Why Megger Tests Are Performed
State of electrical insulation
The insulation resistance meter test method for determining the condition of electrical insulation has been widely used as a general non-destructive test method for many years.
One major limitation of this test is that it is an operating voltage of 500 to 1,000 volts does not always detect isolation points, whereas the higher voltages used by high voltage testers for DC voltage detect these points.
The insulation resistance meter test shows the following parameters:
(a) Relative amount of Moisture in the insulation,
(b) Leakage current across dirty or damp surfaces of the insulation and
(c) Deterioration or failure of the winding due to insulation resistance against time curves.
Description of the test
A direct voltage of 500 or 1,000 volts is applied. The insulation and the readings relate to the insulation resistance against time. The data should be recorded every 1 minute and 10 minutes, as well as several other intermediate times.
The hand-cranked insulation resistance meter has been the standard instrument for checking insulation resistance for many years. The hand-cranked instrument is satisfactory for "spot checks" but is not recommended for routine dielectric absorption testing because very few men can crank for 10 minutes without fatiguing and slowing the cranking speed towards the end of the period.
Motorized or electronic insulation resistance testers are available that use a 115-volt AC power source or a self-contained battery.
Because of this, some of the smaller test instruments may not be suitable for testing large generators or transformers that draw high dielectric absorption current.
For occasional calibration checks and for the proper functioning of the insulation test equipment, it is recommended that a resistor in the range of 100 megohms be attached to the inside of the device cover, which serves as a test standard. It is recommended that the same tester be used for each periodic test on a particular device, as differences in device output characteristics can affect the shape of the dielectric absorption curves, especially at the lower end.
Dielectric absorption curve
The insulation resistance is not a definitive measure; the voltage can withstand the insulation, but if correctly designed it provides a useful indication of the suitability of the winding for continuous operation. It should be noted that the values of the insulation resistance can vary over a wide range even with identical machines and under identical conditions.
Changes in the insulation resistance are more significant than certain absolute values. This curve is called the dielectric absorption curve.
Any such curve that is constant and lower than normal in about 3 minutes or less indicates a high leakage current (since the leakage current is large in proportion to the absorption current) and the winding should be thoroughly cleaned and rechecked or further inspected. This cleaning should preferably precede all insulation resistance tests.
If the insulation is very damp, the dielectric absorption curve can start up and then decrease to a value lower than at the start of the test.
Minimum values of the insulation strength of the machine
"Recommended Practice for Insulation Testing Resistance of Rotating Machinery" (IEEE Standard No. 43, November 1974) states that the recommended minimum insulation resistance Rm for armature and field windings of AC and DC machines can be determined using the following equation:
Rm = recommended minimum insulation resistance in mega-ohms at 40 ° C of the entire machine winding
Vt = rated terminal potential of the machine from terminal to terminal in effective value in kilovolts
The winding insulation resistance is obtained by applying a direct potential to the entire winding for 1 minute must be corrected to 40 ° C to be compared with the recommended minimum value Rm. The insulation resistance of one phase of a three-phase armature winding, in which the other two phases are grounded, is approximately twice as high as the entire winding.
If protection circuits are not used on the two phases in the test If each phase is tested separately, the observed resistance of each phase should be divided by three to obtain a value that, after correcting for temperature, can be compared to Rm. For insulation in good condition, insulation resistance values of 10 to 100 times the value of Rm are not uncommon.
It should be remembered, however, that these declining insulation resistance values obtained from periodic testing are indicative of insulation deterioration rather than low values. Machines rated at 10,000 kV-A or less should have either a polarization index or insulation resistance (at 40 ° C) at least as high as the minimum recommended values to be considered in suitable conditions for operation or for overpotential testing. Machines with more than 10,000 kVCA should have both a polarization index and an insulation resistance that is above the recommended minimum values.
When the end turns of a machine are treated with a semiconducting material for the purpose of corona removal, the insulation resistance can be slightly lower than without such treatment.
Transformer insulation resistance
The preceding paragraphs apply more, especially for generator and motor windings, they generally also apply to transformers, with the exception that no insulation values have been specified for transformers. The technology for measuring the insulation resistance of the transformer is also not known or standardized. If the transformer windings are not immersed in oil, the insulation resistance behaves similarly to the insulation resistance of the generator.
The insulation resistance becomes lower after adding the oil because the insulation resistance of the oil is in parallel with part of the solid insulation. Therefore, insulation resistance values alone cannot be used to indicate the progress of the drying out of the winding, since the winding and the oil resistors cannot be separated.
Whether the slope of these temperature correction curves is influenced by the moisture content in the oil and is not fully known. In the current state of the art, it is believed that the power factor test provides a better indication of the transformer insulation condition than the insulation resistance test. Tests should be grounded between each winding, between each winding and ground with the other windings, and between each winding and ground with the protection circuit connected to the other windings but not grounded.
Cable insulation resistance
The most commonly used test for high voltage, for cables, insulation resistance is measured with an insulation resistance meter. The most meaningful test for high voltage cables is the DC high voltage test, which is modified to combine moderate voltage resistance with insulation current / voltage measurement.
The temperature of the cable is important and should be recorded with the insulation resistance. This is difficult when the cable is partly indoors and partly outdoors, partly underground, partly above ground, partly exposed and partly in conduits.
The temperature may need to be estimated from the various lengths and a calculated weighted average. Tests should be performed between each conductor, between each conductor and earth with other earthed conductors, and between each conductor and earth with other conductors connected to the protection circuit but not earthed.
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