INDEX OF POLARISATION
The index of polarisation represents one of the most significant parameters to monitor the health condition in a motor, with particular care to the conditions of its insulators.
It is directly proportional to the insulation resistance of the insulators. It is defined as the resistance value offered by an insulator when is subjected to a continuous electrical field.
The current that is derived it is defined as insulation current and it is mainly the resultant of the 3 following elements:
- capacitance current
- dielectric absorption current
- superficial or leakage current.
When a continuous voltage is applied to an insulator, the insulation current immediately reaches a high level and then it gradually decreases in time to reach a stable value.
This behaviour it is partially due to the capacitance current (which becomes non influential after few seconds) and partially to the absorption current which becomes non influential after a period that can start from 10 minutes and can go up to several hours (for the insulation test purposes the decrease of value after 10 minutes can be ignored).
The curve, which describes the insulation current (or the insulation resistance) in the time, is known as dielectric absorption curve.
The steepness of the curve, at a specific temperature value, is the index that represents the condition of the insulator (with reference to the humidity, pollution, ageing, etc.).
The steepness it can be described as the “INDEX OF POLARISATION” and it can be defined as follows:
R10 I 1
INDEX OF POLARISATION = ——- = ——- at constant voltage.
R 1 I 10
R 10 =Insulation resistance after 10 minutes
R 1 = Insulation resistance after 1 minute
I 1 =Insulation current after 1 minute
I 10 = Insulation current after 10 minutes
It can be verified that, for new insulators, the index of polarisation is higher than the index obtained for aged insulators or insulators working in particularly stresses or polluted conditions (either thermal or chemical).
It is therefore evident the importance of performing this test to evaluate the state of the insulators used on electric machines that are operating in field for a certain period of time. This allows to forecast the probable residual life and to program any extraordinary maintenance intervention to avoid possible but unexpected faults.
The INDEX of POLARISATION test must be performed using the same continuous voltage values used for the verification of the insulation resistance (500Vdc for machines using rated voltage up to 2400V, 1000Vdc for machines using rated voltage above 2400V) for a period of 10 minutes.
The test is performed applying the test voltage between the windings connected together and ground. After 1 minute, the instrument measures and stores the value in M that is detected. The test is continued to the end of the 10 minutes, and the value measured at the end of the test is compared to the value stored after 1 minute.
The minimum value recommended by the regulations for the INDEX of POLARISATION on class B and superior insulators is 2 (IEEE Std 43, sec. 9.2 and IEEE Std 432, App A2).
It can certainly be said that:
- When the INDEX of POLARISATION is above 2 the insulator is good.
- For values between 1,5 and 2, the insulator is on the limit of its correct operating conditions.
- For values lower than 1,5 the insulator is in a critical condition. Urgent maintenance is required.
It must be also said that, for resistance values above 5000 Mohm (after 1 minute), the test is not significant (IEEE 43) because when values are so high they cannot be measured properly, but the device under test is certainly good.