[RE-wrenches] P1 micro performance

William Dorsett wmdorsett at sbcglobal.net
Fri Mar 22 07:52:17 PDT 2013


OK, if the upper limit if the curve is "flat topped" do we get increased
problems with harmonic noise at the knee as you would in modified "square"
wave?

 

Bill Dorsett

Manhattan, KS

 

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of David
Brearley
Sent: Friday, March 22, 2013 8:43 AM
To: RE-wrenches
Subject: Re: [RE-wrenches] P1 micro performance

 

Thanks for the clarification. 

 

FWIW: Flat topping is exactly what occurs. Inverter limiting clips the
inverter output power curve (not the voltage or current wave forms). The
chart below has one data point for every hour of the year. The
clipped/flat-top area is the result of the 225 kW inverter limiting the
power output of a 385 kW array: 

 

 



 

On Mar 22, 2013, at 1:07 AM, boB wrote:





On 3/21/2013 9:59 PM, Exeltech wrote:


Wrenches,

I'm probably a lone voice on this .. and not intending to get overly picky.



No, two lonely voices, Dan.

I associate clipping with audio waveforms which stops
the negative or positive voltage peaks flat.  Also called
flat-topping.

Limiting is like turning down the volume.  The waveform
stays the same and does not distort as it would if it
were being flat topped (and flat bottomed)

Thanks !
boB









Could we call power limiting what it is .. "limiting", and not "clipping"?

Clipping implies distortion, which isn't the case here.  Limiting is just
that.
The inverter output is limited to some maximum value -- not "clipped".

The output power curve flattens when integrated over time, but this still
isn't
distortion in the waveform.  It's simply a point in the output where the
derivative
is zero.  Not increasing, not decreasing.  Just .. zero.  No additional
increase
in the output for an increase in available energy at the input.  Think
"governor"
on an engine....

Thanks.


Dan Lepinski, Sr. Engineer
Exeltech / Exeltech Solar Products


--- On Thu, 3/21/13, David Brearley
<mailto:david.brearley at solarprofessional.com>
<david.brearley at solarprofessional.com> wrote:




From: David Brearley  <mailto:david.brearley at solarprofessional.com>
<david.brearley at solarprofessional.com>
Subject: Re: [RE-wrenches] P1 micro performance
To: "RE-wrenches"  <mailto:re-wrenches at lists.re-wrenches.org>
<re-wrenches at lists.re-wrenches.org>
Date: Thursday, March 21, 2013, 11:37 PM

Thanks for sharing the screen capture, Marco.

 

Interesting issues to think about here. This is actually prime clipping
season in many places (not sure about Hawaii) due to the cool weather. While
there are more sun-hours in the summer, the cell temperatures are often high
enough that you won't tend to see rated power out of the modules. 

 

While I'm not running performance models for work, the people who do are
routinely increasing dc-to-ac ratios, often as high as 1.4-to-1. Having said
that, most inverters aren't installed on a roof. (Not yet anyway.) 

 

I'd probably lean to a more conservative sizing ratio for micros. While I
can imagine some scenarios where I'd be comfortable with a 215 W micro on a
265 W module-like a flat roof install in Vermont, which reportedly doesn't
see 1,000 W/m^2 very often-I wouldn't try that here in Texas.





 

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