Polarization Index (PI) Test is performed on High Voltage electrical machine especially to determine healthiness, dryness and cleanliness of the insulation.
To ensure the optimum performance and condition of costly electrical machinery, it is most important to conduct certain tests that can prevent costly repairs and unexpected downtime. Similarly, Polarization index (PI) testing is one of these techniques that can help machines to function properly.
PI value with Insulation Resistance (IR) Test is conducted mainly to the judge the service condition of the insulation of machine (Transformer, Reactor, Motor, Generator etc).
Generally, PI value more than 2.0 is considered as good for most of the machines. Let’s discuss in details everything of PI Value Test. Shall we?
What is Polarization Index (PI) Test?
Polarization Index Test is an extended form of Insulation Resistance (IR) measurement test designed to check the healthiness of insulation, moisture & deterioration of insulation of HV Electrical Machine.
PI is a ratio of Insulation Resistance after 10 minutes divided by the same after 1 minute.
PI = R10/R1Where,
R10 = Insulation Resistance after 10 minutes
R1 = Insulation Resistance after 1 minute
A Polarization Index (PI) test is generally conducted at the voltage level same as Insulation Resistance (IR) test. In reality, PI is the ratio of leakage currents assuming constant voltage:
We know the Ohm’s Law, Resistance (R) = Voltage (V) / Current(I)
So, the PI Formula reduced to, PI = I1/I10
Where I1 & I10 is the current at 1 min divided by current at 10 min.
What is DA or DAR Test?
Dielectric Absorption Ratio i.e. DAR is the ratio of the IR at 1 minute divided by IR at 30 seconds.
It is basically performed when the leakage current measured stabilizes within 60 seconds. If this happens, the 10 minutes PI value test is useless, the ratio becomes 1, and the DAR should be used instead if a ratio test is required or useful.
How to Conduct a PI Test?
Performing Polarization Index Test is a very easy task. Below is the step by step guide for conducting PI Test on Transformer, Reactor. You just need to do is to perform the following steps.
Step 1: Take the Insulation Resistance (IR) tester or “Megger” with their all their Cables (Line, Ground & Earthing).
Step 2: Open the Connection that connects Neutral Bushing of Transformer/Reactor and Earth.
Step 3: Connect the Line Cable of Megger to the top of Neutral Bushing, Ground Cable to Earthing and the Earthing Cable to the Body of Machine.
Step 4: Turn ON the Megger, Select voltage level (generally 5KV) and then Set the TIME 15 sec, 1 min & 10 min.
Step 5: Finally, Press the Test button and wait for 10 minutes for the test. Megger will be injecting the Voltage and show the IR value in M-Ohm or G-Ohm (you can note down the reading of IR value of 15 sec, 1 min & 10 min for calculating PI ration manually yourself).
Step 6: Considering 10 min is over…! You will get PI value on your Megger screen.
Congratulation, You have learned how to perform PI Test…!
Note: This is the most common procedure for PI testing and may slightly differ for different make Megger. So, make sure you read the manual of your Megger before performing the PI Test.
Permissible Value of PI (Polarization Index) Ratio?
There might be different PI value for the different electrical machine to judge their insulation condition. However, the following are the guidelines for evaluating Transformer insulation using Polarization Index values.
|Polarization Index||Insulation Condition|
|Less than 1||Dangerous|
|1.0 - 1.1||Poor|
|1.1 - 1.2||Questionable|
|1.25 - 2.0||Fair|
|2.0 - 4.0||Good|
The philosophy behind PI Test?
As we know, Insulation is an important part of the machine. As the resistive value of an insulator may also vary with temperature. only measuring insulation resistance by megger may not always give a reliable result.
This difficulty is partially solved by introducing the PI value test. We will discuss the philosophy behind PI test, below.
The characteristics of the insulation are such that the current and it is not constant and it varies with time.
The purpose of PI Test can be understood by understanding the flow of the different currents in the insulator. When a DC Voltage is applied across the winding, it has mainly four component. It has mainly four components. There are four currents in the insulator and these are:
- Capacitive Current (la)
- Conduction Current (IR)
- Surface leakage current (IL)
- Polarization current (Ip)
Will be updated soon……!