Failure Investigations on Electrical Motors and Transformers Dr Morris Lockwood Technical Manager, Power systems, ERA Technology Ltd

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1 Failure Investigations on Electrical Motors and Transformers Dr Morris Lockwood Technical Manager, Power systems, ERA Technology Ltd 1

2 Summary Intended audience What is a failure and the consequences of failure The time element : Early life failures Mid life failures End of life failures The evolution of arcing failures Investigating failures 2

3 Intended audience The talk is aimed at : Brokers Underwriters Claims personnel from Insurers I have been told that the audience will be particularly interested in the Energy sector and offshore equipment Most of the examples are land based but similar equipment is used offshore I have assumed that at least some of the audience are not familiar with the engineering aspects of electrical equipment The talk will contain a minimum of technical details. I will be happy to expand on any particular points you wish to raise 3

4 What is a failure and the consequences of failure In the context of this talk a failure is an equipment malfunction which may give rise to an insurance claim. The malfunction might be due to an inherent defect due to poor materials, design or workmanship, or due to wear and tear etc The consequences of the failure often far outweigh the value of the failed element. The talk will look at electrical failures with an emphasis on transformers and rotating machinery (motors and generators) An example of chiller motor problem. 4

5 Chiller motor problem- Part 1 Case history multiple Motor Soft Start failures. Used for starting 415V, 250 kw chiller motors in a prestige building complex in S.E. London. LOTS of failures about to become a legal dispute. Epidemiology indicated a problem with a newly redesigned electronics board opto-isolators were failing on over-voltage indicating voltages in excess of 1 kv 5

6 Chiller motor problem- Part 2 The chiller motors were refrigerant immersed. There had not been any change in design of the motor nor the refrigerant. Build-up of static charge was suspected. This was confirmed using a vane type electrostatic voltmeter. Detailed examination of the circuit diagrams for the old and new circuit boards revealed that 3 resistors which gave an earth reference had been omitted! 6

7 Chiller motor problem- Part 3 Conclusions All first failures should be investigated Two similar failures is almost a guarantee of further failures. Multiple failures can destroy business confidence. 7

8 The time element Early life failures: Manufacturing failures non-eu manufactured transformers examples. Materials defects East European steel example. Design failures hospital generator example. Commissioning failures Dutch CT example System issues Emirates transformer example Mid life failures fewer but still important : Manufacturing failures Voltage transformer examples. End of life failures: Long term degradation transformer insulation aging etc. Motors and generators end winding failure due to multiple starts / load transients 8

9 Early life failure transformer example Manufacture of transformers relocated to Turkey and other countries ABB, Alstom, Siemens have all done this The result is a very high rate of failure. 9

10 Mid life failure 27.5 kv voltage transformer example Manufacturing defect allowed moisture to enter bushing led to very explosive failure 10

11 Mid life failure 27.5 kv voltage transformer example - continued 11

12 End of life failure overheated reactor 12

13 End of life failure overheated reactor site of arcing 13

14 Offshore example Angola sunrise FPSO 14

15 Emergency Power battery room fire Large array of NiCad cells. Each a metal can in a plastic pouch. One leak detectable and not a major problem. Two leaks creates a short circuit and fire. The monitor alarms were ignored TWICE two fires! 15

16 Arcing failures Most power electrical failures result in arcing Arcs are lightweight tubes of incandescent atoms. Very high currents can flow. Currents produce magnetic fields A current in a magnetic filed produces a force Arc forces are proportional to the square of current Therefore the direction of force is independent from the polarity of the current flow. The forces are always away from the energy source. 16

17 Arcing along switch panel busbars 17

18 Arcing along switch panel busbars - continued 18

19 Arcing along switch panel busbars - continued 19

20 Investigating failures preservation of evidence Most companies do not have procedures in place to handle failures. Critical evidence can be lost. Suppliers or manufacturers have a vested interest and should not be allowed near the failed equipment unaccompanied. 20

21 Investigating failures preservation of evidence 21

22 Investigating failures preservation of evidence 22

23 Investigating failures preservation of evidence Witness statements, both written and verbal can be critical. Cultural differences can make things difficult in overseas investigations. Loss adjusters should prevent disturbance of the failure site as much as practical. An example : 23

24 Case History Failed super-grid transformer 24

25 Case History Failed super-grid transformer 25

26 Bailing out! 26

27 Bailing out! 27

28 Bailing out! 28

29 Investigating failures The process Before looking at any physical evidence get a clear briefing on the circumstances leading up to the failure. In particular, had there been any changes or odd things happening? Look for evidence from protective devices in the system. Look at the failure without preconceptions. Get a feel for the equipment structure and power flows. Identify hypothetical failure mechanisms. Look for confirmatory evidence. REPORTING! 29

30 Partial discharge tracking trees and worm holes - Part 1 Electrical stress in mixed dielectric systems is in the inverse ratio of the relative permittivities The relative permittivity of air is 1 The relative permittivity of insulation is of the order of 5 Therefore voids in insulation material concentrate stress If the dielectric strength of the void is exceeded, there is a tiny spark a partial discharge 30

31 Partial discharge tracking trees and worm holes - Part 2 The partial discharge relieves the excess stress but carbonises the material near the void. This tends to add to the stress concentration and the process repeats. The carbonisation grows in trees and wormholes until a catastrophic failure occurs. 31

32 Partial discharge tracking trees and worm holes - Example 32

33 Contacts Hot stuff Electrical contacts depend on plastic deformation of the contact materials. Actual conduction area is tiny. Pressure is required to penetrate the surface contamination oxides, sulphates etc. Lack of pressure or excessive contamination lead to small area of contact followed by over-heating, oxidation, relaxation, arcing and failure. See example. 33

34 Contacts Example Hydrogen Sulphide contamination 34

35 Contacts Example Hydrogen Sulphide contamination 35

36 Blowing in the wind a case of poor house keeping. A prestigious bank had dual redundant UPS back-to-back. The UPS company were carrying out a routine maintenance check. The technician opened the door to one of the two UPS. The other UPS suffered a serious arcing failure. The two UPS shared a common under-cabinet void. Inspection of the void revealed : Cleanliness is next to godliness 36

37 Foreign bodies Washers Metal foil Tape Swarf Plastic fitments and steel nuts and washers 37

38 The smoking gun 38

39 Blowing in the wind a case of poor house keeping - continued. Around a quarter of failures in electrical equipment are due to foreign bodies or other visually obvious causes like water ingress, chemical contamination, biological contamination. Cleanliness is next to godliness 39

40 Evolution of a transformer failure photograph taken some time after the explosion (or whoosh) 40

41 Evolution of a transformer failure This was a 140 kva, 33 kv to 400 V three phase oil-filled transformer. The manufacturers had had a series of failures mainly associated with poor quality high voltage windings. The fault was so severe the tank was split along two edge seams. The reaction force of the expelled oil moved the unit about 0.5 metre. 41

42 Evolution of a transformer failure Current traces I A I B

43 The time element Early life failures: Manufacturing failures non-eu manufactured transformers examples. Materials defects East European steel example. Design failures hospital generator example. Commissioning failures Dutch CT example System issues Emirates transformer example Mid life failures fewer but still important : Manufacturing failures Voltage transformer examples. End of life failures: Long term degradation transformer insulation aging etc. Motors and generators end winding failure due to multiple starts / load transients 43

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