[RE-wrenches] array overloading of an inverter

[Chris Mason]

In high temperature locations such as the Caribbean, where we are, the
modules run at about 40C during midday. Combine the temperature derate with
losses for dust, cabling, azimuth and tilt, it makes sense to add ten to
twenty percent more PV.
Given that the string design is never perfect, I would rather go a little
over than way under. When we were installing 240W modules on European Sunny
Tripower 20KW units using 1,000V PV designs, we could either go with three
strings = 18KW or 4 strings = 24KW. The three strings would have seriously
underpowered the inverter.

On Wed, Jul 13, 2016 at 3:02 PM, Kirk Herander <vtsolar at icloud.com> 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 <http://www.vermontsolarnow.com>*
>
> dba Vermont Solar Engineering
>
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>
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>
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-- 
Chris Mason
NABCEP Certified Solar PV Installer™
Solar Design Engineer
Generac Generators Industrial technician

www.cometsolar.com <http://www.cometenergysystems.com>
264.235.5670
869.662.5670
Skype: netconcepts
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