DC HI-POT TEST

Abstract - Overvoltage tests, commonly referred to as high potential or "Hi-Pot" tests, are essential in verifying the minimum dielectric strength of insulation systems. These tests are non-destructive and do not compromise material integrity. They are executed on various parts of the winding to evaluate the grounding condition of the insulation in rotating electric machines.

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Understanding DC Hi-Pot Testing

Routine application of DC tests (Hi-Pot in DC) is customary among users of motors and generators. This practice allows for the on-site identification of insulation system deficiencies, enabling repairs during scheduled downtimes. Additionally, these tests are standard procedure to confirm the status of ground insulation during the manufacturing or repair phases of machines. It is advisable to apply these tests phase-wise, ensuring that the other two phases and any auxiliary devices are grounded to ensure accurate readings.

DC Hi-Pot testing is effective in identifying latent or advancing weaknesses in the insulation structure, which could potentially result in failures during operation. The test serves as a diagnostic tool for detecting a range of issues including: cracks, moisture ingress, separation of insulating materials, contamination, insufficient curing, and internal separation in the epoxy/mica layers of the winding.

A common concern among maintenance managers is whether Hi-Pot tests are damaging to equipment. The definitive answer is NO; such tests do not significantly degrade machinery with a robust insulation system. Instances of equipment failing during a Hi-Pot test have consistently been traced back to pre-existing insulation deficiencies, verified through post-failure analysis. Conversely, high-quality insulation typically withstands voltage levels two to three times higher than those used in Hi-Pot DC tests.

When DC voltage is applied to the stator of rotating electric machines, it serves to evaluate the ground insulation by creating a current 'I' between the winding and the frame (ground). This current is generated from two primary factors:

• Surface Leakage Current: This current is stable over time and usually occurs over the surface of the stator winding's end-turns or between exposed conductors and an insulated rotor body. Factors such as temperature and the presence of conductive materials like moisture or contamination significantly influence the magnitude of this current.
• Absorption Current: This current arises from molecular polarization and electron movement, diminishing over time with a decreasing rate from an initially high value down to nearly zero. Its value depends greatly on the type and state of the insulation system.

The recommended DC Hi-Pot test voltage is contingent upon the nominal machine voltage, which is amplified by a factor reflecting the relationship between the DC test voltage and the alternating voltage (RMS). The guidelines for determining the maximum test voltage are as follows:

1. Acceptance Test: Conducted for new and rewound machines either in the factory, workshop, or on-site. The test voltage should be set at 1.7 times [2E + ] V (where E represents the line-to-line voltage).
2. Maintenance Test: For machines in service, the testing voltage must lie between 65% and 75% of the 1.7 times [2E + ] V value.

The voltage introduced to the winding must be increased gradually to avoid surpassing the prescribed maximum test voltage. The test usually lasts for one minute but can extend to a maximum of ten minutes, beginning once the peak voltage is reached. Upon test completion, the voltage source is brought down to zero, and the winding must be safely grounded.

Experts recommend conducting Hi-Pot DC tests on operational machines every 3-5 years, ideally timed with major maintenance protocols, to reaffirm the unit's condition. A successful test outcome indicates that the insulation system remains strong enough to ensure reliable operation.

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REFERENCES

1. B. Gupta, G. Stone, and J. Stein, "Stator Winding Hipot (high potential) Testing," in Electrical Insulation Conference EIC, no. June, pp. 409-413.
2. IEEE Std 95 Recommended Practice for Insulation Testing of AC Electric Machinery (V and Above) With High Direct Voltage, USA, pp. 1-57.
3. G. Stone, "Stator Winding Hipot Testing," Iris Power, no. February, Toronto, pp. 1-4.
4. IEEE Std 43 Recommended Practice for Testing Insulation Resistance of Rotating Machinery, USA, pp. 1-26.