GE Industrial Solutions Galaxy Pulsar Edge - Manual (Page 171)

Galaxy Pulsar Edge Power System Controller
Issue 3 October 2011
171
Appendix C: Battery Functions
Float Mode
Float mode is the default operation mode of the power system. The system voltage, while in float
mode, is determined by the configuration parameter Rectifier Float Set point (fsp) and may be
adjusted by the Battery Thermal Compensation circuit, if active. No individual adjustment of
plant rectifiers is necessary and load sharing among plant rectifiers is automatic in all plant modes
and will take effect within several seconds of a new rectifier being added to the system.
The Rectifier Float Set point should be set per the battery manufacturer's recommendations. Note
that the actual Rectifier Float Set point measured on the plant may differ from the value set by the
user if battery thermal compensation (STC) is enabled.
Slope Thermal Compensation
The Pulsar Edge has a flexible battery 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 attached to the
system batteries. Thermal Compensation can lower or raise the system bus voltage from the Float
Set Point voltage. For monitored battery temperatures above an ideal battery temperature the
controller can lower the system voltage per programmed linear curve. Lowering the plant voltage
helps keep the batteries at their optimum state of charge while protecting them from thermal
runaway. For monitored battery temperatures lower than the ideal battery temperature the
controller can raise the system voltage per programmed linear curve. Raising the system voltage
helps keep the batteries at their optimum state of charge. Increasing the system voltage with
decreases in battery temperature also causes more current to flow into the batteries which results
in electrolysis of the water in the batteries. This reaction is exothermic and also serves to keep
batteries warm. Thermal Compensation is generally used in systems utilizing sealed or valve-
regulated maintenance free batteries. Thermal Compensation is automatically enabled from the
factory when Valve-Regulated Lead Acid batteries are the system battery type.
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 may be unable to dissipate the internal heat generated by
their charging current and as a result will experience an increase in internal temperature. By
having the controller lower the float voltage as the battery temperature increases, system tries to
keep the float current lower to a point where destructive battery behavior can be avoided. If cell
failure is imminent and the battery temperature continues to rise above the threshold configured
for Battery Thermal Voltage Step Down Temperature, the controller has a feature that will force
the system DC bus voltage to drop in a single step to a level which should keep the remaining
cells in the string from overcharging and being damaged. The following Figure provides a
graphical view of the Pulsar Edge’s Battery Thermal Compensation feature and the relationship
of its various set points.