Insulation failure

1. FAILURE & TESTING OF
INSULATORS

2. CONTENTS
 Introduction
 Literature Survey
 Causes for failure of Insulators
 Testing of Insulators
 Conclusion
 References

3. INTRODUCTION
An electrical insulator is a material whose internal electric
charges do not flow freely; very little electric current will flow
through it under the influence of an electric field.
An insulator performs dual functions:
 Mechanically, it holds the conductor or bus bar at a certain
distance from ground.
 Electrically it also provides the necessary insulation.

4. CLASSIFICATION OF INSULATORS
 Porcelain Insulator
 Glass Insulator
 Polymer Insulator

5. LITERATURE SURVEY
 Ravi.S.Gorur, “ Failure modes of porcelain and
toughed glass suspension insulators”,2011 Electrical
Insulation Conference, Maryland,5& 8th june 2011.
 C.de Tourreil, “Failure rates of high voltage line insulators”,
Insulator News & Market report, vol 8,No 3,May/Jun2000.
 E.A Cherney , “Cement Growth failure of porcelain
suspension insulators”, IEEE Trans,PAS,vol102,pp,2765-
2771,1983.

6. CAUSES FOR FAILURE OF INSULATOR
1)Radial Cracking 2) Pin Corrosion
3) Brittle Fracture

7. 5) Spontaneous shattering 6)Sheath Cutting
7) Non brittle fracture type 8) Bird Pecking

8. 8)Explosive Shattering 9) Shed Tearing
10) Erosion

9. Tests on Insulators
The tests on insulators can be divided into three groups.
 Type Test.
 Sample Test.
 Routine Test.

10. Type Tests
These tests are done to determine whether the particular design is suitable for
the purpose.
(a)Withstand Test:
 Specified voltage is applied.
 Flashover or puncture should not occur. [otherwise permanently damaged].
 The test is repeated five times for each polarity.
(b) Dry One-minute Test
 The insulator (dry) at prescribed voltage should be gradually brought up and
maintained for one min.
 There shall not be puncture or flash-over during the test.

11. c) Dry flash-over test:
 The voltage shall then be increased gradually until
flash-over occurs.
 This is repeated ten times. There shall be no damage to
the insulator.

12. (d) One-minute Rain test:
 The insulator is made wet at temp 10 ̊c.
 The prescribed voltage is maintained for one min.
Wet flash-over test:
 The voltage shall then be increased gradually until flash-over
occurs.
 This is repeated ten times. There shall be no damage to the insulator.

13. Impulse Frequency Flashover Voltage Test of Insulator
The overhead outdoor insulator must be capable of sustaining
high voltage surges caused by lightning etc. So this must be tested
against the high voltage surges.
1. The insulator is kept as it would be used practically.
2. Then several hundred thousands Hz very high
impulse voltage generator is connected to the insulator.
3. Such a voltage is applied to the insulator and the spark over voltage is
noted.

14. Sample Tests
The sample is tested fully, up to and including the point of
breakdown. This is done only on a few samples of the insulator.
a) Mechanical loading test:
 Insulator , mechanically loaded up to the point of failure.
 When failure occurs, the load should not be less than 2000 lbf.
(b) Electro-mechanical test:
 The insulator is simultaneously subjected to electrical and
mechanical stress.
 There should be no damage caused.

15. (d) Puncture Voltage Test of Insulator
 The insulator is first suspended in an insulating oil.
 Then voltage of 1.3 times of flash over voltage, is applied to the
insulator.
A good insulator should not puncture under this condition
(e) Porosity test:
 The insulator is first broken into pieces.
 Then these broken pieces of insulator are immersed in a 0.5 % alcohol
solution of fuchsine dye under pressure of about 2000 PSI for 24 hours.
 After that the sample are removed and examine.
 The presence of a slight porosity in the material is indicated by a deep
penetration of the dye into it.

16. Routine Tests
Mechanical Routine Test:
 Mechanical load of 20% more than working load, applied after
suspending the insulator for one minute.
 There should be no mechanical failure of the insulator.
Corrosion Test of Insulator
In corrosion test of insulator,
1. Insulator is suspended into a copper sulfate solution for 1 min.
2. Then the insulator is removed from the solution and wiped, cleaned.
3. Again it is suspended into the copper sulfate solution for one minute.
4.The process is repeated for four times.
Then it should be examined and there should not be any corrosion on it.

17. Insulator Tester

18. Conclusion
 It is important to focus on modes of failure and its impact
on utility operations than on failure rate of insulators.
 Testing of insulator is very essential to know the ageing
characteristics and their performance in different
environmental conditions.

19. REFERENCES
• Ravi.S.Gorur, “ Failure modes of porcelain and toughed glass suspension
insulators”,2011 Electrical Insulation Conference, Maryland,5& 8th june
2011.
• A.P. Mishra,R.S. Gorur & S.Venkataraman, “ Evaluation of Porcelian and
Toughed Glass suspension insulators removed from service”,IEEE trans
DEIS,vol 15, No 2,pp,467-475,2008.
• C.de Tourreil, “Failure rates of high voltage line insulators”, Insulator
News & Market report, vol 8,No 3,May/Jun2000.
• E.A Cherney , “Cement Growth failure of porcelain suspension
insulators”, IEEE Trans,PAS,vol102,pp,2765-2771,1983.

20. THANK YOU