Three phase transformer for connecting 480/277V inverters to a 208/120V service

[ggunn]

From comments I have read in here as well as from advice I got years ago from a rep at a transformer or inverter manufacturer I have been using transformers that are delta on the primary (208V) side and wye on the secondary (480V) side in the PV systems I design. I get that a wye secondary with the neutral grounded is to give a 0V reference for the 277V lines to the inverter(s), but in full disclosure I confess that I do not have a concrete reason for using a delta primary.

A colleague of mine (who is a project manager at my company and who is probably reading this) is developing design rules for projects for which we farm out the designs to third parties, and he has asked me for an explanation of why a delta primary is necessary. What say you?

 

[hillbilly1]

From what I have read over the years, wye wye does not have as stable of voltage pattern as a delta wye.

 

[ggunn]

From what I have read over the years, wye wye does not have as stable of voltage pattern as a delta wye.

Thanks, but I am looking for something a little more concrete.

Anecdotally, we once built a residential project years ago where we used a pair of 240/120V to 2000V single phase transformers to bring power from a remote ground mounted PV system. Both transformers were center tapped on the low voltage side, and we connected the neutrals both to the inverters and back to the service. We used eGauge for monitoring on the service end of the run, and it showed a large imbalance between the current on L1 and L2 coming from the PV system until we disconnected the neutral on the service end.

 

[pv_n00b]

Either Ygd or Ygyg will work fine for a grid tied PV system. The advantage of the Ygyg is that the inverters can sense a ground fault on the utility side of the transformer. For larger PV systems utilities typically call for Ygyg so we use a lot of those with no problem. But there is no technical or NEC reason to favor one over the other for LV transformers.

 

[wwhitney]

Either Ygd or Ygyg will work fine for a grid tied PV system.

Slightly confused by your notation--with a step-up transformer as in the OP, is the primary side listed first, or the high voltage side? I was expecting the primary side, which would mean the discussion is about DYg vs YgYg.

Cheers, Wayne

 

[pv_n00b]

Slightly confused by your notation--with a step-up transformer as in the OP, is the primary side listed first, or the high voltage side? I was expecting the primary side, which would mean the discussion is about DYg vs YgYg.

Cheers, Wayne

Generally, the HV side is listed first and gets a capital letter for the winding, the LV side gets a lowercase letter for the winding. The notation does not indicate primary or secondary orientation.
In the 2020 NEC 705.30(C) they added that the primary side is the side that faces the higher fault current source. Up till 2020 people just rolled their own as to which side was which.

 

[synchro]

In the 2020 NEC 705.30(C) they added that the primary side is the side that faces the higher fault current source. Up till 2020 people just rolled their own as to which side was which.

I've seen transformer manufacturers call the outside windings of the transformer the primary, and the inside windings closer to the core the secondary. Your reference to 705.30(C) in the 2020 NEC is consistent with this manufacturer designation because the windings further from the core would experience less inrush current if energized with a low impedance source.

 

[ggunn]

OK, but back to the original question: Why delta (why not wye) on the primary side? Some of us were very vocal about that in other threads.

 

[pv_n00b]

I've seen transformer manufacturers call the outside windings of the transformer the primary, and the inside windings closer to the core the secondary. Your reference to 705.30(C) in the 2020 NEC is consistent with this manufacturer designation because the windings further from the core would experience less inrush current if energized with a low impedance source.

Yes, the transformer design results in one side having less inrush current when energized than the other. It's important that the primary side be oriented to face the side that will energize the transformer. While transformers will transfer power in either direction, connecting the secondary to the side that gets energized can result in OCPD operation when using the required NEC OCPD ratings. I'm not sure how common the knowledge is today but it was thought for years that it did not matter in PV which side of the transformer faced the utility. That did cause problems.

 

[pv_n00b]

OK, but back to the original question: Why delta (why not wye) on the primary side? Some of us were very vocal about that in other threads.

A grounded WYE facing the utility will probably end up being a source of zero sequence current to the utility. That means that even with nothing connected to the secondary of the transformer we will measure current on the primary neutral, sometimes a fairly high current depending on the condition of the utility distribution system. I remember talking to a Satcon engineer back in the day when transformers were part of a central inverter. They were seeing 50A of neutral current in the early designs and had to ground the neutral through an impedance to prevent that. So that's one strike against the primary WYE. Another is that since it is providing zero sequence current to the distribution system, that could result in the protection at the local substation not getting the full fault current on a ground fault and that will desensitize the protection system. Not specifically a problem for the PV installer but still something with large transformers on MV interconnections.
The downside of the delta is that it blinds the built-in inverter protection from sensing ground faults or loss of phase on the primary side of the transformer. On larger systems, it can result in not passing the loss of phase test during commissioning so we don't get PTO from the utility.

 

[wwhitney]

A grounded WYE facing the utility will probably end up being a source of zero sequence current to the utility.

Can you clarify which of the following arrangements, with no secondary load, would have the same issue? I'm trying to understand the underlying physics and remove the unnecessary features.

(A) 3 separate iron core-inductors wired in a wye pattern (like 3 single phase transformers, but no secondary coils)
(B) 3 inductors on a figure 8 iron core (like a 3-phase transformer with no secondary)
(C) 3 single phase transformers wired as wye-wye (primary-secondary)
(D) 3 single phase transformers wired as wye-delta
(E) A three phase wye-wye transformer
(F) A three phase wye-delta transformer

Cheers, Wayne

 

[pv_n00b]

Can you clarify which of the following arrangements, with no secondary load, would have the same issue? I'm trying to understand the underlying physics and remove the unnecessary features.

(A) 3 separate iron core-inductors wired in a wye pattern (like 3 single phase transformers, but no secondary coils)
(B) 3 inductors on a figure 8 iron core (like a 3-phase transformer with no secondary)
(C) 3 single phase transformers wired as wye-wye (primary-secondary)
(D) 3 single phase transformers wired as wye-delta
(E) A three phase wye-wye transformer
(F) A three phase wye-delta transformer

Cheers, Wayne

It can happen with a Ygd 3ph transformer configuration, with the HV side facing the utility. That configuration allows zero sequence current to flow on the utility side with no connections to the secondary side of the transformer. In the Ygd the zero sequence goes flows from L-N on the WYE side and circulates in the delta on the delta side. The delta side does not need to be connected to anything.
A Ygyg, Ygy, Yy, or Yyg would not allow the zero sequence current from the utility to return through the transformer.

 

[ggunn]

It can happen with a Ygd 3ph transformer configuration, with the HV side facing the utility. That configuration allows zero sequence current to flow on the utility side with no connections to the secondary side of the transformer. In the Ygd the zero sequence goes flows from L-N on the WYE side and circulates in the delta on the delta side. The delta side does not need to be connected to anything.
A Ygyg, Ygy, Yy, or Yyg would not allow the zero sequence current from the utility to return through the transformer.

To be clear, my question is specifically regarding connecting 480/277V inverters to 208/120V services. The HV side does not face the utility.

 

[jaggedben]

It can happen with a Ygd 3ph transformer configuration, with the HV side facing the utility. That configuration allows zero sequence current to flow on the utility side with no connections to the secondary side of the transformer. In the Ygd the zero sequence goes flows from L-N on the WYE side and circulates in the delta on the delta side. The delta side does not need to be connected to anything.
A Ygyg, Ygy, Yy, or Yyg would not allow the zero sequence current from the utility to return through the transformer.

It doesn't matter which side is HV, right? Any wye-delta with the wye facing the utility would do it, right?

But this would not be a problem with the delta side facing the utility, like ggunn normally does?

 

[ggunn]

It doesn't matter which side is HV, right? Any wye-delta with the wye facing the utility would do it, right?

But this would not be a problem with the delta side facing the utility, like ggunn normally does?

Again to be clear, the question is wye - wye or delta - wye. The transformer will always be wye on the inverter side.

 

[LarryFine]

"WYOY, your philosophical radio station."

 

[electrofelon]

OK, but back to the original question: Why delta (why not wye) on the primary side? Some of us were very vocal about that in other threads.

Ok here is one approach. Why NOT use a Delta? Wye primary transformers are very rare in <600v systems. Instead of asking for a reason why the primary should be Delta, maybe we should ask what would be the reason for using a wye? And I can't think of one.

 

[winnie]

I much prefer the nomenclature where the 'primary' is the side of the transformer intended to be energized first. Generally this means that the coils have higher leakage inductance (to reduce inrush) (often done by having the coils on the 'outside', further from the core), and the coils have the taps for setting the volts/turn ratio. IMHO it doesn't matter which way energy flows during normal use, nor does it matter which side is higher voltage.

My understanding, using the above nomenclature, is that the 'primary' side should face the utility, because even though energy flows from the PV inverters to the utility, this doesn't happen until after the transformer is energized from the utility, and it is the utility that is setting the voltage of the system.

Why should the primary be delta?

If the primary is wye, you have a situation where the primary neutral will be vying with the system neutral, where the PV transformer will be creating a neutral and holding it relative to the phase conductors. If this transformer's neutral is different than the system neutral, then you will see circulating currents as the two neutrals try to equalize.

If the primary is delta, there is no neutral for this to be a problem.

Sometimes the utility requires a primary wye for reasons of 'effective grounding'. In this case it is the utility stating that the derived neutral is providing a desirable benefit, and the transformer needs to be sized to handle the potential loads associated with the dueling neutrals.

-Jon

 

[wwhitney]

If the primary is wye, you have a situation where the primary neutral will be vying with the system neutral, where the PV transformer will be creating a neutral and holding it relative to the phase conductors. If this transformer's neutral is different than the system neutral, then you will see circulating currents as the two neutrals try to equalize.

Does that happen with wye-wye, or just wye-delta (using the nomenclature where primary/utility side is first)? I'm not seeing why it would happen with a wye-wye.

Cheers, Wayne

 

[tortuga]

From comments I have read in here as well as from advice I got years ago from a rep at a transformer or inverter manufacturer I have been using transformers that are delta on the primary (208V) side and wye on the secondary (480V) side in the PV systems I design. I get that a wye secondary with the neutral grounded is to give a 0V reference for the 277V lines to the inverter(s), but in full disclosure I confess that I do not have a concrete reason for using a delta primary.

A colleague of mine (who is a project manager at my company and who is probably reading this) is developing design rules for projects for which we farm out the designs to third parties, and he has asked me for an explanation of why a delta primary is necessary. What say you?

Do the wye-wye transformers have a neutral run to both sides?
If so i'd say 250.24(A)(5) is your reason for no.