ABB RELION REX640 - Page 1921

- a CT with a higher nominal primary current I
1n
(but the same rated
burden) can be chosen
Example 2
Assuming that the actions according to alternative two above are taken in
order to improve the actual accuracy limit factor:
F
IrCT
IrTR
F
a n
= ×
(Equation 422)
IrTR 1000 A (rated secondary side current of the power transformer)
IrCT 1500 A (rated primary current of the CT on the transformer secondary side)
F
n
30 (rated accuracy limit factor of the CT)
F
a
(IrCT / IrTR) * Fn (actual accuracy limit factor due to oversizing the CT) =
(1500/1000) * 30 = 45
In TR2PTDF and TR3PTDF, 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. 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 for the settings of the function block.
1MRS759142 F
Requirements for measurement
transformers
REX640
Technical Manual
1921