Whats the deal with "Solar-Rated" transformers?

[Matt Donovan]

I've heard of "solar-rated" transformers, but I don't see much info out there on this type of transformer. Anyone have experience with these?

- are "solar rated" transformers comparable in cost to other transformers?
- can anyone describe specifically what is meant by "solar rated"? Is there an ANSI or UL spec?
- Anyone have guidance on how to size transformers more intelligently, with or without "solar ratings"? Is specing 65oC rise a solution to allow 100% loading? Are k-rated transformers appropriate?

Many designers will load 1000kW of inverters on a 1000kVA transformer, but inverter manufacturers recommend against this because transformers are not intended for continuous operation at full-load. Solectria, for one, recommends oversizing to at least 105%, others to 110%. I've seen 100% loading cause problems in the field, resulting in over-heating, over-voltage, and systems shutting down.

FYI, my firm designs primarly larger commercial (>500kW) and small utility (<10MW) projects. I'm not talking about residential, but also not talking about massive farms.

 

[iwire]

transformers are not intended for continuous operation at full-load.

I have never heard that before, as far as I know transformers are rated for 100% continuous loading.

 

[ron]

It is a marketing term that I've heard before.

The claim is that they are more robust than regular (maybe a higher BIL for its voltage class) to handle poor power quality from lower end inverters. It has also meant that it has a dual winding low voltage primary, so you can connect (2) 500kVA inverter outputs to a single 1MVA transformer to step up to connect to the grid.

 

[SolarPro]

Have you seen this SolarPro article article by Blue Oak Energy?

Distribution and Substation Transformers

pv_n00b likely has some practical insights into this topic. Otherwise, applications engineers for inverter and transformer vendors are probably the best resources for getting into the nitty gritty.

I suspect this rating has to do with the amount of time the transformer is expected to operate at full power in a PV application. I don't believe that there is a special ANSI standard or similar, but I could be wrong. These solar-rated transformers might also have a higher than average efficiency or come with set of features typically required or desired for solar farms, such as an environmentally friendly oil. These transformers There are some inverters that require specially wound transformers that certainly cost more that a conventional pad-mounted distribution transformer, but this is more a function of the inverter design than anything else.

 

[pv_n00b]

It's just a marketing term and has no engineering basis that I know of. There is no transformer specification that makes it better for PV applications.

 

[Engineer#4827502]

I believe Solectria's recommendation of oversizing transformer has to do with harmonics, not load factor.

I remember an article a while back (can't find it though) that talked about undersizing transformers because solar is only at 100% for part of the day so they would have time to cool overnight. I have not seen anyone do this in practice though, I imagine you risk shortening service life to save a small amount on transformers.

 

[GoldDigger]

I believe Solectria's recommendation of oversizing transformer has to do with harmonics, not load factor.

I remember an article a while back (can't find it though) that talked about undersizing transformers because solar is only at 100% for part of the day so they would have time to cool overnight. I have not seen anyone do this in practice though, I imagine you risk shortening service life to save a small amount on transformers.

You might get away with that for oil filled transformers, but the time scale does not seem right for dry type.

 

[pv_n00b]

For larger central inverters that do not come with a transformer but expect one to be supplied to connect to a medium voltage interconnection, those transformers can have some fairly specific requirements. But still nothing to do with "solar rated."

I remember talking to one company that was really undersizing their oil filled MV transformers saying that they are not fully loaded for that long with the PV system. They are one of the few I have run into and I don't know what the long term result was. You can mess around with oil filled transformer loading, temperature rise, etc. to run them at over their rated specification, utilities do it all the time. They do things that seem crazy to me but they also constantly monitor their transformers while they are pushing them to detect any problems before the transformer fails. It seems like with PV systems people just want to build them and not be constantly doing oil sampling and testing.

 

[ggunn]

It seems like with PV systems people just want to build them and not be constantly doing oil sampling and testing.

I would agree with that. Design it, build it to last, move on.