Active Front End (AFE)
vacon • 47
Local contacts: http://drives.danfoss.com/danfoss-drives/local-contacts/
4
4.15
Paralleling
The power of the input group can be increased by connecting several Active Front End units in
parallel. Paralleling refers to Active Front End units connected in the same input transformer. Active
Front End units of different power ratings can also be connected in parallel. No communication bet-
ween the units is required; they work independently. Our standard LCL filters must be used for
paralleling. If filters other than our standard LCL filters are used in Active Front End units connected
in parallel, too large circulation currents may be generated between the Active Front End units.
Parameter P2.1.4 Parallel AFE must be set to “1/yes” for all parallel AFE units. This parameter will
also set DC Drooping to 4%. The value of DC Drooping can be also modified manually with parameter
P2.2.2.
Each Active Front End unit connected in parallel must have its own short-circuit protection on AC
and DC sides. The fuses are selected in accordance with Section 4.11. When paralleling, attention
must be paid to the sufficient short-circuit capacity of the system.
The derating of Active Front End units connected in parallel is 5% of the DC power; this should be
taken into account when selecting the input unit.
If a device is to be isolated from the AC and DC voltages, and other Active Front End units connected
in parallel are also to be used, separate isolators are required in the AC input and DC output. The
AC input can be isolated using a compact circuit-breaker, an ordinary circuit-breaker or a fuse
switch. Contactors are not suitable for isolating the AC input because they cannot be locked in the
safe position. The DC output can be isolated using a fuse switch. The pre-charging circuit must also
be isolated from the AC input. A load isolation switch or safety isolation switch can be used for this.
The device can also be connected to mains even when the other devices connected in parallel are
already connected and running. In such a case, the isolated deice must first be pre-charged. When
that is done, the AC input can be switched on. After this, the device can be connected to the
interediate DC circuit.
4.15.1
Common pre-charging circuit
In case of paralleled Active Front End units, one common pre-charging circuit can be used, see
Figure 19. Standard pre-charging circuits can be used if the capacitance of the intermediate circuit
not exceeds maximum value. For example if three FI10 Active Front End units are connected
parallel, the pre-charging circuit for FI13 Active Front End unit can be used. If all paralleled Active
Front End units have a common circuit breaker, the breaker can be controlled by one of the Active
Front End units. If each paralleled Active Front End unit has its own circuit-breaker, each Active
Front End controls it’s own circuit.
The circuit diagram for control, see Appendix 74 and Appendix 76.
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