Designing large solar power system, transformer upgrade needed

[PVguy]

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.

 

[ggunn]

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.

PV AC kW (nameplate power rating of the inverter(s)) is kVA, since its power factor is virtually 1.0. The utility may be able to tolerate a bit more throughput than the 13kVA rating of the tranny, but you'll have to ask them if and how much. And yes, I have had to replace transformers.

It's possible you could make a case to the utility that they need to upsize the transformer because of the size of their customer's service. How big is it?

 

[PVguy]

PV AC kW (nameplate power rating of the inverter(s)) is kVA, since its power factor is virtually 1.0. The utility may be able to tolerate a bit more throughput than the 13kVA rating of the tranny, but you'll have to ask them if and how much. And yes, I have had to replace transformers.

It's possible you could make a case to the utility that they need to upsize the transformer because of the size of their customer's service. How big is it?

He originally had a 200A main panel with a 200A main breaker, we have upgraded that to a 400/200 for the 175A of PV backfeed we need.

 

[PVguy]

Also, it appears that the $20,000 figure includes "changing out the pole." Why would our customer be obligated to pay for that, and why would they call for such a repair?

Thank you very much for your help so far, by the way.

 

[iwire]

Also, it appears that the $20,000 figure includes "changing out the pole." Why would our customer be obligated to pay for that, and why would they call for such a repair?

Thank you very much for your help so far, by the way.

This is all typical, the customer wants to be upgraded they have to pay for it

As far as the pole they likely have to install a stronger, thicker pole to support larger transformers

Why would you expect they would do this for free?

 

[PVguy]

I have designed many systems around this large and have never run into anything like this before, I guess I'm just a bit surprised. I was under the impression that the utility had some obligation to upgrade its equipment to the needs of its customers.

 

[iwire]

I have designed many systems around this large and have never run into anything like this before,

I work for a company that does large scale commercial PV all the time and utility upgrade costs are common.

I was under the impression that the utility had some obligation to upgrade its equipment to the needs of its customers.

Each utility operates under different regulations, you could look into what your area requires of the power companies but nothing you describe seems out of the ordinary.

 

[ggunn]

He originally had a 200A main panel with a 200A main breaker, we have upgraded that to a 400/200 for the 175A of PV backfeed we need.

Is it 240V split phase? Going by his main breaker that's 48kVA. Of course, the POCO can get away with a smaller supply tranny than that since no one's service is ever fully loaded. Unfortunately, the output of a PV system has to be figured on the maximum output of the inverter and it has to be considered continuous. You may be stuck.

I have run into this before. We once put PV on two buildings with separate services only to find out later that both were supplied by the same transformer, and our two systems together were bigger than the tranny. We had to pay for the upgrade, which ate up a lot of our margin on the two systems. Lesson learned; we check this stuff out thoroughly now when we bid a system. Look at it this way: you are lucky to be finding this out now instead of at final inspection.

PS: You will also be wanting to find out the ampacity of his service conductors.

 

[jaggedben]

I could be wrong about the nitty-gritty legal stuff, but I believe that if you hadn't upgraded the service then you (and your customer) wouldn't be obligated to pay anything. The utility is obligated to support the existing service and if they hadn't provided adequate support in the past that's their problem. However since you upgraded the service they now get to hit you with the full cost of supporting the solar backfeed, based on the fact you're changing the need. So now it's your problem. I do wonder a bit if there might have been a miscommunication since you actually didn't increase the size of the main breaker. (Perhaps a supply side tap could have avoided the issue?)

I also believe the rules in California change at 30kW, although it may depend on the utility. (I know there's a different review process for PG&E.) So that may disqualify some of the statements above and trigger your responsibility for the upgrade. BTW, I'm surprised you need as much as 175A of PV backfeed for a 30kW system, but maybe your understating the size.

I would probably try to contest having to pay for 'changing out the pole' or anything else that appears to be maintenance and not upgrade. Good luck.

We've had a couple projects where permission to operate was delayed because the utility said they had to upgrade a transformer, but in neither case did they ask us to pay for it. Both were well under 30kW.

 

[pv_n00b]

I had one project where the client had an 1000A 240/120V 3ph commercial service. When the utility changed out the old transformers years ago they put in transformers that would not support a 1000A service, and never discussed this with the client. The building had been renovated and the new use required much less power so we think they based the transformer size on the recorded usage and not the installed service. We were installing some new HVAC and we needed more power, less than the service but more than the utility had set up to supply. The utility wanted the client to pay to upsize the transformers and service conductors. When the client told them he would take them to court over it, and they knew they had nothing to stand on, they paid for the work. So if you have a 200A service approved by the utility but the utility is only supporting a 100A service it's up to them to pay for the upgrades. This is completely separate form the PV system interconnection.


PV interconnection is a bit like a negotiation. The utility's first offer is usually that the customer pays everything, and it's not unusual for the utility to try to slip in some costs that don't really have much to do with the PV system. So you go over their cost estimate and look for things that might benefit other customers and the utility. Those can usually be argued to be shared costs. Also look for things that might just be utility maintenance, for instance the old transformers won't go in the trash, they will probably go the the maintenance yard to be reused later. That means they have residual value that needs to be subtracted from the cost of the upgrades. A lot of people miss the residual value angle. If the pole was scheduled to be replaced as part of a maintenance cycle and they are trying to get the client to pay for that give it a little push back.

Of course at the end of the day, like the AHJ, the utility can ask for whatever it wants. It all comes down to how hard you are willing to push back and what the merits are.

 

[ggunn]

I had one project where the client had an 1000A 240/120V 3ph commercial service. When the utility changed out the old transformers years ago they put in transformers that would not support a 1000A service, and never discussed this with the client. The building had been renovated and the new use required much less power so we think they based the transformer size on the recorded usage and not the installed service. We were installing some new HVAC and we needed more power, less than the service but more than the utility had set up to supply. The utility wanted the client to pay to upsize the transformers and service conductors. When the client told them he would take them to court over it, and they knew they had nothing to stand on, they paid for the work. So if you have a 200A service approved by the utility but the utility is only supporting a 100A service it's up to them to pay for the upgrades. This is completely separate form the PV system interconnection.


PV interconnection is a bit like a negotiation. The utility's first offer is usually that the customer pays everything, and it's not unusual for the utility to try to slip in some costs that don't really have much to do with the PV system. So you go over their cost estimate and look for things that might benefit other customers and the utility. Those can usually be argued to be shared costs. Also look for things that might just be utility maintenance, for instance the old transformers won't go in the trash, they will probably go the the maintenance yard to be reused later. That means they have residual value that needs to be subtracted from the cost of the upgrades. A lot of people miss the residual value angle. If the pole was scheduled to be replaced as part of a maintenance cycle and they are trying to get the client to pay for that give it a little push back.

Of course at the end of the day, like the AHJ, the utility can ask for whatever it wants. It all comes down to how hard you are willing to push back and what the merits are.

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.

 

[Pharon]

I was under the impression that the utility had some obligation to upgrade its equipment to the needs of its customers.

They are obligated to upgrade their equipment to fit the needs of their customers, but not for free. If the customer was adding LOAD, the utility company might be willing to negotiate a lower price for the new transformer, because it would be able to count on new kWh revenue. But with PV, you're adding a source, not a load -- and in that case, the utility company doesn't really gain anything.

 

[electrofelon]

...... they based the transformer size on the recorded usage and not the installed service. We were installing some new HVAC and we needed more power, less than the service but more than the utility had set up to supply. The utility wanted the client to pay to upsize the transformers and service conductors. When the client told them he would take them to court over it, and they knew they had nothing to stand on, they paid for the work. So if you have a 200A service approved by the utility but the utility is only supporting a 100A service it's up to them to pay for the upgrades.

I disagree. Every utility I have been involved with goes off the loads you provide them and/or historic usage - they could care less what the nec service size is. I am not aware of any legal responsibility to support the nec service size.

 

[ggunn]

I disagree. Every utility I have been involved with goes off the loads you provide them and/or historic usage - they could care less what the nec service size is. I am not aware of any legal responsibility to support the nec service size.

That's what we found out. When we pointed out to the utility that the rating of the transformer feeding the two services was far smaller than the sum of the two services it supplied (in an attempt to get some relief on the cost of the necessary upgrade because of the PV systems we installed), they pointed out that they can connect any service to any transformer they want based on their estimate of what the real load will be. Their engineer was a bit arrogant about it. Like I said, since the systems were already installed we had no bargaining power.

 

[coop3339]

That's what we found out. When we pointed out to the utility that the rating of the transformer feeding the two services was far smaller than the sum of the two services it supplied (in an attempt to get some relief on the cost of the necessary upgrade because of the PV systems we installed), they pointed out that they can connect any service to any transformer they want based on their estimate of what the real load will be. Their engineer was a bit arrogant about it. Like I said, since the systems were already installed we had no bargaining power.

They have an obligation to keep the voltage Within a certain percentage of the nominal 120. You could try turning everything that could normally run together on and then cycle a large load off and on like an AC unit and see if you can get the voltage to drop below their contractual obligation. if you can, have the customer call and complain about dimming lights and low voltage readings maybe they also had an appliance fail recently. They will put a chart recorder on the service for a few days to check for issues. During this time turn everything on again and cycle the ac unit again. They will see the voltage drop and may change the transformer for you.

 

[electrofelon]

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.

I know utilities' tarrifs and policies vary, but 20 grand does seem like an awful lot. A client recently paid not much over that for a 500 kva commercial set up from the ground up. It sounds like they are quite solar unfriendly. 13KVA is odd, it must be either a 10 or a 15. Around here, the utility installs 25 KVA by default for residences, and they have a flat $300 charge for an upgrade for resi solar.

 

[PVguy]

I know utilities' tarrifs and policies vary, but 20 grand does seem like an awful lot. A client recently paid not much over that for a 500 kva commercial set up from the ground up. It sounds like they are quite solar unfriendly. 13KVA is odd, it must be either a 10 or a 15. Around here, the utility installs 25 KVA by default for residences, and they have a flat $300 charge for an upgrade for resi solar.

It's a 10 kVA transformer, they can overload by 30% it seems. At this point I'm exploring a number of different options, such as downsizing the system or changing the inverters to reduce the AC system size. I'm not sure what the cost split is here, how much of the cost is replacing the pole vs just the transformer. Also the utility engineer is very difficult to communicate with. The system is already built on the house, just a very irritating situation all around. Does anyone know what transformer sizes they have access to? is it 10, 15, 20, 25, 30 kVA?

 

[Iron_Ben]

The standard sizes for single phase pole top units are 10, 15, 25, 37.5, 50..... As far as the 30% overload, that's appropriate but conservative. We *routinely* loaded ours to 150%, with very few issues (typical load curve assumed). Oh, and as far as that $20k number, that's nuts. I'll tell you this - there was not one 40 or 50 foot pole on our system where the weight increase in going from, say, a 15 kva pot to a 37.5 or 50 would necessitate a pole change. Now, if when looking at the job, it was determined that the pole was 40 years old and rotten, then okay, changing it out is prudent. But you should not pay for that. I'd go back to the poco for some relief and/or negotiation.

 

[ggunn]

The standard sizes for single phase pole top units are 10, 15, 25, 37.5, 50..... As far as the 30% overload, that's appropriate but conservative. We *routinely* loaded ours to 150%, with very few issues (typical load curve assumed).

How did you figure that (over)loading? PV typically can run full out at the nameplate rating of the inverter or close to it for several hours a day.

 

[Iron_Ben]

In the case of single phase pole top transformers feeding one or more houses in residential neighborhoods, we had in house software that made guesses based on typical load curves. Pretty much all you can do, when the meters give you only energy used (kwh) and no demand info (kw). Of course the new smart meters capture a wealth of information, but I retired shortly after we got all of ours installed. I hope they are taking advantage of the data available to make even better loading decisions.

Of course we had many (non residential) customers that were demand metered. So it was easy to compare peak kw to transformer kva. Although, we didn't capture the power factor at all such installations, so while customer peak kw was always known, not so with kva. Sounds shaky I suppose but that's the way it was.

I'll close by saying that years of experience confirmed for me that the 150% guideline is a good one, and probably even a little conservative for pole tops, again, assuming a normal or normal-ish load curve. 175% would probably be pushing it right up to the limit. And for pad mounts, maybe something closer to 125% or so.

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.....