Transformers in PV System

[mlackna]

Presently working on a 150KW Grid Tied Solar System in Los Angeles - We are using a 150KVA transformer to step up the utility voltage from 208Y to 480Y for the SMA 24 TL Inverters. The transformer is installed outdoors with a 200A Fused AC Disconnect within 10Ft of it, protecting the secondary conductors and a 600A/208V/3P Fused AC Disconnect in the electrical room, approximately 30ft, out of sight of the transformer, protecting the primary conductors. My inspector is requiring me to add a second AC Disconnect, close to the transformer, to protect the 208V Conductors [viewed interchangeably as primary or secondary conductors according to him per 690.9(B)]. I believe the installation satisfies the requirements of 450.3 for transformers, and 240.21 for the protection of primary and secondary conductors. The transformer is installed outdoors so the length of primary or secondary conductors shouldn't matter (at least in my opinion).
The crux of the matter seems to be the intent of 690.9(B). In a PV Application, which is primary and secondary of the transformer? Is PV powered produced (480 side) the primary or is it the utility 208 side? Are the primary and secondary interchangeable depending on how it is viewed? Is the additional AC Disconnect Required or not?

 

[pv_n00b]

The guidance is in 690.9(F). When looking at overcurrent protection for a power transformer you perform the 450.3 design twice, once assuming the primary is on the utility side and then again assuming the primary is on the inverter side. Keeping in mind that the main contributor of fault current is going to be the utility and the exception typically requires no overcurrent protection from the inverter side current.

 

[don_resqcapt19]

The following is only based on the general rules and I have not looked to 690 for guidance.

The NEC does not require a local disconnect for the primary of the transformer so, the installation is compliant where the 208 side is the primary.

If the 208 side is the secondary, than the issue is the protection of the secondary conductors per one of the rules in 240.21(C). Depending on where the inside connection is made, you may be able to apply 240.21(C)(4).

 

[jaggedben]

Presently working on a 150KW Grid Tied Solar System in Los Angeles - We are using a 150KVA transformer to step up the utility voltage from 208Y to 480Y for the SMA 24 TL Inverters. The transformer is installed outdoors with a 200A Fused AC Disconnect within 10Ft of it, protecting the secondary conductors and a 600A/208V/3P Fused AC Disconnect in the electrical room, approximately 30ft, out of sight of the transformer, protecting the primary conductors. My inspector is requiring me to add a second AC Disconnect, close to the transformer, to protect the 208V Conductors [viewed interchangeably as primary or secondary conductors according to him per 690.9(B)]. I believe the installation satisfies the requirements of 450.3 for transformers, and 240.21 for the protection of primary and secondary conductors. The transformer is installed outdoors so the length of primary or secondary conductors shouldn't matter (at least in my opinion).
The crux of the matter seems to be the intent of 690.9(B). In a PV Application, which is primary and secondary of the transformer? Is PV powered produced (480 side) the primary or is it the utility 208 side? Are the primary and secondary interchangeable depending on how it is viewed? Is the additional AC Disconnect Required or not?

I gather you are asking about a disconnect only because you intend to use a fused disco as your overcurrent device.

My personal opinion is that he's going overboard since fault between the utility side disco and the transformer will be fed primarily by the utility. Moreover, the primary (utility side) conductors will never be overloaded by the inverter if they are sized properly. An additional overcurrent device would do nothing to protect the conductors; it would not trip before the existing overcurrent device did. But that's not an argument that speaks to what the code requires or intends.

 

[McMac]

I gather you are asking about a disconnect only because you intend to use a fused disco as your overcurrent device.

My personal opinion is that he's going overboard since fault between the utility side disco and the transformer will be fed primarily by the utility. Moreover, the primary (utility side) conductors will never be overloaded by the inverter if they are sized properly. An additional overcurrent device would do nothing to protect the conductors; it would not trip before the existing overcurrent device did. But that's not an argument that speaks to what the code requires or intends.

Ther is plenty of code to support that. 705.31 covers location of OPC for supply-side taps // 240.21(B) covers feeder tap rules for various scenarios. And then of course the previously mentioned 690.9(F) help to fill more of the OCP picture for this scenario. And if you have 705.12 covered as well, then you should be "In Like Flinn" IMHO.

 

[electrofelon]

The guidance is in 690.9(F). When looking at overcurrent protection for a power transformer you perform the 450.3 design twice, once assuming the primary is on the utility side and then again assuming the primary is on the inverter side. Keeping in mind that the main contributor of fault current is going to be the utility and the exception typically requires no overcurrent protection from the inverter side current.

Just lets be clear that 450.3 is for transformer protection. Its seems the inspector was talking about the additional disco for conductor protection.

 

[GoldDigger]

And when sizing any OCPD on the POCO side of the transformer keep in mind that the magnetizing current in the transformer will come from the POCO side so the OCPD will have to withstand that inrush If the transformer is installed "in reverse" with the nominal primary facing the inverter, the energizing inrush may be as much as 10 times what the inrush current would have been on the primary as a multiple of the full load amps for that winding.

 

[winnie]

As far as transformer selection/design is concerned, GoldDigger is right on the money. The utility side of the transformer is 'primary'.

But for purpose of protecting that transformer in a PV system, 690.9(f) requires you to run the ocpd design twice, once assuming the utility side is primary, then assuming the PV side is primary. Inrush is still from the utility side but power does get to flow in the opposite direction.

Jon