Direct Current High Voltage (Potential) Testing

Direct current high voltage testing can be divided into two categories: Proof testing and Controlled Over-Voltage testing.

Proof testing qualifies an insulation system to hold a specific voltage. By definition, it is a pass-or-fail evaluation – there is no diagnostic value. Acceptance proof testing is performed on new stator winding as part of in-process and final acceptance testing. Maintenance proof testing is performed on existing equipment, typically at 75% of the Acceptance test voltage. Although proof testing is pass/fail, the microampere readings for each individual phase should be measured and recorded, and gross variations should be noted and investigated further. An initial failure may be the fault of the test setup, as high resistance leakage to the atmosphere or to ground can occur. The quality of the insulation system should be questioned only if corrections to the test setup do not result in improved test results.

In Controlled Over-Voltage testing, the applied voltage is either “stepped-up” manually and incrementally, or “ramped-up” automatically and fluidly, over time. This provides a degree of diagnostic value since the measured current can be graphed, phase-by-phase, for comparative analysis. Controlled Over-Voltage testing affords the ability to salvage a winding that might otherwise fail in Proof testing. In plotting the measured current in real time, the operator may witness an exponential increase in current (indicating an impending insulation breakdown) and terminate the test.

DC stepped voltage and ramped voltage test results can be evaluated on a pass/fail basis, but important results can be obtained by a more thorough examination. The associated plotting might show evidence of a weakness. The higher the voltage level that presents the indication, the better the quality of the dielectric. Comparison phase-to-phase might show that one or more are comparatively high in resultant current.  Most importantly, routine testing and comparatives can provide insight into the degree of insulation aging, and help predict end of useful life.

All of these tests should be performed in accordance with IEEE Standard 95™-2002, IEEE Recommended Practice for Testing of AC Electric Machinery (2300 V and Above) With High Direct Current.

Polarization Index (PI) Test

A Polarization Index (PI) test is generally performed at the same voltage as the Insulation Resistance (IR) test.  Where the IR test is performed for a period of one minute, the PI test is performed over a period of ten minutes. This gives the absorption (polarization) current ample time to decay, and reveals a more detailed indication of the total leakage and conduction current. As such, PI is a good indication of winding contamination, moisture ingress (leakage currents), and/or bulk insulation damage (conduction currents).

Polarization Index testing is generally performed with an Insulation Resistance (IR) test set (commonly known as a Megger), immediately after performing the IR test. However, the test can also be performed utilizing a DC high potential (hipot) test set. The readings produced by the two instruments are different. A Megger commonly gives readings in ohms of resistance. A hipot registers the amount of current (typically in microamps (mA). One microamp is equal to 1 x 10-6 amps, or 0.000001 amps.

The Polarization Index is derived by the ratio between the one minute reading and the ten minute reading. Recommended minimum PI results for suitability for service (or implementation of high voltage testing) is widely accepted as 2:1 or greater. Any reading lower than this minimum value is a concern. The windings would be presumed to be wet, contaminated, and/or compromised in some fashion. Conversely, vintage windings (varnish cambric, asphalt mica, etc.) may produce an unusually high Polarization Index ratio.  The insulation may be void of binder content, thus making it dry and brittle.  According to IEEE standards, if the insulation resistance reading after the voltage has been applied for one minute is greater than 5,000 megohms the resulting polarization index may or may not be indicative of the true insulation condition and is therefore not recommended as a means of assessment.

Polarization Index testing should be performed in accordance with IEEE Standard 43-2000(R2006), IEEE Recommended Practice for Testing Insulation Resistance of Rotating Machinery. A Megger brand model number BM25 (or its replacement or comparable) is recommended if a Megger is used. A High Voltage, Inc. PTS Series DC high potential test set is recommended if a hipot is used.

Insulation Resistance (IR) Test

The Insulation Resistance Test measures the integrity of the generator’s winding insulation, and therefore the likelihood of developing a ground.  A test voltage is applied to the generator and the current flow required to maintain that voltage is measured over a period of time (typically one minute).  In simplest terms, the less current flow, the higher the resistance value, and the better the insulation.

An IR test should be performed immediately following any type of event that is suspected of over-stressing an insulation system, prior to the generator being placed back into service. This is the first test that should be performed. The results will indicate the ability of the insulation system to withstand  any more searching and/or strenuous testing. The IR test also measures the effect of contamination from water, oil, carbon, and other such undesirables. If a separate high voltage proof test is performed, an IR test should be performed both before and after the proof test. This in order to assure that the proof test itself has not compromised the insulation.

The Insulation Resistance test must be performed by a well trained and experienced technician.  Incorrect procedures can materially affect the results. The test should be performed to IEEE Standard 43-2000 (R2006), using a late model Megger brand machine such as Model MIT1025, or comparable.  It is best practice to test at the main and neutral leads of the stator, as close to the windings as possible.  Stator slot RTD’s should be disconnected from the terminal board and grounded. Surge capacitors should be disconnected.  The water or oil should be drained and completely evacuated from liquid inner-cooled windings, typically by vacuum-processing. Stator windings should be tested one phase at a time, with the other two phases grounded. In this manner, the windings are stressed both phase-to-ground and phase-to-phase.

The results should be interpreted by an experienced technician. Your final report should reflect that environmental conditions and even the age and configuration of the machine were taken into consideration.

If you have any questions concerning the application of this test or the interpretation of the results, please contact Mr. Turbine.