ABB RELION REX640 Technical Manual page 734

Section 4
Protection functions
728
I n I I m I
( )
= ⋅ + + ⋅
prim rs u m
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I The magnetizing current per current transformer at U
m
I The primary current level at which the protection is to start
prim
Operate value setting
I The value of
rs
I The leakage current through the VDR at U
u
n Turn ratio of the current transformer
m The number of current transformers included in the protection per phase (=2)
The I value given in many catalogues is the excitation current at knee point voltage.
e
Assuming , the value of
U ≈ ⋅ 2 U
kn s
170.
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 I
also affects the magnitude of I
2. The rated primary current I
the machine. The choice of the CT also specifies R
3. The required U is calculated using
kn
enough, another CT has to be chosen. The value of the U
manufacturer in case of a Class X current transformer or it can be estimated using
Equation 171.
4. The sensitivity I
prim
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 U
Sn

U F I R
0 8 .
= ⋅ ⋅ ⋅ +
kn n n  in
2

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F The rated accuracy limit factor corresponding to rated burden S
n
I The rated secondary current of the CT
2n
R The secondary internal resistance of the CT
in
S The voltage-ampere rating of the CT
n
The formulas are based on choosing the CTs according to
167 which results an absolute stable scheme. In some cases, it is
possible to achieve stability with knee point voltages lower than the
voltage
s
I
e
I
≈
gives an approximate value for
m
2
. Normally the I
kmax
of the CT has to be higher than the rated current of
1n
Equation
is calculated using Equation
kn


2
I
2 n

1MRS759142 C
voltage
s
value for motors is 6 · I
kmax
.
in
167. If the U of the CT is not high
kn
is given by the
kn
170. If the achieved sensitivity
is calculated using Equation
n
Technical Manual
(Equation 170)
Equation
n
.
n
171.
(Equation 171)
Equation
REX640