ABB 615 Series ANSI, Technical Manual

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1MAC050144-MB C
Section 4
Protection functions
615 series ANSI 331
Technical Manual
2. A current transformer with a higher nominal primary current I
1n
(but the same rated
burden) can be chosen.
The alternative 2 is more cost effective and therefore often better although the sensi-
tivity of the scheme is slightly reduced.
Example 2:
Assuming that the actions according to alternative two above are taken in order to
improve the actual accuracy limit factor
Equation 34
Where
I
R
CT rated primary current of the CT say 1500A
I
R
Motor rated current of the motor under protection say 1000A
Fn rated accuracy limit factor of the CT say 30
Fa actual accuracy limit factor due to oversizing the CT, substituting the values in Equation 34,
Fa = 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 equal as possible. Should 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 to the settings of the function block.
Connection of current transformers
The connections of the main current transformers are designated as Type 1 and Type 2. In
case the grounding of the current transformers are either inside or outside the area to be
protected, the setting
CT connection type is of “Type 1” as shown in Figure 156. In case
the grounding of the current transformers are both inside and outside the area to be
protected, the setting
CT connection type is of “Type 2” as shown in Figure 157.
n
R
R
a
F
Motor I
CT I
F







