Hello from san Diego

[Markemac]

Hi all, new here, and new to solar,
I put together a ground mount solar system at my house 36 samsung panels, with enphase micro inverters and combiner box.
SDG&E, Won't let me turn it on because the transformer is too small and old until they replace it.

 

[Ozark Tinkering]

So their transformer is good enough to supply power to you but not to receive the same power from you? Sounds like a stall tactic since you're paying $.50kWh? What part of San Diego are you in? I spent most of a year converting entire neighborhoods to underground fed services for SDG&E back in '05 in the areas of El Cajon and Clairmount. I'm surprised they haven't gotten farther along by now, the original plan was to put everything in the city underground and delete all poles. That would include upgrading transformers.

 

[Hedges]

It would at first seem that adding PV might reduce load on a transformer, not increase it. But, I could see a situation where a transformer (and the wires it feeds) are protected by fuses on primary side, and additional power applied to secondary could allow other consumers to draw excess current through utility drop wires or from one or more windings. (Backfeeding 240V into the 120/240V secondary winding of a transformer would deliver additional power, beyond what the fuse-protected primary provides, and it might be overloaded by 120V load from customers.)

Hey Ozark, do utility transformers every have multiple isolated secondaries? The one in a vault in my front yard feeds half a dozen houses, with spring-loaded connections for each wire. But I don't know whether we're connected in parallel or isolated. Of course if isolated, 240V PV backfeed adds to what is available for other secondaries, could exceed capacity of transformer. Are fuses meant to prevent excess current that could cause saturation? I think thermal fuses always have some time-delay while saturation is instantaneous. So I think saturation causing short-circuit current causing fuses to blow would be the sequence of events.

 

[Ozark Tinkering]

Hey Ozark, do utility transformers every have multiple isolated secondaries?

I won't tell you they don't exist but I have no experience with such a beast.

I'm failing to see why the transformer or line cares from which direction that alternating current originates. I just see less load on the line upstream from wherever the excess power from a grid-tied system is finding the least resistance? In my case I'm the last transformer on my grid circuit and across the road upstream on the primary side is another transformer that feeds down to a hay barn. Any excess power that isn't consumed at my house from my grid tied system gets put back ont the primary 7200 line and makes it's way to the barn loads. Beyond that upstream on the 7200 line, all it knows is the line is running cooler due to the reduced load but stands ready to fill any need should need arise.

But I'm still teachable.

 

[Markemac]

I am in Alpine (east county area) still kind of wilderness, I am on 11.5 acres, the transformer is 10k and very old. They are worried it will blow and start a fire.
I was wondering about adding batteries, is it worth it, would I need a million of them? Is the cost too high? Still learning

 

[Ozark Tinkering]

Cool... I spent 18 months on the Viejas Rez and another year out on Japatul Valley Rd in my 5th wheel from '04-'06 when we were doing all that work for SDG&E. I was there in the summer of '06 when the fire went through there and pretty much surrounded us, but we lucked out and didn't have to move our trailer. I think it was started by illegal migrants coming up through the Cleveland forest from Hwy. 94. We rented a spot about halfway between Alpine and I-8 from a fella that had 5 acres on the south side of the road and had set up 2 rv sites below his house in the scrub oaks, so we had lots of privacy...snakes and coyotes too. I know that area really well all the way to Jacumba. The best sunrises on the planet are just west of Jacumba and the best sunsets are in Alpine just 50 miles apart! I can see why they haven't gone underground in your area, that won't happen anytime soon. What do you mean "10k"?

I am in Alpine (east county area) still kind of wilderness, I am on 11.5 acres, the transformer is 10k and very old.

 

[Ozark Tinkering]

I think thermal fuses always have some time-delay while saturation is instantaneous.

A couple of years ago a squirrel climbed my transformer pole out onto the fuse link and was instantaneously saturated and blew the fuse link right off the transformer with him on it. I know he was saturated because it went in his nose and leaked right out his left foot...instantaneously.

 

[Hedges]

I'm failing to see why the transformer or line cares from which direction that alternating current originates.

But I'm still teachable.

That's good
I have this habit of walking into a room full of experts who've been on a project for a decade, and telling them what they've done wrong.
Oh, did I say I am a rocket scientist?
(Although it is more common for me to do that to an academic hired by industry, than to people with actual hands-on experience in the field.)

We all know the neutral of a transformer carries difference between current in L1 and L2. Except, I've come to realize that is only true if source of power is the primary winding. For an auto-transformer, it can be the sum of L1 and L2.

Imagine a 10kW PV system backfeeding 40A, 240V. And a neighbor drawing 5kW 40A 120V on L1. Also a second neighbor. We're now using transformer secondary as an auto-transformer, sending our 10kW 40A 240V to loads drawing 10kW 80A 120V. Probably so far so good.
Now let another customer draw additional power, say 5kW 40A 120V also on L1 (which means 5kW comes from grid). Neutral wire from transformer and along the poles is now carrying 120A, which utility never would have though possible for the 10kVA transformer if primary (or L1 and L2 of secondary) fused appropriately.

It takes a big PV system or a small transformer to get much of an overload. I've grumbled about the 120% rule for PV breakers in a panel, said 200% would be fine. But I now realize neutral busbar and wire from grid can carry that 120% or 200%.

It's just Kirchhoff's current law, but it does take a bit of thinking to understand how the auto-transformer does this.

 

[Markemac]

What do you mean "10k"?

The transformer rating (as i understand it) is 10kva

 

[Ozark Tinkering]

The transformer rating (as i understand it) is 10kva

So your service must be an original 60amp with bussman screw-in glass fuses? Yikes!
Did you also upgrade your service when you installed solar or did they stop you from doing that too?
How long is SDG&E saying it will be before they replace your transformer?

 

[Ozark Tinkering]

Imagine a 10kW PV system backfeeding 40A, 240V. And a neighbor drawing 5kW 40A 120V on L1. Also a second neighbor. We're now using transformer secondary as an auto-transformer, sending our 10kW 40A 240V to loads drawing 10kW 80A 120V. Probably so far so good.
Now let another customer draw additional power, say 5kW 40A 120V also on L1 (which means 5kW comes from grid). Neutral wire from transformer and along the poles is now carrying 120A, which utility never would have though possible for the 10kVA transformer if primary (or L1 and L2 of secondary) fused appropriately.

Yep and had I not been the 2nd post in this thread I would have had an explanation without any further questioning. Notice how I able to ferret out the fact that SDG&E transformer is a 10kva transformer as opposed to a transformer that would cost him $10k to replace? I never assume someone who is thinking about adding 1,000,000 batteries to an Enphase system because the transformer is too small or old, that the initial information is complete or correct. 1 wrong or misunderstood word can change any answer. I actually did that for your benefit...making it easy for you to do the calculations and tease me.
You're welcome.

 

[Ozark Tinkering]

Oh, did I say I am a rocket scientist?

Why yes, yes you did. hehehe

 

[Markemac]

So your service must be an original 60amp with bussman screw-in glass fuses? Yikes!

It was on the first house that burned in the late 60's this one was built 1970, it had a 100a panel i upgraded to a 225, and the county made reduce the main breaker to 200 (i think ) for the solar system

 

[Markemac]

What would it take to add a battery backup system?

 

[Hedges]

There are several models of Enphase micro-inverter. Which are yours?

Enphase sells a battery add-on. It is a box with multiple micro-inverters each connected to a battery.

If the micro-inverters do the "frequency-watts" optional feature of UL-1741-SA, then they can interact with some battery inverters from SMA, Schneider, Outback. (If they do not have the feature of ramping down wattage output in response to increasing frequency, they would just cycle on/off every 5 minutes, possibly functional but not good, may be stressing to various part of your system.

 

[Ozark Tinkering]

It was on the first house that burned in the late 60's this one was built 1970, it had a 100a panel i upgraded to a 225, and the county made reduce the main breaker to 200 (i think ) for the solar system

@Hedges care to calculate the potential load on the transformer should @Markemac decide to load it down to his potential on that 200amp main that the county and SDG&E allowed? I might be inclined to test that transformer integrity myself were it my place. Where's that 50,000 watt toaster I set aside for bench testing transformers anyway? If it burned down then I guess SDG&E would just have to replace it?

 

[Ozark Tinkering]

Oops...toast is ready.

 

[Hedges]

225A busbar and 200A main breaker allows 70A PV breaker under 120% rule. But utility might not allow 240V x 70A x 0.8 = 13.4kW of PV.

If their transformer is fused in the primary against overload, his loads even with 200A main breaker would just blow their fuse. But backfeeding 13.4kW with neighbors drawing power and same old 10kW transformer would be a problem.

 

[Ozark Tinkering]

If their transformer is fused in the primary against overload, his loads even with 200A main breaker would just blow their fuse. But backfeeding 13.4kW with neighbors drawing power and same old 10kW transformer would be a problem,

In my thinking his 10kW transformer doesn't have anything to do with his neighbors and their usage. They are on their own crappy under sized transformers connected to the well. The only thing SDG&E cares about is having to spend $thousands to replace that transformer so they no longer get to charge OP 50 cents per kWh. Plain and simple. The excess power just absorbs into the infinite well potential of the primary line back to the substation looking for a path of least resistance along the way. We aren't disagreeing that the transformer is too small and old and should be replaced or that a 13kW system could/would do harm to that transformer in the event that somehow it produced at 100% capacity while the house was under a no load condition. My position is that putting the transformer under a house load that would max out but not trip the county inspected and approved 200amp breaker would also damage the transformer and it would require immediate replacement by SDG&E thereby solving OP's initial problem in an expedited manner. If it were me and SDG&E had a problem with replacing my transformer I guess I'd just have to cook some toast even at 50 cents a kWh. I'd be pretty hot under the collar after dropping $30k-40k for a ground mount Enphase setup and THEN the utility told me they couldn't accommodate my little system because they hadn't upgraded the service in over 60 years and 2 houses with 3 services? Give me a break.
I guess some of it was my fault for not checking with SDG&E engineering downtown about whether they could handle a system like the one that cute gal from Enphase was selling me. She said she would handle everything and I would love this trouble free system and low electric bills and gave me 60 free LED light bulbs too.

 

[Markemac]

There are several models of Enphase micro-inverter. Which are yours?
Mine are iq7