GUID-EA4FE2BC-4E93-4093-BD14-F20A4F33AEF2 V1 EN (Equation 62)
R
s
the resistance of the stabilizing resistor
U
s
the stabilizing voltage of the IED
I
rs
the value of the
Operate value
setting in secondary amps.
The stabilizing resistor should be capable to dissipate high energy within a very
short time; therefore, the wire wound-type resistor should be used. Because of the
possible CT inaccuracy, which might cause some current through the stabilizing
resistor in a normal load situation, the rated power should be 25 W minimum.
If U
kn
is high or the stabilizing voltage is low, a resistor with a higher power rating
is needed. Often resistor manufacturers allow 10 times rated power for 5 seconds.
Thus the power of the resistor can be calculated with the equation
GUID-93E59545-7530-408D-8ECF-2D3D9CF76C13 V1 EN (Equation 63)
The actual sensitivity of the protection is affected by the IED setting, the
magnetizing currents of the parallel connected CTs and the shunting effect of the
voltage-dependent resistor (VDR). The value of the primary current I
prim
at which
the IED operates at a certain setting can be calculated with the formula
I n I I m I
prim rs u m
= × + + ×( )
GUID-2A742729-7244-4B1C-A4DF-404BDD3A68D9 V1 EN (Equation 64)
I
prim
the primary current at which the protection is to start
n the turn ratio of the current transformer
I
rs
the value of the
Operate value
setting
I
u
the leakage current flowing through the VDR at the U
s
voltage
m the number of current transformers included in the protection per phase (=4)
I
m
the magnetizing current per current transformer at the U
s
voltage
The I
e
value given in many catalogs is the excitation current at the knee point
voltage. Assuming U
kn
≈ 2 x U
s
, the value of
gives an approximate value
for Equation 64.
The selection of current transformers can be divided into procedures:
Section 4 1YHT530004D05 D
Protection functions
438 615 series
Technical Manual
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