wye-delta dilemma

[Open Neutral]

So on the attached SLD, I have a dilemma on X2. But first, I should fill in some details.

Poco supplies power from upper left, via their own HV:480/277wye xfmr. The load comes off via the 200A shunt trip in H1 & down a long $$ cable to a 112.5KVA delta-wye, and some other 3ph 480 loads. No 277 loads.

Also on H1 is an array of grid-tie inverters via the 40A breaker.

When Reddy Kilowatt goes away [too frequently], ATS1 throws and the backup inverters start. The generator offscreen right will start/stop as needed.

My dilemma is X2. I believe we need wye on the LV side because I can't get a statement from the mfgr that they can [not?] drive a delta winding, and because I have local 120VAC loads fed from SL1. [True/false?]

When poco is feeding; their HV secondary provides the ground reference so the 480 legs are not floating wherever they wish vs ground; they are 277 above, period.

But in the other mode, what does that? It's gotta be X2's HV side is a wye.

But I at least was taught that wye-wye was evil... err asking for real problems, harmonic & otherwise, without major spells and chants, invoked under a full moon, etc.

To add complexity, the grid-tie inverters are not just able to produce when the poco is up; but also after the backup units are up & stable. Any surplus shall recharge the backup batteries via the output, now input of the backup inverters. So then X2 is then acting as a stepdown transformer.

I've read of "wild leg" 3ph schemes that ground a corner or a centertap, and of zig-zags, but know only that much. A magnetics guru told me of wye-wye xfmers with an aux. delta winding solely to cancel the harmonics.

So what's the answer here?

 

[kingpb]

Why not use a 480delta - 208Y/120V?

The fact that you don't have any 277V loads is inconsequential to the transformer.

Use the EGC coming from the utility for anything on the 480V side of the transformer.

 

[Smart $]

Appears to me that X2 is a 208 delta-configured primary to 480Y/277 secondary step up xfmr.

The primary is connected to Panel SL1 through what appears to be a 4 pole RV Starter????

Your 480Y/277 secondary is an SDS which requires a grounding electrode system (GES; that's the symbol extending downward off the xfmr enclosure), which is what provides reference to earth. This GES gets connected to the H0 terminal, either directly or indirectly though the system bonding jumper SBJ.

 

[Open Neutral]

Appears to me that X2 is a 208 delta-configured primary to 480Y/277 secondary step up xfmr.

That's what the print says, but the words DRAFT should be all over this. I didn't do the drawing; someone else did & he used words/terms I would not have. I don't see how X1 can be correct.

The primary is connected to Panel SL1 through what appears to be a 4 pole RV Starter????

That is really the secondary...most of the time.

The inrush for the downstream 112.5KVA xfmr load is way about the instantaneous current limits for the inverter; it would just fold. So we use series resistance in each leg to limit the current to 50A, for the first 5-10 cycles until the rush is over; then we shunt them out. It's 3-pole.

Your 480Y/277 secondary is an SDS which requires a grounding electrode system (GES; that's the symbol extending downward off the xfmr enclosure), which is what provides reference to earth. This GES gets connected to the H0 terminal, either directly or indirectly though the system bonding jumper SBJ.

But that's the gotcha. I think I need the LV side to be wye; I have an inverter system feeding it that is 3x120V L-N units, and I have 120V loads off the SL1 panel.

 

[Open Neutral]

Why not use a 480delta - 208Y/120V?

Because then I have a 208wye:480D source feeding a long cable to the 480D:208wye 112.5KVA load; so the 480 side is floating with no reference above ground. That strikes me as ...undesirable....

 

[Jraef]

The only thing I see that I think is wrong s that X2 is mislabeled, the primary is 480V and should be Delta, the secondary is the 4W 120/208V, confirmed by the fact that SL1 is shown as a 120/208V panel, MTS1 is also shown as 120/208V and the two disconnects are shown as 250V.

So the only other issue is what do you do about grounding on the 480V side of everything when on the generator / inverters right? Wouldn't that usually be done in the ATS? That's where I would do it.

 

[Open Neutral]

The only thing I see that I think is wrong s that X2 is mislabeled, the primary is 480V and should be Delta, the secondary is the 4W 120/208V, confirmed by the fact that SL1 is shown as a 120/208V panel, MTS1 is also shown as 120/208V and the two disconnects are shown as 250V.

I'm with you.

So the only other issue is what do you do about grounding on the 480V side of everything when on the generator / inverters right? Wouldn't that usually be done in the ATS? That's where I would do it.

HOW will you reference the 480 to ground is what I don't know. When fed with a wye, any wye, it's assumed and automatic. Without that....

Do I buy a small 480Y:anything delta and not connect that delta?

 

[Jraef]

Sorry, I was wrong; looks like you would ground at the generator. From the fact that he specified a 4 pole ATS I would assume it is a separately derived system. See Section 702.10, (A) and (B).

 

[Open Neutral]

Sorry, I was wrong; looks like you would ground at the generator. From the fact that he specified a 4 pole ATS I would assume it is a separately derived system. See Section 702.10, (A) and (B).

The generator is on the LV side; what keeps the 480 from floating?

[Further, the generator may not be there; if it is cycled down or out of fuel, it will drop out of the picture]

He's convinced it must be a switched neutral; but that's a separate issue.

 

[Smart $]

That's what the print says, but the words DRAFT should be all over this. I didn't do the drawing; someone else did & he used words/terms I would not have. I don't see how X1 can be correct.



That is really the secondary...most of the time.

The inrush for the downstream 112.5KVA xfmr load is way about the instantaneous current limits for the inverter; it would just fold. So we use series resistance in each leg to limit the current to 50A, for the first 5-10 cycles until the rush is over; then we shunt them out. It's 3-pole.



But that's the gotcha. I think I need the LV side to be wye; I have an inverter system feeding it that is 3x120V L-N units, and I have 120V loads off the SL1 panel.

In the SLD picture you posted, there is no X1 nor is there a 112.5kVA xfmr, if they are not one and the same.

What I don't get is where you see a problem. If X1 is feeding ATS1 through the meter main, when the utility power goes down, ATS1 kicks over to the non-utility source, feeding ATS1 form the right and subsequently Panel H1 below. There is no connection between inverter/generator system and utility system.

The power of the transformer is going right to left. Right side is a 208 delta-configured primary and left is a 480/277 wye-configured secondary.

Your primary system is grounded, probably at the inverter's and generator's disconnecting means since MTS1 is a 4-pole transfer switch (but doesn't appear to be shown on the SDL, at least for the inverter-supplied system.

Your secondary is an SDS, and the SLD shows it is to be grounded at the transformer.

I simply don't see anything wrong with the SLD (didn't check disconnect or ocpd ratings, though).

 

[Open Neutral]

In the SLD picture you posted, there is no X1 nor is there a 112.5kVA xfmr, if they are not one and the same.

Many things are off page. X1 is incidental and not shown. The BIG xfmr, X3, is off page down, fed via the 200A shunt trip breaker in the H1 panel.

What I don't get is where you see a problem. If X1 is feeding ATS1 through the meter main, when the utility power goes down, ATS1 kicks over to the non-utility source, feeding ATS1 form the right and subsequently Panel H1 below. There is no connection between inverter/generator system and utility system.

The power of the transformer is going right to left. Right side is a 208 delta-configured primary and left is a 480/277 wye-configured secondary.

If I stick with that (wye high side, delta low) I am feeding [right to left] that delta with the 5KWA inverters [and maybe the generator, if in use...]. I'm concerned (perhaps needlessly) about balance between phases.

Then in recharge mode, I'm pushing power left-to-right and I don't know how they will like it. If they were were hooked to a wye winding, I see no problem; with a delta I'm not as confident.

In both cases I have loads off of SL1 that would to some extent unbalance things.

Your primary system is grounded, probably at the inverter's and generator's disconnecting means since MTS1 is a 4-pole transfer switch (but doesn't appear to be shown on the SDL, at least for the inverter-supplied system.

Your secondary is an SDS, and the SLD shows it is to be grounded at the transformer.

The center of the inverter outputs would be the neutral, and grounded.

 

[Smart $]

Many things are off page. X1 is incidental and not shown. The BIG xfmr, X3, is off page down, fed via the 200A shunt trip breaker in the H1 panel.



If I stick with that (wye high side, delta low) I am feeding [right to left] that delta with the 5KWA inverters [and maybe the generator, if in use...]. I'm concerned (perhaps needlessly) about balance between phases.

Then in recharge mode, I'm pushing power left-to-right and I don't know how they will like it. If they were were hooked to a wye winding, I see no problem; with a delta I'm not as confident.

In both cases I have loads off of SL1 that would to some extent unbalance things.



The center of the inverter outputs would be the neutral, and grounded.

In what your are calling "recharge mode", the SLD shows no power going left to right through X2.

 

[Open Neutral]

Sorry, I must have mis-edited that part out. Grr, I remember typing it....

The right side inverters are backup one; battery driven. Also at H1, on the 40A breaker, is 21KW of grid-tie inverters. When on grid, they feed loads and sell-back to the PoCo.

When the grid fails, they go off line, per code. But once the backup inverters come up, the grid-ties will soon sync up and also source power. When their output exceeds the demand; the backups backcharge their battery bank, saving hours of generator time. (I have no clue how they accomplish this. The only bidirectional converters I'd read of before this were in the NYC subway system; and they were rotary machines...)

When that's going on, power flows from left to right; and X2 acts as a stepdown wye:delta unit, driving the 120v loads in SL1 and the backup inverters.

 

[Smart $]

Sorry, I must have mis-edited that part out. Grr, I remember typing it....

The right side inverters are backup one; battery driven. Also at H1, on the 40A breaker, is 21KW of grid-tie inverters. When on grid, they feed loads and sell-back to the PoCo.

When the grid fails, they go off line, per code. But once the backup inverters come up, the grid-ties will soon sync up and also source power. When their output exceeds the demand; the backups backcharge their battery bank, saving hours of generator time. (I have no clue how they accomplish this. The only bidirectional converters I'd read of before this were in the NYC subway system; and they were rotary machines...)

When that's going on, power flows from left to right; and X2 acts as a stepdown wye:delta unit, driving the 120v loads in SL1 and the backup inverters.

Well, as they say, the devil is in the details (Don't know exactly why "they" say this, but it seemed fitting ).

I don't know enough about the inner workings of inverters to know how they will respond. I'd say the manufacturer is the only entity qualified to provide you with the answer you seek.

 

[Open Neutral]

I don't know enough about the inner workings of inverters to know how they will respond. I'd say the manufacturer is the only entity qualified to provide you with the answer you seek.

Nor do I. But considering my client wants to spend a few bux with them, you'd think they would be more helpful......