Polarisation Index,the better way to IR test.
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 environment 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:
- Capacitive Current (la)
- Conduction Current (IR)
- Surface leakage current (IL)
- Polarization current (Ip)
- Capacitive Current: Insulator behaves as a capacitor when a DC voltage is applied to a capacitor, a high charging current first flows and then it decays exponentially. The size of the capacitor and the internal resistance of the voltage supply, typically a few hundred kilo ohms, set the currents decay. In case of generator or motor windings, the current effectively decays to zero in less than 10 seconds. Since the capacitive current contains little diagnostic information, the initial IR is measured once the capacitive current has decayed. Hence the first IR measurement has been set as one minute to ensure that this current does not distort the IR calculation.
- Conduction Current: This current is due to the flow of electrons between the copper and the core. This is galvanic current through ground wall. Such a current can flow if the ground wall has absorbed moisture, which can happen on the older thermoplastic insulation systems. The current may also flow if there are cracks, cuts or pinholes in the ground insulation and some contamination is present to allow current to flow.This current is constant with time. With modern insulation this current usually is zero (as long as there no damage to the insulation).
- Surface leakage Current: This is constant DC current that flows over the surface of the insulation. It is caused by conductive contamination (oil or moisture mixed with dust, dirt, insects, chemicals etc) on the surface of the windings. This current is also constant with time.
- Polarization Current: Electrical insulation is hygroscopic in nature and presence of moisture will be there either in low quantity or in excess. Water molecules are very polar. When an electric field is applied across the insulation start absorbing electrons from the hydrogen molecules causing ionization of hydrogen. In other words, the molecules constituting water align in the electric field, just as magnetic field. The energy required to align the molecules comes from the current in the DC test voltage supply. This current is called polarization current. The water becomes completely polarized when the absorption of electron from hydrogen merging with oxygen is completed. Once the molecules are all aligned, the current stops. The approximate time for complete polarization is 10 minutes. That is why the IR is measured after 10 minutes of applying voltage.
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
Effect of Temperature on IR
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.
Importance of PI
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|