ABB RELION 620 Series Technical Manual page 891

1MAC504801-IB E
620 series ANSI
Technical Manual
Requirements for measurement transformers
a few tens of watts because of the possible CT inaccuracy which might cause some current
through the stabilizing resistor in a normal load situation.
If V is high or V is low, a resistor with a higher power rating is needed. The resistor
kn s
manufacturers often allow 10 times rated power for five seconds. The power of the resistor
can be calculated with Equation
2
V
kn
R
10
s ⋅
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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
relay operates at a certain setting can be calculated with
(
I n I I m I
= ⋅ + + ⋅
prim rs u m
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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 V
u
m the number of current transformers included in the protection per phase
I the magnetizing current per current transformer at the V
m
The I value given in many catalogs is the excitation current at the knee point voltage.
e
Assuming V ≈ 2 · V , the value of
kn s
The selection of current transformers can be divided into procedures.
1. The rated current I
n
magnitude of I .
kmax
2. The rated primary current I
feeder.
58.
)
s
I
e
I
≈
m
gives an approximate value for
2
of the feeder should be known. The value of I
of the CT must be higher than the rated current of the
1n
Section 13
at which the protection
prim
Equation 59.
voltage
voltage
s
also affects the
n
(Equation 136)
(Equation 137)
Equation 59.
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