ECN Forum Forums General Topics of the Trade General Discussion Area Delta from wye generator

We've got a wye generator feeding a delta switchbard. Common sense is just to ground T0 to the ground bus. Is it that simple, or is a different method required? I worry there is potential for 1200A+ fault current flowing through the #3/0 ground cable.

Turns out my instinct was right- Caterpillar confirmed that grounding the neutral is the right way to do this, and furthermore that they ground the neutral inside the generator assembly at the factory and have to remove the ground in the field if a floating delta is desired.

Steve......could you drop me a line. I have a related question.

sandrosandro@santinoelectric.com

Please remove one "sandro" from the email address.

This has come up again- my technician still feels very strongly that the neutral should be floating as opposed to grounded and wants to remove it. His reasoning is that there is 25' of unprotected cabling before the first OCP, and an L-G fault would fry the generator. (I'm not entirely convinced this would destroy the generator, but do agree an unprotected L-G fault is bad.)

I don't like that the L-G voltages could be all over the place and than an L-G fault might go completely unnoticed, and feel that a floating delta could potentially create other issues down the line.

How do you all feel about floating vs centered delta?

[This message has been edited by SteveFehr (edited 09-26-2006).]

You are calling this delta, but I am _guessing_ that this really is a wye source in which you are not using the neutral. Is it possible that you are _required_ to ground the neutral?

Have you considered resistance grounding this source? A suitable grounding resistance would limit the current flow in a L-G fault to an easily tolerable level, while keeping L-G voltages nice and stable during normal operation. Suitably sized, this would permit continuous operation even with a ground fault. Smaller resistances would need to trigger some sort of shutdown. There are many design issues that cannot be addressed here, but a possibility to consider.

-Jon

I am a ground the neutral kind of guy or at the very least resistive grounding but there are other things to consider. Is this an emergency generator or standby for a building that works on a power failure? Is it a power supply for an industrial operation? Both of these may require an adjustment in the grounding of the generator. If this is used with a floating ground then you are also going to need ground detection lights so someone will know there is a ground falt. I like to see a breaker installed at the generator so the generator does not have to work on a fault but not grounding it to protect the generator is a dangerous reason to float the ground, because if you get a ground fault on a phase then another the explosion will be just as loud and damaging as if it faults to a solid ground.
Is there a transfer switch involved? open neutral or closed? A 3 pole transfer switch would require you to lift the neutral from the chassis in the generator so as to prevent a second ground on the buildings service.
There are a few things to consider before you decide grounded or open.

Jon- this is a wye source with a delta load. There is no neutral wire anywhere and no neutral bus in the switchboard, just 3 hots and a ground. A resistance ground is an interesting thought! Would solve a number of the issues... I don't think it will work out this time, but I'll file that one in the toolbag, thanks!

mikesh- this is an emergency stand-by generator. There's no transfer switch, but there are some automated breakers in the switchboard (all 3-wire delta) doing the same function. The main load is a UPS, which is another angle I was looking at this morning. It turns out that the UPS requires a balanced center-grounded delta input. Per the UPS manufacturer's engineering rep, the MOVs and static switch are not designed to cope with fluctuating line voltages (with respect to ground) and could potentially fault, especially during a ground fault which would raise the other two 277V lines to 480V.

And overcurrent protective relaying built into the generator should theoretically protect it from an L-G fault by shutting it down, which was our other main concern.

[This message has been edited by SteveFehr (edited 09-26-2006).]

[This message has been edited by SteveFehr (edited 09-26-2006).]