounded terminals are not measured. UST mode is used to isolate an individual section of insulation and test it without measuring other connected insulation.

The formulas used for calculating the PF or DF are illustrated by using an example with insulation of PF = 1.0%. The value of PF is given by the cosine of the θ angle, or the equation can be written as

PF = cosθ = dielectric losses ÷ charging volt-amps = watts (W) ÷ volt-amps (VA) = 0.01 (1%)

= arc cos (0.01) = 89.43°

Also, PF is approximately equal to DF when PF and DF <10.0%, that is

 cos θ = tan δ = cotan θ

cos(89.43) = tan(0.57) = 0.01(1%)

Therefore, PF test results are comparable with DF test results up to 10.0%.

Factors That Influence PF Measurements

The PF measurement is a searching diagnostic tool for evaluating insulation conditions. 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. Two environmental variables temperature and humidity cannot be controlled easily. Depending on the cleanliness of the insulation and relative humidity, surface leakage current can also affect the PF measurements. 

The electrical characteristics of most insulation materials vary with temperature. 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 the 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 a 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 the 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 the variables that affect 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 such as 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.

Power Factor Test of Transformer

The PF test as applied to transformers is the most comprehensive test for detecting insulation degradation, usually caused by moisture, carbonization, and other forms of contamination.

Power and distribution transformers may be either single-phase or three-phase and may be either dry-type or oil or synthetic-liquid filled. Transformers may be installed indoors or outdoors depending upon the application. Depending on the type, size, and voltage rating of the transformer, the PF test may be performed as an overall transformer PF test, or on individual components of the transformer to localize the dielectric circuit for effective analysis of the test results; that is deterioration in the solid winding, bushing, and liquid insulation can be localized by separate tests on these components. 

Generally, it is common practice to perform PF tests of the bushing and the solid winding together on medium-voltage transformers that have solid porcelain-type bushings. On HV transformers with condenser-type bushings, the PF tests are performed on the individual bushings by the UST method. On all other bushings, hot-collar tests are performed by the GST method. When performing PF tests on transformers, the following conditions should be observed: 

  • Transformer is de-energized and completely isolated from the power source. 
  • Transformer housing is properly grounded. 
  • All bushing of HV and LV winding, including the neutral are shorted to make them into equivalent HV and LV bushings. Neutrals must be ungrounded. 
  • Transformers equipped with load-tap-changers should be set to some position off neutral, and this position should be noted on the test datasheet. 

Two-Winding Transformers: In a three-phase or a single-phase two-winding transformer, the transformer winding insulation system comprises of three insulation systems; that is, CH HV winding insulation, CL LV winding insulation, and CHL high-to-low winding insulation. The three insulation systems are shown in Figure.

Insulation circuit of two winding transformer.

When performing PF tests, the HV bushing of the three phase units are shorted together to make them into an equivalent single bushing. Similarly, the LV bushing of three phase units are also shorted together to make them into an equivalent bushing.

Four PF tests of the windings are made as shown in Table. As shown in Table, results of test 1 minus test 2, and test 3 minus test 4 are calculated to validate that the PF test have been made correctly. 

Calculated results of Table: test 1 minus test 2

  1. Subtract charging current of test 2 from test 1
  2. Subtract watt loss of test 2 from test 1
  3. Then calculate the PF, that is

[CH+CHL] – [CH] = CHL

Power Factor Test of Three-Winding Transformer

The equivalent circuit of a three-winding transformer insulation system is shown in Figure given below. The insulation system in a three-winding transformer is similar to that of the two-winding transformer except that there is an additional winding in the transformer.

Insulation circuit of three-winding transformer.

The standard test procedure for a three-winding transformer is given in following Table. This test technique for three-winding transformers is an extension of the two-winding transformers.

PF Tests for Three-Winding Transformers

Power Factor Test of Autotransformers

The equivalent circuit of an autotransformer insulation system is shown in following Figure.

Insulation circuit of an autotransformer.

The PF tests that are conducted on an autotransformer are shown in following Table. For test purposes, an autotransformer is considered the same as the two winding transformer with the exception that the HV is a combination of HV and LV winding which cannot be separated physically.

PF Tests for an Autotransformer

To short circuit the HV winding on a three-phase unit all seven bushings are shorted together when performing PF tests; H1, H2, H3, X1, X2, X3, and H0, X0.

Power Factor Test of PTs

PTs are used on HV systems for metering and relaying applications. Because of LV rating of the PTs, the PF tests are routinely performed only on the primary winding of the PTs. Care should be exercised when performing PF tests on PTs to ensure that the PT is completely and effectively isolated from the power source before any tests are conducted.

The test connections for conducting PF tests are shown in above Figure. The tests that are conducted on PTs are shown in Table given below.

PF Tests for PT

Notes:

  • On tests 1, 3, 4, and 5 limit the test voltage to the rating of the H0 bushing, usually 5 kV; check the literature of the manufacturer of the PTs.
  • Tests 4 and 5 must be performed at the same voltage.
  • Remove ground from H0 when performing tests.
  • Secondary winding of PTs need not be shorted; ground one end of the secondary winding only.
  • Tests 1, 2, 3, and 6 are standard tests for PTs.
  • Correct measured PF values to 20°C; use air temperature.

Evaluation and Grading of PF and DF Test Results

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 Insulationcondition is suitable for continued service.
  2. Deteriorated Insulationcondition is satisfactory for service but should be checked within six months to see if the condition has further degraded.
  3. Marginal Insulationcondition 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 the 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 the correlation of known test values at which equipment insulation has failed in service. Insulation in deteriorated conditions may operate for some 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.

Whenever the test results are questionable or marginal, it is generally recommended to perform tests on a more frequent basis to keep abreast of the condition and to establish a trend. A gradual and consistent increase in PF may be due to contamination, deterioration, or normal aging, whereas a sudden increase in the PF is a cause of immediate concern even when the absolute PF value is not considered excessive. Whenever an increasing trend is established, then the equipment should be removed from service as soon as practical for inspection to determine the cause of the problem and to make appropriate repairs.

The electrical characteristics of the insulation vary with temperature. To compare PF and DF test results periodically of given equipment or apparatus, the PF and DF values should be converted to the base temperature of 20°C.

Oil-Filled Power and Distribution Transformers: The overall PF test results of oil-filled power and distribution transformers indicate the insulation condition of the solid windings, oil, barriers, bushings, etc. The overall PF value for individual windings-to-ground and interwinding insulation of modern oil-filled transformers should be 0.5% or less, corrected to 20°C.

Service-aged power transformers will have somewhat higher PF values due to normal aging, loading (heat), and voltage stress. A PF value as high as 1% is considered acceptable for older transformers when previous history or knowledge of the PF value of the transformer, or similar transformers is not available.

However, when the PF value of one insulation system of the transformer is higher than the others, for example, HV winding insulation PF is higher than the LV and interwinding insulation PF, then causes of the higher PF should be investigated.

A PF value of 2% for extremely old power transformers may be considered acceptable. In the case of older transformers that utilized varnished-cambric or varnish insulation, these transformers may have normal PF values in the range of 4% to 5% at 20°C.

Transformers that are subjected to excessive internal forces due to possible through-faults or other causes may have windings that are physically distorted, that is the core–coil assembly configuration has changed from its original design configuration. If this should happen, then the capacitance of the winding-to-ground and interwinding (i.e., CH, CL, and CHL) would have changed. Therefore measuring the capacitance of the individual winding is also important in judging the condition of the winding insulation.

Also, the transformer excitation current test is extremely effective in revealing damage to the core, core–coil assembly, tap changer, and short-circuited turn-to-turn insulation. In evaluating excitation current test results for three-phase transformers, evaluation is based on a normal pattern of excitation current readings, that is either a two high and one low reading, or two low and one high reading depending on the transformer winding connections. A change in this pattern is a cause for investigation. Also, if benchmark data, such as factory or field acceptance data are available, then the evaluation should be based on comparing the test results with the benchmark data.

Dry-Type Power and Distribution Transformers: The PF of dry-type transformers varies over a relatively wide range due to the insulation of support insulators, bus work, and insulation materials. Corona is a greater possibility in HV dry-type transformers and the test procedure should include provisions for checking it.

This can be done by making PF tip-up tests on the dry-type transformers. An abnormal increase in the tip-up value may be an indication of excessive corona or voids in the insulation. A comparison of the PF and PF tip-up test values with benchmark data if available, or with test results of similar units tested under similar conditions is recommended in evaluating the insulation condition of the dry-type transformers.

It is not unreasonable to expect a PF of 2% or less for new modern dry-type transformers. However, PF may increase with the age of the transformer and may increase to 5% – 8%. PF values substantially higher than the values discussed here should be investigated to determine the cause of the high PF.

A better approach for evaluation of transformer insulation is to use the PF data recorded during the initial tests on the transformer, such as acceptance testing, as a benchmark for comparison with subsequent test results.


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