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 transforme
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 the knee point
voltage.
Assuming U
kn
≈ 2 x U
s
, the value of
gives an approximate value for
Equation
216
.
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
is calculated with
Equation 213
. 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 217
.
4. The sensitivity I
prim
is calculated with
Equation 216
. If the achieved sensitivity is
sufficient, the present CT is chosen. A CT with a bigger core is chosen if a better
sensitivity is needed.
If a Class X CT is not used, an estimate for Ukn is calculated using
Equation 216
.
U F I R
S
I
kn n n in
n
n
= × × × +
0 8
2
2
2
.
(Equation 217)
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
Equation 213
,
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.
1MRS759142 F
Protection functions
REX640
Technical Manual
813
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