Metering 3-phase Wye without Neutral?

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BretHeilig

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Location
Brooklyn NY
I have this specialized inline meter that I'm supposed to install to monitor the output of a PV array I'm building. It's a regular 7-jaw meter topology, but there is no neutral conductor anywhere in the system (the inverter instructions, for example, say to just bond the inverter's neutral lug to ground).

The meter manufacturer says to just run a ground conductor to the neutral lug of the meter socket, but I think it's possible that they aren't telling me the whole story, because they just don't want to rock the boat and deal with me returning the equipment (I'd have to buy a different meter entirely if their method doesn't fly). Is there a code or other safety reason not to do this?

If there isn't, couldn't I just also bond neutral to chassis ground in the meter socket and achieve the same result? Is there a section of the code that addresses this situation?

Thanks for your help!
 

gar

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Ann Arbor, Michigan
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EE
160129-1651 EST

If there is no neutral, then you have a delta.

Your title and description don't correspond.

From Stout's book on "Basic Electrical Measurements" on p 447
"In fact, there is a thorem for which a generalized proof can be given that the power can be determined in an n-conductor system under any condition of unbalance by (n-1) wattmeters." Total power is the sum of the individual meters taking sign into account.

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jaggedben

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The general contractor tells me it's a three-phase Wye without a neutral and I am pretty sure those exist.

I'm pretty sure they don't exist. However...

in either case my question is still the same - can I safely meter three phase power through a seven-jaw meter without a neutral conductor?

If the inverter is genuinely approved by the manufacturer to be connected to a wye system without a connection to the neutral (some are, some aren't) - or if the system is truly a delta - then I believe you should have no problem with your meter, since there would be no neutral current for the meter measure in any case. I see no reason for connecting the meter's neutral terminal to ground, although I suppose it wouldn't hurt?

It's not really a question of safety, unless you mean keeping your contract obligations safe. ;)

...(the inverter instructions, for example, say to just bond the inverter's neutral lug to ground)...

I'm skeptical you're reading that correctly. What model inverter?
 

GoldDigger

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In the same way that a 120/240 single phase meter connects only to L1 and L2 and does not measure either neutral current or voltage can *approximate* the exact power (exact if voltages are exactly balanced), a three phase wye meter can approximate the power without a connection to the neutral. An exact measurement requires a voltage but not as current connection.
The connection to ground is a safety bond, not part of the metering.
 

BretHeilig

Member
Location
Brooklyn NY
I'm pretty sure they don't exist. However...

But my link to All About Circuits says they do.. is that wrong? Genuine question. This isn't really my area of expertise.


I'm skeptical you're reading that correctly. What model inverter?

It's an ABB Trio 27.6kw TL-US. I went back and read the manual and I think you might be right.. this was just something my GC had told me *he'd* read in the manual. I'll have to discuss this with him further..

I appreciate your help!
 

gar

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BretHeilig:

but there is no neutral conductor anywhere in the system (the inverter instructions, for example, say to just bond the inverter's neutral lug to ground).

Does the inverter really have a neutral (the center point of a wye) or is what they refer to as neutral simply an EGC point?

Excluding the inverter you you say there is no neutral in the system. This could mean the source is a floating delta, a corner grounded delta, a delta grounded in some other way, or a grounded wye with the center point not being distributed. All of these except the floating delta have a specific relationship to ground (earth). Floating deltas are not like to be present other than in manufacturing plants.

You can somewhat determine the system source by voltage measurements to ground. A floating delta could show substantial unbalance to ground. A corner grounded delta will be obvious, but a floating delta could look like a corner grounded one with one line shorted to ground. A grounded wye will look fairly well balanced.

The only safety problem with your power meter will relate to its voltage rating and your system voltages to ground.

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This could mean the source is a floating delta, a corner grounded delta, a delta grounded in some other way, or a grounded wye with the center point not being distributed.
.

Just as an academic question, but why couldnt it be an ungrounded wye? Is that just an assumption that a utility wouldnt do that? If I wanted a 480 ungrounded, isnt it conceivable that the utility would use a wye connection so they could use the same transformers they would for a regular 480/277?
 

GoldDigger

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The general contractor tells me it's a three-phase Wye without a neutral and I am pretty sure those exist. in either case ....

By definition a wye circuit (transformer secondary) must have a star point whether it is used by loads or not.
And that wye point is by definition a neutral whether it is grounded or not.

But in most cases where a wye supply exists the neutral must, according to the NEC, be the grounded conductor. And if the service is electrically a wye the neutral conductor from the POCO side must be brought at least to the service disconnect. It does not need to be extended from there in any branch or feeder circuits which support only line to line loads.

There is no safety issue in using a three phase wye meter without a neutral connection.
 

mivey

Senior Member
Just as an academic question, but why couldnt it be an ungrounded wye? Is that just an assumption that a utility wouldnt do that? If I wanted a 480 ungrounded, isnt it conceivable that the utility would use a wye connection so they could use the same transformers they would for a regular 480/277?
From an academic point, it could be.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
It's an ABB Trio 27.6kw TL-US. I went back and read the manual and I think you might be right.. this was just something my GC had told me *he'd* read in the manual. I'll have to discuss this with him further..

Both the data sheet and manual say you need a neutral connection. This inverter is for a 3phase 4wire system (not including ground).

If you really don't have anywhere to connect the neutral then you should call ABB and ask them what to do, get a letter saying if the neutral is not necessary.

If it is really a wye without any neutral conductor being used, but the wye point is grounded, then connecting the inverters neutral terminal to ground will probably get the inverter to turn on. And yet it without that letter from ABB then there could be warranty or inspection issues.

If the wye point is not grounded, or if it is really a delta, then you probably won't get this inverter to turn on. Again, call ABB.

160129-2131 EST
Does the inverter really have a neutral (the center point of a wye) or is what they refer to as neutral simply an EGC point?

It really has a neutral terminal. It is used only for voltage and phase sensing, but UL listing may require that it actually be a neutral connection.

Excluding the inverter you you say there is no neutral in the system. This could mean the source is a floating delta, a corner grounded delta, a delta grounded in some other way, or a grounded wye with the center point not being distributed. All of these except the floating delta have a specific relationship to ground (earth). Floating deltas are not like to be present other than in manufacturing plants.

Good summation. To repeat, the blue is the only one that this inverter is likely to be able to function with. And listing requirements may dictate that the neutral get distributed to the inverter.
 

mivey

Senior Member
I think I remember a thread from a while back and one of the utility guys on here saying that an "ungrounded delta" is often served by a wye connected secondary.
You can certainly serve it that way. What happens in the source windings is different. Even a uni-grounded wye starts to look more like a delta as you get a long way from the source.
 
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