7/28/2019 Power Factor Testing

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Why Perform Power Factor Testing?Most failures of electrical equipment are the result of abreakdown in the insulation system. Power factor testingis an effective way to detect defective electricalequipment insulation. Power factor is the ratio of thecapacitive or charging current (measured in volt-amperes) to resistive or leakage current (measured in

watts). It is a means of measuring insulation integritysince the smaller the power factor, the better theinsulator.

Insulation may be deteriorating, however, the equipmentmay not fail for some time. Although this conditionrepresents a serious hazard, if deteriorated insulation isdetected before failure, it can often be reconditionedinstead of being replaced. This allows corrective actionto be taken during a planned outage before extensiveequipment damage results from insulation failure.

Power factor testing also makes it possible to detect

equipment insulation problems without making aninternal visual inspection. The result is a significantdecrease in maintenance cost and downtime. Powerfactor testing is one of the best testing techniques forpredictive and preventive maintenance.

What are the Test Procedures?Once the equipment is taken out of service, an ACvoltage at or below the normal rating is placed on theequipment conductors. At the same time, the dielectriclosses to ground are measured. After correcting thefield measurements to standard conditions, the test

results are then compared to guides which list propervalues for specific types of equipment and insulations.These guides are contained in an extensive data librarycompiled by testing thousands of similar types ofequipment in the field and at the factory.

If an abnormal condition is indicated, a number of specialtests may be used to localize the problem. These includehot guard tests, hot collar tests, ungrounded specimentests, cross check tests, oil tests, and dielectric loss testson wood members.

What Types of Equipment are Tested?

The most frequent applications of power factor testinginclude medium and high voltage transformers andcircuit breakers. Other equipment such as bushings,

reclosers, switches, cables, capacitors, lightningarrestors, liquid insulation, potheads, rotatingmachinery, and voltage regulators may also be powerfactor tested.

Power factor testing is used as a factory test and in thefield for acceptance testing, preventive maintenance,and emergency diagnostic testing.

What are the Advantages?Power factor tests can detect the presence of poor

insulation even if there is a layer of good insulation inseries. However, DC tests usually show a high insulationresistance under this condition.

At a given voltage, the charging current and the dielectricloss increase as the quantity of insulation increases.However, the ratio, power factor, remains the same nomatter how much insulation is present. This makes itpossible to establish simplified test values for particulartypes of insulation.

Power factor testing provides a measure and evaluationof the overall condition of medium and high voltage AC

apparatus insulation under simulated normal-frequencyoperating conditions. This is independent of the durationof the test. DC tests not only require more time, theyalso do not simulate normal insulation operatingconditions. Furthermore, the results are affected bythe duration of the test.

Power factor tests provide a benchmark which may becompared over periods of time in an organized mannerand are used to identify any trends on apparatusinsulation. It provides a more complete picture of overallinsulation conditions.

Used in conjunction with capacitance and ACresistance, it provides comprehensive data of thenature of the insulation.

Power Factor Testing

PowerFactorTesting

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