ABB RELION REX640 Technical Manual page 686

Section 4
Protection functions
680
• Small power transformers: I
infinite grid)
• Large power transformers: I
infinite grid)
• Generators and motors: I
Where I = rated current and z
n
object
2. The rated primary current I
the machine.
The choice of the CT also specifies R
3. The required U is calculated with
kn
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
with Equation 133.
4. The sensitivity I
prim
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
S

U 0 8 . F I R
= ⋅ ⋅ ⋅ +

kn n 2 n in
I

GUID-DB41ED8A-DC16-4C55-8E15-1BDD76C7F7D7 V1 EN
F the rated accuracy limit factor corresponding to the rated burden S
n
I the rated secondary current of the CT
2n
R the secondary internal resistance of the CT
in
S the volt-amp rating of the CT
n
The formulas are based on choosing the CTs according to
129, 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. The conditions in the network, however, have to be
known well enough to ensure the stability.
1. If U
2. If U
slightly prolonged and should be studied case by case.
If U
Another CT has to be chosen.
The need for the VDR depends on certain conditions.
= 16 x I
kmax
= 12 x I
kmax
= 6 x I
kmax n
= short circuit impedance of the protected
k
of the CT has to be higher than the rated current of
1n
.
in
Equation
is calculated with Equation

n

2
2 n

≥ 2 x U , fast protection relay operation is secure.
k s
≥ 1.5 x U and < 2 x U
k s
< 1.5 x U , the protection relay operation is jeopardized.
k s
1MRS759142 C
(corresponds to z
n k
(corresponds to z
n k
129. If the U of the CT is not high
kn
is given by the
kn
132. If the achieved sensitivity is
is calculated with the equation
kn
n
, protection relay operation can be
s
Technical Manual
= 6% and
= 8% and
(Equation 133)
Equation
REX640