Solar panel transformer 240.21(C)

[hhsting]

I have 150kva transformer 208V delta three phase utility side and 480V wye solar panel side three phase.

All install indoor. The 208V side disconnect with fuse is 80 feet away while 480V disco is less than 10 feet away.


Question:

1. I am not sure how this would work which is primary and which is secondary. However does NEC 2014 section 650 or 240.21(C) or any other code section apply and 208V side disco which is 80 feet away needs to be less than 10 feet away from transformer?

2. Grounding electrode conductor size transformer should be based on what 480V conductors or 208V conductor? Does NEC 2014 address? If so then which code section?

3. Is their solar transformer as 208V delta to 480/277V wye? If their is such a thing then 208V side need neutral?

 

[hhsting]

I have 150kva transformer 208V delta three phase utility side and 480V wye solar panel side three phase.

All install indoor. The 208V side disconnect with fuse is 80 feet away while 480V disco is less than 10 feet away.


Question:

1. I am not sure how this would work which is primary and which is secondary. However does NEC 2014 section 650 or 240.21(C) or any other code section apply and 208V side disco which is 80 feet away needs to be less than 10 feet away from transformer?

2. Grounding electrode conductor size transformer should be based on what 480V conductors or 208V conductor? Does NEC 2014 address? If so then which code section?

3. Is their solar transformer as 208V delta to 480/277V wye? If their is such a thing then 208V side need neutral?

What I am asking is post #1:

#1 solar transformer secondary conductor protection tap rules for solar panel does NEC 2014 section 240.21(C) or some other code section apply? Of course which is secondary and which is primary

#2 post #1 transformer grounding electrode size depend on 480V conductor or 208V conductor?

#3 Is their solar transformer as 208V delta to 480/277V wye? If their is such a thing then 208V side need neutral?

 

[electrofelon]

What I am asking is post #1:

#1 solar transformer secondary conductor protection tap rules for solar panel does NEC 2014 section 240.21(C) or some other code section apply? Of course which is secondary and which is primary

#2 post #1 transformer grounding electrode size depend on 480V conductor or 208V conductor?

#3 Is their solar transformer as 208V delta to 480/277V wye? If their is such a thing then 208V side need neutral?

Just think of the PV inverter as a regular load. Just ignore that the average energy flow going the other way. Just try to envision the inverter being a big milkshake machine making milkshakes. Just focus on the energy going into that delicious milkshake and let your thoughts go to that happy place.

The only special thing to consider is 705.30(B), but note this is for your 450.3 TRANSFORMER protection, not the conductors.

Is there a transformer that is 208V delta to 480/277V wye?

yes it is a common transformer (note I fixed some errors for you).

If their is such a thing then 208V side need neutral?

Im still trying to figure out how to connect a neutral to a delta connected transformer. Perhaps this can be done in a parallel universe, where there are also these "service feeders" we sometimes hear about.

 

[electrofelon]

Renewable Energy - Hammond Power Solutions

The renewable energy generation market has grown remarkably in the past decade, and it is an expanding part of HPS's product development focus.

www.hammondpowersolutions.com

 

[jaggedben]

2. Grounding electrode conductor size transformer should be based on what 480V conductors or 208V conductor? Does NEC 2014 address? If so then which code section?

The grounding electrode conductor will be based on the 480V conductors because that is the separately derived system that would be grounded at the transformer. (The 208V side will be grounded upstream, at the service or wherever else it's derived.) See 250.66 and note how it includes the phrase 'at a separately derived system'.

If their is such a thing then 208V side need neutral?

Presumably the 208V side has a system neutral elsewhere, but there is no general reason that it needs to be brought to the solar transformer, and it is generally not the practice to do so. Solar inverters usually put balanced current on the ungrounded conductors, with neutral current ranging from zero to insignificant. Many single phase inverters do not require a neutral connection at all. And in any case, differences in phase currents on the wye side just transfer to the delta side phases and this in itself is not a problem, any more than unbalanced load. (If someone wanted the 208V neutral brought to the solar transformer for some reason then obviously they'd have to make the transformer a wye-wye instead of a delta-wye. My understanding is that this is neither necessary nor a good idea for most commercial scale transformers, whether used for a load or backfed by PV.)

 

[hhsting]

Just think of the PV inverter as a regular load. Just ignore that the average energy flow going the other way. Just try to envision the inverter being a big milkshake machine making milkshakes. Just focus on the energy going into that delicious milkshake and let your thoughts go to that happy place.

The only special thing to consider is 705.30(B), but note this is for your 450.3 TRANSFORMER protection, not the conductors.

yes it is a common transformer (note I fixed some errors for you).

Im still trying to figure out how to connect a neutral to a delta connected transformer. Perhaps this can be done in a parallel universe, where there are also these "service feeders" we sometimes hear about.

So their is no solar transformer conductor protection rule and how many feet overcurrent protection be located from solar transformer?

What about NEC 2014 section 750.30 first paragraph does that not qualify solar transformer conductor protection: “conductors shall be protected in accordance with Article 240. Equipment and conductors connected to more than one electrical source shall have a sufficient number of overcurrent devices located so as to provide protection from all sources”?

 

[electrofelon]

So their is no solar transformer conductor protection rule and how many feet overcurrent protection be located from solar transformer?

Yes. 240.21(C)

What about NEC 2014 section 750.30 first paragraph does that not qualify solar transformer conductor protection: “conductors shall be protected in accordance with Article 240. Equipment and conductors connected to more than one electrical source shall have a sufficient number of overcurrent devices located so as to provide protection from all sources”?

I cant think of a situation where anything extra would be required over what a regular milkshake machine would require.

 

[Carultch]

What I am asking is post #1:

#1 solar transformer secondary conductor protection tap rules for solar panel does NEC 2014 section 240.21(C) or some other code section apply? Of course which is secondary and which is primary

#2 post #1 transformer grounding electrode size depend on 480V conductor or 208V conductor?

#3 Is their solar transformer as 208V delta to 480/277V wye? If their is such a thing then 208V side need neutral?

I agree with Electrofelon. Due to the fact that the transformer is initially energized by the utility/building grid, assign the utility/building grid side as the primary. That is the side where the dominant source of fault current will come from, and everything else that motivates the circuit sizing and protection requirements. The inverters produce voltage and current that is subordinate to an existing grid that is already within the tolerance, and will not continue to feed a faulted grid after an upstream OCPD gets tripped.

To answer the questions:

#1: yes, apply 240.21(C) to the conductors between the solar side of the transformer, and the first disconnect on that side.

#2: the GEC that grounds the inverter-side neutral to the grounding electrode, should be based on the inverter side circuit of the transformer. It is a separately derived system. NEC 250.30 applies to its sizing.

#3: You don't connect the primary-side neutral, and there is no need to do so. There is no place to do so anyway. The phase wires alone are capable of carrying balanced current back to the source. The topology with a delta primary, will generate current among the phase wires that vectorially adds up to zero by design, even if the secondary has an imbalance.

You have the option of either using a WYE-to-WYE transformer for this application, or a WYE-to-DELTA transformer, with the delta on the building grid side. WYE-to-DELTA has the advantage of harmonic mitigation, is a more economical topology to build, and does not need the neutral on the delta grid side. WYE (inv)-to-DELTA (bldg) is therefore what I would recommend in general, when adapting inverter voltage to a different building grid voltage. An application where you would need to specify WYE-to-WYE, is if your solar is intended to operate with storage, and work as a grid-forming device when the utility power is in blackout.

In any case, you have to be careful with procurement and approvals of any transformer used in this application. Someone not paying attention to product specs might overlook which side is WYE and which side is DELTA. 480V delta to 120/208V wye transformers are a lot more common, than the other way around as you likely need for solar. The reason it is common, is that it is the way to fulfill the minority of loads requiring 120V, when you have a 480V service. So reinforce this spec, with "wye topology req'd on 480V side", and pay attention to detail with product submittals. If your inverters need a neutral, or otherwise need a wye grid on their side, which is common, you don't want to end up with a transformer that is DELTA on the inverter side.

 

[electrofelon]

I agree with Electrofelon. Due to the fact that the transformer is initially energized by the utility/building grid, assign the utility/building grid side as the primary. That is the side where the dominant source of fault current will come from, and everything else that motivates the circuit sizing and protection requirements. The inverters produce voltage and current that is subordinate to an existing grid that is already within the tolerance, and will not continue to feed a faulted grid after an upstream OCPD gets tripped.

Perhaps it would be prudent for the NEC to define primary and secondary, or put in an informational note somewhere, as this question does come up frequently.