Transformer IR and PI Test: A Practical Guide
Insulation Resistance is a basic Test used to check the soundness of the Transformer Insulation i.e winding Insulation.
This Test is carried out when the Transformer in Newly Installed or when the Transformer is under Fault and it is a Diagnostic Test also i.e it carried out every year too.
PI is the Ratio of R @ 10 Min and R @ 1 Min.
Procedure:
- Take the Transformer Out of Service and Isolate it completely.
- Connect all HV Bushing connectors and Neutral connectors by a suitable conductor wire.
- Connect all LV Bushing Connectors with each other by suitable conductor wire.
- Note down Oil Temperature and Winding Temperature of both Windings.
- Make the Test Connections as shown in below table:
| IR Between | Red Lead | Black Lead | Guard |
| HV Winding to Earth | HV Bushing Stud | Transformer Earth Point | Open |
| LV Winding to Earth | LV Bushing Stud | Transformer Earth Point | Open |
| HV Winding to LV Winding | HV Bushing Stud | LV Bushing Stud | Open |
- Select the PI Mode and keep the Test Voltage at 5 KV.
- Press and hold the ‘Test’ button till, the red Led above starts blinking, which means that the Test has started.
- After 10 Minutes, we get the PI Value,
Results:
| Time | HV to LV | HV to E | LV to E |
| 15 Sec | |||
| 1 Min | |||
| 10 Min | |||
| PI= R@10Min/R@1Min |
For a 220/33 KV, 50 MVA Power Transformer, Minimum Desired IR Value at 1 Minute at 30 Deg C is around 500 M Ohm
Evaluation of Test Results:
IR Value @ 1 Minute should be Greater than or equal to 500 M Ω
All the Results must be corrected to a standard Temperature of 20 deg C.
The Value of IR doubles for every 10 deg C drop in Temperature.
If Insulation Resistance is 100 M Ohm @ 30 Deg C, its value will be 200 M Ohm at 20 deg C.
Lets say the Room Temperature is 30 Deg C,
HV to E is say 500 M Ohm
At Standard Temperature of 20 Deg C,
HV to E will be 1 G Ohm.
As IR Value Doubles for every 10 Deg C Drop.
| Sr. No. | Polarization Index | Insulation Condittion |
| <1 | Dangerous | |
| 1 to 1.1 | Poor | |
| 1.1 to 1.35 | Questionable | |
| 1.35 to 2 | Fair | |
| 2 to 4 | Good | |
| > 4 | Excellent |
Why is the PI Test more Reliable than the IR TEst?
An advantage of the index ratio is that all of the variables that can affect a single Megohm reading such as temp and humidity, are essentially the same for both the 1 min and 10 min readings