Insulation of the system prevents winding faults. The stator winding is generally designed to achieve a
satisfactory service life of typically 20+ years. But it all depends on the enviroment of operation.
Generally insulation is in the form of organic compounds that contains water as a part of chemical
make-up. Excessive temperature rise dehydrates and oxidizes and makes the insulation brittle.
Subsequently it disintegrates under vibration and shock.
As the life of a generator or motor mainly depends on the insulation, the condition of the insulation
should be ascertained at a regular interval. Insulation Resistance (IR) and Polarization Index (PI) are
two universally accepted diagnostic tests for insulation tests. These have been in use for more than 75 years.
The IR test measures the resistance of the electrical insulation between the copper conductors and
the core of the stator or rotor. Ideally the value of this resistance is infinite since the purpose of the
insulation is to block current flow between the copper and the core. But in practice, it is not possible.
However, the resistance should have a high value to avoid any appreciable leakage current. Lower
value of IR indicates that the insulation has deteriorated.
PI is a variation of the IR test. It is the ratio of 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 value of PI indicates that the windings may have been contaminated with oil, dirt etc or absorbed
moistures. In the test, a relatively high DC voltage is applied between the copper conductor and the
stator or rotor core usually between the winding and ground as the machine core & body are
grounded). By applying Ohm's law, IR (Rt) at time t is then,
R1 = V / It
V is the DC Voltage applied and It is the current flowing in the circuit.
The characteristics of the insulation are such that the current. It is not constant and it varies with time.
The purpose of measuring PI can be understood by knowing the flow of the different currents in the insulator. There are four currents in the insulator. There are four
currents that may flow when a DC voltage is applied to the winding.
These four are:
Now, the total current is the sum of all above currents, i.e.
It = Ic + IR + IL+ 1p
As we have analysed, after one minute, lc is zero.
So It (1 minute) = IR + IL + Ip
As we have seen that, after 10 minutes, Ip is zero,
So It (10 minute) = IR + IL
PI= Ir + Il + Ip / Ir + Il = R10 / R1
One may argue that by measuring argue that by measuring IR after one minute, one can diagnose the condition of the insulator. If it is less, the insulation will be considered to have been deteriorated.
Unfortunately, just measuring IR has proved to be unreliable, since it is not tenable over time. The reason is that IR is strongly dependent on temperature. A 10°C increase in temperature can reduce IR by 5 to 10 times. When readings of temperature and insulation resistance are plotted on ordinary equally divided co-ordination, a curved characteristics is obtained. On the other hand if graph paper is used on which the insulation scale is laid out in logarithmic division, the graph becomes a straight line. Further, the effect of temperature is different for each insulation material and type of contamination. Although some temperature correction graphs and formulae are given in the IEEE-43 and some other books, they are acknowledged as being unreliable for extrapolation by more than 10°C. The result is that every time IR is measured at different temperatures, one gets a completely different IR. This makes it impossible to define a scientifically acceptable IR value over a wide range of temperatures.
PI was developed to make interpretation of results less sensitive to temperature. PI is the ratio of two IR at two different times. Temperature of the winding does not rise during the test period of 10 minutes. So it is fairly assumed that both R10 and R1 are measured at same winding temperature. Then the temperature correction factor will be same for both cases and will be cancelled during the calculation of Pl. Thus PI is relatively insensitive to temperature. Further in the formula of PI, the polarization current is used as a 'yard stick' to see if the leakage and conduction currents are excessive. If these later currents are much larger than the polarization current, the ratio will be about one. It is known from the experience that, if PI is about one, leakage and conduction currents are large enough that electrical tracking will occur. Conversely, if the leakage and conduction current are low compared to polarization currents, PI will be greater than 2, and experience shows that electrical tracking problems are unlikely. Thus during test, if we see the decay in the total current or rise in the IR in the interval between 1 minute and 10 minutes, then this must be due to polarization current ( since the leakage and conduction currents are constant with time) which implies that the leakage and conduction currents are low.
Interpretation of Polarisation Index results
|PI||Condition of item under test|
|1 - 1.5||Bad|
|1.5 - 2.0||Doubtful|
|2.0 - 3.0||Adequate|
Please see the Electrical Testing menu above for a complete description of Electrical Testing & safety in the place of work and in your home .In particular how and why we test the way we do. We are situated on the Kent, Sussex and Surrey borders.This electrical testing site has been designed and built by GadSolutions and is hosted on one of their servers.
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