ABB 615 Series ANSI, Техническое руководство, Страница: 372 / 713

1MAC050144-MB C
Section 4
Protection functions
615 series ANSI 365
Technical Manual
When the values are substituted in the equation, the result is:
If the actual burden of the current transformer (S
a
) in
(Equation 44)
cannot be reduced
low enough to provide a sufficient value for F
a
, there are two alternatives to deal with the
situation:
• a CT with a higher rated burden Sn can be chosen (which also means a higher rated
accuracy limit Fn)
• a CT with a higher nominal primary current I1n (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:
(Equation 46)
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 87T, it is important that the accuracy limit factors Fa 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 pickup 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.
CT connections and transformation ratio correction
The connections of the primary current transformers are designated as "Type 1" and "Type
2". If the positive directions of the winding 1 and winding 2 IED currents are opposite, the
CT connection type setting parameter is "Type 1". If the positive directions of the winding
1 and winding 2 IED currents equate, the CT connection type setting parameter is "Type
2". The default is "Type 1".
F
IrCT
IrTR
F
a n
=
*