PV Transformer Configuration and Inrush

msteiner

Member
I'm reviewing a 289kW PV design with three inverters outputting at 480V. The building's service is 208V, so a 500kVA transformer is being used. Two questions: 1) What should the wiring configuration be for the xfmr (Y-Y?). The solar designer has shown a neutral in both the primary and secondary circuits - is that typical? 2) Is inrush current a consideration on the primary side (the inverter output side)? On the secondary (utility) side?

electrofelon

Senior Member
I'm reviewing a 289kW PV design with three inverters outputting at 480V. The building's service is 208V, so a 500kVA transformer is being used. Two questions: 1) What should the wiring configuration be for the xfmr (Y-Y?). The solar designer has shown a neutral in both the primary and secondary circuits - is that typical? 2) Is inrush current a consideration on the primary side (the inverter output side)? On the secondary (utility) side?
500kva seems big. What is the sum of the inverters output currents? Anyway on to the specific question: You want 480 wye, 208 Delta transformer. Inrush should not be a problem if you size the primary OCPD to 125% of FLC (or even 250%, but you are getting into some major conductors for that) Per table 450.3(B).

msteiner

Member
What is the sum of the inverters output currents?
Inverter output is 360A at 480V, 3ph = 299.3kVA.
How can I connect a delta secondary to the existing 208/120V building service, without running a neutral? Doesn't the unbalanced current need a path to the source?

electrofelon

Senior Member
Inverter output is 360A at 480V, 3ph = 299.3kVA.
How can I connect a delta secondary to the existing 208/120V building service, without running a neutral? Doesn't the unbalanced current need a path to the source?
Nope. No neutral required (nor possible) for a Delta.

msteiner

Member
You'll have to explain to me how the PV source can be stepped down through a delta secondary xfmr and provide service to a building that's wired 208/120V, 3PH, 4W.

jaggedben

Senior Member
You'll have to explain to me how the PV source can be stepped down through a delta secondary xfmr and provide service to a building that's wired 208/120V, 3PH, 4W.
The utility transformer still handles the unbalanced current on the neutral.

This is generally true for grid tied systems even when there is no transformer and a neutral connects to the inverter. Inverters usually don't put current on the neutral. It's mainly there to measure voltage and phase angle.

msteiner

Member
And no concern with xfmr inrush on either side of the xfmr, assuming OCPDs sized at least 125% FLA?

electrofelon

Senior Member
You'll have to explain to me how the PV source can be stepped down through a delta secondary xfmr and provide service to a building that's wired 208/120V, 3PH, 4W.
An unbalanced L-N current in the wye side, appears as L-L current on the Delta side. So even if there was imbalance on the 480 side it wouldnt matter. There is no need to use or put current in to the utility neutral conductor, so all you need is the three phases. Hope that makes sense.

electrofelon

Senior Member
And no concern with xfmr inrush on either side of the xfmr, assuming OCPDs sized at least 125% FLA?
The utility side is the only side where inrush would be a concern. I think you will be ok at 125% unless the impedence is really low. Maybe investigate using a breaker that has a high mag setting. I would try to avoid protection at 250% as that will be a huge cost adder. Maybe others will chime in with their thoughts

Has using a 300kva been investigated? FWIW A utility wouldn't hesitate to max out their transformer, and transformers don't need a 125% continuous adder. The 500 would give you a bit less voltage drop, but I can't think of any other reason to go with 500. Might want investigate efficiency of both, that could be a factor.

winnie

Senior Member
The terms 'primary' and 'secondary' are very confusing in this particular situation. Transformers happily move power in either direction, what determines primary and secondary are things such as grounding and which side of the transformer gets initially energized.

When a transformer is first energized, you can see 'inrush' current as the magnetic flux establishes a balanced cycle around zero flux. Inrush is only a concern on the side that gets energized first, and gets damped by leakage inductance. Since you have 2 coils with one having higher leakage, you make the one with higher leakage your primary.

Generally voltage adjusting taps are also on the primary coils, so that you can adjust to get proper core saturation for different utility voltages.

Generally the primary of the transformer is a delta, and for grounded installations a wye secondary is preferred.

For common grid connected solar installations, the _utility_ provides the voltage reference, and the inverters adjust relative to what the utility provides in order to supply power to the grid. On top of this the transformer is generally connected to a grounded source on the utility side and is the ground reference on the solar side. This means that the utility side of the transformer is the one that is considered the primary, and the solar side is the one considered the secondary, even though power is flowing from the solar side to the utility side.

So in general for this application you will have a 208V delta primary and a 480/277V wye secondary. On the 208V side of things the utility transformer will provide the ground reference for any unbalanced current that the facility produces. If you had a wye connection, with the neutral connected, on the utility side of things you might see significant circulating currents between the two transformer neutrals as they try to 'fight' to set the neutral reference.

If you were designing a solar power system for off-grid application, where the inverters were providing the voltage reference and the 208V side of the transformer was deriving the neutral for the user, then you would need a 208/120V wye on the user side, and the 480V side would be the primary. But I doubt that is the installation you are asking about.

With that said for some installations the utility _requires_ a wye:wye transformer and uses a grounded wye in exactly the way described above as a 'no-no'. This if for something that the utility calls 'effective grounding' and is totally separate from NEC considerations for grounding. I don't understand it well myself, so if you are curious about that issue hopefully someone else will explain it.

-Jon

Last edited:

pv_n00b

Senior Member
It's a confusing issue. Let me see if I can help.

A lot of people get confused connecting a delta transformer to a 4 wire WYE service. We do not need a 4 wire WYE transformer to connect to a 4 wire service. No neutral is run to the delta side, it works fine. WYE - WYE transformers have problems with neutral current on the utility side from imbalances in the utility system and that is one of the reasons why it is not a standard configuration.

Grid tied 3 phase PV inverters generally provide only balanced 3 phase output current. The current is not changing to match a building's loads. Any imbalance between the phases is picked up by the utility. The inverters just convert energy in the array to 3 phase AC and pump it out, it goes where the impedance is lowest. Inverters are like honey badgers, they don't care. Unless you are doing something fancy like a non-export system that actively controls the inverter output to prevent export.

Transformers are designed to reduce the inrush current on the primary side and not the secondary side. So you want to specify your transformer with the primary side facing the utility, in this case the primary will be 208/120V. Since the transformer in a PV system will always be energized by the current from the utility side first there will be no inrush when the PV inverters start generating energy. If you do put a transformer secondary towards the utility side don't be surprised if it trips the OCPD from the inrush which will be several times the normal value for a primary side connection.

msteiner

Member
Thanks for all the replies. So we're landing on:
Primary: 208V delta
Secondary: 480Y/277V

Yes?

winnie

Senior Member
Thanks for all the replies. So we're landing on:
Primary: 208V delta
Secondary: 480Y/277V

Yes?
I'd suggest that this is the most common solution and you should question anything else. However you specifically say that the designer brought a neutral to the 208V side, and they may have a specific reason for doing so.

-Jon