Remedy for switching transients- 4160V motors 5

[rbulsara]

We suspect switching voltage transients have caused insulation failures in two 4160V motors and a cable. Motors are controlled by vacuum contactor starters.

More concrete data will have to be collected by further monitoring.

I would appreciate any input as to the remedies available to take care of the transients. Also has anyone run into similar situation? One remedy is to have the motor windings insulation of proper rating that I know.

We are contacting starter manufacturers to see if they have any recommendations.

 

[Skogsgurra]

Yes, the non-arcing nature of the vacuum breaker is responsible for the overvoltage. An arcing contactor will absorb the inductive energy in the arc and thus limit the overvoltage. Surge arresters are a solution, but upgrading the insulation level seems to be better - if there is room enough in the slots.

Gunnar Englund

http://www.gke.org

 

[dpc]

I agree that the vacuum contactors could be causing high transient voltages. Surge protection should be added, preferably at the motor, but probably more easily at the starter. Surge capacitors will take the edge off the voltage spike that occurs during switching.

 

[edison123]

How old are these windings ? If new, then poor insulation quality/materials. If old, they were due for rewind anyway.

VCB's, in theory, are supposed to produce what is called as voltage doubling depending on the time of closure / opening in a voltage cycle. A good winding insulation is supposed to withstand this.

* Anyone who goes to see a psychiatrist ought to have his head examined *

 

[rbulsara]

Thanks everyone for the input.

Edison: Some motors are very old (1960's) and some are new like 2 years old. One burnt is old. One new was assumed burnt.

It is interesting that new one is later found to be not burnt or damaged. Only its fuse (miscoordinated) appeared to have blown.The Owner assumed the motor had failed. The new starters were installed 2 years ago, I was stumped as to why the new motor, which supposedly should have proper insulation, will get affected by the transients.

At any rate I think this site still needs protection for the transients.

 

[GordS]

If you add surge protection AND you are on a high resistance grounded system, be aware of the amount of charging current vs the resistor let thru current. Surge caps will be a source of charging current. If the sum of charging current on the system exceeds the neutral resistor let-thru, you can expect overvoltages on arcing ground faults with the potential of multiple failures. In my case, I lost 2 motors and a surge arrestor within about 45 sec. The surge caps were removed about 3-5 yrs ago with no noticeable ill effects so far( we also use vacuum contactors).

One of the factors used to justify removal of the surge caps was the cable length between starter and motor was long enough to provide enough capacitance to lower the rise time of any pulses to that defined by standards. The effect of motor feeder length needs to be considered when deciding if the problem is transients. In my case, about 70 m of cable was adequate.

If I didn't have a high resistance grounded system, I would have left the surge caps in because they did a better job of lowering the pulse rise time than cable capacitance.

 

[PowerfulStuff]

Another though is the selection of vacuum contactor. I have read a bit on vacuum circuit breakers (but assume similar things for contactors). The contact faces are a hard material but with particles of soft metal (e.g. copper) embedded and made in a particular shape. The shape and copper mean that conduction continues up to the zero crossing to provide soft switching rather than the quick interruption that is possible. Are vacuum contactors made the same way - or does it depend on the manufacturer?

Regards,
PowerfulStuff

 

[electricpete]

Not much to add except on the subject of specifying a motor to be surge-resistant.

NEMA MG-1 allows 2 surge levels: 2.0 p.u. and 3.5 p.u. If you don't specify anything, I think you are left with 2.0 p.u.. So it's better to specify 3.5. p.u. and also put coil and motor surge test requirements in your spec. Some users also add specific requirements for types of materials to be used in strand, turn, and ground insulation. EPRI has a pretty detailed spec in this regard.






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[jraef]

This was discussed in the past as well, some good information is shown, but you have to scan down towards the bottom of the thread, especially tmahan's response.

thread237-42676

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[RalphChristie]

Interesting stuff to read:

http://www.electricity-today.com/et/issue0401/i04_refiner.htm

http://www.joslynclark.com/downloads/VacuumTechnologyOverviewwithQA.pdf

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[rbulsara]

Thanks again. I had "refiner" article's copy, but was on hard drive and could not link it..thanks ralph.

 

[jraef]

More relevant info I recently came across for sharing.

http://www.nojapower.com.au/techdoc...kV Motor Interruption by A Vacuum Contact.pdf

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