[RE-wrenches] array overloading of an inverter

Solar dahlsolar at gmail.com
Sun Jul 17 17:25:36 PDT 2016


This thread show how this idea is really a site by site issue. Two winters ago we had something like 40+ days with temps below zero (-45F the coldest)  while in the summer temps tend to be near STC very often.  My Tigo system at home shows the DC side of my system (210W modules) producing 220-250W regularly during those cold winter months. I try take that into account when designing a system. 

Jesse Dahl

NABCEP PV Installation Professional
IBEW Local 292 - Electrician 
Electrical/Solar PV Instructor - HCC

Sent from my iPhone

> On Jul 13, 2016, at 3:37 PM, Kirk Herander <vtsolar at icloud.com> wrote:
> 
> Of the many Enphase systems I've done, clipping is a regular issue in
> wintertime Vermont, especially with the older micros whose output wattage
> ran way below the max. panel wattage. I've seen clipping for 3 hours or more
> on super-cold,  crystal clear days, with customer complaints about not being
> able to harness the peak . The DC:AC ratio of greater than 1 may also not be
> advantageous when using module based electronics, ala Solaredge, since the
> claim of 5 -7 % greater dc to ac conversion over a string inverter appears
> to hold true in my experience. You may be already getting the gains which
> overloading will accomplish, making overloading unfeasible economically. 
> 
> -----Original Message-----
> From: RE-wrenches [mailto:re-wrenches-bounces at lists.re-wrenches.org] On
> Behalf Of Matt Partymiller
> Sent: Wednesday, July 13, 2016 4:16 PM
> To: RE-wrenches
> Subject: Re: [RE-wrenches] array overloading of an inverter
> 
> Kirk,
> 
> I would recommend you consider using a tool like PVsyst or Helioscope to
> model DC:AC ratios.  A quick modeling of roughly 200 kW on three Solectria
> 50 kW inverters shows about 1.3% clipping here.
> 
> It then comes down to analysis of your costs for additional modules, dc
> wiring, and inverters.
> 
> Admittedly I sit on the opposite side of the fence.  I try to provide at
> least a 1.2 ratio to minimize cost per kWh.  I am not aware of any large
> scale studies comparing PVsyst or Helioscope clipping projections to real
> world performance.  I can tell you that our arrays with monitoring and a
> ~1.2 DC:AC ratio perform as well as our older 1:1 systems.  Not that I use
> Enphase often but they have some research/whitepapers on the topic since
> their micros tend to undershoot 60 cell module power ratings.
> 
> Good luck!
> 
> 
> Matt
> 
> Matthew Partymiller
> Solar Energy Solutions LLC
> (877) 312-7456
> matt at solar-energy-solutions.com
> 
> 
>> On Wed, July 13, 2016 3:02 pm, Kirk Herander wrote:
>> Hello folks,
>> 
>> 
>> I am composing a response to a question a potential customer asked me. 
>> It  seems a competitor is trying to talk him into a 20% larger KW 
>> array than the inverter AC max output rating is. The idea of course is 
>> to generate more power on either side of peak output, but at a cost I 
>> don't feel is justified. My opinion of this particular big-box 
>> installer I will keep to  myself. My response to the customer, trying to
> keep it simple:
>> 
>> 
>> 
>> "On the DC array input side, most inverters do allow an overload factor.
>> For
>> instance, a 10kw AC inverter may allow for 12 kw of DC array as an input.
>> Whether or not this is a good idea boils down to economics and 
>> technical reasons.
>> 
>> On a sunny day, the inverter generates power as a typical bell curve.
>> Power
>> output rises in the morning, peaks at noon, declines in the afternoon. 
>> In my example, the inverter can't output more than 10 kw AC. What 
>> overloading the input will do will widen the bell curve, i.e. 
>> generating more power in the morning and afternoon, BUT clipping the 
>> peak at 10kw on either side of noon. So there is power to be gained in 
>> morning and afternoon, but peak power is lost(if conditions allow the 
>> peak output to be reached), since the 12 kw array can never be converted
> to more than 10 kw of AC power.
>> Depending upon
>> time of year(ambient / cell temperature) and weather conditions, the 
>> peak may be clipped at 10 kw for several hours a day. So you are both 
>> gaining and losing power using this method. And typically the 
>> inverters are only overloaded in this manner on large-scale farms 
>> where the economics are favorable.
>> 
>> In your case, if you could actually put 200kw of DC array into 150 Kw 
>> of inverter, the economics would never justify it. That extra 50 kw of 
>> array would cost you $100k of more, and the dollar payback for the 
>> power that extra 50Kw would generate will take 2 - 3x the time that 
>> the array size does that stays inside the output limit of the 
>> inverters. This is why I'm not a big fan of dramatically overloading 
>> the inverters, if at all,  in your case. Any KW portion of the array 
>> which is above the nameplate kw rating of the inverter is going to 
>> have a longer payback for these reasons. "
>> 
>> I feel my reasoning is sound, But I don't want to be too loose with 
>> the facts. Comments are appreciated. Thanks.
>> 
>> 
>> 
>> Kirk Herander
>> 
>> 
>> Owner|Principal, VT Solar, LLC
>> 
>> 
>> Celebrating our 25th Anniversary 1991-2016
>> 
>> 
>> www.vermontsolarnow.com
>> 
>> dba Vermont Solar Engineering
>> 
>> NABCEPTM  2003 Inaugural Certificant
>> 
>> 
>> VT RE Incentive Program Partner
>> 
>> 
>> 802.863.1202
>> 
>> 
>> 
>> 
>> 
>> 
>> 
>> 
>> _______________________________________________
>> List sponsored by Redwood Alliance
>> 
>> 
>> List Address: RE-wrenches at lists.re-wrenches.org
>> 
>> 
>> Change listserver email address & settings:
>> http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org
>> 
>> 
>> List-Archive:
>> http://www.mail-archive.com/re-wrenches@lists.re-wrenches.org/maillist
>> .ht
>> ml
>> 
>> List rules & etiquette:
>> www.re-wrenches.org/etiquette.htm
>> 
>> Check out or update participant bios:
>> www.members.re-wrenches.org
> 
> 
> 
> 
> 
> _______________________________________________
> List sponsored by Redwood Alliance
> 
> List Address: RE-wrenches at lists.re-wrenches.org
> 
> Change listserver email address & settings:
> http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org
> 
> List-Archive:
> http://www.mail-archive.com/re-wrenches@lists.re-wrenches.org/maillist.html
> 
> List rules & etiquette:
> www.re-wrenches.org/etiquette.htm
> 
> Check out or update participant bios:
> www.members.re-wrenches.org
> 
> 
> _______________________________________________
> List sponsored by Redwood Alliance
> 
> List Address: RE-wrenches at lists.re-wrenches.org
> 
> Change listserver email address & settings:
> http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org
> 
> List-Archive: http://www.mail-archive.com/re-wrenches@lists.re-wrenches.org/maillist.html
> 
> List rules & etiquette:
> www.re-wrenches.org/etiquette.htm
> 
> Check out or update participant bios:
> www.members.re-wrenches.org
> 



More information about the RE-wrenches mailing list