polarisation index

What is Polarization Index? What is its significance? What is its minimum value?

Please read this article, its taken from

Copyright Material IEEE Paper No. PCIC-2004-XX

Greg C. Stone IEEE Fellow Iris Power Engineering 1 Westside Drive, Unit 2 Toronto, Ontario M9C 1B2 Canada

II. IEEE 43 – INSULATION RESISTANCE AND POLARIZATION INDEX

^{IEEE 43 – INSULATION RESISTANCE AND POLARIZATION INDEX}

This is probably the most widely used diagnostic test for motor and generator rotor and stator windings. It can be applied to all machines and windings, with the exception of the squirrel cage induction motor rotor winding, which does not have any insulation to test. This test successfully locates pollution and contamination problems in windings. In older insulation systems, the test can also detect thermal deterioration. Insulation resistance (IR) and polarization index (PI) tests have been in use for more than 70 years. Both tests are performed with the same instrument, and are usually done at the same time. The last revision to IEEE 43 was in 1974.

A. Purpose and Theory

The IR test measures the resistance of the electrical insulation between the copper conductors and the core of the stator or the rotor. Ideally this resistance is infinite, since after all, the purpose of the insulation is to block current flow between the copper and the core. In practice, the IR is not infinitely high. Usually, the lower the insulation resistance, the more likely it is that there is a problem with the insulation.

PI is a variation of the IR test. PI is the ratio of the IR measured after voltage has been applied for 10 minutes (R10) to the IR measured after one minute (R1), i.e.:

PI = R10/R1

A low PI indicates that a winding may be contaminated with oil, dirt, insects, etc. or soaked with water. In the test, a relatively high DC voltage is applied between the winding copper and the stator or rotor core (usually via the machine frame). The current flowing in the circuit is then measured.

The insulation resistance (Rt) at time t is then:

Rt = V/It

which is just Ohm's law. V is the applied DC voltage from

the tester, and It is the total current measured after t minutes. The reference to the time of current measurement is needed since the current is usually not constant.

There are four currents that may flow when a DC voltage is applied to the winding. These four are:

1. Capacitive current. When a DC voltage is applied to a capacitor, a high charging current first flows, then decays exponentially. The size of the capacitor and the internal resistance of the voltage supply, typically a few hundred kilohms, sets the current decay rate. A motor stator winding may have a total capacitance of about 100 nF. Thus this current effectively decays to zero in less than 10 seconds. Since this capacitive current contains little diagnostic information, the initial insulation resistance is measured once the capacitive current has decayed. This time before taking the current reading has been set as one minute to ensure that this current does not distort the insulation resistance calculation.

2. Conduction current. This current is due to electrons or ions that migrate across the insulation bulk, between the copper and the core. This is a galvanic current through the groundwall. Such a current can flow if the groundwall has absorbed moisture, which can happen on the older thermoplastic insulation systems, or if a modern insulation has been soaked in water for many days or weeks. This current also flows if there are cracks, cuts or pinholes in the ground insulation (or magnet wire insulation in random wound machines), and some contamination is present to allow current to flow. This current is constant with time, and ideally is zero. With modern insulation, this current usually is zero (as long as there are no cuts, etc) since electrons and ions cannot penetrate through modern epoxy -mica or film insulation. Older asphaltic mica insulations always had non-zero conduction currents, since such insulation systems absorb moisture. If this current is significant, then the winding insulation has a problem.

3. Surface leakage current. This is a constant DC current that flows over the surface of the insulation. It is caused by partly conductive contamination (oil or moisture mixed with dust, dirt, fly ash, chemicals, etc.) on the surface of the windings. Ideally this leakage current is zero. However, if this current is large, it is likely that contamination–induced deterioration (electrical tracking) can occur. This current can be large in round rotor windings where

The polarization index (PI) was developed to make interpretation less sensitive to temperature. PI is a ratio of the IR at two different times. If we assume that R10 and R1 were measured with the winding at the same temperature, which is usually very reasonable to assume, then the 'temperature correction factor' will be the same for both R1 and R10, and will be ratioed out. Thus PI is relatively insensitive to temperature. Furthermore, PI effectively allows us to use the absorption current as a 'yard stick' to see if the leakage and conduction currents are excessive. If these latter currents are much larger than the absorption current, the ratio will be about one. Experience shows that if the PI is about one, then the leakage and conduction currents are large enough that electrical tracking will occur. Conversely, if the leakage and conduction currents are low compared to the absorption current after 1 minute, then PI will be greater than 2, and experience indicates that electrical tracking problems are unlikely. Thus, if we can see the decay in the total current in the interval between 1 minute and 10 minutes, then this decay must be due to the absorption current (since the leakage and conduction currents are constant with time), with the implication that the leakage and conduction currents are minor.

Table 1 Guidelines for dc voltages to be applied during

C. Interpretation

What constitutes a 'good reading' and a 'bad reading' depends on the nature of the insulation system and the component (stator or rotor) being tested. Until 2000, the minimum R1 and the acceptable range for PI was essentially the same for all types of stator winding insulation. However, it has been recognized that the modern insulation materials in random wound and form wound stators have essentially no conduction current (as long as there are no cracks or pinholes).

Thus it is possible for a clean, dry, form wound stator winding to have an R1 that is essentially infinite – greater than 100 GW. With an R1 of infinity, calculations of a realistic PI are dubious. Such high R1's are not likely in systems made before the 1970's. Consequently, the maintenance person needs to establish the type of insulation used in the winding, or at least the approximate age of the winding, before interpreting IR and PI results.

Table 2 summarizes how to interpret IR and PI results in stator and rotor windings. The distinction between older and modern insulation systems was set at 1970, although this is somewhat arbitrary. Of note in this table:

1. If R1 is below the indicated minimu m, the implication is that the winding should not be subjected to a hipot test, or be returned to service, since failure may occur. Of course if historical experience indicates that a low R1 is always obtained on a particular winding, then the machine can probably be returned to service with little risk of failure.

2. The minimum R1 is the value corrected to 40oC.

Unfortunately, any more than 10-20oC correction is unlikely to be valid.

3. The minimum acceptable R1 is much lower for old stators than new stators, and it depends on voltage class. For modern stators, the minimum acceptable R1 only depends on whether it is a form wound or random wound stator.

4. For modern form wound stators, if a very high R1 is measured (say greater than 5 GW), then PI is not likely to indicate anything about the winding. Thus, one can save time by aborting the test after the first minute of testing.

5. If the IR or PI is below the minimum in a modern

stator winding, it is only an indication that the winding is contaminated or soaked with water.

6. If a high PI result is obtained on an older stator winding, then there is a possibility the insulation has suffered thermal deterioration. This occurs because thermal deterioration fundamentally changes the nature of the insulation, and thus the absorption currents that flow. The insulation has changed in an asphaltic mica winding if the asphalt has been heated enough to flow out of the groundwall.

Table 2-Recommended minimum insulation resistance values at 400C (all values in MW )

In general, the IR and PI tests are an excellent means of finding windings that are contaminated or soaked with moisture. Of course the tests are also good at detecting major flaws where the insulation is cracked or has been cut through. In form wound stators using thermoplastic insulation systems, the tests can also detect thermal deterioration. Unfortunately, there is no evidence that thermal deterioration or problems such as loose coils in the slot, can be found in modern windings