Failure Investigations on Electrical Motors and...

© ERA Technology Limited Failure Analysis 1

Failure Investigations on Electrical Motors and

Transformers Dr Morris Lockwood – Technical Manager, Power systems, ERA Technology Ltd

© ERA Technology Limited Failure Analysis 2 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


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


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 ….


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



• 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!



• Conclusions

• All first failures should be investigated

• Two similar failures is almost a guarantee of further failures.

• Multiple failures can destroy business confidence.


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


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.


Mid life failure – 27.5 kV voltage

transformer example

• Manufacturing defect allowed moisture to enter bushing – led to very

explosive failure


Mid life failure – 27.5 kV voltage

transformer example - continued


End of life failure –

overheated reactor


End of life failure –

overheated reactor – site of arcing


Offshore example –

Angola sunrise FPSO


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!


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.


Arcing along switch

panel busbars


Arcing along switch

panel busbars - continued


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.




• 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 :


Case History – Failed super-grid

transformer


Bailing out !



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 !


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”



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.



and worm holes - Example


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 .


Contacts – Example

Hydrogen Sulphide contamination


Contacts – Example Hydrogen

Sulphide contamination


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 …


Foreign bodies

Plastic fitments

and steel nuts

and washers

Tape

Washers

Metal foil

Swarf


The smoking gun


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 …


Evolution of a transformer failure –

photograph taken some time after

the explosion (or whoosh)


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.


Evolution of a transformer

failure – Current traces

0

100

200

300

400

500

600

700

800

900

1000

1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6

IA

IB


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

Failure Investigations on Electrical Motors and...

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