Generator Behavior To 3-phase Short Circuit

What has been observed so far?

.......or if it was on board a vessel.

Pity about any safey devices! Who needs them????

Boem

If you are lucky you are not standing near the generator and can take nice pictures.

It is normally complicate to detect an overcureent due to short circuit in synchronous machines.

The reason is that for medium and small generators the short circuit current has very small time constants, leading it rapidly to the steady state.

During the steady state, the synchronous impedance is very large(1 p.u.) and not rarely the short circuit current will run below the nominal current.

What must be done?

Several means to detect short circuits in synchronous machines.

1)install two sets of current transformers upwards and downwards the generator and connect them to a differntial relay

2) an inexpensive way to detect faults in synchronous machines of small rating(under 400kVA) is to connect an undervoltage relay, directly or by means of a voltage transformer

3)there are some sofisticate means to rotect a generator. If you are realy interested in such kinds of protections, please refer to one of the following sites:

www.selinc.com

www.abb.com

www.ge.com

And stay at large to contact me if you think I would be able to help you in any way.

Kind regards

Emmanuel

http://nearingzero.net/screen_res/nz105.jpg

Okay with no short circuit protection this is what could happen.

The short circuit will be a short circuit current, roughly I = R/V (0.001/V). As the current increases dramatically, the voltage will drop. The Automatic Voltage Regulator (AVR) will try to make up the voltage and won't be able to. The engine on the other hand has just had a very large electro-magnetic brake applied with an immense amount of torque and needs to return to normal speed to reset the frequency to nominal levels, and hence the voltage. The best case for the engine is that it will stall and stop, but it might act like confetti, in which case you need to change your underpants, AFTER calling your Fire & Rescue heroes.

The moral to this story is check your trips; short-circuit, reverse power, under-voltage, under-frequency, with the electrical and governor overspeeds at regular intervals, and don't tamper with anything.

The problem is that the windings of the generator do not survive the time needed to drain all the inert energy from the rotation. The forces on the winding wires are immence and the thing justs explodes within the time a normal circuit breaker has to react.

in which case you need to change your underpants, AFTER calling your Fire & Rescue heroes.

This is O.K. ashore, but at sea, you (the crew) are the fire brigade, so its probably better to put the fire out first, as well. However, you may be uncomfortable for a while, and if you do not succeed in extinguishing the fire you wash your underpants when you jump over the side................you also hope the life raft inflates and doesn't catch fire.

Been there, done that - the last time I'd seen the CO2 go in the engine room we were able to walk down the gangway like gentlemen...I'd stopped wearing skiddies long before then.

This little piccie was a result of a collapsed DE bearing on a shaft genny. The shaft is about 8" diameter, the end plate of course did become confetti along with some other scrap that buried itself in cable about 30 feet away.

I say that's a nifty little shaft. Have you thought of patenting it.

I wonder if it will catch on!!

The first time it happened wasn't so picturesque because we stopped the engine when we could smell paint burning. The second time was the 'big bang' theory in practice, ship stopped, force 10, very dark, a lot of smoke, overalls tucked into my socks - all at an unreasonable 4am of course.

Why not. Isn't it amazing how all these delightful things take place during daylight hours, force 1 or2 breeze and the sea like a mill pond.

OH! if only it was all like that. As you so correctly pointed out, a force 10 gale, no lights, plenty of smoke, etc., Oh, how that I wish I was back at sea, how I miss it< NOT. I think that I prefer the teaching, I only have to dream of those things.

As for memories, it's mostly nightmares now the EU has opened up - if you get my drift. That Berlin Wall should have stayed up!

.............let's rig up emergency lighting!!!!