Synchronous Generator Short Circuit - Hazards

[Ragnar Landet]

What are the possible hazards and causes of short circuit internally in a synchronous generator?

 

[anorlunda]

The most likely cause it's insulation failure. You do mean stator short, correct?

Hazards? It is hard to put an upper limit on that. How big is the generator? Is it hydrogen cooled? How will the short be cleared? How much is the per unit short circuit current?If you have only a little short and are thinking of ignoring it, contact the manufacturer immediately.

Ping @jim hardy

 

[jrmichler]

I witnessed a 100 hp induction motor short internally. Summer job in a motor rebuild shop, motor came in with no paperwork, we put it on the test stand, started it up, and watched it run perfectly for a couple minutes. Then there was a "poof", some smoke came out, and the motor started to coast to a stop. It blew two 60 amp fuses in the test stand, and two more 100 amp fuses on the wall at the far end of the building. There was a small spherical volume inside the motor that was missing. That volume included copper, insulation, and some stator steel.

I would expect similar inside a synchronous generator, with the amount of damage dependent on the short circuit protection (size and type of fuses) and grounding.

Electrical shorts in motors and generators are a function of electrical and mechanical stress. Electrical stress comes from the winding design. Adjacent turns have low voltage between them, first and last turns have high voltage between them. Mechanical stress comes from differential thermal expansion between copper and steel every time the machine warms up and cools down.

 

[jim hardy]

@jrmichler's description is very plausible. Not to mention he's seen it.

Answering @anorlunda's questions would enable a better educated guess regarding your question.

However much current can flow will flow.
.......

Shorts in the stator:

A phase to phase fault will be spectacular for the current that can flow into the fault is the sum of the machine's capability and the capability of the source.
A phase to neutral fault will be almost as spectacular.

How much current flows into a winding to ground fault depends on how the machine is grounded.
A solidly grounded machine allows however much current is available from machine and source to flow and will likely be spectacular.
Big central station machines are typically grounded through a resistor that limits ground fault current to a small value that won't make an explosion, ten amps or so.
Once upon a time somebody left a big Crescent wrench inside one of our machines.
When it fell down onto a terminal and grounded a phase the machine tripped. No harm done thanks to the current limiting ground resistor.

..........

shorts in the rotor

You'll likely not notice until they progress pretty far.
Since they just short out part of the DC electromagnet, there's no pyrotechnics.
They distort the magnetic field causing uneven magnetic attraction between rotor and stator so your vibration instrument readings change with excitation.
Your operators will notice the machine requires more field current than it used to.
The increased vibration is hard on the stator wedges that hold the windings in place. Loose stator windings abrade their insulation.
Here's a writeup by some specialists
http://www.generatortech.com/B-Page2-Theory-Effects.html

...............................
old jim

 

[jrmichler]

I designed the test program for a liquid cooled servomotor. You can see the motor at the bottom right of the second page in this brochure: https://marquipwardunited.com/docs/marquipliterature/dual-rotary-dd-knife-english.pdf?sfvrsn=4. The motor is 9.9" diameter, 16" long, and (at the time) was rated 225 ft-lbs RMS torque and about 600 ft-lbs peak torque. The first generation motor had a high failure rate, which was mostly cured by adding insulation between phases in the second generation motor. The third generation motor had further improvements. It lasted so long that we ended up failing the test stand twice before stopping the test. While designing the test protocol, we learned that thermal cycling the motor correctly duplicated field failures, but at an accelerated rate. Thus my remark above about differential thermal expansion.

jim hardy's link mentions shorted turns causing vibration. I saw a motor-generator set that lost a phase. It vibrated so bad that it broke mounting bolts and ripped itself off the foundation. When power was cut, the vibration stopped instantly while continued to spin.

 

[jim hardy]

Thanks for sharing real world experience. It's how we confirm (or tweak) our understanding of the formulae.

old jim.

 

[jim hardy]

You can see the motor at the bottom right of the second page in this brochure: https://marquipwardunited.com/docs/marquipliterature/dual-rotary-dd-knife-english.pdf?sfvrsn=4.

wow is that for making plywood ?

Glad you joined the forum. PF needs a few more "Motor guys" . old jim

 

[Ragnar Landet]

Thanks for all the useful advice. I might want to specify further. We are typical talking about machines of size 1500-3400kVA 400V ph ph. Starpoint of alternator is directly grounded with no resistor.

The question is asked mainly to understand the potential risks of putting into operation a machine after installation. What are the risks if a wrench or metallic debree is left inside the machine? What will happen if let's say if a wrench would cause a short in a IP21 alternator. Will there be a risk of arc's around the machine? In such a scenario the only way to break the short is to stop the machine.

I think it is important that personel putting newly installed machines into operation understand the risks, but I find it hard to find any sources describing the actual risks of said scenario.

Any help understanding actual risk is highly appreciated.