[RE-wrenches] Trace SWs in Energy Management Mode

Brian Mehalic brian at solarenergy.org
Fri Oct 4 12:04:36 PDT 2013


Have a friend that is on an electric co-op where peak $/kWh is 4x the
off-peak rate.  Peak hours are 6-10 am in the winter and 12-6 pm in the
summer.

He has relocated an older system from his previous house to his new one;
circuits were separated out to sub-panel that is connected to the inverter
AC output.  The reinstallation is very professional.

Two Trace SW4024 in series (04/96)
120/240 Vac Grid connected to AC1 of each inverter
575 Ah battery bank
Two C40s with digital displays, ~ 4.5 kW STC array total
Trimetric

The goal is to not use any utility energy during peak times, unless the low
battery voltage setting is reached, and in general to run as an off-grid
system, minimizing charge time from the grid during off-peak hours as well.
 The assumption (I know it's a tough one, but I'm coming in after the
transition) is that the PV array should at least be able to provide the
bulk of the load; using the grid to charge the batteries to ensure they are
regularly filled and to maximize their life expectancy.  The customer is
also managing loads actively.

It's been a few years, but I used to work on these all the time; however it
was usually off-grid with a genset, or fixing poorly maintained Y2K systems
which were always in float. I'm attempting to use the energy management
mode.

Inverter settings:
Clocks: Time is accurate
Grid AC1 amps AC: 60
Max charge amps AC: 10 (each inverter)

Grid usage: FLT
Set start charge time: 23:00
Start bulk time: 23:10
Set end charge time: 05:00
Set float volts DC: 25.4
Bulk volts DC: 28.8
Absorb time: 1.5 hours
Low battery cut-out: 22
Low battery cut-in: 25.4

Okay, programmed the system, both inverters are inverting, AC1 is blinking
green, looks good.  Then a well pump kicks on, registers 70 A DC on the
Trimetric.  This causes inverter 1 to go into bulk charging and pass
through about 10 additional amps from the grid to the load (inverter 2
continued to inverter about 10 amps for the load).  Inverter 1 soon cycled
into float, then went back to what invertering (amber light) and green
blinking AC1 (which is what I want it to be at).

Had to leave, both the customer and I had other obligations.

He took a picture of the inverter sat 11:15 pm, and they were in bulk,
battery voltage rising...exactly as planned.  But then in the next set of
pictures, taken at 7 am, after the end of the charge time, they were in
float, not inverting, even though, based on the Trimetric readings at 11
(64%, 85A charging), and the fact that charging would have been active all
night, they were not in anyway at a low voltage condition (Trimetric
readings am readings were 85%, trickle charge, 25.6 V).

Something seems amiss.  Any suggestions or insight?

Thanks!

Brian Mehalic
NABCEP Certified Solar PV Installation Professional™ R031508-59
IREC ISPQ Certified Affiliated Instructor/PV US-0132

PV Curriculum Developer and Instructor
Solar Energy International
http://www.solarenergy.org
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