4.7 Paralleling of Active Filter Units
The VLT Active Filter is designed to be installed in
networks with other active in-feed current suppliers and
operate in conjunction with other active filters, UPSs and
AFE drives. There are no limitations to the maximum
allowed units to be installed. Four filters are allowed to be
connected to the same CT input and run in a master-
follower configuration. The master unit is activating the
individual followers according to mitigation demand in a
cascade network. This keeps the switching losses as low as
possible and thus improves the system efficiency. The
master unit will automatically allocate a new follower in
case a unit is out due to service or has unintentionally
tripped.
4.7.1 CT wiring for Parallel Filter
Connection
VLT Active Filter is designed to allow up to four units to
run in parallel allowing for a harmonic and reactive
compensation extension to four times the individual filter
rating. The parallel installed filters uses the same current
input and so only one external set of CTs have to be
installed. If additional filtration is needed, additional filters
have to use separate current transformers installed up or
down stream to the CT signal and injection point of the
paralleled installation.
PCC1
130BB713.10
AAF
AAF
AAF
AAF
Figure 4.42 Two sets of AAFs in Master-Follower.
PCC1
130BB714.10
AAF
AAF
AAF
AAF
Figure 4.43 Four AAFs in Master-Follower
The parallel connected filters must have the CT input
signal wired in serial according to
13
0B
B7
15
.1
0
F2
F1
L1
12
12
12
M
Figure 4.44 Single phase CT connection diagram for master and
follower.
CAUTION
All CT wires must be shielded for correct EMC installation.
Unshielded cables can lead to noise on the CT wire and
result in incorrect harmonic filtration.
The current transformers VA limitation still have to be kept
for filters in parallel and so total wires length has to be
limited according to wire type and CT VA rating.
[M] = ([VA]-1,25) / (25*[Ohm/M])
See
for more details.
How to Install
VLT Active Filter AAF 00x
4-34
MG.90.V2.22 - VLT
®
is a registered Danfoss trademark
4
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