[RE-wrenches] 3 Phase With 2 Inverters?

[Dave Click]

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, penobscotsolar at 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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