ABB RELION 620 Series Technical Manual page 571

1MRS757644 F
620 series
Technical Manual
U
s
R
=
s
I
rs
GUID-EA4FE2BC-4E93-4093-BD14-F20A4F33AEF2 V1 EN
R the resistance of the stabilizing resistor
s
U the stabilizing voltage of the IED
s
I the value of the Operate value setting in secondary amps.
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 is
kn
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
GUID-93E59545-7530-408D-8ECF-2D3D9CF76C13 V1 EN
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
operates at a certain setting can be calculated with the formula
I n ( I I m I
= × + + ×
prim rs u
GUID-2A742729-7244-4B1C-A4DF-404BDD3A68D9 V1 EN
I the primary current at which the protection is to start
prim
n the turn ratio of the current transformer
I the value of the Operate value setting
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 voltage.
e
Assuming U ≈ 2 x U , the value of
kn s
89.
The selection of current transformers can be divided into procedures:
)
m
I
e
I
≈
m gives an approximate value for
2
Section 4
Protection functions
at which the IED
prim
voltage
s
voltage
s
(Equation 87)
(Equation 88)
(Equation 89)
Equation
565