Transformer size with PV in play

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mobo

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Help me understand please.......let's say you have a 500 KVA transformer at a customer's site. Then let's say a customer plans to have a PV system installed, let's say a 100KW system. Let's say the 500 KVA transformer is operating at near full capacity. Does the 100 KW output of the PV system have to be added to the load on the 500 KVA transformer? Would the 500 KVA transformer have to be upgraded to a bigger rating? Or does it not matter since the PV will be flowing in the opposite direction?
 
The basic answer is that the PV is not a load, plain and simple. So it is never added to the load. When operating it subtracts from or reverses the load. But since you can't count on its being there 100% of the time, you don't subtract it from the load either for this type of calculation. If it is a customer owned transformer, it probably stays the same size. If the PV size were closer to the full capacity of the transformer, that might be less sure of a thing, since the PV may operate more continuously than the load in the middle of sunny days.

The longer more complicated answer involves the impedence of the local grid and the utility's obligations to other nearby customers. For example, if it's a utility owned transformer that serves multiple customers, and they all have solar systems, the total solar backfeed in the middle of a sunny day might exceed the load that an existing transformer was designed for. The impedence of the transformer also affects the local voltage, and the utility calculations might call for a change for that sort of reason.
 
It is really the same as a line side connection from an electrical viewpoint, and 705.11(A) permits the connection of a PV system having an output rating that is equal to or less than the rating of the service.
 
don_resqcapt19 said:
It is really the same as a line side connection from an electrical viewpoint, and 705.11(A) permits the connection of a PV system having an output rating that is equal to or less than the rating of the service.
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Yes, but you should be careful. A few years ago we were commissioned to install PV systems on two car dealerships that were close to each other. Each system was rated less than its respective main service disconnect, but what we didn't know at the time (lesson learned the hard way) was that both facilities were fed from the same transformer and the sum of the two PV systems' AC ratings exceeded the kVA rating of the transformer. We protested that the transformer was undersized to begin with, but it fell on deaf ears. Since the PV systems were already built our negotiating position was pretty weak. We bought a transformer.
 
ggunn said:
Yes, but you should be careful. A few years ago we were commissioned to install PV systems on two car dealerships that were close to each other. Each system was rated less than its respective main service disconnect, but what we didn't know at the time (lesson learned the hard way) was that both facilities were fed from the same transformer and the sum of the two PV systems' AC ratings exceeded the kVA rating of the transformer. We protested that the transformer was undersized to begin with, but it fell on deaf ears. Since the PV systems were already built our negotiating position was pretty weak. We bought a transformer.
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Yes, that could be an issue where the service size is larger than the utility transformer, and that is common.
 
mobo said:
Help me understand please.......let's say you have a 500 KVA transformer at a customer's site. Then let's say a customer plans to have a PV system installed, let's say a 100KW system. Let's say the 500 KVA transformer is operating at near full capacity. Does the 100 KW output of the PV system have to be added to the load on the 500 KVA transformer? Would the 500 KVA transformer have to be upgraded to a bigger rating? Or does it not matter since the PV will be flowing in the opposite direction?
Click to expand...

Consider the extreme possibilities:
1. The PV is not running at all, and the loads are at full capacity.
2. The loads diminish to zero, and the PV is at full capactity, and is therefore exporting everything it can possibly export.
3. The loads are running at full capacity, and so is the PV system

In your example:
Possibility #1 would mean a 500 kVA of load on the transformer.
Possibility #2 would mean a 100 kVA of load on the transformer
Possibility #3 would mean 400 kVA of load on the transformer

This means that a 500 kVA transformer would be all that is needed.

It is common that utilities undersize their transformers, compared to what you'd have to do when sizing a customer-owned transformer serving the same purpose. This may restrict your system size as well, as many utilities limit you to the size of their transformer.
 
ggunn said:
Yes, but you should be careful. A few years ago we were commissioned to install PV systems on two car dealerships that were close to each other. Each system was rated less than its respective main service disconnect, but what we didn't know at the time (lesson learned the hard way) was that both facilities were fed from the same transformer and the sum of the two PV systems' AC ratings exceeded the kVA rating of the transformer. We protested that the transformer was undersized to begin with, but it fell on deaf ears. Since the PV systems were already built our negotiating position was pretty weak. We bought a transformer.
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Was this a utility transformer? shouldn't the utility have caught that when the application was submitted?
 
electrofelon said:
Are you saying Gunny was a naughty boy and started the system before all approvals and paperwork? I have never done such a thing 😉
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Well, in all seriousness, for a given size system in a given location that may or may not get you in a lot of trouble. One needs to know how the rules work wherever one is installing.
 
jaggedben said:
The basic answer is that the PV is not a load, plain and simple. So it is never added to the load. When operating it subtracts from or reverses the load. But since you can't count on its being there 100% of the time, you don't subtract it from the load either for this type of calculation. If it is a customer owned transformer, it probably stays the same size. If the PV size were closer to the full capacity of the transformer, that might be less sure of a thing, since the PV may operate more continuously than the load in the middle of sunny days.

The longer more complicated answer involves the impedence of the local grid and the utility's obligations to other nearby customers. For example, if it's a utility owned transformer that serves multiple customers, and they all have solar systems, the total solar backfeed in the middle of a sunny day might exceed the load that an existing transformer was designed for. The impedence of the transformer also affects the local voltage, and the utility calculations might call for a change for that sort of reason.
Click to expand...
Great input, thanks for your reply!
 
ggunn said:
Yes, but you should be careful. A few years ago we were commissioned to install PV systems on two car dealerships that were close to each other. Each system was rated less than its respective main service disconnect, but what we didn't know at the time (lesson learned the hard way) was that both facilities were fed from the same transformer and the sum of the two PV systems' AC ratings exceeded the kVA rating of the transformer. We protested that the transformer was undersized to begin with, but it fell on deaf ears. Since the PV systems were already built our negotiating position was pretty weak. We bought a transformer.
Click to expand...
Definetly something to consider! Thanks for your input!
 
Carultch said:
Consider the extreme possibilities:
1. The PV is not running at all, and the loads are at full capacity.
2. The loads diminish to zero, and the PV is at full capactity, and is therefore exporting everything it can possibly export.
3. The loads are running at full capacity, and so is the PV system

In your example:
Possibility #1 would mean a 500 kVA of load on the transformer.
Possibility #2 would mean a 100 kVA of load on the transformer
Possibility #3 would mean 400 kVA of load on the transformer

This means that a 500 kVA transformer would be all that is needed.

It is common that utilities undersize their transformers, compared to what you'd have to do when sizing a customer-owned transformer serving the same purpose. This may restrict your system size as well, as many utilities limit you to the size of their transformer.
Click to expand...
Ah, gotcha, that helps a lot. Thanks so much for your time in replying!
 
electrofelon said:
Was this a utility transformer? shouldn't the utility have caught that when the application was submitted?
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Yes, and that's what we said, but the utility (ONCOR) refused to take any responsibility for not catching it. We were new to commercial PV at the time (this was several years ago), and we thought that the size of the main service disconnects alone defined the maximum size of the PV systems we could install. As I said, the fact(s) that all the PV equipment was installed, and both customers were impatient to get their systems turned on, and we hadn't been paid yet combined to put us in a weak bargaining position when we tried to get the utility to pay for a larger transformer. We lost; we paid our tuition in the school of experience.

In fairness, I have to say that these systems were on separate applications and someone from ONCOR would have had to look at them both together to see that there was a problem. Maybe that was too much to expect.
 
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