ABB RELION 620 Series Technical Manual page 422

Section 4
Protection functions
416
 
I CT
R
F =  ⋅ F

a n
I Motor
 
R
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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
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.
Connection of current transformers
The connections of the main current transformers are designated as Type 1 and Type 2. If
the earthings 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
earthing 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
1MAC504801-IB E
(Equation 41)
a
of the phase
a
Figure 188. If the
Figure 189.
620 series ANSI
Technical Manual
= 45