AC Voltage Rise / Buck Boost Transformers

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KY Solar

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Owensboro, Kentucky, USA
My first post on MH.

We are wrapping up a 30kW solar array that is based on (100) ABB 300W micro-inverters :slaphead:.
The main feed wire is MCM250 Cu and travels a distance of 365'. Nominal load is 120A.
The wire feeding each trunk cable is 12-3 UF-B. Average length is 100'. Nominal load is 7.5A.
Nominal "solar off" voltage is 250 at the meter can.
We anticipated some voltage rise but the solar array is driving up voltage at the meter can by about 5 volts to 255.
The underground wiring adds another 5 volts to 260.
Although the micros are rated to 264 volts, we are seeing some instability and inverters dropping out during mid day at 260 volts.
We have opened up the voltage parameters within the micro-inverters to the max allowed (132 / 264 volts).

Two options:
1.) Get the local utility to install a new 25kVA transformer with taps, dropping all secondary voltage on site by 2.5%.
2.) Install Buck Boost transformers on the main AC feed wires, dropping the voltage to the solar array by 5%.

Questions:
  • What would you do to reduce the voltage as seen at the micro-inverters?
  • The micro-inverters depend on a well balanced Neutral for proper function. Do we need two (2) buck boost transformers to maintain a good Neutral?
  • Who makes the best quality outdoor rated Buck Boost transformers? (probably about 2kVA).

Thanks!
 
KY Solar said:
My first post on MH.

We are wrapping up a 30kW solar array that is based on (100) ABB 300W micro-inverters :slaphead:.
The main feed wire is MCM250 Cu and travels a distance of 365'. Nominal load is 120A.
The wire feeding each trunk cable is 12-3 UF-B. Average length is 100'. Nominal load is 7.5A.
Nominal "solar off" voltage is 250 at the meter can.
We anticipated some voltage rise but the solar array is driving up voltage at the meter can by about 5 volts to 255.
The underground wiring adds another 5 volts to 260.
Although the micros are rated to 264 volts, we are seeing some instability and inverters dropping out during mid day at 260 volts.
We have opened up the voltage parameters within the micro-inverters to the max allowed (132 / 264 volts).

Two options:
1.) Get the local utility to install a new 25kVA transformer with taps, dropping all secondary voltage on site by 2.5%.
2.) Install Buck Boost transformers on the main AC feed wires, dropping the voltage to the solar array by 5%.

Questions:
  • What would you do to reduce the voltage as seen at the micro-inverters?
  • The micro-inverters depend on a well balanced Neutral for proper function. Do we need two (2) buck boost transformers to maintain a good Neutral?
  • Who makes the best quality outdoor rated Buck Boost transformers? (probably about 2kVA).

Thanks!
Click to expand...
You could double up on your 250kcmil conductors to reduce the voltage drop/rise in the lines. Pricey, yes, but compare it to the cost of the transformer(s).
 
The system does seem a bit lossy: you have about 2% at full output just in the 365' run, then there is feeders (if applicable) and trunk cables losses. Off the top of my head, I think the autotransformer option would be quite affordable.
 
Your statement that the nominal is 250V at the meter seems odd to me. If 250 is really the nominal voltage, then another option should be to consult ABB on how to widen the inverters operating range to be higher.

If the nominal voltage is really supposed to be 240, then option 1 to fix the utility transformer and reduce grid impedance would seem to be best.

FWIW, using buck-boost for this application can be kind of mean to the utility and neighboring customers on the same utility transformer, not to mention your customer. Remember, part of the reason the inverters shut off when the voltage gets too high is to protect loads from over-voltage. If you put in buck-boost the loads will still see the high voltage, perhaps higher. So to a large extent, masking the out-of-range voltage from the inverters using buck-boost is defeating a grid protection mechanism. It's the sort of thing that if everybody did it could create large scale problems with distributed generation and make the utilities really unhappy with us.
 
jaggedben said:
Your statement that the nominal is 250V at the meter seems odd to me.
Click to expand...

I dont see that as abnormal - maybe a hair on the high side. I have "healthy" voltage at my house, usually high 240's. Seems like the voltage window could be a bit higher.

IMO that is part of the design process, to calculate wire losses not only for loss of revenue (or the right balance of loss of revenue vs capital cost), but also for excess voltage rise at the inverters.
 
Thanks

Thanks

Thank You Gentlemen,

Yes, the local utility considers their nominal voltage to be a full 250.
I was really surprised that the voltage at the meter base goes up a full 5 volts when the solar is operating. What causes this? Impedance in the (old and cruddy) meter base, riser and utility transformers?

The micro-inverters are UL listed and that precludes them from exceeding 264 volts. The manufacturer (ABB) tells me that the inverters will operate erratically at 260 volts and above.

The utility is suggesting that we buck the voltage to the micro-inverters down 5% to 247. This should not change the voltage at the meter or what the power line sees…. other than the 5 volts we are driving up voltage.

The local utility says that all of their solar net metering customers experience higher voltage when the solar is operating. Is this typical?

How much current will be lost in a typical 3kVA buck-boost transformer?
 
KY Solar said:
Thank You Gentlemen,

Yes, the local utility considers their nominal voltage to be a full 250.
I was really surprised that the voltage at the meter base goes up a full 5 volts when the solar is operating. What causes this? Impedance in the (old and cruddy) meter base, riser and utility transformers?

The micro-inverters are UL listed and that precludes them from exceeding 264 volts. The manufacturer (ABB) tells me that the inverters will operate erratically at 260 volts and above.

The utility is suggesting that we buck the voltage to the micro-inverters down 5% to 247. This should not change the voltage at the meter or what the power line sees…. other than the 5 volts we are driving up voltage.

The local utility says that all of their solar net metering customers experience higher voltage when the solar is operating. Is this typical?

How much current will be lost in a typical 3kVA buck-boost transformer?
Click to expand...

You do not exactly lose current in a buck boost transformer.
You get a different current/voltage pairwhich represents roughly the same amount of power from the inverter.
A good question is how much power is lost in the buck transformer, both when operating and when idle all night.
The answer is probably in the small single digit percentage area, but worth calculating for a given set of transformer specs.
 
KY Solar said:
The micro-inverters are UL listed and that precludes them from exceeding 264 volts. The manufacturer (ABB) tells me that the inverters will operate erratically at 260 volts and above.
Click to expand...

I'm not an expert on the UL standard but that doesn't sound exactly right. I've definitely heard of manufacturer's adjusting parameters for grid profiles that are nominal different from the datasheet. It's unfortunate if ABB can't do it, I'm pretty sure Enphase could, and at least in the past they would. Maybe drive at ABB a little harder to confirm if they really gave you the correct answer. It's probably not the cheapest option to replace the inverters, but then it again it might actually be close enough to other options to not dismiss out of hand, if you can get someone to confirm they'll support the capability.

If the utility is okay with the buck-boost solution then I guess it's fine. After all, they are the one's with the somewhat non-orthodox nominal grid voltage.
 
electrofelon said:
I dont see that as abnormal - maybe a hair on the high side. I have "healthy" voltage at my house, usually high 240's. Seems like the voltage window could be a bit higher.

...
Click to expand...

Abnormal for a reading it is not, in the least, I agree. But I'm surprised it's the nominal voltage. I wonder how many utilities are like that.
 
Another Problem

Another Problem

GoldDigger said:
You do not exactly lose current in a buck boost transformer.
You get a different current/voltage pairwhich represents roughly the same amount of power from the inverter.
A good question is how much power is lost in the buck transformer, both when operating and when idle all night.
The answer is probably in the small single digit percentage area, but worth calculating for a given set of transformer specs.
Click to expand...

Yes, that is the question - what do we lose in the magnetizing current of the transformer? The primary winding would be energized 24/7.

There is also another unrelated problem - the micros are drawing 340x the published standby power requirement. (This was replicated and confirmed with a separate test array wired directly to a breaker at the meter base). The published standby power spec is <50mW and each inverter is drawing 17 watts at night.

So, maybe isolating the solar array and the buck-boost transformer at night is where this will end up.
 
Standby Watts

Standby Watts

I have no idea.
I just used a combination of several CT's along with a data logger.
All measure the same - each about 0.065 Amps @ 250 VAC.
What did you guys have in mind?

Thanks!
 
You have about 1.4% voltage drop in the #12 feeders to the trunk cables and about 2.1% in the #250 conductors. How feasible is it be to upsize those conductors? Do you know how many other customers share the transformer? Can the utility drop the voltage down? I am guessing not if they are suggesting that you drop it down on your side.
 
Voltage Blues

Voltage Blues

PWDickerson said:
You have about 1.4% voltage drop in the #12 feeders to the trunk cables and about 2.1% in the #250 conductors. How feasible is it be to upsize those conductors? Do you know how many other customers share the transformer? Can the utility drop the voltage down? I am guessing not if they are suggesting that you drop it down on your side.
Click to expand...

I agree with your voltage drop calculation. Thanks!
Unfortunately, all of the feeders are buried and would be very difficult and expensive to up-size.
The utility can install a 25kVA transformer with taps and take the (farm operation) volts down 2.5%.
The other option would be to install two buck-boost transformers on the solar array circuit to drop 5%.
How much effeciency would we loose to the buck-boost approach?
Keeping in mind that the magnetizing current is now on the customer side of the meter.

Thanks
 
jaggedben said:
What Golddigger said. Inverter standby power flows through capacitors (to my understanding) so power factor is very low.
Click to expand...

THD is also very high at night, harmonics of about the same magnitude with no fundamental frequency. if it's not a true RMS meter this will cause a discrepancy too.
 
KY Solar said:
I have no idea.
I just used a combination of several CT's along with a data logger.
All measure the same - each about 0.065 Amps @ 250 VAC.
What did you guys have in mind?

Thanks!
Click to expand...

Yeah, measuring volts*amps in this situation is not going to be accurate. You need to put a true watt meter on it.
 
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