Why VLF testing is preferred than conventional DC testing in Diagnostic field?

Testing of cable with conventional method is "DC hipod" testing.A considerable rise in premature failures was observed when polymeric cables (XLPE, EPR, etc.) were introduced into the electric grid and readily being tested with DC.

This failure with DC hi-Pot testing is cause of trapped charge within the defects of Insulation due to intense charge migration via Unidirectional DC field

When DC test completed and AC supply applied to the cable this charge lead to rapid generation of electrical tree and subsequent failure.

VLF testing benefits

Potential failures should happen during the actual test so that repairs can be made immediately because test time of VLF testing is more about 15 min to 60 min.
Lower power and current requirement for testing which significantly reduced size/weight to allow for a very portable high voltage tester.

Both benefits describes briefly as below,

Under DC test, test object remain less time under voltage stress than VLF test.This allow the breakdown of the test specimen in VLF test which allow to repair the specimen before it is in service.Because Failures in the cable during service result in higher costs for the utilities and are a nuisance for the power consumer.Second ,here no issue of trapped charge like DC test because of we applied C test voltage which reduce the considerable reduce in premature failures occurred during DC test
Power for capacive circuits P = 2πfCV2

where f = applied test frequency, C = capacitance of the test object, and V = applied test voltage.

From the equation its seen that decreasing the applied test frequency will decrease the power required to apply a voltage, and since P = IV (where I = current), the amount of current required to apply a voltage is also lowered.

Insted of applying power frequency 50Hz we apply 0.01 Hz to the test specimen it clearly shows that we required 500 times less power.This less requirement of power make the size of test kit compact and portable.

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