Advantages of DC hipot test compare to AC test

One of the advantages of using a DC test voltage is that the leakage current trip can be set to a much lower value than that of an AC test voltage. This would allow a manufacturer to filter those products that have marginal insulation, which would have been passed by an AC tester.

When using a dc hipot tester, the capacitors in the circuit could be highly charged and, therefore, a safe-discharge device or setup is needed. However, it is a good practice to always ensure that a product is discharged, regardless of the test voltage or its nature, before it is handled.

It applies the voltage gradually. By monitoring the current flow as voltages increase, an operator can detect a potential insulation breakdown before it occurs. A minor disadvantage of the dc hipot tester is that because dc test voltages are more difficult to generate, the cost of a dc tester may be slightly higher than that of an AC tester.

The main advantage of the DC test is DC Voltage does not produce harmful discharge as readily occur in AC. It can be applied at higher levels without risk or injuring good insulation. This higher potential can literally “sweep-out” far more local defects.

The simple series circuit path of a local defect is more easily carbonized or reduced in resistance by the dc leakage current than by ac, and the lower the fault path resistance becomes, the more the leakage current increased, thus producing a “snow balling” effect which leads to the small visible dielectric puncture usually observed. Since the dc is free of capacitive division, it is more effective in picking out mechanical damage as well as inclusions or areas in the dielectric which have lower resistance.

Carrying out High Voltage or High Potential (Hi-Pot) DC testing on XLPE insulated cables has a tendency to 'polarize' the insulation. It can also lead to capacitive space charges within the tiny anomalies in the insulation. These space charges will eventually form water trees which will seek a path to earth. The ideal test would be a frequency (50Hz/60Hz) test but the test equipment is size and weight restrictive, especially on long lengths of cable because of the charging current required. A VLF (Very Low Frequency) test is something of a compromise, a true sine wave (ac) test but low frequency. Therefore none of the destructive qualities of a dc test.

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