Multiple sources and switched neutral

[Open Neutral]

Given a system with the following multiple feeds:

1. 277/480Y from PoCo xfmr
2. same from 3ph inverters
3. pad mounted gen feeding inverter line side [1]

All sitting cozy & swapping tales with each other....

Normally, I believe, you'd need a 4 pole Transfer Switch to move from PoCo feed to the backup inverters. But suppose all share the same single point of neutral bond to ground, the building's Ufer+whatever rods? Is neutral switching still required?





1] The inverters have "line" and "load" connections. If/when there's a generator feeding the line side, they kindly sync up to its output and assist, or consume gen. power to recharge their battery plant.

 

[LEO2854]

Given a system with the following multiple feeds:

1. 277/480Y from PoCo xfmr
2. same from 3ph inverters
3. pad mounted gen feeding inverter line side [1]

All sitting cozy & swapping tales with each other....

Normally, I believe, you'd need a 4 pole Transfer Switch to move from PoCo feed to the backup inverters. But suppose all share the same single point of neutral bond to ground, the building's Ufer+whatever rods? Is neutral switching still required?





1] The inverters have "line" and "load" connections. If/when there's a generator feeding the line side, they kindly sync up to its output and assist, or consume gen. power to recharge their battery plant.

read 455.8 page 651 NEC HANDBOOK 2008
Transfer switches 708.24/100.1/700.6/701.7/702.6 and ARTICLE 705 That may help you

 

[hurk27]

Given a system with the following multiple feeds:

1. 277/480Y from PoCo xfmr
2. same from 3ph inverters
3. pad mounted gen feeding inverter line side [1]

All sitting cozy & swapping tales with each other....

Normally, I believe, you'd need a 4 pole Transfer Switch to move from PoCo feed to the backup inverters. But suppose all share the same single point of neutral bond to ground, the building's Ufer+whatever rods? Is neutral switching still required?





1] The inverters have "line" and "load" connections. If/when there's a generator feeding the line side, they kindly sync up to its output and assist, or consume gen. power to recharge their battery plant.

switching the neutrals would depend upon where the main bonding jumper is installed, and if there is any point in the system where the grounding would parallel the neutral, I myself do not like to switch the neutral as it just adds cost to the installations but there are times you cant avoid it, also generator must not have the neutral bonded to ground in the generator if the neutral is not switched, the other thing is if the transfer switch ever has the neutral contacts fail you could end up with allot of damage equipment. but other like to switch neutrals, but not me, I have seen the damage cause by one failed neutral transfer switch contact that did about $40k worth of damage to electronic equipment, that was all it took for me to avoid it as much as I can.
when two or more transfer switches are installed for more then one building using a single generator is about the only time I can see where a system can't be designed to avoid from switching a neutral, other wise it almost always can be as far as I know.

 

[__dan]

Look at the load side

Look at the load side

I would attack it from the load side. If you can arrange and guarantee that all the loads are delta, zero neutral connected loads, then neutral current in parallel over the grounding system is less of an issue, should be zero except for misconnections.

Solidly connected neutrals and a single point of N to G bonding jumper can be a clean system at that point, with all L to L loads. The cleanliness of the ground and neutral voltage is an issue but any, "all", neutral connected loads would be fed from a downstream delta to Y transformer, separately derived with a new N to G bonding jumper at that point. That's where your clean neutral is.

The utility transformer probably (surely) has a N to G connection and one at the service makes two total minimum, which is why zero neutral connected loads at the service and tie busses may be the only way to really clean it up.

You will have to look closely at the line - load circuit path of the inverter. It will probably have an isolating filter transformer at the output and be nominally separately derived. The N to G jumper at the service and serving everything else will be noisy. A large system will load the inverter at 480v delta, after the output transformer, and step down with another downstream delta to Y transformer close to the load.

 

[Open Neutral]

The utility transformer probably (surely) has a N to G connection and one at the service makes two total minimum, which is why zero neutral connected loads at the service and tie busses may be the only way to really clean it up.

The PoCo xfmr will be right there. My [however flawed] vision was about that: "...given the transformer shall be right there, can the same single point ground [which will be Ufer+rods] be used by the stuff on our side of the meter? If so, what does that mean re: other issues?"

where the issue that came to mind first was switching neutral; was there a way we could avoid it?

There are multiple sources and users housed in the one 20*8 shed, with the poco transformer abutting. I assume we could make both generators have 5 wire connections; as this appears to require.

The output from the building is via a 1000 ft cable terminating in a delta-wye stepdown; so no neutral there. There are various single phase loads in the building...

 

[__dan]

The place to start is with the drawing, if you can post one it would help to grasp what you are trying to do, details. kVA size, load types, and distances all make a difference. Also the UPS inverter, I was confused when you said it would 'assist'. It's probably either fully online double conversion type or it's in standby in parallel and swaps in with a fast static switch. The inverter probably does not parallel with the other source for load carrying.

I would want the drawing first and the factory literature for the inverters. Without knowing either, just to throw some ideas around:

Assuming the utility service is solidly grounded, the other sources can be connected as 'not separately derived' with solidly connected neutrals, 3 pole switches, because the "system", in sum, is solidly grounded. The neutral would be insulated from ground everywhere, insulated neutral terminations, except at the common grounding, bonding, and neutral busbar at the service where the single N to G bonding jumper is also. That's the five conductor feeder, L, L, L, insulated neutral, EGC.

You would need switched neutrals when connecting more than one 'separately derived' system together because of the presence of more than one neutral to ground connection from each separately derived source.

So, you're correct to focus on the placement and essence of the N to G bond jumper. You will have to look closely at the inverter for a factory jumper from neutral to ground, which will make the system hairy, a parallel path for neutral current over the ground paths.

 

[Open Neutral]

The place to start is with the drawing, if you can post one it would help to grasp what you are trying to do, details. kVA size, load types, and distances all make a difference. Also the UPS inverter, I was confused when you said it would 'assist'. It's probably either fully online double conversion type or it's in standby in parallel and swaps in with a fast static switch. The inverter probably does not parallel with the other source for load carrying.

These shall be SMA "Sunny Island" and "Sunny Boy" inverters. Both the literature and conversations with the rep. have me believing that yes, the Sunny Islands shall sync up with a generator fed through it. The Sunny Boy's are grid-tie units, but I've been assured that they will "follow" not just poco {and thus sell back..}, but also the Sunny Island's. In that case, when load is less than the SB output; they will recharge the SI's battery string.

I would want the drawing first and the factory literature for the inverters.

Attached/linked; note this draft has many bells & whistles shown. Also, some reality is not shown; the 750' is more likely to be 1000'....

Without knowing either, just to throw some ideas around:

Assuming the utility service is solidly grounded, the other sources can be connected as 'not separately derived' with solidly connected neutrals, 3 pole switches, because the "system", in sum, is solidly grounded. The neutral would be insulated from ground everywhere, insulated neutral terminations, except at the common grounding, bonding, and neutral busbar at the service where the single N to G bonding jumper is also. That's the five conductor feeder, L, L, L, insulated neutral, EGC.

That was per my thinking.

You would need switched neutrals when connecting more than one 'separately derived' system together because of the presence of more than one neutral to ground connection from each separately derived source.

Suppose the sources had no N-G connections; i.e. we bring in 5 leads from each? Then the only N-G connection is the same, regardless of source...

Hmmm the pdf attachment failed twice; here is a public URL <http://www.sendspace.com/file/uyvegh>