ABB 615 Series - Page 586

F
I CT
I Motor
F
a
R
R
n
=






×
GUID-560620F9-50B1-4DA4-AF3F-2B24212080D9 V2 EN (Equation 115)
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
• 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
291 and 292.
• 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
293 and 294.
• The default value of the CT connection type setting is "Type 1".
Section 4 1MAC059074-MB A
Protection functions
580 615 series ANSI
Technical Manual