Danfoss VLT AAF00x, Operating Instructions Manual, Page: 30 / 103

Table 4.5 shows the minimum CT burden for different wire
gauge at wire length of 50m and standard wire resistance
value:
Wire Gauge
[mm2 / AWG]
Resistance
[Ohm/Km] Wire length
[meters / feet]
Minimum CT
burden [VA]
1.5 / #16 13.3 50 / 164 >16.6
2.5 / #14 8.2 50 / 164 >10.2
4 / #12 5.1 50 / 164 > 6.3
6 / #10 3.4 50 / 164 > 4.2
10 / #8 2 50 / 164 > 2.5
Table 4.5 Minimum CT Burden
For a fixed CT burden the maximum allowed wire length
can be calculated as:
[M] = ([VA]-1.25) / (25*[Ohm/M])
Below the maximum wire length of CT with 2.5mm
2
wires
and resistor value equal 8.2 Ohm/km:
Wire Gauge
[mm2 / AWG]
Resistance
[Ohm/Km] Minimum CT
burden [VA]
Wire length
[meter / feet]
2.5 / #14 8.2 5 <18m / 60
2.5 / #14 8.2 7.5 <30m / 100
2.5 / #14 8.2 10 <42m / 140
2.5 / #14 8.2 15 <67m / 220
2.5 / #14 8.2 30 <140m / 460
Example
Calculation example for correct current transformer for an
application with:
RMS= 653Amp, Distance between filter and CT’s of 30m.
RMS=653*1.1= 719A, CT RMS = 750Amp. Burden :
30m@2.5mm
2
wire => 25*0.0082*30+1.25=7.4 => 7.5 [VA].
Current transformer installation
The unit only supports three CT installations. External CTs
should be installed on all three phases to detect the
harmonic content of the grid. The flow direction of the
sensor is in most cases indicated by an arrow. The arrow
should point in the direction of the current flow and so
towards the load. In case the flow direction is incorrectly
programmed the polarity can be changed via filter
300-25 CT Polarity . 300-25 CT Polarity can program the
polarity of all three phases individually.
L1
L2
L3
K L
K L
K L
K L
K L
K L
1
2
91 92 93
L1 L2 L3
95
130BB510.11
1 or 5 Amp CT Set-up
To allow for possible reuse of already present CT
transformers, the VLT Active Filter allows use of either 1
Amp or 5 Amp CTs. The filter is as standard set-up for 5
Amp CT feedback. If the CTs are 1 Amp, redirect the CT
terminal plug from slot MK101, pos. 1, to MK108, pos. 2,
on the AFC card, see
Illustration 4.29 .
130BB950.10
1
3
4
5
6
2
Illustration 4.29 AFC Board
Individual or group compensation
The compensation of the filter depends on the signal that
is returned from the current transformers. The point of
installation for these sensors is so determine the loads that
are corrected.
How to Install VLT Active Filter AAF 00x
MG.90.V2.02 - VLT® is a registered Danfoss trademark 29
4 4