We were called in on a system similar to this, and we ended up just
installing an AC transfer switch. Normally the GT array sold back to
the grid side of the system, but in an extended outage, the customer had
the option to switch the connection to the back up load side (AC coupled)
He was made aware that battery regulation could be an issue, and to
switch off the GT array when the batteries got even close to full.
Obviously this would not be acceptable in a full time off grid system,
but this option could save your bacon in the very occasional long outage.
It boils down to: Is it better to let the batteries remain discharged
and the customer to have no power, or take a chance on over charging the
batteries in an emergency?
Essentially, the system would remain full proof until they manually
switched in the AC coupled system. I look at this sort of like any
other manual over ride switch: don't use it unless you understand the
consequences, and really need it. Add strongly worded warning stickers:
" Danger Battery damage could result, Emergency use only!"
The other options you are considering would certainly be better, but at
significantly more cost.
R.Ray Walters
CTO, Solarray, Inc
Nabcep Certified PV Installer,
Licensed Master Electrician
Solar Design Engineer
303 505-8760
On 1/30/2016 3:21 PM, Allan Sindelar wrote:
Wrenches,
I am serving as a consultant for a homeowner with an array of solar
equipment that doesn't do what he wants, a common story. He wants
independence in the event of an extended utility outage. To cut to the
chase:
He has one array of 3,100 watts feeding through a controller into a
48V battery bank. No issues with this.
A second array of 4,230 rated watts (2 9-module strings of 235W Sharp
modules) currently feeds a Fronius IG-4000 inverter and the grid. In
the event of an outage this system shuts down, of course. He is
looking at replacing his existing pair of old SW5548s with a single
Schneider XW system, or possibly a Radian.
I would like to give him the ability to switch from the Fronius
inverter to a 600V charge controller if either a) the grid went down
or b) the Fronius failed.
I looked at Morningstar's TS-MPPT-600 controller with the DC transfer
switch, but its DC output is rated as limited to 60A, less than the
4.2kW output of the array. The spec sheet gives a nominal maximum
wattage of 3,200W, although a note adds that "input power can exceed
Nominal Maximum Operating Power, but controller will limit and provide
its rated continuous maximum output current into batteries. This will
not harm the controller."
I looked at Schneider's XW MPPT80 controller, which has the necessary
higher capacity. But beside lacking any built-in or optional input
metering, it lacks the transfer switch available with the Morningstar.
So questions:
- Is there a good transfer switch to use with the Schneider unit, to
allow manual switching of the PV input between the Fronius (when the
grid is up) and the controller and batteries (when the grid is down)?
- Should I use the Morningstar unit, recommending that the periodic
flatlining due to the excess wattage be simply accepted as a small
concession to being able to use the array during an extended outage?
- Is AC coupling simply a better approach?
- Any other solutions?
Thanks, Allan
--
*Allan Sindelar*
al...@sindelarsolar.com
NABCEP Certified PV Installation Professional
NABCEP Certified Technical Sales Professional
New Mexico EE98J Journeyman Electrician
Founder (Retired), Positive Energy, Inc.
*505 780-2738 cell*
**
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