MX60 Max Array Voltage? [RE-wrenches]

Tue Jul 1 17:25:04 PDT 2003

Ray,

There is no problem putting an SP70 in series with two SP140s in series. It
is simply half the number of cells in series as an SP140. Same cells so you
have a 180 cell string instead of a 216 cell string (that you would get with
3 SP140s in series). Looks is the only reason for not doing it.

Bill.

-----Original Message-----
From: Ray Walters [mailto:remotech at taosnm.com]
Sent: Tuesday, July 01, 2003 4:30 PM
To: RE-wrenches at topica.com
Subject: RE: MX60 Max Array Voltage? [RE-wrenches]

SOoo, what I'm reading here is that its not quite UL listed (which isn't a
problem for this job) but it should work. And if conditions are just right
and it does go a volt or 2 over 140 it won't matter because there is plenty
of headroom. What is the automatic shutoff set at? 145 Voc? What is the
ultimate Voc that the transistors start blowing? You mentioned it varies,
what is the lowest you've seen? Are there other factors like array current
or controller temperature? Since the controller isn't switching, when does
it see Voc? As it tapers the finishing charge? Should we put in a dump load
to spare the controller at finishing charges?
  Which Sharp modules were you using 3 in series? The 185 shows a Voc of
44.9, which would be 6.3 V higher for the series total  than the modules I
was considering. Also, a look at IV curves confirms Bill's concern of
higher Voc at higher irradiance.
I know the different modules in series idea is generally problematic, but
we're talking about SP140s with an SP70. They're identical in all respects
but voltage (Same cell, Asc, Apmax, etc). This is why I'm asking, its a
tough call: mix different modules, suffer some significant line losses, or
push the envelope on the new MX60 at my expense. (which We are willing to
do as long as I'm not voiding the warranty too)

I love this stuff,

Ray

At 06:58 PM 7/1/2003 +0000, you wrote:
>Ray Walters wrote:
>
> > What is the collective experience with running the MX 60 with a high
> > voltage array to a 48 vdc nominal battery bank. The manual shows a Voc
> > max of 140 which looks like I could go with a 72 V nom. array. I'm using
> > 140 watt modules with a Voc of 42.8 - Times three puts me at 128.4 Voc
> > at 25C. I know I need to add some Voc for cold weather, but my
> > experience is that if the sun is shinning on the modules enough to
> > produce power, it will produce heat too. I usually have measured about
> > 30 to 40 degreesF higher cell temp than ambient. I know air speed
> > humidity etc play a part too. Basically, The lowest temp I would
> > anticipate in good sun would be 20F or -7C. Using Outback's chart (1%
> > increase inVoc for each 2.5C below 25C) and assuming a 20F warmer module
> > (cell temp=40F = 5C) then I should add 8% to 128.4Voc which would be a
> > Voc max of  138.7F. Boy that's close.
>
>We have had OutBack MX60s running with three 24 vdc sharp modules in
>mild climates for over 8 months now without any problems.  As far as
>installations in cold climates - we will find out this winter I guess...
>The newer MX60s do record internally the maximum open circuit voltage
>ever seen within the microprocessor itself - so we can conduct autopsies
>if required.
>
> > What happens Chris, if the array does exceed 140Voc? Smoke and fire, or
> > temporary shut down?
>
>The 140 vdc limit has additional headroom still in it - we also will
>turn off the controller if the voltage gets too high and will wait until
>the modules warm up and the open circuit voltage drops.  We have tested
>the MX60 well above the 140VDC limit and have found the limit varies
>slightly between units - when you reach the ultimate limit the
>transistors fail - but not much exciting happens other than the
>controller won't work.
>
> > Did I mention the array will be over 300 ft away? Now I know supposedly
> > Voc is open circuit and that means no current therefore no voltage drop,
> > but wouldn't the meter still see some discernable volt drop in real
> > world conditions? ( meter has to pull at least a few micro amps to
> > measure voltage) Is the controller switching the array off temporarily
> > to check Voc every few seconds? Will the Voc measured at the controller
> > for this split second really be the same as at the array or would it be
> > a volt or 2 less?
>
>We DON'T check the open circuit voltage every 8 seconds - we use a real
>maximum power point tracking method where we actually measure the power
>being produced by the PV array and maximize it by moving up or down with
>the operating voltage.
>
>The open circuit voltage will be the same at the controller as the PV
>array 300 feet away - no significant voltage drop will occur.
>
> > Also, in the MX 60 Manual, they show a chart with various array voltages
> > labeled 40, 65, and 95 V Nominal. Is that a misprint and should that be
> > labeled V at Max Power instead?
>
>These values on the performance graph were just the simulated PV array
>voltage that we used for the test conditions for efficiency
>measurements.  The charts for system design/wire sizing are included in
>the manual have "real" PV array voltages - such as 15V, 30V, 60V and 72V
>nominal.
>
> > We're even considering running two 24 v modules in series with a 12 v of
> >  similar current rating, but the racking and look would be awkward.
>
>Mixing PV modules types or models within the same series string is not
>the best option in my experience (if that is what you meant...).
>
>Christopher Freitas
>OutBack Power Systems, Inc.
>cfreitas at outbackpower.com  www.outbackpower.com
>Tel 360 435 6030  Arlington WA USA
>
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