Chase Fell
Technical Education Committee Chair – EASA
Jay Industrial Repair
High potential (hipot) testing procedures for motor and generator windings are usually performed with 50/60 Hz AC or DC as the power source. Hipot testing is a critical step in validating the quality of new windings. AC and DC hipot tests are also useful to understand the condition of aged insulation for machines in service. DC hipot testing is widely used in motor repair because the equipment is portable, and the steady state test current comes mostly from leakage through the insulation.
When a breakdown occurs, DC causes less damage to material adjacent to the fault when compared to AC tests. A disadvantage of DC testing is the voltage is not distributed in the same way as what the winding sees with AC. Specifically, the DC test stresses the end turns much higher.
AC hipot testing is much more consistent with the voltage stress of the machine in service. Studies have shown that the AC hipot test can reveal insulation defects that are left undetected with DC tests. An AC test can better detect voids and delamination inside the insulation system. The disadvantage of the AC test at power frequency is when the size of the test set and/or complexity of the setup becomes problematic in motor repair and in the field.
The larger the motor, the greater the capacitance of the winding. The current for the AC hipot test is related to magnitude of capacitance (impedance). Impedance is related to the frequency of the test voltage and the capacitance of the winding under test. The lower the frequency of the tester, the higher the capacitive reactance of the capacitive load. See the equation below.
(For more, refer to “Benefits of the AC Hipot for New Form Coil Stator Windings” by Mike Howell in the August 2013 issue of Currents.)
Higher capacitive reactance means lower AC test current will flow. With the very low frequency (VLF) test, it takes 600 times longer for the current to reach its maximum when compared to power frequency. And charging current is 600 times less. See Figure 1.
The VLF test set has been successfully used to bridge the test gap between AC and DC hipot tests. VLF technology has been used for many years for the testing of long cable runs. The VLF device outputs a sinusoidal voltage wave at a frequency of 0.1 Hz. Laboratory studies have shown that the stress distribution of VLF is not significantly different from that of the 60-cycle AC test. The VLF test set is portable and easy to implement; it would be especially useful in wind energy and marine applications where AC 60-cycle test apparatus just does not fit.
Some VLF test sets feature integrated power factor meters. VLF with power factor is useful to detect voids, incomplete curing, delamination, and aging of an insulation system. This test would also be valuable in evaluating the quality of the groundwall system and resin Vacuum Pressure Impregnation (VPI). Tests have shown that power factor measurements for VLF are higher when compared to 60-cycle power factor tests.
When compared to standard 60-cycle AC hipot test, the voltage for VLF must be adjusted up as is standard practice for DC hipot. IEEE Standard 433 is the IEEE Recommended Practice for Insulation Testing of AC Machinery with High Voltage at Very Low Frequency. The suggested value for VLF is 115% of the AC test voltage. VLF voltage is measured at crest value, where 60-cycle tests are measured at Root-Mean-Squared (RMS). Therefore, the RMS voltage must be increased by a multiplier of 1.414 to arrive at crest voltage. This makes the multiplier for VLF 1.63 times the AC 60-cycle test. Example: If AC hipot voltage is 9000V AC RMS, then the equivalent VLF test would be (9000)(1.414 ) (1.15)=14,634 Volts (Peak).
| Hipot Voltage For New Windings | |
| AC | 2E + 1000V AC (RMS) |
| DC | (2E + 1000)(1.7) V DC |
| VLF | (2E + 1000)(1.63) VLF (Peak) |
Conclusions
AC hipot testing with VLF is an effective way to perform testing on new and used windings. The AC test gives a more real-world voltage distribution in the winding when compared to DC. Power factor tests with VLF can give important information about coil quality, VPI impregnation and aging of the insulation system. The VLF is portable and the set-up is minimal. See Figure 2. VLF bridges the gap between the risk of destructive faults with AC power frequency hipot testing and the risk of a fault going undetected with DC hipot testing.
Copyright © 2021, EASA, Inc., St. Louis, Missouri. Used with permission.
