GE Industrial Solutions CPS6000 - Manual (Page 191)

CPS6000 –48V Indoor/Outdoor Power Shelf
Issue 21 January 2008
191
Low Temperature Compensation
This feature is disabled by default, and can be enabled only if Temperature Slope Thermal
Compensation is enabled. The following are the associated parameters.
Low Comp Limit: Low temperature thermal compensation can be set from -5°C to 20°C.
The factory default setting is 0°C.
Increase: The low temperature slope (rate of increase) can be set from 1mV to 10mV in 1mV
increments. The factory default setting is 3mV.
The QS840A has a flexible Thermal Compensation feature which provides voltage
compensation from that level established by the Plant Float Set-Point (fsp) or Boost Set-Point
(bsp), dependent on the highest temperature monitored by the QS873A VT-Probes located at
the plant batteries. Thermal Compensation should be used in a plant containing “sealed” or
valve regulated “maintenance free” batteries. Note that Thermal Compensation is
automatically enabled if the system controller detects the presence of a VT-Probe. Refer to
the Installation Instructions for more details on wiring and configuring this feature.
Thermal Compensation lowers plant voltage from the fsp for monitored battery temperatures
which are above the ideal temperature established during configuration as the Battery
Thermal Slope Nominal Temperature (ntt). (The items in parenthesis are the user
configurable points referred to in the graph shown below.) Lowering the plant voltage helps
to keep the batteries at their optimum state of charge while protecting them from thermal
runaway. Thermal runaway is a complex sealed battery phenomenon where, for one or more
of a number of reasons, one or more cells in a string are unable to dissipate the internal heat
generated by their charging current and experience an increase in internal temperature. By
lowering the float voltage as cell temperature increases, the float current is lowered to a point
where this destructive behavior can be avoided. If a cell failure is imminent and the cell
temperature continues to rise above the threshold configured for Battery Thermal Step
Temperature (stp), plant voltage drops in a single step to a level which keeps the remaining
cells in the string from overcharging and being damaged. Refer to Figure 6-2 for a graphical
view of Battery Thermal Compensation and the relationship of its various set points.
The QS840A can also increase plant voltage above that set by the fsp or bsp for colder
environments, again seeking to keep batteries at their optimum charge state. Batteries will
lose capacity as the battery temperature drops below their optimal operating temperature.
Increasing the plant voltage for decreases in temperature causes more current to flow into the
batteries. This results in electrolysis of the water in the batteries. Since this reaction is
exothermic, it also serves to keep batteries warm. This feature results in an increase in plant
voltage, and is required to be enabled during controller configuration.