Advise sought for large off grid PV system; 480 volt 3-phase [RE-wrenches]

[Larry Crutcher]

Hi Jeff, Thanks for your input. I was told by outback that they can only support 208/3-phase with one inverter per leg. How can you get 32.4kW? 

 

Since the distribution system is already there and trenching of undisturbed soil may not be an option, I have to work with what I have. There are six 480 volt transformers ranging from 25 to 117 kva. That can not change either. If someone made a 480 volt inverter, I would be camping at their doorstep.  I am looking real hard at the Exeltech MX inverters: 60kW, 3-phase, 208. Several people have reassured me (off list) that highly efficient step-up transformer to make the 480Y will not be a problem; perhaps 3% loss there.

          

So far, the design I have will not be paralleling the generator. An isolated PV/bat/inverter system will take over all loads. The generator will only operate to support the load demand through high current chargers. A transfer switch will send loads back to the generator when necessary. The problem with this, as I see it, is not getting 100% use of the PV. There are only two circuits leaving the generator building to the village. I could split this and use the PV for the lesser loads during summer but this could be problematic. 

 

I really appreciate all the info so far. People on this list are extremely helpful, kudos to you.



Larry

  ----- Original Message ----- 
  From: Jeff Oldham 
  To: RE-wrenches at topica.com 
  Sent: Thursday, March 29, 2007 2:53 PM
  Subject: Re: Advise sought for large off grid PV system; 480 volt 3-phase [RE-wrenches]




  I'm sorry that I don't have more time to go in depth on this, it is indeed a complex issue that I have and continue to deal with. A few comments -
  Transformers can be killer if they mount in numbers much and I've found wire to be cheaper than tare losses. As a very rough rule of thumb figure on at least a 3% loss at each transformer and a 1% tare loss 24/7 even without a load. Large transformers can also give fits to inverters. Gensets have a limit to a backfeed from inverters and other sources. The rule of thumb on this one is 30% of genset rating. For your 60kW units this is only 20kW of PV.
  AC coupling gensets w/PV is a tough nut on efficiency, they won't couple unless the genset is running or you have additional battery based inverters to couple to. If the genset is running as it seems it will be here, the battery based inverters with transfer and charge the batteries and in most cases fully charge them before the PV had much chance to do so because the genset is likely producing double the charge rate of the PV. It can be sickening to see the PV at Float by noon and the gensets running all morning!
  The o-t-g world is in serious need of large battery based inverters THAT ARE EFFICIENT to tackle these challenges. As far as I know only Outback fills the bill with the key word "efficient". Outback is limited to 10 inverters in a single (120/240 split) phase for 36kW and 9 in 3 phase 208VAC for 32.4kW. The only way to make it work is as others have suggested - break up the loads and distribution, maybe point-of-use smaller systems. In addition, you should not connect more than 10 Outbacks to a single battery bank. 
  Good industrial batteries are cheaper than diesel and pollute less while increasing the life of the gensets and holding them at optimum loading.
  I'm 4 years into a 17,000 s.f. o-t-g "home" in Mexico, I'm using 28 Outback inverters on 3 battery banks and 3 seperate distribution lines and 3 66kW Whisperwatt gensets. The EE designed the original 400M distribution system w/transformers and 3 phase 480VAC like a good EE would, but when I demonstrated the economics of spending a small fortune on copper to save a massive fortune on solar he jumped on a transformerless distribution system! The project has tripled it's load since my original design a few years ago. I've spent 90% of these 4 years controlling loads, I've specified 98% of all appliances, motors and misc. loads and have given my blessings to the rest. I don't think we can make this palace any more efficient.
  I'm echoing the others here and the Wreches Mantra - put most of your efforts into efficiency and controlling loads, THEN look at solar and other RE's.
  Good luck and keep us posted....
  -jeff o

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