Danfoss VLT AAF006 Operating Instructions Manual page 22

Electrical Installation
Rated power/ 5 7.5
burden [VA]
Impedance of ≤ 0.15 ≤ 0.25
current CT [Ω]
Table 4.5 Rated Power/Burden
NOTICE
All other technical data such as dynamic rated current,
maximal permitted operating voltage, thermal
dimensioning of continuous current, thermal
dimensioning of short-time current, overcurrent limit,
isolation class, working temperature range etc. are
specific values of the system and have to be defined
during the project planning phase of the equipment.
RMS specification
The minimum RMS is determined by the total current that
passes through the current transformer. It is important that
the current sensor is not too small, leading to saturation of
the sensor. Add 10% margin and pick the next following
bigger standard RMS rate. Use current transformers that
have an RMS rating close to the maximum current flowing
through it to allow the highest possible accuracy of the
measurement and so an ideal compensation.
CT burden
To ensure that the current transformer performs according
to specifications, the rated burden should not be above
the true current requirement of the active filter. The
burden of the CT depends on the wire type and the cable
length between the CT and the filter CT connection
terminal. The filter itself contributes with 2 mΩ.
NOTICE
The accuracy of the CT is depending on wire type and
length of the cable between filter and current
transformer.
Calculate the required (minimum) CT burden as:
[VA]=25*[Ω/M] *[M]+1.25
[Ω/M] being the cable resistance in Ω/meter, [M] being the
cable length in meters.
Table 4.6 shows the minimum CT burden for different wire
gauge at wire length of 50 m and standard wire resistance
value:
Wire Gauge Resistance
2 [Ω/km]
[mm /AWG]
1.5/#16 13.3
2.5/#14 8.2
4/#12 5.1
6/#10 3.4
10/#8 2
Table 4.6 Minimum CT Burden
MG90V302
Operating Instructions
10 15 30
≤ 0.35 ≤ 0.55 ≤ 1.15
Wire length Minimum CT
[meters/feet] burden [VA]
50/164 >16.6
50/164 >10.2
50/164 > 6.3
50/164 > 4.2
50/164 > 2.5
Danfoss A/S © 09/2014 All rights reserved.
For a fixed CT burden, calculate the maximum allowed
wire length as:
[M]=([VA]-1.25)/(25*[Ω/M])
Below the maximum wire length of CT with 2.5 mm
and resistor value equal 8.2 Ω/km:
Wire Gauge Resistance
2 [Ω/km]
[mm / AWG]
2.5/#14 8.2
2.5/#14 8.2
2.5/#14 8.2
2.5/#14 8.2
2.5/#14 8.2
Table 4.7 Maximum CT Wire Length
Example
Calculation example for correct current transformer for an
application with:
RMS=653 A, Distance between filter and CTs of 30 m.
RMS=653*1.1=719 A, CT RMS=750 A. Burden: 30 m@2.5
mm 2 wire =>25*0.0082*30+1.25=7.4=>7.5 [VA].
Current transformer installation
The unit only supports 3 CT installations. Install external
CTs on all 3 phases to detect the harmonic content of the
grid. The flow direction of the sensor is indicated by an
arrow in most cases. The arrow points in the direction of
the current flow and so towards the load. If the flow
direction is programmed incorrectly, the polarity can be
changed via active filter parameter 300-25 CT Polarity,
which can program the polarity of the CTs in each of the 3
phases individually.
L1
K L
L2
K L
L3
K L
1
91 92 93
L1 L2
Illustration 4.5 CT Connections
2
wires
Minimum CT Wire length
burden [VA] [meter/feet]
5 <18/60
7.5 <30/100
10 <42/140
15 <67/220
30 <140/460
K L
K L
K L
2
95
L3
19
4 4