INSULATION RESISTANCE&

POLARIZATIONINDEX TEST

Prepared by :Amit Kumar

INTRODUCTION Rotating machines –

One of the most important component in the Power System.

Always under the influence of electrical, mechanical, thermal and environmental stress, e.g., heat, cold, moisture, dirt, oil, vibration etc. which causes the degradation of insulation quality and ultimate failure.

Degradation Processes in the stator winding of rotating machines include – coil looseness, slot and end winding discharges, de-lamination of insulation etc.

Stator Winding – designed to achieve a satisfactory service life of typically 20 to 40 years, depending on the operating conditions and periodic maintenance of the windings.

NEED OF INSULATION RESISTANCE (IR) & POLARIZATION INDEX (PI) TEST

They are excellent means of ascertaining winding conditions that are contaminated or soaked with water.

Good for detecting major flaws where the insulation is cracked or has been cut through.

Life of a generator or motor mainly depends on the insulation, and IR and PI are two universally accepted diagnostics tests for insulation system. These tests help us to check for suitability for operation of the machine.

Helps in assuring safety of the personnel and to reduce or eliminate down time.

Helps to schedule repair work such as: vacuum cleaning, steam cleaning, drying and rewinding and also to evaluate quality of repairs before the equipment is put back into operation.

Depending on the environmental and surrounding conditions, it is recommended to perform these tests annually.

INSULATION RESISTANCE TEST

Measures the resistance of the electrical insulation between the copper conductors and core of the stator or rotor.

Insulation resistance should have a high value to avoid any appreciable leakage current.

Lower value of Insulation Resistance(IR) indicates that the insulation has been deteriorated.

POLARIZATION INDEX TEST Polarization Index (PI) is a variation of IR test.

Polarization Index measurement :

PI = (R / R )₁₀ ₁Where R = IR measured after the voltage has been ₁₀

applied for 10 minutes;And, R = IR measured after one minute.₁

PI test was developed to make interpretation less sensitive to temperature. As, temperature of the winding doesn’t rise during the test period of 10 minutes. So it is assumed that both R and R are measured at same ₁₀ ₁winding temperature. Hence, PI is relatively insensitive to temperature.

TEST METHOD (PI Test) Instrument Required :

Mega – Ohmmeter , with a timed test function.

Test Voltage :

WINDING RATED VOLTAGE ˜

TEST VOLTAGE (DC) in VOLT

<1000 500

1000 – 2500 500 – 1000

2501 – 5000 1000 – 2500

5000 – 12000 2500 – 5000

>12000 5000 – 10000Refer IEEE Std. 43-2000

˜ Rated line–to–line voltage for 3-phase AC machines, line–to–ground voltage for 1-phase machines.

TEST PROCEDURE :

IR & PI tests shall be done simultaneously. All the external connections to the machine are

removed and windings are completely discharged to the grounded machine frame.

DC voltage between the windings and ground is applied using a direct indicating, mega-ohmmeter.

DC voltage is applied continuously for 10 minutes, and insulation resistance is measured after 1 minute and 10 minutes.

Power supply to the meter is switched off and windings are completely discharged to the grounded machine frame.

Polarization Index is measured by dividing the 10-minute insulation resistance by 1-minute insulation resistance.

If the test is carried out only on one winding of three phase equipment, then other windings should be grounded during the test.

NOTE: For modern form wound or cage type motors, if the stator has a very high value of IR measured (say greater than 5 GΩ) then PI is not likely to indicate anything about the winding. Thus, the test may be stopped after one minute to save time.

TEST PROCEDURE :

INTERPRETATION OF PI VALUE

PI CONDITION OF WINDING

MEASURES TO BE TAKEN

<1.0 Hazardous Dry Winding

1 – 1.5 Bad Dry Winding

1.5 – 2.0 Doubtful Drying is recommended

2.0 – 3.0 ˜ Adequate

3.0 – 4.0 Good

>4.0 Excellent

˜As per O.I.S.D. , Polarization Index of H.V. Stator winding shall have a value of at least 2.

ANALYSIS & CONCLUSION

If the PI value is below the minimum value in a modern stator winding, it is an indication that the winding is contaminated or soaked with water. Therefore, in general IR and PI test are excellent means of ascertaining winding conditions and detecting major flaws relating to insulation.

REFERENCES

O.I.S.D. Standard – 137 O.I.S.D. Recommended Practice – 149 IEEE Standard 43 – 2000