High Leg Delta

[steve66]

I have an existing wastewater treatment plant with a 240/120 volt high leg service.

The process engineer wants to add some packaged blower units. It's basically a blower, VFD, ventilation fan, and controls inside a cabinet

The manufacturers info. indicates their VFD's require a "symmetrical service". They basically want a center grounded wye for the electrical supply. (The also want a ground wire the same size as the phase conductors, but that's not really a problem.)

I'm wondering if I can just add a second transformer supplied from the service transformer, and supply the blowers from the second transformer

The second transformer would be a delta primary to wye secondary transformer, with the center of the wye grounded.

Does anyone see any issues with this plan? The one possible issue i see is that I'm not sure if the existing service is an open or closed delta. (It's supplied with a pad mounted transformer, and I don't think I have ever even seen a high leg service supplied with a padmount before).

If it is an open delta, then I would be closing the delta when I connected to the new transformer. What effect would that have, or is there a way to avoid that?

 

[tortuga]

If your asking if you can add a 240D : 240/139 wye drive isolation transformer to a delta supply the answer is yes its done often.
if the loads are mostly un attended 240V (no occupants) the other thing you could do is ask the utility if they can convert the service from hi-leg to 240/139 wye and then you need a auto transformer for the 120V loads like the GFCI and lights (unless you have 240V lighting) and dont have the losses from the drive isolation transformer. Here the utility charges you for a custom transformer, however you'd buy a transformer either way.
EDIT all that is presuming simply converting to 208/120 is out of the question, which tends to be the case.

 

[jim dungar]

...the existing service is an open or closed delta.

This is not a concern.
You can always feed a closed delta set of windings from an open delta source. If not you would need special motors.

 

[tortuga]

Another thing you can do is ask the utility for a pad mounted 208/120 with a +5% tap set so you get 218/126 that gets you withing spec of most 230V motors and keeps your 120V usable, I have seen that many times here.
There are also 220/127 pad mounts available for the utility market.
If you make the change on the utility side your not paying 24/7 for core losses, and they often want to get rid of delta's anyway.
the other option is tell the VFD vendor to supply you with a VFD that works off a hi-leg delta.

 

[JoeStillman]

...the other option is tell the VFD vendor to supply you with a VFD that works off a hi-leg delta.

Never been able to do that. Have you?

Δ-Y FTW

 

[tortuga]

Never been able to do that. Have you?

Δ-Y FTW

Unfortunately no, and not from a lack of trying.

 

[winnie]

Are these blowers small enough to work with the _single phase_ portion of the high leg delta? That is a symmetrical supply right there.

 

[ggunn]

Wouldn't it be cheaper to put in an isolating transformer on the high leg secondary? You wouldn't have to deal with MV.

 

[Jraef]

You can get VFDs that will allow you to use them on a Delta service, typically from North American based mfrs like AB, ABB, even Siemens, but you must remove some of the protective circuit connections internally, which exposes them to damage if there is a ground fault anywhere on the service. Asian based mfrs typically do not allow modification of their drives for use on Delta services. But if you do, you should make sure you also add Zero Sequence GF protection in your feed to the VFD.

It's usually just simpler to add the Drive Isolation Transformer (240D to 240/139Y) ahead of it, and that also takes the place of needing a Line Reactor, so it is actually less expensive that the ZSGF + LR.

 

[Joethemechanic]

If it's a high leg open delta on the primary side it might only be two phases and the MGN coming in. I'd check that before I made plans to change transformer configurations.

 

[steve66]

If it's a high leg open delta on the primary side it might only be two phases and the MGN coming in. I'd check that before I made plans to change transformer configurations.

Yes, I assume that's what is probably there. Isn't that why utilities usually provide a 240/120V 3 phase system? And its way out in the country - a mile of the supply looks like this:

This is not a concern.
You can always feed a closed delta set of windings from an open delta source. If not you would need special motors.

I never thought of that. Makes sense.

But now I realize I wasn't thinking about the whole transformer. Is it the primary or secondary that is usually wired in an open delta? I can't seem to find anything that shows how both are wired. Again, I think the whole point was the utility only provides 2 phases to supply the transformer, but now I'm drawing a blank for exactly how that would work.

If your asking if you can add a 240D : 240/139 wye drive isolation transformer to a delta supply the answer is yes its done often.

I should have mentioned I don't have anything that really needs 240V. Was planning on just using a 120/208V secondary.

You can get VFDs that will allow you to use them on a Delta service, typically from North American based mfrs like AB, ABB, even Siemens, but you must remove some of the protective circuit connections internally,

I had no idea that was a thing. VFD's don't usually even have the neutral connected, and they just convert the input AC to DC, so I didn't think they would care about anything other than getting the right 3 phase voltage.

And these VFD's are part of the blower package, so there is no chance in getting them to be changed to accept a delta supply.

 

[Joethemechanic]

Isn't that why utilities usually provide a 240/120V 3 phase system?

In your case 100% why. Saves them running a mile of primary. They also did it a lot to save on one pole pig too.

If it is only 2 phases and a MGN, going out there, you are stuck with the open delta.

In that case this is the easiest and cheapest by a long shot

It's usually just simpler to add the Drive Isolation Transformer (240D to 240/139Y) ahead of it, and that also takes the place of needing a Line Reactor, so it is actually less expensive that the ZSGF + LR.

 

[Joethemechanic]

can't seem to find anything that shows how both are wired.

Notice the primary connection is basically a WYE with one transformer missing

 

[Birken Vogt]

Yes, I assume that's what is probably there. Isn't that why utilities usually provide a 240/120V 3 phase system? And its way out in the country - a mile of the supply looks like this:

That picture shows a single phase primary. You would need one more hot up top to get 3 phase open delta.

 

[winnie]

I had no idea that was a thing. VFD's don't usually even have the neutral connected, and they just convert the input AC to DC, so I didn't think they would care about anything other than getting the right 3 phase voltage.

As you note, VFDs convert the input AC to DC, and then take that DC to generate AC. If you ignore voltage to ground, then they shouldn't care about anything other than the L-L voltage. The VFD doesn't use a connection to neutral, even one designed to work on a high leg system.

The problem is specifically about the voltage to ground. Many VFDs have L-G (not L-N, L-G) filters and transient suppression components. Put these VFDs on a corner ground or high leg system and these L-G components get hit with sustained overvoltage.

The DC+ and DC- to G voltage has different ripple characteristics when you don't have a balanced supply. This impacts the stresses on the winding-ground insulation system of the motor. I doubt this is significant on a 240V system of any flavor, but it is another issue.

Since VFDs go from AC to DC and back to AC, they have some single phase to three phase capacity, thus my question about using a single phase 240V supply.

 

[Jraef]

If it's a high leg open delta on the primary side it might only be two phases and the MGN coming in. I'd check that before I made plans to change transformer configurations.

But if you add the DIT ahead of the VFD(s) it doesn’t really matter.

 

[steve66]

Ok, so it sounds like a drive isolation transformer is the way to go.

But I'm still not sure how they are getting 3 phase from the 2 wires on the pole, even with an open delta.

It is a pad mounted transformer, not the typically POTS I see on a high leg, and I don't know exactly where the underground run to the transformer goes, so maybe its actually fed from somewhere else.

 

[tortuga]

Well there you go open delta pad mount.

usually just simpler to add the Drive Isolation Transformer (240D to 240/139Y) ahead of it, and that also takes the place of needing a Line Reactor, so it is actually less expensive that the ZSGF + LR.

Thats the answer ^^

Wouldn't it be cheaper to put in an isolating transformer

Yes the 240/139Y isolation transformer is the lowest initial cost and the fastest solution, other than running the VFD off single phase.

The bigger picture is as you know customer side transformers are not free to operate, they consume kWH 24/7 and they can start to pile up you can end up with several at each site then extend that over 20 sites, then people that run wastewater systems / water systems tend to do things like 25 year plans, energy audits, and energy reviews.
So if the bigger picture is to not just one un-attended pump site gets a new blower but like 20 sites they will want to plan ahead, and use design patterns and If all your 240 equipment going forward is going to really want 240/139Y and converting to 480 (or 208) is out of the question It would be in the best interest of the wastewater operator suits to have conversation with the utility suits in their districts about the feasibility of supplying 240/139Y service as things are upgraded where 3 primaries exist, as coincidentally on the utility side they may also due to technical reasons want to get rid of delta customer configurations on their primary network.
In the end though you still have no 120V, so you still need a autotransformer for 120V GFCI's and line reactors on the drive but that is less vampire loss.

 

[Joethemechanic]

But if you add the DIT ahead of the VFD(s) it doesn’t really matter.

Nahhh, kill two birds with one stone that way. And probably the only lead time is the shipping.

If the service transformer is sufficient in KW, I'd leave it alone.

Just not seeing changing a mile of primary from 3 wire to 4 wire and the service transformer would ever work out to saving money. And personally if I was doing the transformer I'd just do the whole place in 480

 

[Jraef]

Nahhh, kill two birds with one stone that way. And probably the only lead time is the shipping.

If the service transformer is sufficient in KW, I'd leave it alone.

Just not seeing changing a mile of primary from 3 wire to 4 wire and the service transformer would ever work out to saving money. And personally if I was doing the transformer I'd just do the whole place in 480

You don't replace the service transformer with the DIT, you just mount it next to the VFD, sized only for that drive, just like you would with a Line Reactor. No need to run 4 wires either, you just ground the X0 and leave it that way.