Application Guide - NP900 Series
333 (504
)
A996A
These values were calculated with
R
Cond
=
ρ
×
l
A
formula.
Suggestion is that for calculating the CT burden the worst case scenario is used. For most
cases these 75
℃
values can be used. If in your application ambient temperature is higher
than 75
℃
, then the resistance should be calculated for that temperature.
Important is also to know the wiring of the CTs in that point that is there common return
wire used or are the CTs both ends wired to the terminal connector. If the case is as usual
that four wires come from the CTs to terminal then length per phase is distance from the
CT to relay added with the distance to common coupling point. If from all CTs are both
sides wires connected to relay or to the terminal then the length of the wiring is two times
the distance from the CTs to relay. In case if the connection is mixture of these then the
length can be estimated by increasing the distance by proportion of the six wires / four
wires connection. For example if 30% of the wiring is made from the CTs to terminal with
six wires and from the terminal wiring continues with four wires then the wire length
estimate should be 1.3 times the distance between of relay and CTs.
Next loading factor is the resistance of the relay measuring input. In NP900 relays it is
0.0005
Ω
for current input. This gives about 0.001VA with 1A current.
Now how to calculate the accuracy limit factor first the CT nominal accuracy limit factor
needs to be known. As mentioned before that number in the CT rating (after the P) gives
the current overload as a factor of nominal rated value which can still be said that the CT
output will be in its rated accuracy 5% (5P) or 10% (10P) gives the accuracy limiting factor
applicable at that overload of the CT.
Actual accuracy limit factor can be calculated as follows (this is common method):
ALF
ACT
= ALF
RATED
×
�
S
CTRN
+ S
Rated
S
CTRN
+ S
Actual
�
, where
ALF
RATED
= The “factor after P”. The rated accuracy limit
factor.
S
CTRN
= Internal burden of the CT secondary
S
Rated
= Volt-Amp Rating of the current transformer
S
Actual
=Actual taken power from the CT
In this formula the S values are in VA. Biggest problem in this equation is to know the
internal resistance of the CT secondary for calculation of the S
CTRN
.
The internal resistance is related to what is the CT rating and how long is the winding
length and also what is the used winding wire dimension. However the internal burden of
the ct should be considered minority in the calculations since majority of the CT burden in
