ABB RELION REX640 Technical Manual page 798

Protection functions
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.
=
F
a
I CT
R
I Motor
R
F
n
F
a
In differential protection it is important that the accuracy limit factors F
of the phase 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.
Connection of current transformers
The connections of the primary current transformers are designated as Type 1 and
Type 2.
• If the positive directions of the winding 1 and winding 2 protection relay currents
are opposite, the
"Type 1" are as shown in figures
• If the positive directions of the winding 1 and winding 2 protection relay currents
equate, the
examples of "Type 2" are as shown in figures
• The default value of the
798
 
I CT
R  ×

F
n
 I Motor 
R
rated primary current of the CT, for example, 1500A
rated current of the motor under protection, for example, 1000A
rated accuracy limit factor of the CT, for example, 30
actual accuracy limit factor due to oversizing the CT, substituting the values in
the equation, F
CT connection type is of "Type 1". The connection examples of
CT connection type setting parameter is "Type 2". The connection
= 45
a
Figure 452
and
Figure 454
CT connection type setting is "Type 1".
1MRS759142 F
(Equation 211)
Figure 453
.
Figure 455
and
Technical Manual
a
.
REX640