MX60 Max Array Voltage? [RE-wrenches]

Christopher Freitas - OutBack Power cfreitas at outbackpower.com
Tue Jul 1 18:06:17 PDT 2003


Ray Walters wrote:
> 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? 

Yep - it is set at 145 VDC +/- a little.  

> 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?

The failure voltage of the FETs is not that easy to come up with without 
having a PV array that can be set incrementally.  Doing it on a power 
supply isn't the same - it has a lot of output capacitance to "nail" the 
FET with once it starts going.  A PV array at open circuit by definition 
has no output current until the voltage starts to drop - so it is a 
little different.  

Technically, when the controller has been shutdown the transistors 
should be able to stand-off 300 vdc or more.  The input capacitor 
however is 160 vdc rated - so don't get too excited. 

>   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.

We used the Sharp 165W modules - 43.1 VDC open circuit x 3 = 129.3 vdc. 

The published IV curve data is with the PV cell being keeped at 25C or 
is done with a "flash" type tester - not real world conditions.  My 
experience says it doesn't increase much - but I could be wrong or maybe 
some don't and some do...  

> 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)

We added the recording of the maximum open circuit voltage to allow us 
to warrant any failures below the 145 limit - we already have had our 
share of "oops - I connected 168 VDC to it and it doesn't work now" 
calls.  

We will be doing more testing to qualify the compatibility with various 
modules as we can.  So far the testing says no problem - even with the 3 
in series sharp PV modules being sprayed with cold water (40-50F).  Its 
hard to tell however at this time of year - Maybe I can convince Robin 
to let me hang out at the top of nearby Mount Baker to do some 
testing... yah... that's the ticket...    

> I love this stuff,
> 
> Ray

Thanks - we do also...  

Christopher Freitas
OutBack Power Systems, Inc.
cfreitas at outbackpower.com  www.outbackpower.com
Tel 360 435 6030  Arlington WA USA

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