Hi-pot Test is a high voltage test that is used to check the integrity of insulation for high voltage equipment such as bus bars, cables, motors etc. The term 'Hi-pot' is the shortened form of High Potential. The Hi-pot test is used to ensure that an insulation can withstand a high potential without risk of failure.
However, the hi-pot test carries with it the potential failure of the insulation during the testing process itself. Weak insulation can fail during the test. Hence, many equipment owners avoid conducting this test. The hi-pot test certifies that the insulation is sufficient to withstand excess voltage during operation. This is significant in situations where the failure of a machine in service can cause serious damage or downtime as compared to a failure during the testing procedure.
The Hi-pot test is alternatively known as Dielectric Withstand test. The test involves the application of a high voltage usually about two times the
operating voltage. Thus a 6.6kV equipment will be tested at a voltage of 13kV.
The test is conducted for 1 minute or five minute. If the hi-pot test is conducted on a transformer winding or an alternator winding, the test is conducted on individual phases. The phases are separated and those phases which are not subjected to the hi-pot voltage are grounded.
Test Procedure
Prior to commencing the hi-pot test, it is necessary to get the Insulation Resistance and the Polarization Index values for the insulation. This ensures that the winding are free of any moisture or contamination. A wet or contaminated winding is more vulnerable to fail during the test.
The hi-pot test voltage is applied to the winding terminals to be tested. The voltage is sustained for one minute or five minute and then reduced. The current during the test period is also studied. Should there be a failure during the testing. There wile be a surge in the current which will cause the MCBs in the hi-pot test kit to trip.
There are two methods of raising the voltage to the value of the test voltage. They are
Step Test
In this method, the test voltage is raised gradually in small incremental steps. This enables the tester to abandon the test if he suspects that any current increase which may indicate a weak winding.
Ramp test
In this method, the test voltage is raised gradually or ramped up at a specific rate. The voltage can be increased to the rated voltage along with constant monitoring of the current. The ramp method is the most effective test as it can avoid any insulation failure during the test by identifying potential weaknesses in the winding early on.
The Hi-pot test does not offer scope for analysis such as the Insulation Resistance or the Polarization Test. It is simple a pass-fail kind of test. It is significant in that it gives operators the confidence that the equipment is strong enough to withstand the operating voltage and transient over voltages in the system.
The high voltage used during the test calls for high standards of safety. The area around the test location should be cleared of all items not related to the test(clutter). The area needs to be cordoned off to prevent the entry of unauthorized persons. Personnel should be stationed at the main power switch so that the switch can be turned at the first sign of any abnormality. The personnel conducting the test should be properly trained with awareness of emergency first-aid procedures in the event of an electric shock. The device which is being tested should be grounded after the test to discharge the capacitance.
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