Magnetic Fields from Large Standby Generators

[sewerratt]

We need to install a large standby generator. 800kw. Not sure of the amp rating. The most convenient location is just outside our building next to the computer services offices. The walls are brick, approx 1 ft thick with windows. The genset might be as close as 20ft from the wall. Will the magnetic field from the generator be an issue, and if so, how far away will it be? And the conductors coming into the building come in under the same computer offices where the mainframe and servers are. How much concern should this cause. I'm not an EE PE, I usually handle civil site work, so speak slowly and use simple words.

thanks

 

[dpc]

Well, when it's not running, it's no problem at all, obviously.

At 20 feet, I wouldn't be worried at all. The generator's magnetic field is surrounded by steel, which will provide an excellent shield for any stray flux. Generators are designed to keep the magnetic field inside the machine - that is what makes them work.

The only issue might be the large power cables entering the building from the generator and location of automatic transfer switch and panelboard. Routing the power feeders in steel conduit will help a great deal. Locating the transfer switch and panelboard away from the computers is also advisable.

The computers themselves are probably generating a lot more electrical noise than the generator will be providing.

Assume you thought about vibration and real noise?

 

[ijl]

As dpc writes, the power cables may be a source of nuisance. We once had a test set-up with a computer near single phase wires carrying a current of the order of 100A or so. The distance between the CRT screen of the computer and the current carrying wires was about 10 feet. The current made the picture on the screen to jitter unpleasantly, probably due to the magnetic field.

 

[EEJaime]

If as DPC suggests, the incoming feeders are routed in steel conduit, and the routing is not in the same location as copper station cables and work stations, you should minimize any effects.

An 800 kW generator providing power at 480 Volts, 3 phase has a main distribution feeder capability of aroung 1200 Amps. This is a substantial amount of current and care should be taken to route this away from sensitive equipment.

 

[scottf]

Magnetic interference should really only be a concern for CRT monitors. Most computer rooms are probably using LCDs, which would not be affected by magnetic fields.

All-in-all, I doubt it'll be a problem.

 

[Skogsgurra]

Yes, scottf.

There's a lot of unnecessary concern about magnetic fields. Those who know the least are the most concerned.

But, there are cases where magnetic fields from motors and generators are a problem. When I do measurements on big motors/generators, it is convenient to put the scope between a couple of the cooling ribs on the motor/generator housing. The scope usually works quite well, but now and then it self-resets (the Fluke 199 seems to be more sensitive than others).

If I move the scope a foot or two away from the machine, the self-reset never happens.

Circuitry in a scope is largely the same as in a computer nowadays.

So a safe distance seems to be like one yard or so.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[ScottyUK]

On an 800kW set I would be more concerned about vibration and noise than magnetic field problems. The mag field on a big machine certainly has a fair spread, especially around the stator ends. I have seen previously reliable test instruments go crazy when used in close proximity to a running generator, but only on multi-MW machines. Our elderly Fluke 123 scopemeter has real problems in that environment. Curiously the LeCroy DSO has never faltered even when the interfering field has been so strong as to make the CRT unreadable. If the trace is frozen by the single-shot capture and stored the catured waveform is very clear (I think!).


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[Skogsgurra]

That's right. I have a LeCroy with LCD screen. Can use it on top of any machine - or rest it on the connection box. I sometimes think that Fluke is more "Dirt&Knock rugged" than "EMI rugged".

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[sewerratt]

Thanks much. The generators distance and the thick ancient brick wall should help there. The conductors still concern me a little so I'll push for steel conduit. I think that PVC had been discussed. The switchgear and buses are located in the story/floor directly below my office, but I have an LCD monitor so I should be OK. But I pity the poor HR people with the old CRTs! Hopefully it wont be used much. As for vibration and noise, it is planned to be set and anchored on a large concrete slab, 4ft thick or so, about 20 ft from the building footings. Not much we can do about location due to site constraints. Perhaps we'll issue rubber mats for the equipment and earmuffs for the employees. Joking aside, the genset is specified with a noise attentuated enclosure rated to a certain decible level a certain distance from it. The details escape me now, but that was a design consideration.

 

[scottf]

How is the brick wall going to help against magnetic interference?

 

[Skogsgurra]

Brick walls are very good at stopping people and equipment from moving close to a genset. I think that's what was on sewerrat's mind. ;-)

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...