ABB RELION REX640 Technical Manual page 719

1MRS759142 C
REX640
Technical Manual
K = 1/(1-0.4) = 1.6667
r
Equation 162 with these values gives the result:
F K Ik 0 9 . ( T
> ⋅ ⋅ ⋅
a r max
GUID-E4669D94-4DDB-4BF4-81FC-52FA7CEDBD18 V1 EN
If the actual burden of the current transformer S
cannot be reduced low enough to provide a sufficient value for F
alternatives to deal with the situation.
1. A current transformer with a higher rated burden S
means a higher rated accurate limit F
2. A current transformer with a higher nominal primary current I
rated burden) can be chosen.
Alternative 2 is more cost-effective and therefore often better, although the sensitivity
of the scheme is slightly reduced.
Example 2
Here the actions according to alternative 2 are taken to improve the actual accuracy
limit factor.
I CT
 
R
F F
=  ⋅
a  n
I Motor
 R 
GUID-507670DA-4AE6-40DB-AD0A-0BB23AB4CF18 V1 EN
I CT rated primary current of the CT, for example, 1500A
R
I Motor rated current of the motor under protection, for example, 1000A
R
F rated accuracy limit factor of the CT, for example, 30
n
F actual accuracy limit factor due to oversizing the CT, substituting the values in the equation, F
a
= 45
In differential protection it is important that the accuracy limit factors F
current transformers at both sides correspond with each other, that is, the burdens of
the current transformers on both sides are to be as close to each other as possible. If
high inrush or start currents with high DC components pass through the protected
object when it is connected to the network, special attention is required for the
performance and the burdens of the current transformers and the settings of the
function block.
T
−
m
T
( 1 e ) 1 ) 24
dc
ω
⋅ ⋅ − + ≈
dc
a
).
n
Section 4
Protection functions
in the accuracy limit factor equation
, there are two
a
can be chosen (which also
n
(but the same
1n
a
(Equation 164)
(Equation 165)
a
of the phase
713