ABB RELION REX640 Technical Manual page 766

Protection functions
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
which the protection relay operates at a certain setting can be calculated with the
formula
I
prim
I
prim
n
I
rs
I
u
m
I
m
The I
voltage. Assuming U
Equation 185
The selection of current transformers can be divided into procedures:
1.
2.
3.
4.
If other than Class X CTs are used, an estimate for U
equation
U
kn
766
n ( I I m I )
= × + + ×
rs u m
the primary current at which the protection is to start
the turn ratio of the current transformer
the value of the
the leakage current flowing through the VDR at the U
the number of current transformers included in the protection per phase (=4)
the magnetizing current per current transformer at the U
value given in many catalogs is the excitation current at the knee point
e
≈ 2 x U
kn
.
In principle, the highest through-fault should be known. However, when the
necessary data are not available, approximates can be used:
- Small power transformers: I
infinite grid)
- Large power transformers: I
infinite grid)
- Generators and motors: I
Where I = rated current and z
n
object
The rated primary current I
of the machine.
The choice of the CT also specifies R
The required U is calculated with
kn
high enough, another CT has to be chosen. The value of the U
manufacturer in the case of Class X current transformers or it can be estimated
Equation 186
with .
The sensitivity I is calculated with
prim
sufficient, the present CT is chosen. If a better sensitivity is needed, a CT with a
bigger core is chosen.

S
= × × × +

0 8 . F I R

n 2 n in
2
I

2
Operate value setting
I
≈
m
, the value of
s
= 16 x I
kmax n
= 12 x I
kmax n
= 6 x I
kmax n
= short circuit impedance of the protected
k
of the CT has to be higher than the rated current
1n
.
in
Equation 182
Equation 185

n



n
(Equation 185)
voltage
s
voltage
s
I
e
2 gives an approximate value for
(corresponds to z = 6% and
k
(corresponds to z = 8% and
k
. If the U of the CT is not
kn
is given by the
kn
. If the achieved sensitivity is
is calculated with the
kn
(Equation 186)
Technical Manual
1MRS759142 F
at
prim
REX640