Designing large solar power system, transformer upgrade needed

[jaggedben]

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And a solar inverter would give me pause, since it could produce full output for what, 8 hours continuous in some cases? I would make some allowances for that to be sure. Long answer to a short question.....

8 hours continous would be quite unlikely for a residential system, unless there was a tracker.

 

[ggunn]

8 hours continuous would be quite unlikely for a residential system, unless there was a tracker.

8 hours I agree would be unlikely, but three hours, i.e. continuous use, is certainly possible. We have to size our conductors with that in mind; it seems to me that transformers should be similarly evaluated.

 

[pv_n00b]

What we learned, the hard way, is that if you find out after you have installed a PV system that its output is more than the rating of the transformer feeding the service with which your system is interconnected, you have little to no bargaining power, even if the size of the transformer is already badly matched to the size of the service. Be warned.

That project was for an additional load and not a PV system, and the utility reduced the size of the service from what it was in the past without informing the customer. It's the only situation like that I have ever run across.

Once the customer lawyered up the utility caved. The situation might have been different if it were a PV system install, but the customer was a big property owner in the area and not afraid to load up the lawyer-o-matic and point it at anyone who got in his way. He tended to get what he wanted. It's cheaper for the utility to spend a few thousand on equipment upgrades than a few 10's of thousands on lawyers.

 

[Electric-Light]

I am a pv designer and I'm having an issue with a particular job, one I've never dealt with before so I'm looking for advice. It's a relatively large residential system, about 30 kW DC, and the utility is saying we need to spend ~$20,000 to upgrade the customer's transformer which is currently rated at ~13 kVA. How much would we need to downsize the system to fit the transformer, down to 13 kW AC? If we do go for the upgrade isn't the utility (SCE) obligated to pay for some or all of it? The system is so large because the customer owns a large property, it seems odd that the transformer would be so small. Does anyone else have experience dealing with this sort of thing?

Thanks for any and all help.

Study the tariff. That's the governing document that says what and how much you can hook up to their system. You'll need to hire a consultant to help you locate and understand if you can't do it yourself.

PoCo transformers are calculated differently. They're meant to be overloaded regularly by a significant amount like a motor with generous service factor. Even though efficiency drops while you're in overload range, total energy use can be less than having a larger transformer with more baseline.

A transformer is like your car engine. You have to turn it off or have it idling to have any amount of power available and it consumes energy just to keep it idling. If you upgrade the engine to increase the hauling capacity that is only used 5% of the time, there's obviously the cost of upgrade, but also the added fuel baseline fuel consumption for increased idling fuel usage. The added wattage is 24/7 around the year and the added kWh could actually remove quiet a bit from social benefit these solar things are pitched to provide.

What they're asking is perfectly reasonable. If you were a vegetable retailer obligated to buy things grown by customers at their pace at retail price and asked to upgrade your truck to fit what they're selling even though they buy almost nothing, wouldn't you think that's BS?

 

[jaggedben]

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What they're asking is perfectly reasonable. If you were a vegetable retailer obligated to buy things grown by customers at their pace at retail price and asked to upgrade your truck to fit what they're selling even though they buy almost nothing, wouldn't you think that's BS?

I'd say it's certainly mitigated by the fact there's a guaranteed resale of their goods.. And a trucking metaphor is far from perfect, since a truck can be used to serve multiple customers and companies with trucks amortize the cost of them over all their customers. We should probably not go down this road though. Whether $20,000 is a reasonable cost for the work and materials in this case is probably answerable in a lot of different ways more pertinent to the actual job than discussing net-metering policy in general.

 

[Electric-Light]

I'd say it's certainly mitigated by the fact there's a guaranteed resale of their goods.. And a trucking metaphor is far from perfect, since a truck can be used to serve multiple customers and companies with trucks amortize the cost of them over all their customers. We should probably not go down this road though. Whether $20,000 is a reasonable cost for the work and materials in this case is probably answerable in a lot of different ways more pertinent to the actual job than discussing net-metering policy in general.

It would also need to buy out the cost of remaining life, because it is getting taken out prior to calculated lifespan for discretionary toy for some guy with a big house.

If you just spent $3,000 installing carpet with 10 year life expectancy and you want to change it because you want different color in 5 years, $1,500 is literally flushed down the drain, oh and half of whatever cost associated with removal too, because, that too was only supposed to happen every 10. So, there's that. The transformer may have re-purpose value, but there's no question that the labor value of the original install for the remaining expected life is burned to nothing. The cost to install new one is likely higher when it has to be scheduled for one time job rather than a part of crew assembled to service multiple ones in a planned cycle.