Polarization Index Test (PI Value Test) along with Insulation Resistance Test (IR Value Test) is conducted on HV electrical machine to determine service condition of the insulation. IP test is conducted especially to determine the dryness and cleanliness of the insulation.
In insulation resistance test, a high DC voltage is applied across the insulator. This applied voltage is then divided by current through the electrical insulator to obtain resistive value of the insulator. Since, as per Ohm’s law,
Without using separate source for direct voltage, voltmeter and ammeter for measuring corresponding voltage and current, we can use direct indicating potentiometer which is also locally called megger.
Megger gives required direct (DC) voltage across the insulator, and it also shows the resistive value of insulation directly in M – Ω and G – Ω range. We generally use 500 V, 2.5 KV and 5 KV megger depending upon the dielectric strength of the insulation. For example, we use 500V megger for measuring up to 1.1 KV rated insulation. For high voltage transformer, other HV equipment and machines, we use 2.5 or 5 KV megger depending up on the insulation level.
As all electrical insulators are dielectric in nature, they always have a capacitive property. Due to that, during application of the voltage across the electrical insulator, initially, there will be a charging current. But after few instants when the insulator becomes totally charged, the capacitive charging current becomes zero. Because of that it is recommended to measure insulation resistance at least after 1 minute (sometimes 15 seconds) from the instant of application of voltage across the insulator.
Only measuring insulation resistance by megger may not always give the reliable result. As the resistive value of an electrical insulator may also vary with temperature.
This difficulty is partially solved by introducing polarity index test or in short PI value test. We will discuss the philosophy behind PI test, below.
When we apply a voltage across an insulator, there will be a corresponding current through it. Although this current is tiny and it is in milliampere or sometimes in microampere range, it has mainly four components.
- Capacitive component.
- Conductive component.
- Surface leakage component.
- Polarization component.
Let us discuss one by one.
When we apply a DC voltage across an insulator, because of its dielectric nature, there will be an initial high charging current through it. This current decays exponentially and becomes zero after some time. This current exists for initial 10 seconds of the test. But it takes nearly 60 seconds to decay totally.
This current is purely conductive in nature flows through the insulator as if the insulator is purely resistive. This current is a direct flow of electrons. Every insulator has this component of electric current. Since, in practice, every material in this universe persists some conductive nature. This conductive current remains constant throughout the test.
Surface Leakage Component
Due to dust, moisture and other contaminants on the surface of solid insulator, there is one small component of current flows through the outer surface of the insulator.
Every insulator is hygroscopic in nature. Some contaminant molecules mainly like moisture in the insulator are very polar. When an electric field is applied across the insulator the polar molecules align themselves along the direction of electric field. The energy required for this alignment of polar molecules, comes from voltage source in form of electric current. This current is called polarisation current. It continues until all the polar molecules allied themselves along the direction of electric field.
It takes around 10 minutes to align the polar molecules along electric field, and that is why if we take megger result for 10 minutes, there would be no effect of polarising in megger result.
So, when we take megger value of an insulator for 1 minute, the results reflects, the IR value which is free from the effect of the capacitive component of current. Again when we take the megger value of the insulator for 10 minutes, the megger result shows the IR value, free from effects of both capacitive component and polarisation component of the current.
Polarisation index is the ratio of megger value taken for 10 minutes to the megger value taken for 1 minute.
The significance of polarization index test.
Let I be the total initial current during polarisation index test or PI test.
IC is the capacitive current.
IR is resistive or conductive current.
IS is surface leakage current.
IP is polarization current of the insulator.
Value of insulation resistance test or IR value test, i.e. value megger reading just after 1 minute of the test, is-
Megger value of 10 minute test, is
Therefore, result of polarization index test, is
From the above equation it is clear that, if the value of (IR + IS) >> IP, the PI of insulator approaches to 1. And large IR or IS or both indicate unhealthiness of the insulation.
The value of PI becomes high if (IR + IS) is very small compared to IP. This equation indicates that high polarization index of an insulator implies healthiness of insulator. For good insulator resistive leakage current IR is very tiny.
It is always desired to have polarisation index of an electrical insulator more than 2. It is hazardous to have polarisation index less than 1.5.