Jamie,
The 6kVA imbalance limit, as you point out, is for 120V/240 split phase services, not 3-phase. It comes from California’s Rule 21 interconnection rules adopted by each California utility. As the technical facilitator of the development of Rule 21, I had a lot to do with the 6kVA number. It was based on the largest 120-Volt inverter that could be placed on the smallest available service. It JUST happened to coincide with the size of my Trace Engineering SW5500 inverter—funny how that works out. Bill. From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Jamie Johnson Sent: Friday, December 10, 2010 12:14 PM To: RE-wrenches Subject: Re: [RE-wrenches] feed-in imbalance Marco, I believe some of the discussions in the past had to do with the old 6kW imbalance limit that is used by several utilities on a 120/240 small residential single phase transformer. (don't remember the CA utility that came up with this) I had this discussion with our local utility (FPL) recently and they look at it on a case by case basis due to the many variables involved with 3 phase power distribution, such as the size and type of transformer(s), 3 phase loads on the premises, the harmonic distortion if any on a shared 3 phase transformer, core saturation, load balancing, etc. If you install a system in the FPL service area that is not balanced and either the harmonics or voltage is out of spec as a result (yes, they do test them), then you basically have 2 choices, balance the system or pay to upgrade the service drop and transformer. There has been talk of testing various 3 phase transformer(s) to determine the maximum/safe out of balance configuration by our local utility, however, again there are a lot of variables, so it would be difficult to cover every scenario. And this usually is only an issue on small 3 phase transformer(s). I think that the 8,9,10TL 208v inverter has a load limiting ability that is incorporated in the inverter when you install 2 or more inverters, and it is programmable to limit the output of a single inverter to 6kW. I believe you have to purchase the additional communication cables from SMA (cables available after the first of the year? or so I have been told). Also, the design parameters seem a bit limited on the TL's. Jamie -------- Original Message -------- Subject: [RE-wrenches] feed-in imbalance From: "Marco Mangelsdorf" < <mailto:ma...@pvthawaii.com> ma...@pvthawaii.com> Date: Fri, December 10, 2010 2:00 pm To: "'RE-wrenches'" < <mailto:re-wrenches@lists.re-wrenches.org> re-wrenches@lists.re-wrenches.org> Now that SMA has come up with their new 8, 9 and 10 kW transformerless line of single-phase 120/208V inverters, it brings up the question once again about how practical and wise it is to have one or two of these inverters feed into a three-phase service. That is, is such an unbalanced feed much, if any, of a big deal? I spoke to my consulting E.E. about this and he said that the large majority of utility-provided transformers are so large these days that an unbalanced feed of, say, a 10 kW inverter that only has output power through L1 and L2 should not present a problem. Any other thoughts out there on this matter? Thanks, marco _____ _______________________________________________ List sponsored by Home Power magazine List Address: <mailto:RE-wrenches@lists.re-wrenches.org> RE-wrenches@lists.re-wrenches.org Options & settings: <http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org> http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: <http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org> http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org List rules & etiquette: <http://www.re-wrenches.org/etiquette.htm> www.re-wrenches.org/etiquette.htm Check out participant bios: <http://www.members.re-wrenches.org> www.members.re-wrenches.org
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