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