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
I CT
I Motor
F
a
R
R
n
=
×
(Equation 68)
I
R
CT rated primary current of the CT, for example, 1500A
I
R
Motor rated current of the motor under protection, for example, 1000A
F
n
rated accuracy limit factor of the CT, for example, 30
F
a
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
a
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
CT connection type
is of "Type 1". The connection examples of
"Type 1" are as shown in figures
Figure 260
and
Figure 261
.
• If the positive directions of the winding 1 and winding 2 protection relay currents
equate, the
CT connection type
setting parameter is "Type 2". The connection
examples of "Type 2" are as shown in figures
Figure 262
and
Figure 263
.
• The default value of the
CT connection type
setting is "Type 1".
Protection functions
1MRS757644 H
512 620 series
Technical Manual
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