PG&E transformer upgrade: why?

I was part of the NEM 2.0 rush in April 2023. My interconnect application indicated a system size of 5.4kW DC, 3.84kW AC (Enphase microinverters, no storage). My monthly household usage is about 1,000 kWh. I DIYed the system, and successfully tested it in September.

I was approved for NEM 2.0, but recently received notice that PG&E needs to do a transformer upgrade in my neighborhood before they can give PTO. They aren't making me pay for it, unlike last year when I tried to upgrade from a 100-amp main to 200, and they said I would have to pay for the transformer (of course I abandoned that upgrade). But they say it'll take until Summer 2024 to replace the transformer, and obviously I'm annoyed to leave a new PV system idle for almost a year.

I asked the NEM team how in the world a 3.84kW backfeeding system could be a concern for an overloaded transformer. Here's my grade-school math:

Option 1: I never turn on the system and continue to draw 1,000 kWh/month.

Option 2: I turn on the system and generate the absolute maximum (let's say 6 full hours per day at 3.8kW, or about 680 kWh/month). I offset my own consumption, so now the load on the grid is 320 kWh/month. Better for the transformer.

Option 3: I go completely bonkers and stop using electricity entirely. I still generate the same PV power as in Option 2. I'm feeding 680 kWh monthly to the grid. Load is smaller than Option 1, but higher than Option 2 (albeit in the reverse direction). Better for the transformer.

Under all these scenarios, the load on the transformer is equal to or less than my current usage.

What is PG&E's deal? Is reverse power harder on transformers than forward power? Or are they just saying no because they can?

(Just to clarify, I'm not quibbling with the transformer upgrade; I assume that's legit. I'm wondering why they're blocking PTO on the upgrade.)

Transformer only minimally cares about the average monthly load; it cares about amps flowing through it. 1,000kVA/month is an average of 1.3kW; your system would go from consuming 1.3kW to exporting 2.5kW during the day, and for the transformer you would go from consuming power your neighbors produce to exporting additinoal power via the transformer.

The only thing you can do is set your Enphase system up for zero export, which might save you ~300kWh per month. That is why batteries are such a big deal today.

sowbug said:

Is reverse power harder on transformers than forward power? Or are they just saying no because they can?

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I have never understood why that would delay a PTO. As far as I know any transformer should be able to work bidirectional. There are issues at substations that were designed with control devices that are only unidirectional. The reality is the power you export will go to your neighbors on the same transformer, effectively reducing the load on the transformer.
I had a similar situation several years ago with an install that required a change in the underground feed which was delayed by my installer. I turned on the system and charged my EVs as much as I could during the day. That took almost fifteen months to resolve and I never heard from PG&E during that time so I would suggest you do the same, which is Option 2

I don't think 3.8kW PV needs transformer upgrade. 16A, 20A breaker in your 100A panel.

What price did they quote you for upgrade from 100A to 200A panel? My request is outstanding. Their estimate is maybe $3500. There's also something about $1900 credit.

I don't think upgraded transformer to be shared by multiple customers should cost us money.
My request for 3-phase transformer (one pole is 200' from 12kV 3-phase) was given a verbal quote of $150k so I passed.

sowbug said:

What is PG&E's deal? Is reverse power harder on transformers than forward power? Or are they just saying no because they can?

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I don't know if PG&E knows it, but I've figured out that backfeeding from PV and using their transformer as autotransformer could put 200% of rated current in neutral wires. In fact, if I put 200A at 240V into my own breaker panel, I could pull 400A on neutral.

I have never seen a settled explanation for this and I've seen the question on several forums.

Seems like whatever engineering book they use applies a much bigger safety margin to backfeeding compared to load because they know it's going to be constant and sustained.

Of course the load budget and backfeed budget would be independent of eachother, so it's also possible your neighbors already have 200 amps or whatever of backfeed subscribed and yours is the straw on the camel's back, but it seems like that wouldn't be as common as it is for people to run into a transformer upgrade requirement.

Anyway you got in 2.0, your locality accepted your DIY install, and PG&E's not charging you for the upgrade, so I'd consider it a pretty lucky problem. Summer will be here again quick.

Ampster said:

I have never understood why that would delay a PTO. As far as I know any transformer should be able to work bidirectional. There are issues at substations that were designed with control devices that are only unidirectional. The reality is the power you export will go to your neighbors on the same transformer, effectively reducing the load on the transformer.
I had a similar situation several years ago with an install that required a change in the underground feed which was delayed by my installer. I turned on the system and charged my EVs as much as I could during the day. That took almost fifteen months to resolve and I never heard from PG&E during that time so I would suggest you do the same, which is Option 2

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Let me go back to basic and my apologies if I'm completely off mark here.

Let's say inside the transformer there are two B fields fighting one another and secondary coil can only take so much resistive loads.

If he and his neighbors use too much power, this is essentially a short and incurs a "do not compute" situation. The more power demand, the B fields fight is more intense and this causes exhaustion in the primary coil and draining power from the grid.

So if he pushes out power, he's not part of the team that creates a short. He's only pushing power to the neighbors OUTSIDE of the transformer.
In this case there's a lot of fighting of the B fields depending on sunlight. When he's producing, the B field in the secondary coil is less and the fight is easy for the primary coil and the grid is drained little.

If he pushes out too much power then he supplies that secondary coil and turns it into a heater. This is where an upgrade is needed.

The primary coil is under an enormous pressure from the high voltage line and whatever he does to the 2nd coil won't harm it.

So just look at the output of the transformer and sum up all suppliers and if they're less then upgrade is not required.

hwy17 said:

I have never seen a settled explanation for this and I've seen the question on several forums.

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A lot of the rationale is downright wrong, but a very real issue is that the final transformers themselves get overloaded regularly and to the point where it has a drastic impact on life.

Less material issues also exist, but if transformer load is kept in check it isn't a huge issue. Things like overloading distribution lines in addition to the transformers, as there is no protection, and to a lesser degree certain protection relays malfunctioning.

Hedges said:

What price did they quote you for upgrade from 100A to 200A panel? My request is outstanding. Their estimate is maybe $3500. There's also something about $1900 credit.

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I never found out. They wanted a $1,500 advance for an engineering study. They said it could be applied toward the work if I chose to proceed. But when I asked if they had any estimate for the cost, be it $100, $1,000, or $10,000, they declined to provide it, and said I'd have to pay the $1,500 to find out. It was already seeming strange that I might have to pay for a transformer shared across many people in my neighborhood, so I just said no to the whole thing. This was almost two years ago, so your $1,900 might be the same thing.

Shimmy said:

Things like overloading distribution lines in addition to the transformers, as there is no protection, and to a lesser degree certain protection relays malfunctioning.

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Is part of that because the control mechanisms were only designed for one way flow? I seem to recall that was an issue in Hawaii, which caused them to fundementally reduce the number of solar installs. It did give batteries a big kick since many people went to a model of self consumption with batteries and went without selling back to the grid.

Overhead Upgrade:




"You may be eligible to deduct a $1,918 allowance from the total cost of your electric service if you are upgrading your panel."

I have 100A service, might be allowed 20A PV breaker 3.8kW.
PG&E has approved my NEM 2.0 reservation for 15.6kW, said no upgrades needed.
That would have 80A OCP. I see a few PV systems around the neighborhood.

My plan is to install a meter and 200A main breaker only box, then use Polaris to fan out to multiple boxes and PV disconnect switch. I could install any size breaker, 200A came in the box and I have a Square D 150A that would fit. Think they're available from 100A to 225A in 25A increments.

Up to some amperage I expect PG&E to just cut the old wires and crimp onto my new ones. Maybe they need larger gauge for the 50' drop to supply 200A.

I told them of plans to add electric heating appliances and EV charger, but that would be later. Need the service upgrade first, and need the service upgrade to install the PV.

I don't like the idea of paying any money for them to move wires to a different connection, or even to drop a larger gauge. I get paying for long distance that serves just one customer.

To give me 3-phase, maybe I didn't qualify for 120/208Y but I did for 120/240 high-leg delta, it would have been a transformer to supply at least 55 kVA, 200' of wire to the 12kV lines, and 50' drop to my house. Explaining the $150k they said 2 trucks with crews for 2 days, priced at $25k per truck per day. (Then I guess there was another $50k for the $7000 transformer?)

hwy17 said:

Of course the load budget and backfeed budget would be independent of eachother, so it's also possible your neighbors already have 200 amps or whatever of backfeed subscribed and yours is the straw on the camel's back, but it seems like that wouldn't be as common as it is for people to run into a transformer upgrade requirement.

Anyway you got in 2.0, your locality accepted your DIY install, and PG&E's not charging you for the upgrade, so I'd consider it a pretty lucky problem. Summer will be here again quick.

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That is probably my guess, max PV back feed is more than likely limited to the xfrm name plate.

As for delay in getting the xfrmr replaced, blame the supply chain, huge huge huge delays in xfrmr manufacturing these days. Regular pole tops seem to be not that bad but step transformer and pad mounts are unobtainium. There are massive housing developments being placed on hold/delayed as utilities can’t get brought transformers in time.

This article is about a year old but is even more valid today than ever.

Ampster said:

Is part of that because the control mechanisms were only designed for one way flow? I seem to recall that was an issue in Hawaii, which caused them to fundementally reduce the number of solar installs.

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Some of the protection relays can be impacted by distributed generation; reverse power was often considered a fault. Most of the concerns are overblown as long as the DG systems have anti-islanding protection though.

Mainly Hawaii used it as an excuse though; they could see the writing on the wall for their bread and butter. The transformer serving my (and 20-some neighbors) has obvious scortch marks due to gross overloading, but HECO has failed to do much of anything for 10 years (except watch the problems get worse).

Hedges said:

To give me 3-phase, maybe I didn't qualify for 120/208Y but I did for 120/240 high-leg delta...

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I don't think PG&E will install a high-leg delta service to a residence (for good reason). You might have been able to get a 240V corner-grounded delta though as a separate secondary service.

That's what I could have had, if I agreed to highway robbery.
I wanted 120/208Y

"If PG&E determines that a single residential or nonresidential building or structure justifies a 75-kVA transformer at 120/208 V, or a 5 horsepower or larger motor at 120/240 V for an overhead service, then installing three-phase service is an option for the applicant"

Hedges said:

I don't know if PG&E knows it, but I've figured out that backfeeding from PV and using their transformer as autotransformer could put 200% of rated current in neutral wires. In fact, if I put 200A at 240V into my own breaker panel, I could pull 400A on neutral.

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Actually, on a 240 volt feed, the Neutral wire is only carrying the imbalance between the two legs. Since microinverters only generate 240 volts, they don't even have a neutral connection just a red and a black. Your panel will have four wires coming into it. A Black and Red Hot. A White Neutral. A green (or bare copper) ground.

It is running 120 volt loads on your system that can cause an imbalance and result in current on the neutral wire. If you put a 5 amp load between the the red and the white, there will be 5 amps on the white. Add another 5 amp load between the black and the white, and you ZERO amps on the white. That's because the + and - amps cancel each other out. That's why a 240 volt circuit only needs the two hots. An AC compressor circuit is wired with 12/2 cable rather than 12/3. The white wire in this case is actually a hot wire (and it should be wrapped with colored tape to show it is not a neutral).

That's what YOU (and almost every body else) thinks. It is taught in school.
Question Everything.

Yes, "I am certain." So long as someone has a 120V load.

Analyze an auto-transformer. If your head doesn't explode first, you will come to the conclusion that center-tap ("Neutral") carries the sum of transformer's L1 and L2. NOT the difference, the imbalance.

You can figure this out with pencil and paper by applying Kirchhoff's Current Law or by simply reasoning on power flow. You can analyze it with SPICE. You can measure it with a clamp ammeter.

If we consumers only draw power from the grid, utility transformer is an isolation transformer. But if we backfeed power while anyone draws single-phase load, we use it as an auto-transformer (plus isolation for any power flowing to or from primary side.)

Hedges said:

If we consumers only draw power from the grid, utility transformer is an isolation transformer. But if we backfeed power while anyone draws single-phase load, we use it as an auto-transformer (plus isolation for any power flowing to or from primary side.)

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Well stated; I never really thought of it that way.

I am trying to wrap my head around how it works if you mix an AIO with microinverters. Also, if there is net load on the transformer what is happening?

I'm also trying to figure out if that is why my IoTaWatt is inaccurate on my main CTs.

Most AIO don't have a transformer (I don't think), so two switchers transfer power. That's how my stacked transformer type 120V Sunny Island work. but effect on current per conductor is the same.

If your AIO has an auto-transformer for split-phase and microinverters downstream, you could potentially overload neutral wire.
(Or even if two switchers, it may still be able to overload the wire. Hmm, mine share a single neutral, so have that issue.)

@Hedges would you be so kind as to draw a simple diagram of what you mean and where the double-neutral current is flowing? I am picturing what you are saying in my head, but, I'm coming up with different numbers than you are.

Hedges said:

That's what YOU (and almost every body else) thinks. It is taught in school.
Question Everything.

Yes, "I am certain." So long as someone has a 120V load.

Analyze an auto-transformer. If your head doesn't explode first, you will come to the conclusion that center-tap ("Neutral") carries the sum of transformer's L1 and L2. NOT the difference, the imbalance.

You can figure this out with pencil and paper by applying Kirchhoff's Current Law or by simply reasoning on power flow. You can analyze it with SPICE. You can measure it with a clamp ammeter.

If we consumers only draw power from the grid, utility transformer is an isolation transformer. But if we backfeed power while anyone draws single-phase load, we use it as an auto-transformer (plus isolation for any power flowing to or from primary side.)

Click to expand...

If you are feeding the utilities transformer with a 240 volt 200 amp backfeed all that power will still flow on L1 and L2. You could feed this back to the utility even if you did not have a neutral wire. The neutral is only needed for 120 volt circuits. It's 48,000 watts. The reason is that the current on each leg is 180 degrees out of phase with the other leg. That is why it cancels out. If the current was on the same phase then indeed the 200 amps would add together on the neutral, but that's not how 240 volt current works.

Now if a neighbor were to consume all that power you push into the transformer on a single 120 volt phase, they could draw 400 amps out of the transformer. It would flow out through one of the hot legs and back through the neutral. The transformer is then working essentially as a step down transformer. Half the voltage means twice the current since the power is conserved. 200 * 240 = 400 * 120. But again that 400 amps is not going to flow on the neutral wire going into your house. Your neighbor would need to have a 400 amp service and consume all that power on a single leg in order to get 400 amps on his neutral wire. So 24 * 2000 watt hair driers all on the same Hot Leg would do the job. If your neighbor plugged 12 hair driers into each leg, then he would also have 0 current on the neutral.

You are correct when you say the current on the neutral is the IMBALANCE. When you push a 240 volt source back to the grid, you have zero imbalance. Your neutral will have zero current.