ABB RELION REX640 Technical Manual page 813

1MRS759142 F
I
m
I
prim
I
rs
I
u
n
m
The I
voltage.
Assuming U
216
.
The selection of current transformers can be divided into the following steps.
1.
2.
3.
4.
If a Class X CT is not used, an estimate for Ukn is calculated using
U
kn
F
n
I
2n
R
in
S
n
The need for voltage dependent resistor (VDR) depends on the insulation level for
which the protection relays are tested.
REX640
Technical Manual
The magnetizing current per current transformer at the U
The primary current at which the protection is to start
The value of the
The leakage current flowing through the VDR at the U
The turn ratio of the current transforme
The number of current transformers included in the protection per phase (=2)
value given in many catalogs is the excitation current at the knee point
e
≈ 2 x U , the value of
kn s
The rated current In of the protected machine should be known. The value of In
also affects the magnitude of I
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 217
with .
The sensitivity I is calculated with
prim
sufficient, the present CT is chosen. A CT with a bigger core is chosen if a better
sensitivity is needed.

S
= × × × +

0 8 . F I R

n 2 n in
2
I

2
The rated accuracy limit factor corresponding to the rated burden S
The rated secondary current of the CT
The secondary internal resistance of the CT
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 ≥ 2 x U
k
• If U ≥ 1.5 x U
k
and should be studied case by case.
• If U < 1.5 x U
k
be chosen.
Operate value setting
I
e
I
≈
m
2 gives an approximate value for
. Normally the I
kmax
of the CT has to be higher than the rated current
1n
.
in
Equation 213
Equation 216

n



n
, fast relay operation is secure.
s
and < 2 x U , relay operation can be slightly prolonged
s s
, the relay operation is jeopardized. Another CT has to
s
Protection functions
voltage
s
voltage
s
value for motors is 6 · I
kmax
. If the U of the CT is not
kn
is given by the
kn
. If the achieved sensitivity is
Equation 216
(Equation 217)
n
Equation 213
Equation
.
n
.
,
813