[RE-wrenches] DC to AC derate

Thu Jan 5 18:58:57 PST 2012

Ray, 

I think a few people have already chimed in. You can't use a balancing
transformer because in fact you already have a balancing transformer. Mine
is on the pole next to my house. Some are planted in the front or rear yard,
etc.

If you have an outback X-240, now you sorta have two in parallel (one little
4kw unit and one 25kw or 50kw version), that is not likely a good idea.

Another thing to note, if the first outback sells a few hundred watts, it
wakes up the other sleeper to help. I can't remember the exact figure, so I
looked it up. Each GVFX3648 can sell about 3300 watts, and using the Power
Save feature set P01 (the master to 0 [zero] and set P02 (the slave) to 0
for, I guess, sell all the time, 1 for start to help on or before 15 Amps
AC, 2 for start to help on or before 24 Amps. Maybe even a setting of 3
would delay it even more??? I am not sure. In my home system I only have one
GVFX inverter and one XW inverter, so I can't try it to be sure.

For the sharp modules, I would use a clamp on dc meter to measure amps from
each string at the combiner during full sun and verify that all strings are
a) equal, b) high enough (shoot for 80% of nameplate for 
instance). I have found some strings to be lacking and moved modules around
to get an optimized array. With the string breaker off, you can measure the
individual pv module performance (kinda) with an incandescent bulb and some
test leads in the junction box [or add a dc switch for MC cable]. Of course,
ground mounted arrays are worth their weight in silver for this aspect
alone.

Maverick

P.S. Of course, I think of some weird things at times... with one inverter,
if you have local loads on the red leg, while selling on the black leg,
doesn't the utility transformer run your power back through their meter $$$?
Hopefully, the meter zeros out this cross flow... I digress...

  _____  

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Ray Walters
Sent: Thursday, January 05, 2012 3:34 PM
To: RE-wrenches
Cc: Levi, Dean
Subject: Re: [RE-wrenches] DC to AC derate

I have a related question to this. Does setting up a pair of Outback GVFXs
with the balancing transformer and allowing one inverter to go to sleep at
lower AC output improve efficiency?  It seems we would be reducing
electronic loads, but adding transformer losses.   Any comments? I have a
recently commissioned double Outback system with 200 AH @48v battery bank.
He's reporting efficiency at 77% DC in to AC out.  I'd love to improve that,
as his aging array of Sharp modules seems to be under performing as well,
(4.08 KW STC rated,  3 Kw actual out)  I haven't gone back and done testing
with my insolation meter, so I'm not sure exactly what is happening, but his
overall efficiency (STC rated to AC out) is only 56% .

Ray

On 1/4/2012 7:55 PM, Maverick Brown [Maverick Solar] wrote: 

Please use the 90% of the CEC or PTC rating of the PV module at most.

Among several systems that I monitor daily, I have an example 11.760kw (STC)
system that has a 500Ah battery bank and it peaks at 77-80% of the STC
rating each day.

Peak is defined as the top of  the bell curve that forms from the graphing
watts output versus nameplate watts.

If I use the CEC rating (10.54kw), it peaks at 86-90% of the CEC rating each
day.

If I go to the CEC list and copy & paste into a spreadsheet and enter the
values for the STC versus PTC, I might  find I would loose 10% (or more) in
the PV rating.

http://www.gosolarcalifornia.org/equipment/pv_modules.php (go ahead, find
your modules and divide PTC by Nameplate).

I also have a grid-interactive system that has a very large battery bank
(2600Ah @ 48V, with 9kw STC Array) and it only has a peak efficiency rating
of 69% because of the load of the batteries. The larger the battery bank,
the happier customers are for outage situations, but the larger the battery
bank, the more current goes into "Floating" the batteries during selling.
For that system, I may lower "Grid Support/SellRE" and use Auto EQ to keep
the batteries fresh...

Anyway, keep in mind that you cannot simply assume:

12kw Array x 6 hours of sun =  72kwh of generation each day. The brand of
PV, the size of the battery bank and even the charge controller's Absorb &
Float settings can greatly reduce what the inverter thinks it can sell. With
multiple inverters and charge controllers it gets even more complicated
(worse).

Good luck,

Maverick

  _____  

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of
boB at midnitesolar.com
Sent: Wednesday, January 04, 2012 5:14 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] DC to AC derate

On 1/4/2012 3:01 PM, All Solar, Inc. wrote: 

Wrenches,
Once again I am trying to size a grid tie w/battery system and I struggle a
little with the efficiencies of them.
With PV Watts (v.1), is changing the DC to AC derate an accurate method for
getting the output?

Thanks in advance!

Jeremy

If you keep the batteries charged  to a voltage just above their resting
voltage,
say, 52 volts for a 48V system, and the battery bank isn't too large, (~225
A-hours,
maybe a bit more), about 90%  PV to grid AC efficiency is in the ball park.

That's what we figured in the early days of GFX inverters at OB.  That's
giving
5% for the electronics and 5% for keeping up the batteries.
Different and/or more accurate information may have come along since
that time.

boB

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