Insulation Resistance and Polarization Index Testing of Generators

09/05/2017

Insulation Resistance and Polarization Index Testing of Generators

Megger & P.I. as it’s commonly referred to, is one of the quickest, safest, and simplest tests in the arena of generator electrical testing, but it is also one of the most useful.  It is this simplicity that often makes the test somehow seem less important than some of the “big ticket” tests such as Hi-Pot or ELCID.  Megger and PI is a very quick way to get an overall assessment of the health and cleanliness of an insulation system in just a few minutes’ time.

 

Sidewinders makes engineering evaluations based on the overall analysis of many tests and inspections considered as a whole, but the Megger & PI is considered the front line test when evaluating the condition of an insulation system.  The main goal of a test & inspection job is to verify that all insulating systems properly confines the flow of electricity within the conductors by means of Megger & P.I.; and that all conductors allow the unimpeded flow of current by means of DC resistance testing using a digital low resistance ohmmeter (DLRO).  Copper should pass current freely, and insulation should block the flow of current.

 

The megger test, as with most electrical systems, is best understood when an analogy is made to a piping system.  Take the common 50-foot long garden hose, for example.  Initially, the hose is empty and has zero pressure, and is closed at the far end.  The instant the faucet is turned on, the hose swells up, and you can hear and feel the water current rushing in as it charges the hose to the same pressure as the supply faucet, say 50 PSI.  If the hose doesn’t have any leaks, the current will stop once the hose has fully charged to 50 PSI.  If the hose has some microscopic leaks, there will be a small trickle current which could be measured as the leakage current.  Substituting voltage for pressure, and current for flow, we can make a direct analogy to an electric winding insulation system.  During the megger test, the winding will initially accept large current flows as the unstressed insulation becomes stressed up to the test voltage of say, 5000 volts.  Once the insulation reaches the test voltage, the insulation molecules continue to rotate and orient themselves to be parallel to the electric field lines of force.  After a period of ten minutes, the system is considered to have reached steady state, and any remaining current going into the winding is assumed to be purely due to leakage through imperfections in the insulation system.

The polarization index is a measure of how much the insulation system resistance improves with time.  Let’s return to the garden hose.  A perfect garden hose with the other end closed off would exhibit zero current 10 minutes after the pressure was turned on, and the hose could not expand any further.  If the other end was wide open, the flow at 10 minutes would be exactly the same as it was the moment the hose was turned on.  Taking the 10 minute flow divided by the 1-minute flow would give a ratio of 1.0.  In an electrical winding system, if there is a large amount of contamination, or a major breach in the insulation system, the leakage current would be large compared to the inrush charging current, and the ratio of the 10-minute resistance divided by the 1-minute resistance would be close to 1.0.

In a winding system, most OEM’s recommend a PI value of 1.25 or higher on a rotor winding, and 2.0 or better on a stator winding.  The reason for the difference lies in the fact that most generator rotors are an “open” insulation system, with naked conductors in the end winding potion and thus it is expected to tolerate lower insulation and P.I. values.  In stators, the voltages are far higher, and the insulation system is “closed”, that is, the entire length of the copper windings are entirely enclosed in insulation and thus we would expect higher insulation values.

[latexpage] The polarization index is calculated as  \[PI  = \frac{R_{10}}{R_1}\]

where R10=the ten minute resistance reading, and R1 is the 1-minute reading.

In a perfect world, all winding systems would exhibit both high PI & high resistance values.  But we don’t live in a perfect world—we encounter units that are very old, facing demanding operating conditions, heavy contamination, and environmental effects such as humidity and intrusion of salt mist for seaside units, or industrial contamination such as sulfides or heavy metals for units in caustic environments.  Surface contamination and humidity are the most common causes of low resistance and/or PI values.  If we encounter low test values, the first step is to inspect the system and clean up any areas that appear to have contamination.  Low PI values are most often corrected by cleaning, followed by the application of dry heat for 12-36 hours.  Based on our experience, 95% of units will drastically improve after cleaning and dry out.  Units with electrical defects in the ground wall insulation will usually not improve after these efforts.  In these cases, further investigation is required.

As is often the case, marginal readings where the PI is good, and the Insulation Resistance is lower than we would like to see, we have OEM guidance which helps us make a tradeoff between the two factors.  For example, a unit has 2 gig-ohm resistance, but a low PI of 1.05, we could make the determination that the unit was suitable for service due to the excellent IR values and attribute the low PI to humidity.  Conversely, a resistance value of 25 meg-ohms and a PI of 2.6 would also constitute a unit that is safe to return to service.  In either of these cases, we would recommend performing additional testing to determine the nature of the defect and provide a plan for improving these readings.  Virtually anyone can perform the test; but it takes a highly trained and experienced operator to interpret the data and make the right call on what to do if the readings are less-than-perfect!

Sidewinders takes many factors into consideration, including but not limited to:

  • The OEM of the unit
  • The cooling technology (air-, hydrogen-, or water cooled)
  • The unit vintage
  • Plant-specific factors including environment, temperature, humidity, and altitude

The megger and PI test is one example of how Sidewinders’ experience and technical acumen can be instrumental in helping you ensure the maximum reliability of your generating assets.

HIRE THE EXPERTS –SIDEWINDERS, YOUR RELIABILITY PARTNER!

 

 


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