Rockwell Automation Publication 750-RM100A-EN-P - August 2019 95
Chapter 10
Droop Control for Parallel Operation of
PowerFlex 755TM AFE Bus Supplies
Droop control of the DC bus voltage reference level is an internal process for
sharing the load between multiple AFE bus supplies that are working in parallel
to energize a common DC bus. Droop control provides consistent current and
power sharing between converters so that one converter is not subjected to a
larger share of the load than the other converters. To enable droop control in the
PowerFlex 755T, set parameter 13:45 [DC Bus Ref Sel] to 2 ‘Droop Ctrl.’
Overview
As the load on the bus supplies increases, the droop function manages DC bus
voltage evenly between the parallel bus supplies. The droop function manipulates
the DC bus voltage reference to control the output current from each bus supply.
Droop control manipulates the DC bus voltage reference so that a bus supply can
produce a larger share of current when the load is low and a smaller share of
current when the load is high.
In the motoring direction, droop control increases the DC bus voltage reference
as the active current reference decreases in magnitude. As the active current
reference increases towards full load, droop control decreases the DC bus
reference towards the minimum DC bus voltage. The DC bus voltage reference is
greater when near zero load than it is when near full load. In this way, the AFE
bus supply tends to increase its share near zero load, and is less likely to increase its
share near full load.
In the regenerating direction, droop control increases the DC bus voltage
reference as the active current reference increases in magnitude (becomes more
negative). As the magnitude increases toward full negative load, droop control
increases the DC bus reference. The DC bus voltage reference is lower when near
zero load than it is when near full negative load. In this way, the AFE bus supply
tends to increase its share near zero load, and is less likely to increase its share near
full negative load.
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