I
m
The magnetizing current per current transformer at the U
s
voltage
I
prim
The primary current at which the protection is to start
I
rs
The value of the
Operate value
setting
I
u
The leakage current flowing through the VDR at the U
s
voltage
n
The turn ratio of the current transformer
m
The number of current transformers included in the protection per phase (=2)
The I
e
value given in many catalogs is the excitation current at knee point voltage.
Assuming U
kn
≈ 2 x U
s
, the value of
I
m
≈
I
e
2
gives an approximate value for
The selection of current transformers can be divided into the following steps.
1. The rated current In of the protected machine should be known. The value of In
also affects the magnitude of I
kmax
. Normally the I
kmax
value for motors is 6 · I
n
.
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
. 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 a Class X current transformer or it can be estimated
4. The sensitivity I
prim
. 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 a Class X CT is not 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 111)
F
n
The rated accuracy limit factor corresponding to the rated burden S
n
I
2n
The rated secondary current of the CT
R
in
The secondary internal resistance of the CT
S
n
The volt-amp rating of the CT
The formulas are based on choosing the CTs according to
, which results an absolutely stable scheme. In some cases, it is
possible to achieve stability with knee point voltages lower than stated
in the formulas. However, the network conditions have to be known well
enough to ensure the stability.
• If U
k
≥ 2 x U
s
, fast relay operation is secure.
• If U
k
≥ 1.5 x U
s
and < 2 x U
s
, relay operation can be slightly prolonged
and should be studied case by case.
• If U
k
< 1.5 x U
s
, the relay operation is jeopardized. Another CT has to
be chosen.
The need for voltage dependent resistor (VDR) depends on the insulation level for
which the protection relays are tested.
Voltage U
max
, ignoring the CT saturation during the fault, is calculated with the
equation
1MRS757644 H
Protection functions
620 series
Technical Manual
603
