Design using 208V Delta: 480Y/277V transformer connection

We commonly connect 480Y/277V PV systems to 208Y/120V utility services.

I’ve been told WYE : WYE transformers aren’t necessary, and are known to create circulating currents that can wear out a transformer prematurely.
Furthermore, I’ve heard they are atypical, have longer lead times, and cost more.

Is it reasonable to run a 3-phase 3-wire circuit from the 208Y switchboard to feed a 208D transformer to connect to the 480Y/277V PV system. The neutral on the PV side isn't current carrying, and used primarily for monitoring.

I'm hearing that the delta on the transformer may not sync with the 208Y of the switchboard. Please advise.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
What you want is a 480v Delta primary and a 208Y/120v secondary.
No, what he wants is a 480/277V wye to 208V delta step down transformer.

OP: You are correct; there is no need to carry the neutral from the 208V service to the transformer.
 

winnie

Senior Member
I don't have the expertise to answer questions about the actual system in question, but I am in a position to point out a confusing terminology issue here.

For one of these PV systems, power is flowing from the 480V to the 208V side. But because the inverters are 'line interactive' and designed to adjust their voltage to match the connected mains, you have to think of them as connected loads. This transformer gets 'energized' (first voltage applied) from the mains, on the 208V side. Only after the transformer is powered up and producing 480V can the inverter supply power back to it.

So this transformer is what we would normally think of as a 208V to 480V step _up_ transformer even though power is flowing from the 480V side to the 208V side.

Remember that transformers are bidirectional; what differentiates the primary from the secondary is leakage inductance to limit inrush current when you energize the thing and which coil has the voltage adjustment taps. For this application your inrush is happening on your 208V side and you want to adjust for whatever the utility voltage is on the 208V side.

Re wye:wye versus delta:wye; for normal distribution you generally use a delta:wye transformer. My understanding (not my area of expertise) sometimes the utility will require a wye:wye transformer for 'effective grounding' of their distribution network during fault conditions.

-Jon
 

jaggedben

Senior Member
...

Is it reasonable to run a 3-phase 3-wire circuit from the 208Y switchboard to feed a 208D transformer to connect to the 480Y/277V PV system. ...
Yes.

I'm hearing that the delta on the transformer may not sync with the 208Y of the switchboard. Please advise.
That part sounds like nonsense to me.

...
Re wye:wye versus delta:wye; for normal distribution you generally use a delta:wye transformer. My understanding (not my area of expertise) sometimes the utility will require a wye:wye transformer for 'effective grounding' of their distribution network during fault conditions.

-Jon
Not exactly my area of expertise either, and who knows what utilities will sometimes ask for. But in this example you have a grounded service on one side, and a grounded SDS on the other side where the PV is connected. Any additional request for grounding would be superfluous as far as I can tell. The fact that a grounded conductor isn't brought to the primary (i.e. utility) side of the transformer doesn't mean you lack effective grounding of that system.
 

jaggedben

Senior Member
...

So this transformer is what we would normally think of as a 208V to 480V step _up_ transformer even though power is flowing from the 480V side to the 208V side...

-Jon
I agree. If for whatever reason you want to use one sold as a 'step down', ask the manufacturer first if it's okay to energize it from the 'secondary' side.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I agree. If for whatever reason you want to use one sold as a 'step down', ask the manufacturer first if it's okay to energize it from the 'secondary' side.
Second that the thing to do is to talk to the transformer manufacturer and tell them what you want to do. When I did that for the same reason they told me that I needed a step down tranny and that they were not interchangeable.
 
Thank you for all of the responses. I am new to this posting so bear with me.

We install commercial scale solar systems from 100kW to 5MW and larger using for the most part transformerless 480V string inverters. They have a neutral that isn't current carrying, but available for monitoring purposes.

I am new to the company employing me, and have questioned their use of wye:wye transformers - "it's just the way it's done". Vendors and manufacturers have stated that they cost more, have longer lead times, and can wear out prematurely. They have said that it can produce harmonics on the larger transformers 300kVA and up, and that the neutrals will cause circulating currents.

Much of this is like radio waves. They say they exist, but I cannot see them and fully understand it. The inverter manufacturer said they didn't see any issues with it, but the transformer manufacturer said that this config could create a delta that has shifted 30 degrees out, and that I must scope it to be sure the deltas are in "sync". I don't know how you'd shift the delta if it was. He said check with the inverter manufacturer. Sounds like people are guessing, and pass the buck. Does anyone know for sure, and/or where I can find out.


Thank you,
 

winnie

Senior Member
You are getting confused by transformer phase shift or 'vector group'. In a normal delta-wye transformer the phase angles of the three phases gets 'rotated' by 30 degrees simply because of how the coils are arranged and connected. This is critically important when you have multiple transformers connecting systems together; you can't connect transformers of different phase group in parallel.

This is basically irrelevant when you are talking about systems with a single transformer. Your inverters simply don't care about the absolute phase of the circuit that they are connected to relative to the power grid. The various transformers will introduce their phase rotations, but the local 480V circuit will be 3 phases each 120 degrees apart, and that is all that matters.

Regarding the wye:wye versus wye:delta, all I can say is to look up utility 'effective grounding' requirements. I don't really understand it except that it isn't what low voltage guys think of in terms of proper bonding and grounding. Rather it has to do with the distributed impedance of the power distribution system, and what happens to the voltage when there are faults on the high voltage lines.

-Jon
 

LarryFine

Master Electrician Electric Contractor Richmond VA
No, what he wants is a 480/277V wye to 208V delta step down transformer.
Wouldn't that be 480 Delta to 208/120 wye?

OP: You are correct; there is no need to carry the neutral from the 208V service to the transformer.
Wouldn't that be no neutral to the primary, but from the transformer to the 208v service need the neutral?
 

winnie

Senior Member
I wasn't aware that inverters must receive power to transmit power.
In the most general case they do not. You can design inverters to operate in island mode or to simply generate power.

But many if not most solar inverters are designed so that they automatically adjust to the supplied power conditions. If nothing else the inverter has to adjust its output voltage to control output current and thus power output.

Most people don't know this about their rooftop solar; when there is a power outage their power goes out even when the sun is shining.

I don't know if this applies to the more utility scale systems that the OP is talking about. I guess the 'primary' of the transformer really depends on which side gets energized first.

-Jon
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Most people don't know this about their rooftop solar; when there is a power outage their power goes out even when the sun is shining.
If they don't then the company that sold them their PV system should be severely castigated. It is unscrupulous salesmen who keep that information from their customers that give the industry a black eye. Every one of our customers is made well aware that without batteries their system will shut down when the grid goes down and that batteries are very expensive. This is made clear before any paperwork is signed.
 

jaggedben

Senior Member
Wouldn't that be 480 Delta to 208/120 wye?
OP was very clear that PV need 480/277. That's a wye. Utility interactive inverters don't turn on if you don't connect them to a system that's specified on the datasheet. OPs question is about the system on the other side of a transformer he needs to specify.

I wasn't aware that inverters must receive power to transmit power.
They must 'see' the voltage their specified for to turn on. Whether they draw power from the AC side of DC side in order to boot up and configuring themselves before they start operating depends on the design. I've seen both.
 
I am new to the company employing me, and have questioned their use of wye:wye transformers - "it's just the way it's done". Vendors and manufacturers have stated that they cost more, have longer lead times, and can wear out prematurely. They have said that it can produce harmonics on the larger transformers 300kVA and up, and that the neutrals will cause circulating currents.

Much of this is like radio waves. They say they exist, but I cannot see them and fully understand it. The inverter manufacturer said they didn't see any issues with it, but the transformer manufacturer said that this config could create a delta that has shifted 30 degrees out, and that I must scope it to be sure the deltas are in "sync". I don't know how you'd shift the delta if it was. He said check with the inverter manufacturer. Sounds like people are guessing, and pass the buck. Does anyone know for sure, and/or where I can find out.
No wye:wye is not the way it is done, I think someone is giving you hog wash. In fact I have never seen or heard of a wye wye being used in <600 systems. Utilities of course use wye:wye extensively but that is a different animal.



We install commercial scale solar systems from 100kW to 5MW and larger using for the most part transformerless 480V string inverters.
I understand that there is limited selection of 208 inverters for larger systems, but keep in mind fronius makes a 15 KW inverter that does 208 and 1kv strings and imo that would be preferred over using transformers for systems up to a few hundred KW.
 
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