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re: string sizing for your 2500 watt inverter
18 oct 2002
from: nick pine 
date: fri, 18 oct 2002 11:08:16 -0400 (edt)
to: richard-sma@pacbell.net, techinfo@sma-america.com
cc: mesagts@aol.com, nick@ece.vill.edu
subject: re: string sizing for your 2500 watt inverter

>hello nick,

thanks for the rapid reply, richard.

>as long as you do not exceed the input ratings of the inverter all
>will be fine.

ok...

>the problem is that we have input capacitors rated at 600vdc on the input.

that sounds more like an opportunity. you might put a little inductance in
series, with a simple shunt chopper between to limit the voltage with no
grid connection. or simply short the input when the grid goes down. one more
dumb relay. 

>if these capacitors see greater than 600vdc you may have very undesirable
>results.

sure. btw, it seems unusual to expose a cap with a 600 v rating to 600 v
in a fine german-engineered product. i used to work with siemens. i've
blown up 35 v electrolytics in 24 v power supplies...

>the inverter is also designed according to ul1741 and to be installed
>according to article 690 of the national electric code for use with less
>than 600vdc on the input.

the inverter might assure that, pro-actively. it probably will, under
load, if it's working correctly and driving the grid. seems to me the
nec people can be reasonable and recognize the difference between likely
and rare events. 

can the "varistors on the dc side for surge voltage supression" handle
this infrequent overvoltage condition by providing enough load to lower
the voltage to 600 on a cold day with the inverter output disconnected?

>so we probably will never have a "fix" for the inverter.

please reconsider.

>yes it does make sizing with the sharp panels difficult unfortunately
>this is all we can do at this time.

i like these panels. cheap, with mc cables. that's a large market share
for sma to essentially abandon. what's in the future?
 
>using a thermostat or other means of controlling the voltage of the panels
>when cold is not recommended by us and is probably not very reliable.

how about a load resistor across the whole string? or two of them, for
greater reliability. we only have to short 3 panels to make this work.
how many cycles per year in phila? if we try to short 5 panels with
freezestats, what's the probability of shorting at least 3? this seems
like a kludgey and inefficient approach, compared to building the load
or shorting relay into the sunny boy.

>ultimately it is the system designers decision but over-voltage conditions
>are not covered under the warranty and are stored in the system memory.

aha. threats :-)

>i am sorry for any inconvenience this may have caused.

you might fix this problem, as an alternative.

i hope you will look into it further. 

>richard scott
>sma america
>rscott@sma-america.com
>www.sma-america.com

thanks.

nick

nicholson l. pine                      system design and consulting
pine associates, ltd.                                (610) 489-1475
821 collegeville road                           fax: (610) 831-9533
collegeville, pa 19426                     email: nick@ece.vill.edu

computer simulation and modeling. high performance solar heating and
cogeneration system design. bsee, msee, sr. member, ieee. registered
us patent agent. web site: http://www.ece.vill.edu/~nick 




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