ABB RELION REX640 Technical Manual page 685

1MRS759142 C
REX640
Technical Manual
U
s
R
=
s
I
rs
GUID-A578FB88-D2B1-4EEC-9397-78F3D05823BE V1 EN
R the resistance of the stabilizing resistor
s
U the stabilizing voltage of the protection relay
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
R
10
⋅
s
GUID-FC3F8B7D-3A05-4D9F-8221-45B01933E88B V1 EN
The actual sensitivity of the protection is affected by the protection relay 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
protection relay operates at a certain setting can be calculated with the formula
I n I ( I m I )
= ⋅ + + ⋅
prim rs u m
GUID-AAB428D4-73E0-4E76-BC7B-86D1BA3C4559 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
132.
The selection of current transformers can be divided into procedures:
1. In principle, the highest through-fault should be known. However, when the
necessary data are not available, approximates can be used:
Protection functions
voltage
s
voltage
s
I
e
I
≈
gives an approximate value for
m
2
Section 4
(Equation 130)
(Equation 131)
at which the
prim
(Equation 132)
Equation
679