Saturday, December 20, 2014

DC Testing Methods

After seeing how insulation behaves when DC voltage is applied to it, let us now take a look at the various tests that are conducted with this voltage. Two tests can be conducted on solid insulation with the application of DC voltage:

• Insulation resistance testing
• High-potential (Hi-pot) voltage testing

 Insulation Resistance Testing

This test may be conducted at applied voltages of 100–15,000 V. The instrument used is a megohmmeter, either hand cranked, motor driven, or electronic, which indicates the insulation resistance in megohms. An electronic megohmmeter is shown in Figure 1.1. The quality of insulation is a variable, dependent upon temperature, humidity, and other environmental factors.
Therefore, all readings must be corrected to the standard temperature for the class of equipment under test. The megohm value of insulation resistance is inversely proportional to the volume of insulation being tested. As an example, a cable 100 ft. long would have one-tenth the insulation resistance of cable 1000 ft. long, provided other conditions were identical. This test can be useful in giving an indication of deteriorating trends in the insulation system. The insulation resistance values by themselves neither indicate the weakness of the insulation nor its total dielectric strength. However, they can indicate the contamination of the insulation and trouble ahead within the insulation system if a downward trend continued in the insulation resistance values.

(a) Electronic megohmmeter, 5000 V and (b) 15 kV DC dielectric test set. (Courtesy of Megger, Inc., Valley Forge, PA.)

Insulation resistance measurement values can be accomplished by four
common test methods:
• Short-time readings
• Time-resistance readings (dielectric absorption ratio [DAR] test)
• Polarization index (PI) test
• Step-voltage readings  

High-Potential Voltage Test 

A DC hi-pot voltage test is a voltage applied across the insulation at or above the DC equivalent of the 60 Hz operating crest voltage (i.e., DC value = 1.41 times RMS value). This test can be applied as a step-voltage test. When the highpotential voltage is applied as a dielectric absorption test, the maximum voltage is applied gradually over a period of 60–90 s. The maximum voltage is then held for 5 min with leakage current readings taken each minute. When this test is applied as a step-voltage test, the maximum voltage is applied in a number of equal increments, usually not less than eight, with each voltage step being held for an equal interval of time. The time interval between each step should be 1–4 min. At the end of each interval, a leakage current or insulation resistance reading is taken before proceeding to the next step. A plot of test voltage versus leakage current or insulation resistance can then be drawn to indicate the condition of the insulation system. Routine maintenance tests are conducted with a maximum voltage at or below 75% of the maximum test voltage permitted for acceptance tests, or at 60% of the factory test voltage. 

Dielectric absorption test: The dielectric absorption test is conducted at voltages much higher than the usual insulation resistance test values and can exceed 100 kV. This test is an extension of the hi-pot test. Under this test, the voltage is applied for an extended period of time, from 5 to 15 min. Periodic readings are taken of the insulation resistance or leakage current. The test is evaluated on the basis of insulation resistance. If insulation is in good condition, the apparent insulation resistance will increase as the test progresses.
The dielectric absorption tests are independent of the volume and the temperature of the insulation under test.

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