Confused about utility transformer sizing for PV system

[Equinox Energy]

You clearly need to get someone who knows about solar involved in your setup. With a 20 amp service you cannot back fed the panel with 125 amps--The max back fed breaker allowed is 40 amps on a 200 amp panel with a 200 amp ocdp .

Unfortunately this is not a diy site and I must close this thread

Looks like you completely missed the sentence where OP explained a line side tie in was planned DOOOHHH. In that case, the distribution bus bar rating has absolutely nothing to do with it.

 

[electrofelon]

If the OP is still around, ay updates?

I was just going to add that last summer we had any issue with inverters tripping out from over voltage on 25kw DC system. AC voltage with the PV off was all over the place, so I suspect the customer was near the end of a long/old line. POCO ended up replacing the transformer, I don't know to what size though. I was skeptical this would solve the problem but it did - I guess the new transformer had lower impedance and better voltage regulation that was enough to give us the few volts we needed. Anyway, point is I can see the POCO perhaps coming up with rules like this to try to stave off voltage Rise problems, but 100kva still seems out of the ballpark.

 

[Flicker Index]

Could power quality thing be the reason? Many line commutated solar PV inverters put out a chock full of harmonic currents. You can boil it down just kWh and kW for accounting purposes but quite often solar power is dirty.
Water trickling from faucet and Waterpik shower that shoots water in a machine gun pattern can have the same average quantity of water per minute, but the delivery quantity isn't the same.

 

[jaggedben]

Could power quality thing be the reason? ..

For the utilities application rejection? I'm gonna say no.

 

[Hv&Lv]

I’m real interested in the OPs situation here.
we absolutely HATE net metering, and we don’t even offer it.

Still the POCO request doesn’t make sense to me.

 

[Flicker Index]

For the utilities application rejection? I'm gonna say no.

for the utility setting requirements about xfrmr change and such

 

[jaggedben]

for the utility setting requirements about xfrmr change and such

Still no.

 

[lost_aussie]

Do you have an accurate way to measure your service voltage? If so, try measuring it at your approximate maximum load (during the afternoon when all your work equipment is running) and then during light loading conditions (i.e. early am). I am guessing there is at least a few percent voltage variation due to voltage drop over the transformer and feeder to your house at high load conditions.

I suspect the utilities issue isn't with continuous current rating (transformer is sufficient for the proposed array, as you have already mentioned), but the potential voltage rise that would occur if your PV system were generating at nameplate output >> load. In this instance, your voltage drop will invert and become voltage rise. Likely you will see service voltage outside of acceptable range at this time. If the utility allow this interconnect and then in a year you call them with failing appliances, they are obliged to fix it.

 

[electrofelon]

I suspect the utilities issue isn't with continuous current rating (transformer is sufficient for the proposed array, as you have already mentioned), but the potential voltage rise that would occur if your PV system were generating at nameplate output >> load. In this instance, your voltage drop will invert and become voltage rise. Likely you will see service voltage outside of acceptable range at this time. If the utility allow this interconnect and then in a year you call them with failing appliances, they are obliged to fix it.

Essentially what I said post #22. In that system, POCO voltage was all over the place and flirting with being out of required specs, but without PV no one noticed or cared. Spec'ing/requiring a big transformer may help them not have to resolve the issue with some sort of line improvement.