Three Phase with Common Neutral

onthesolar

Hi, I have a three phase line from the POCO to my home with a common neutral.  I have the circuits in the house divided into 3.  On one of those circuits I have a SW4024.  The SW installation guide states that the neutral from the power company goes to a common neutral bus bar, the neutral going to the input side of the inverter is from the common neutral bus bar, the neutral coming from the output of the inverter is to the common neutral bus bar, the neutral to all the loads are connected to the same bus bar.  Since the loads from all the 3 circuits are sharing the same neutral, is the wiring configuration correct. Thanks.

BB.

There are two major classes of three phase power Wye ("Y") and Delta.

Wye 3 phase power is (generally) 208 VAC / 120 VAC with a common neutral.

Delta 3 phase power is (generally) 240 VAC / 120 VAC with a (not common) neutral to derive 120 VAC power from two of the three phase power leads (the third phase wire to neutral is the "stinger" and is (should be?) 208 VAC.

Normally, in the US we have 120/240 VAC split phase power. It is a single phase transformer with a "Center Tap Neutral". 240 between the Black and Red leads. And 120 VAC from Black or Red to White (neutral) center tap.

And (at least in my area of California), Wye three phase (208/120 VAC) three wire+common neutral is usually found in office and light industrial. Delta power is usually in industrial and where power usage is heavier and/or needs 240 VAC.

https://en.wikipedia.org/wiki/Three-phase_electric_power

So--At this point, I could not give you a good answer. I do not know which type of three phase power you have.

-Bill

mike95490

Or, you may have conventional split phase power and see 3 wires (2 hot & 1 neutral)  and think 3 wires = 3 phase ?

onthesolar

I am located in India.  There are 4 wires from the POCO (over head lines have 4 cables in the street) coming into the house distribution box.  The voltage between phases is 440V and between each phase and neutral is 220V.  From what I understand this is a WYE type 3 phase power.

BB.

I am not sure, but if Wye--then would not your voltages be 381/220 VAC or 440/254 VAC (Wye phase to neutral vs phase to phase should vary by sqrt(3) or ~1.732 factor. Delta is a factor of 2.0 between the two line voltages).

I don't think you can have 220/440 VAC with Wye connected power (only Delta gives you 220/440 VAC power--And not across all phases--More complicated).

It is possible that people have "short hand" talk about 440/220 VAC power and they really mean 440/254 VAC (or some version). 220 is approximately equal to 254 VAC in the US (technically, our "legal" 240 VAC power runs from 212 to 264 VAC). But that is certainly not ideal muddying of the voltage range. (US does not have a "legal" definition of 240 VAC power--It is up to the power company to define--In my area, our voltage range tends to be ~240-250 VAC for our "240 VAC" circuits and that has, I believe, averaged up over the past decades from ~220 VAC as the US standardized power--Europe has similar issues with power standardization across countries).

I don't know anything specifically about power/voltage in India. But should it not be 400/230 VAC @ 50 Hz if Wye system? (In the US, we have both Wye and Delta power from utilities--And what you get is regional/local/application specific from what I have seen).

Do you have a link to the specific SW product you are looking at? In the US we have split phase 120/240 VAC with a ground bonded Neutral.

Many 240 VAC only products (or 230 VAC @ 50 Hz) would not have a ground bonded neutral for carrying power. And in many countries (as I understand), the AC power may not be ground bonded neutral at all, or at least not ground bonded in the home and/or guaranteed polarity (hot/neutral polarity).

Check the SW installation manual for a 230 VAC @ 50 Hz installation and do not look at the US 120/240 VAC @ 60 Hz wiring/configuration manuals--They are (most likely, I think) the incorrect setup for your requirements.

-Bill

onthesolar

I was using the shorthand talk stating 440/220.  I am using the manual that came with the product.

The product link is http://solar.schneider-electric.com/product/conext-sw/

BB.

OK--Assuming I interpret your power connections correctly (always confirm my guesses--I do not work for Schneider or our host, NAWS--just a volunteer poster here)... Here is the manual I am looking at:

http://cdn.solar.schneider-electric.com/wp-content/uploads/2015/09/Conext-SW-Installation-Guide-230VAC-975-0637-01-01_Rev-E_ENG.pdf

Assuming all of your loads you want to run from your AC inverter are 230 VAC @ 50 Hz single phase--And you are using just one SW4024 inverter... You do not have any 3 phase loads to backup power for.

Normally with a Wye based 3 phase system, the AC loads are distributed evenly across the 3 possible phase connections (in the US, smaller commercial installations, as I recall, single phase loads are supposed to be matched within 10,000 Watts or better).

Ideally you would "rewire" / move the loads you want to backup to (for example) Phase A and re-balance the rest of the loads to Phase B and C. (in practice, we are only talking about a maximum of 4,000 Watts of loads here--Re-balancing may not be needed).

You now need to create a "protected power sub-panel" for your loads you want to power from the AC inverter. Move the loads from the main panel to the sub-panel following your local electrical codes/practices. The inverter should only power "critical loads"--Solar/backup power is "expensive" and you do not want to waste power if you can avoid the problem.

Installing the SW4024 is done by connecting the L/N connections (Line and Neutral) to your AC mains Phase A, and the AC Output of the inverter to your protected sub-panel.

You will have to help me here... In the US, "N" is Neutral--And the Neutral wire (white in the US) is tied to your Green wire ground in one location. In the US the Green wire safety ground and White Wire Neutral Power bus are (usually) connected together in the Main Breaker panel for the home. And no other place. Your protected sub-panel AC Neutral Bus must be "isolated" from the metal box/safety ground connections. If you connect the Neutral bus and the green wire ground together in the sub-panel, you can have "circulating ground current"--Which can, in some cases cause the white wire/green wire safety ground to carry too much current and burn out. Also, in some AC inverters, Neutral/Ground bonding on "both sides of the inverter" can cause problems inside the inverter (improper operation and/or damage).

If you have a backup AC genset, you have to disconnect the AC Neutral and AC Green Wire safety ground--Most larger gensets default to bonding Neutral+Ground inside the genset and you need to physical disconnect for your installation.

To identify your AC Neutral. Take an AC volt meter and connect one lead to "safety ground". And measure your 3 phase neutral. If the measured neutral voltage is near zero volts (say 10 VAC or less). You should be fine. You may find your neutral bus is grounded inside your main panel--Or it may be earth grounded on the transformer outside your home (in the US, neutral is bonded in both locations--Don't know about India/your specific region).

If your AC "Neutral" is >100 VAC--Then we may have an issue to fix. Let us know what voltage you measure (and measure the other phase to safety ground voltages).

The SW4024 appears to carry the Neutral wire through the internals as straight wire connection. No fancy Neutral switching between utility and backup power. This works fine if your location has a ground referenced AC Neutral (the center tap of your three phase power).

There is one other question I have about your home's wiring. This a bit difficult to explain. Note in your "three phase" power system, your AC Neutral/Center Tap "balances" current between all three phases. I.e., if Phase A has 10 amps and Phase B and C have zero (0) amps, the Neutral will carry 10 amps.

If, Phase A, B, and C all have 10 amps each--The Neutral Current will be near zero (0) amps.

Now that you have possibly moved all of your AC backup loads to a single phase sub-panel. The single Phase Neural currents all add up... Your SW4024 inverter can output near 20 Amps @ 220-230 VAC--You need to make sure that the Neutral wiring in your home is large enough to handle the to total AC current of your loads...

In the US, we can run each single phase load (hot and neutral) back to the main panel (no shared neutral), or we can run several phases (Red and Black for split phase 120/240 VAC systems, or the 3 hot phases + Neutral for three phase) and a common AC Neutral between the breaker panel and the loads.

The "home run" Neutral (one hot + one neutral from panel to load back to panel)--No issue (Neutral cannot carry too much current--it is always paired with a single hot lead of the same size). However, with "shared neutral" (Phase A+B+C to loads, one neutral back to panel)--Because you are running a single phase (Phase A+A+A + one Neutral)--You may have to do some rewiring to be safe (add "home run Neutral" from loads back to protected sub panel) so that the Neutral does not carry too much current.

Sorry this is so many words--It is difficult to explain and may not even apply in your section of the world--Just trying to be safe.

Does this help/answer your questions?

-Bill

onthesolar

Thanks Bill for a details answer, I believe I understand most of it.  This is for an existing installation and I was not confident that my installer knew what he was doing, hence the question.

You are right I do not have 3 phase loads.  In India we do not bond the neutral to ground in the breaker panel at home.  The neutral is connected to the POCO's neutral which probably is connected to ground in the transformer.  The green ground wire at home is connected to the homes safety ground.  I do not have a genset as part of this setup.  I think the information I was looking is the neutral being pass through in the SW4024, the manual states both the INPUT & OUTPUT neutral be connected to the same neutral bus which caused some confusion. So what is the point of having a separate neutral in the input and output side of the SW4024 if they are one and the same.

BB.

It is just an easy way to connect the Utility and Protected Output Neutrals together. It is alll "one connection" in the entire building/SW/protected wiring neutral side.

There are inverters that do switch the AC Neutral so that it uses the POCO's neutral bond when using utility power. And switches to "local" inverter+green wire ground when "off grid" (running solar or generator power). Switching Neutrals is especially common in the Recreational Vehicle industry.

-Bill

onthesolar

Thanks Bill, all the information you shared really helps with my learning. Appreciate your time.