Effect of future Solar energy on existing transformers

[mehdi1351]

Hi
I am in process of selecting a transformer for a Plant in Texas. There might be on an adding a solar panels on the roof of the plant building to get some Solar energy for the plant. The question is: Should I consider anything on size/type of the transformer or even any special protection for that transformer for this future change?

 

[jaggedben]

Whether you *should* consider it is an economic question that only the person with the pocketbook can answer. It also depends in theory on the goals for the solar and regulations (e.g. net metering or other payment regime) that would govern the solar, as well as the site details. For example if the future solar would be net-metered and attempting to cover 100% of the building's annual usage, then the transformer would likely need to be a few times larger than you would otherwise specificy. However the solar could be limited by another factor, such as the number of panels that could fit on the roof. A future solar system could also be designed around the transformer you choose, but that could possibly (but not necessarily) be a partial lost opportunity for the owner if they wanted a bigger system.

If the client wishes, I'd recommend they pay a qualified solar company a design fee to estimate the optimal size solar system for the facility give the info you have, and go from there. That could potentially save the client many times the cost of the design fee down the road.

I don't believe any special protection would usually be required but that may also depend on the amount of power and voltage involved, as well as the local utility rules.

 

[mehdi1351]

Thanks for the reply. It goes without saying that the Solar would not take the 100% of the total energy consumption for the plant. The current KW required to run the plant is around 4000KW so for sure Solar panels on the roof of the building cannot generate that amount of power. The plan is to install the FV panels on the roof to generate 10-20% of the total energy required for run the plant.
Now would you please tell me whether that amount of energy generated by Solar, may effects on Transformer type/protections?
I am not very familiar with NFPA/NEC which is the dominant codes/standards in US, so it would better to knowing now rather than after placing the order of transformer.

 

[electrofelon]

There are not really any codes that would dictate transformer selection. As long as the transformer size is greater than or equal to the AC output of the solar system, you should be good from a functional standpoint. It is more the distribution equipment that would take some design consideration to be solar ready. The only thing I can think of is utility rules, most likely one that dictates that the winding configuration be a wye-wye to conform to their "effective grounding" requirements.

So you will be taking service from the utility at the MV level and stepping it down to what 277-480?

 

[petersonra]

There is not a realistic scenario where solar generates enough power that it becomes a transformer problem. If it does, when the solar is installed, someone would need to address that issue. The answer could be as simple as adding a transformer just for the solar.

 

[jaggedben]

Thanks for the reply. It goes without saying that the Solar would not take the 100% of the total energy consumption for the plant. The current KW required to run the plant is around 4000KW so for sure Solar panels on the roof of the building cannot generate that amount of power. The plan is to install the FV panels on the roof to generate 10-20% of the total energy required for run the plant.
Now would you please tell me whether that amount of energy generated by Solar, may effects on Transformer type/protections?
I am not very familiar with NFPA/NEC which is the dominant codes/standards in US, so it would better to knowing now rather than after placing the order of transformer.

Producing 20% of the total *energy* consumption may still involve an amount of solar *power* that routinely runs up against the transformer rating during the middle of the day in summer. But I really think it's outside the scope of this forum to give you a proper analysis of that, still too many questions. Like, is 4000kw the plant's peak or average load. I assume you are saying the building roof is too small for 4000kw, but there are buidlings that are big enough for that.

There is not a realistic scenario where solar generates enough power that it becomes a transformer problem. If it does, when the solar is installed, someone would need to address that issue. The answer could be as simple as adding a transformer just for the solar.

I don't think we have enough detail for your first statement. In any case, planning for future now could still save money down the road.

 

[ggunn]

Thanks for the reply. It goes without saying that the Solar would not take the 100% of the total energy consumption for the plant. The current KW required to run the plant is around 4000KW so for sure Solar panels on the roof of the building cannot generate that amount of power. The plan is to install the FV panels on the roof to generate 10-20% of the total energy required for run the plant.
Now would you please tell me whether that amount of energy generated by Solar, may effects on Transformer type/protections?
I am not very familiar with NFPA/NEC which is the dominant codes/standards in US, so it would better to knowing now rather than after placing the order of transformer.

Something to keep in mind is that whatever power is being generated by the PV system that is not being consumed by the plant will reduce, not increase, the current flowing through the utility transformer.

 

[mehdi1351]

There are not really any codes that would dictate transformer selection. As long as the transformer size is greater than or equal to the AC output of the solar system, you should be good from a functional standpoint. It is more the distribution equipment that would take some design consideration to be solar ready. The only thing I can think of is utility rules, most likely one that dictates that the winding configuration be a wye-wye to conform to their "effective grounding" requirements.

So you will be taking service from the utility at the MV level and stepping it down to what 277-480?

Thanks for the respond. Yes the idea is to ask Swepco to install a step-down transformer for our plant based on the calculated loads. We need 3pH 480V, 60HZ and the Solar energy is going to feed some part of the plant, let say 10-20% of the total loads.
So there is no specific clause either at NEC or NFPA which I should be taken into account in this regard?

 

[mehdi1351]

Producing 20% of the total *energy* consumption may still involve an amount of solar *power* that routinely runs up against the transformer rating during the middle of the day in summer. But I really think it's outside the scope of this forum to give you a proper analysis of that, still too many questions. Like, is 4000kw the plant's peak or average load. I assume you are saying the building roof is too small for 4000kw, but there are buidlings that are big enough for that.


I don't think we have enough detail for your first statement. In any case, planning for future now could still save money down the road.

Thanks for the reply.
The 4000KW is the peak load as most of the loads are intermittent, not continuous. The plant is batch-wise process so not all loads are coming into the picture in a same time.

 

[electrofelon]

Thanks for the respond. Yes the idea is to ask Swepco to install a step-down transformer for our plant based on the calculated loads. We need 3pH 480V, 60HZ and the Solar energy is going to feed some part of the plant, let say 10-20% of the total loads.
So there is no specific clause either at NEC or NFPA which I should be taken into account in this regard?

So will this be a customer owned or utility transformer?

Do you have a rough idea how big the solar system might be, just based on available area?

Generally speaking, as long as the transformer is bigger than the PV system, there isn't much to worry about as far as the transformer goes, except as mentioned that effective grounding stuff which might dictate it that it be a Y-Y.

The more likely concerns will be with interconnection points and policies, metering, and distribution equipment designed to accept the solar back feed

 

[SteveO NE]

Thanks for the reply.
The 4000KW is the peak load as most of the loads are intermittent, not continuous. The plant is batch-wise process so not all loads are coming into the picture in a same time.

So then you need to determine your baseload and figure out your worst case vector. Determine how the plant is affected if the PV is offsetting 10%, or 50%, or 90% of load during a given moment during the day, for example. It sounds like you wouldn't have an issue with the transformer being too small, if it is going to be a 4MW transformer and you cannot fit that much PV, but you would want to consider how the PV offset load affects certain things such as how the PV will interact with loads on-site (voltage rise/transformer taps, power factor, etc.). That said, you may have a 4MW MV feeder with smaller distribution transformers at point of use.

At the end of the day, the more likely consideration of what to do to save money now is going to be with your switchgear. If you are primary metered (take service at MV), then, you need to have the provisions now to tie in. MV transformers add up quick so it would make sense to make sure it fits behind a transformer and its feeder. if you aren't taking service at MV and instead you have 480V utility metering, you are going to want to make sure the bus can handle the current of all of the sources when taking in consideration if that utility is hot or cold sequenced metering.

 

[mehdi1351]

So will this be a customer owned or utility transformer?

Do you have a rough idea how big the solar system might be, just based on available area?

Generally speaking, as long as the transformer is bigger than the PV system, there isn't much to worry about as far as the transformer goes, except as mentioned that effective grounding stuff which might dictate it that it be a Y-Y.

The more likely concerns will be with interconnection points and policies, metering, and distribution equipment designed to accept the solar back feed

We are renting the building and the transformer will be installed by Utility company (Swepco) and the owner of the building will be responsible for the maintaining of the transformer. But we have to do the sizing calculation for Swepco to enable them to purchase the transformer.
I am guessing that the PV would be something between 600 -800KW , so based on what you explained, there shouldn't be any worry on transformer size? and the only thing that I have to taken into account is the type of grounding i.e Delta-WYE or WYE-WYE
I guess it is also depends on the existing earthing system on that area, right? i.e grounded or ungrounded system

 

[mehdi1351]

So then you need to determine your baseload and figure out your worst case vector. Determine how the plant is affected if the PV is offsetting 10%, or 50%, or 90% of load during a given moment during the day, for example. It sounds like you wouldn't have an issue with the transformer being too small, if it is going to be a 4MW transformer and you cannot fit that much PV, but you would want to consider how the PV offset load affects certain things such as how the PV will interact with loads on-site (voltage rise/transformer taps, power factor, etc.). That said, you may have a 4MW MV feeder with smaller distribution transformers at point of use.

At the end of the day, the more likely consideration of what to do to save money now is going to be with your switchgear. If you are primary metered (take service at MV), then, you need to have the provisions now to tie in. MV transformers add up quick so it would make sense to make sure it fits behind a transformer and its feeder. if you aren't taking service at MV and instead you have 480V utility metering, you are going to want to make sure the bus can handle the current of all of the sources when taking in consideration if that utility is hot or cold sequenced metering.

Very good point Steve
Thanks for the respond and advice.

 

[SteveO NE]

...and the only thing that I have to taken into account is the type of grounding i.e Delta-WYE or WYE-WYE
I guess it is also depends on the existing earthing system on that area, right? i.e grounded or ungrounded system

There really isn't much to take into account until you understand the implications of each for your specific installation. Even if its a Delta service, most still have grounded conductors with modern installations. The only ungrounded systems I work on are must run applications, and we use ground fault detection instead; so if that is the case you need to make sure your ground fault detection P&C is capable of working with a PV load.

If you are only worried about this transformer, you should let the utility know that you intend to install PV and make sure the transformer they install would be the same one they would install based on their requirements for PV - it may not matter to them if it is customer owned or it may - its worth engaging the utility. Based on the size of that transformer and the size of your PV system you may be required to put in a grounding bank regardless of whether or not the transformer was a delta or a wye. I would doubt for this transformer that it would be ungrounded on the customer side but this depends on where the actual point of metering is...are you renting a piece of a property that has primary metering and they are installing another transformer, or are you the only one behind the meter?

 

[electrofelon]

.
I am guessing that the PV would be something between 600 -800KW , so based on what you explained, there shouldn't be any worry on transformer size? and the only thing that I have to taken into account is the type of grounding i.e Delta-WYE or WYE-WYE
I guess it is also depends on the existing earthing system on that area, right? i.e grounded or ungrounded system

Yeah so since your transformer is much bigger than the pv, no issue with sizing. If they have wye distribution, then the transformer will most likely be a YY.

As Steve touched on, with PV it is sometimes Handy to have a transformer with taps. Maybe request taps if it isn't already included.

Have you told the utility you will likely be doing PV to see if they have any input? I admit it is probably hard to get in touch with someone who knows anything about distributed generation because it's probably a totally different department and you might have to put in an application and plans to even talk to someone. If that is a dead end, perhaps you can dig through their distributed generation requirements, or as has also been mentioned, talk to a PV firm that is familiar with this utility

 

[mehdi1351]

Yeah so since your transformer is much bigger than the pv, no issue with sizing. If they have wye distribution, then the transformer will most likely be a YY.

As Steve touched on, with PV it is sometimes Handy to have a transformer with taps. Maybe request taps if it isn't already included.

Have you told the utility you will likely be doing PV to see if they have any input? I admit it is probably hard to get in touch with someone who knows anything about distributed generation because it's probably a totally different department and you might have to put in an application and plans to even talk to someone. If that is a dead end, perhaps you can dig through their distributed generation requirements, or as has also been mentioned, talk to a PV firm that is familiar with this utility

Thanks for the respond.
I already started discussion with Swepco (Texas utility company) but they are not much responsive. I need to find a local consultancy over the Texas/Longview to talk with the utility company.
My main concern was, at the peak load, let say 3200kw, and at the same time during the summer the PV create 800KW and then the current passing through the transformer going beyond its limitation and I would have thought that I should do some additional consideration on transformer overload protection.

 

[jaggedben]

Thanks for the respond.
I already started discussion with Swepco (Texas utility company) but they are not much responsive. I need to find a local consultancy over the Texas/Longview to talk with the utility company.
My main concern was, at the peak load, let say 3200kw, and at the same time during the summer the PV create 800KW and then the current passing through the transformer going beyond its limitation and I would have thought that I should do some additional consideration on transformer overload protection.

In that situation the PV would reduce the current flowing through the transformer.

Rules around protecting equipment and conductors when solar is involved are in Article 705 of the NEC. As SteveO alluded to above, there is a lot there that would affect switchgear but not so much transformers.

 

[ggunn]

My main concern was, at the peak load, let say 3200kw, and at the same time during the summer the PV create 800KW and then the current passing through the transformer going beyond its limitation and I would have thought that I should do some additional consideration on transformer overload protection.

Current cannot flow both ways simultaneously. Current produced by the PV system subtracts from the current demanded from the utility through the transformer, and if it is more than the demand by the plant, current flows back to the utility.