ABB RELION REX640, Техническое руководство, Страница: 742 / 1960

If the actual burden of the current transformer (S
a
) in
Equation 173
cannot be
reduced enough to provide a sufficient value for F
a
, there are two alternatives.
• A current transformer with a higher rated burden S
n
can be chosen (which also
means a higher rated accuracy limit F
n
) or
• A current transformer with a higher nominal primary current I1
n
(but the same
rated burden) can be chosen.
Example 2
Assuming that the actions according to the current transformer with a
higher nominal primary current I1
n
(but the same rated burden) are taken
to improve the actual accuracy limit factor (F
a
):
I
rTR
= 1000 A Rated secondary current of the transformer
I
rCT
= 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
= (I
rCT
/ I
rTR
) · F
n
Actual accuracy limit factor due to oversizing the CT
F
a
= (1500/1000) · 30 = 45
In differential protection it is important that the accuracy limit factors
(F
a
) at both sides of the phase CTs correspond with each other, that is,
the burdens of the CTs on both sides should be as equal as possible. If
high inrush or start currents with high DC components flow through the
protected object when it is connected to the network, special attention
should be paid to the performance and the burdens of the CTs and to the
settings of the function block.
4.3.3.9 Signals
TR3PTDF Input signals
Table 671: TR3PTDF Input signals
Name Type Default Description
I3P1 SIGNAL - Three-phase currents
1
I3P2 SIGNAL - Three-phase currents
2
I3P3 SIGNAL - Three-phase currents
3
BLOCK BOOLEAN 0=False Block
BLK_OPR_LS BOOLEAN 0=False Blocks operate out-
puts from biased
stage
BLK_OPR_HS BOOLEAN 0=False Blocks operate out-
puts from instantane-
ous stage
Protection functions 1MRS759142 F
742 REX640
Technical Manual