I would agree that the imbalance at these power levels is not a major
issue. If it appears that Phase A is most heavily-loaded, I'd install
the inverters as A-B and A-C to help mitigate the existing imbalance. If
you've got a lot of 1P loads on a 3P service-- lighting, office
receptacles or whatever-- you could easily have a large imbalance on a
regular basis. The 7kVA PV imbalance in your installation would happen
extremely rarely.
If this imbalance still makes you uneasy, then install three inverters
and then a relay system to bring all the inverters down in case any of
them fails. The SMA Power Balancer does this for their inverters and
maybe some third-parties offer them too for these Fronius installs. This
seems to make the most sense for decentralized systems with many string
inverters, to ensure that a low phase voltage doesn't turn off a
fraction of your inverters and cause a >100kVA imbalance.
I've heard that some utilities have a max PV imbalance (calculated from
the inverter ratings) of 6kVA. No utility to my knowledge requires an
interconnection agreement for a 10kVA single-phase commercial water
heater (to steal a Bill Brooks example) that would have a much stronger
impact.
Dave
On 2012/11/27 15:14, penobscotso...@midmaine.com wrote:
Hi Holt,
We installed a commercial system a year ago for the National Park
Service that was designed by a competitor (?) and signed off on by an
engineer. It consisted of 13.2 kw of pv and two Fronius 7000 watt, 208
3 ph. inverters. It sounds a lot like what you are dealing with. I
questioned the engineer on it and he said because the facility was so
large (it was a large research and visitor "campus" for Acadia National
Park with 20+ buildings)the transformer would never see the imbalance
and therefore was not a problem. Fronius agreed, but prefaced that with
the fact that it still was "not recommended". I can say that we have
had no issues or complaints at all since the installation in August of
2011 and they monitor it continuously. It would have been a far better
design to use three Fronius 4.5 inverters at 3 ph. I drafted a letter
(CYA letter) to the powers that be about my concerns (transformer
imbalance) and they chose to go ahead with the installation. Nothing
like government work.......many more stories about this installation
than anyone has time for here.....
Daryl DeJoy
NABCEP Certified PV installer
Penobscot Solar Design
We were asked to inspect a GT system installed by others. Service is 208
3ph with two (?) Fronius IG 4500 208v inverters. The inverter panel is 3ph
with two 2pole breakers - one each side of the buss bar in a A/B, B/C
configuration (??). Called Fronius and they said it would work, but they
"don't recommend it". No other information was given as to why they don't.
My electrician is very uncomfortable with doing work on this system as is
(ME TOO!), so I am wondering if I might be able get some ammo to convince
system owner to add a third inverter to get the system right. What
problems might/will be encountered in this system if not modified? BTW -
inverter LCD screens were crapped out, so we couldn't readily see if
system was functioning properly....from metering, we think not.
Holt E. Kelly
Holtek Fireplace & Solar Products
500 Jewell Dr.
Waco TX. 76712
254-751-9111
www.holteksolar.com
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