1. WELCOME

2. A Presentation on Insulation Testing of Transformer
Dhritiman Nath, ET

3. Contents Transformer Insulation Insulation Tester Insulator Model
Significance of Records

4. Transformer
Transformer is an electromagnetic device that is used to change the electrical parameters keeping the frequency constant
It is the most efficient equipment in the field of electrical engineering having efficiency upto 99 %
Availability of transformer makes the use of alternating current more advantageous than direct current
We have total 104 nos of transformer in 103 nos of sub-station

5. Insulation Insulator prevents the flow of current in a
unwanted direction
Transformers have transformer oil and paper as
it’s insulation
Causes of insulation failure are –
Thermal Breakdown
Chemical deterioration
Effect of oxygen and humidity
Electro-chemical deterioration

6. insulation tester We use insulation tester of megger
D.C. voltage is applied to the insulator under measurement
The voltage levels that are available are –
250V, 500 V, 1000V, 2500V, 5000V
Measurement is taken between H.V. - Earth, L.V.- Earth and H.V.-L.V.

7. Procedure Shutdown the transformer and isolate it
Discharge the winding capacitance
Clean the bushings
Short Circuit the windings
Guard the terminals to eliminate surface leakage over terminal bushings
Connect “LINE”, “EARTH” leads
Transformer neutral bushing is to be disconnected from earth during test
Apply test voltage and note the readings

8. Continued….. Records are taken at an interval of – After 30 secs
After 1 minute
After 10 minutes
The total current is the sum of following –
Capacitive current (IC )
Conduction current ( IR )
Surface leakage current (IL )
Absorptive current ( IA )

9. + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + -
Dipoles

10. INSULATOR MODEL Total Current = Conduction current
C R R1
= Leakage current
= Conduction current
= Capacitive current & absorptive current
Total Current

11. Time of Voltage Application(minutes)
Graph
= Total Current = Absorptive Current = Capacitive current = Leakage Current = Conduction Current
Relative Current
Time of Voltage Application(minutes)

12. Continued…. Reading of 10 Mins Reading of 1 Min Reading of 1 Min
Insulation resistance reading :
After 30 secs the reading is of,
IT = IC+ IA + IL + IR
After 1 minute, the reading is of,
IT = IA + IL + IR
After 10 minutes, the reading is of,
IT = IL + IR
Polarization Index =
Dielectric Absorption Ratio =
Reading of 10 Mins
Reading of 1 Min
Reading of 1 Min
Reading of 30 sec

13. Significance IL + IC + IA IL + IC Polarisation Index :
One minute measurement is not reliable and scientifically
acceptable as IR is temperature dependent
P.I. gives result which is temperature independent and this gives
more accurate result.
P.I. gives better idea of conduction and leakage current
If IL and IC are low compared to IA, then PI will be more than 2
If IL and IC are high compared to IA, then PI will be around 1
IL + IC + IA
IL + IC

14. Continued….. AS per IEEE, Insulation Resistance Polarisation Index
Dangerous
Less than 1
poor
Questionable
Fair
Good
Above 2
Source : grouper.ieee.org

15. Source: www.openelectrical.org
Continued…..
Dielectric Absorption Ratio :
The DAR test is performed only when P.I. is found to be less than 2 even for new materials.Under such cases a minimum DAR value of 1.25 is required.
IC +IA +IL + IR
IA + IL + IR
Insulation Resistance
DAR value
Questionable
Less than 1.25
Adequate
Less than equal to1.6
Good
1.6 and above
Source:

16. Recall
Transformers
Procedure
Significance of data

17. THANK YOU