[RE-wrenches] array overloading of an inverter

[Matt Partymiller]

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
>
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