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Wrenches, Here's one of those threads to save if you expect to do large multi-inverter battery-based systems. This is a thread from last August, with recent updates. Bottom Line: in a large system with Sunny Islands, use a 1,000A/50mV shunt, not a 1000A/100mV shunt. It's counterintuitive. Allan On Tue, Jul 31, 2012 at 6:37 AM, Allan Sindelar <al...@positiveenergysolar.com> wrote:
Hi Allan et al: Twenty years ago I called Deltec and talked to them about amp
ratings for shunts-- at that time they said that they never tested
them themselves-- they just made them "like other manufacturers
did", then assumed the ratings they were OK. My impression is
that the "67% rule" came out of a hat, not out of actual
measurement.
The basic rule is that the shunt material (plates between the
brass blocks) is and alloy called manganin, (designed for very
stable resistance under changing temperature.) should not exceed
140 degrees Celsius (not Fahrenheit) . Incidentally, there is
surprising disagreement on what this temp should be-- I trusted
most a German company that sold the manginin material.) I
actually measured some shunts and their temperature rise and found
the 67% rule to be VERY approximate.
The temperature rise for one given shunt is going to be
proportional to the square of the current, so the temp rise at
100Amps is 30 degrees, for 200 amps it would be 120 degrees. The
bottom line for this is that it is only when the currents get
really high could you seriously overheat them. Some may remember
some so called "200% rated" and even "300% rated" shunts in "the
old days" which were hardly different in size from the 100% rated
ones. Don't know what they were smoking when they came up with
these.
So if you really want to know how much a shunt can handle,
measure the temperature rise right in the center of the plates,
with a steady 200Amps. You can pretty much assume that the temp
rise above that will be proportional to the square of the current.
You don't want the temp in the center of the plates to go above
140 C.
Frankly, maybe you guys know better, but I can't imagine ever
going above 500 amps in any system-- maybe, if systems are really
getting that big, people should be considering going to 96V
systems if they have power requirements that high. I would be
really surprised if any shunt rated at 1000A could really go that
high, continuously, without overheating. (But I could be wrong!)
Lastly, if there is really a demand, we could modify the
TriMetric so it would work for 1000A/50mV shunts-- this would at
least reduce the shunt heating to 50% compared to 1000A/100mV
shunts--and possibly even more. Another possibility for very large
systems could be to provide a cooling fan for the shunt, which
could come on when the amps get high. That should would increase
the rating a good bit. You could use it instead of a 500A/50mV
shunt with a TriMetric (or other similar amp measuring device) but
you get readings 1/2 of the true reading-- so would need to
multiply it by 2 to get actual reading.
Incidentally we sell some "1000A/100mV" Deltec shunts that
we've never measured the capability of. My GUESS is that it might
do 600-700 A-- since the much smaller 500A/50mV shunts are OK to
about 400 A. 1000A only for a very short time. It's about 7
inches long.
Here's a picture (Warning-- it is very high resolution- and may
take forever to load unless you have a fast internet connection)
Yours truly,
Ralph Hiesey, Bogart Engineering
From:
Allan Sindelar [mailto:al...@positiveenergysolar.com] This request for
a product development will come up with four or more Sunny
Islands. With the newest 6000W model, it may even be advisable
with a pair of inverters, or three in 3-phase. SMA tells me that
the SI programming allows selection of a shunt ampacity of
100-1000 amps in 100A increments. However, the only mV choices
are 50mV or 60mV. So for this application, we need to work with
a 1000A/50mV shunt. -------- Original Message --------
I am just
forwarding this to you for your info, it sounds like a
100MV shunt can be used, but they do not recommend it
based on the Email below and it makes sense. Will you
update me when you get word when you can send me the 50Mv
1000Amp shunt. I can send back the one I currently have. From:
Dane Forsberg [mailto:dane.forsb...@sma-america.com] I received an email back from my
colleagues in Germany regarding the battery shunt. The
Sunny Island’s measurement range is 0-120 mV in order to
allow for peak currents (2 x 60 mV). Since it may be possible that you also
see peak currents that are greater than 1000 A, they did not
recommend that you use the 100 mV / 1000 A shunt. Best Regards, Dane Forsberg Senior Technical Service Specialist, Hybrid Energy Solutions 6020 West Oaks Blvd, Suite 300 Rocklin, CA 95765 - 3714 U.S.A. Service Line: +1 877 697 6283 Fax: +1 916 624 2445 Email: dane.forsb...@sma-america.com www.SMA-America.com |
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