[RE-wrenches] Data System Puzzle

[vince at solarenergydesign.com]

Sorry for the long post. I don't know how else to communicate this issue.
We installed an Energy Recommerce data system on a project last year that
we have not been able get working properly. Everything works except the
monitoring of the series string amperage outputs from the solar array.
There are 3 pv source circuit combiner boxes with 11 series strings of 14
modules and 1 with 10 series strings of 14. Each combiner box feeds one
inverter with its own conduit run. The data system shows amperage from
each circuit consistent within each combiner box, typically with a range
of variance of about 1.5 amps DC between series strings.  Each combiner
box will show totally divergent amperages. For example, combiner box 1
might read 2 amps, combiner box 3 reads 10 amps, and at the same time our
clamp meters are reading the actual amperage at around 5 amps consistently
between every box for every circuit. The same combiner boxes will
consistently read either too low or too high. The problem may be in the
interface between the Hall Effect sensors that the circuit board in the
combiner box uses to measure the DC current. The Hall Effect sensors
deliver a calibrated voltage based on the strength of the magnetic field
around the series string circuit as it goes through the box. The combiner
box then calculates the amperage based on the millivolts that the Hall
Effect sensor produces. For example 500 millivolts equals 5 amps.  The DC
output voltage of the Hall Effect sensor varies by about 50 millivolts or
more, so rapidly that it’s hard for our meters to follow it. We have spent
countless hours on the phone with Energy Recommerce technicians trying
everything that anyone could think of to trouble shoot the problem. Their
system uses a MODBUS protocol for transferring the data and there seems to
be no problem with the data transmission or connectivity.  I would love to
hear from anyone that sees a pattern to this problem that might help us
find a solution.