How do you install a battery for backup and arbitrage?

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shockking

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engineer
Say we have a 1000 kVA transformer feeding an appropriate switchgear. It is 75% loaded. We want to install a 750 kW 4hr battery. If the battery is able to charge at full capacity, we would be adding 750 kW of potential load, and when used for arbitrage we could overload our transformer and gear to 150%. Not good. Alternately if we install a smaller 250 kW battery, then the battery is only able to supply a 1/3rd of the demand, defeating the backup function.

Do you have to upsize your transformer and gear to be 2X your load? Can you use controls to limit the charge rate, and create a 750 kW-discharge, 100-kW-charge battery? What am I missing?

TIA
-Tom
 
ron said:
705.12 will have a lot to do with your situation
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Not really, if I understand correctly that he is talking about a transformer on the supply side of the service disconnect. Also not clear if the transformer is customer or utility owned.

I'm curious what '75% loaded' means. Is that NEC calc or actual usage (peak? I hope not average).

Also is there no other source to charge batteries? Solar? This is important.

This sort of amperage is above my pay grade and I don't know how the question is typically handled with projects this size. (@pv_n00b probably would have thoughts.) It hasn't come up for me yet in resi situations that a service needed to be upgraded for battery charging current from the grid. (We also usually promise the utility that the batteries won't charge from grid most of the time, because they'll be charging from solar.)

As far as the export question, limiting export by Power Control Systems is probably the answer. It does not follow that the transformer will be 150% loaded when the battery is used to power the load. However, it's recently come to my attention that an engineer may still consider that a non-export system has an impact on the transformer.
 
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jaggedben said:
However, it's recently come to my attention that an engineer may still consider that a non-export system has an impact on the transformer.
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Can you elaborate on that? I infer you mean a transformer that is upstream (towards the grid) from the non-export control point.

Cheers, Wayne
 
wwhitney said:
Can you elaborate on that? I infer you mean a transformer that is upstream (towards the grid) from the non-export control point.

Cheers, Wayne
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Correct. A utility owned transformer in this case so the NEC is not relevant.

There was already a solar system with permission to export from the site. We added a battery that would not (continuously) export. The utility engineer called for a transformer upgrade. We asked them if they understood that the battery would not export. We received a relatively technical reply that mentioned open loop response time.
 
jaggedben said:
We asked them if they understood that the battery would not export. We received a relatively technical reply that mentioned open loop response time.
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I had to google that one. Apparently the term "open loop" refers to the control method the inverter uses to approximate true non-export. This link talks about it for Enphase:


I only skimmed a few paragraphs, but apparently relevant are: "Utilities like PG&E require energy storage systems to comply to the ESS import only mode with an Open Loop Response Time(OLRT) of less than 2 seconds." And "While the PCS Certification Requirement Decision (CRD) to UL1741 requires an Open Loop Response Time (OLRT) of less than 30 seconds in general, utilities may require a faster response time to consider the storage system as non-exporting (import only)."

It goes on to say that sufficiently recent versions of their product have a 2 second option that can be enabled.

Cheers, Wayne
 
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