ABB RED615 Technical Manual page 444

Section 4
Protection functions
438
U
s
R
=
s
I
rs
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R the resistance of the stabilizing resistor
s
U the stabilizing voltage of the IED
s
Operate value setting in secondary amps.
I the value of the
rs
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 is high or the stabilizing voltage is low, a resistor with a higher power rating
kn
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
2
U
kn
× 10
R
s
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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
the IED operates at a certain setting can be calculated with the formula
I = × n ( I + I + × m I
prim rs u
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I the primary current at which the protection is to start
prim
n the turn ratio of the current transformer
Operate value setting
I the value of the
rs
I the leakage current flowing through the VDR at the U
u
m the number of current transformers included in the protection per phase (=4)
I the magnetizing current per current transformer at the U
m
The I value given in many catalogs is the excitation current at the knee point
e
voltage. Assuming U
kn
for Equation 64.
The selection of current transformers can be divided into procedures:
)
m
I
≈ 2 x U , the value of m
s
1YHT530004D05 D
prim
voltage
s
voltage
s
I
e
≈
gives an approximate value
2
Technical Manual
(Equation 62)
(Equation 63)
at which
(Equation 64)
615 series