ABB RELION REX640 - 4.3.5.7 The measuring configuration; 4.3.5.8 Recommendations for current transformers

The whole scheme, that is, the stabilizing resistor, voltage-dependent resistor and
wiring, must be adequately maintained (operation- and insulation-tested regularly)
to be able to withstand the high-voltage pulses which appear during an internal
fault throughout the lifetime of the equipment. Otherwise, during a fault within the
zone of protection, any flashover in the CT secondary circuits or in any other part
of the scheme may prevent a correct operation of the high-impedance differential
function.
4.3.5.7 The measuring configuration
The external measuring configuration is composed of four current transformers
measuring the currents and a stabilizing resistor. A varistor is needed if high
overvoltages are expected.
The value of the stabilizing resistor is calculated with the formula:
R
U
I
s
s
rs
=
(Equation 179)
R
s
the resistance of the stabilizing resistor
U
s
the stabilizing voltage of the IED
I
rs
the value of the Low operate value setting
The stabilizing voltage is calculated with the formula:
U
I
n
R R
s
k
in m
= +
max
( )
(Equation 180)
I
kmax
the highest through-fault current
n the turns ratio of the CT
R
in
the secondary internal resistance of the CT
R
m
the resistance of the longest loop of secondary circuit
Additionally, it is required that the current transformers' knee-point voltages U
k
are
at least twice the stabilizing voltage value U
s
.
4.3.5.8 Recommendations for current transformers
The sensitivity and reliability of the protection depends a lot on the characteristics
of the current transformers. The CTs must have an identical transformation
ratio. It is recommended that all current transformers have an equal burden and
characteristics and are of same type, preferably from the same manufacturing
batch, that is, an identical construction should be used. If the CT characteristics
and burden values are not equal, calculation for each branch in the scheme should
be done separately and the worst-case result is then used.
First, the stabilizing voltage, that is, the voltage appearing across the measuring
branch during the out-of-zone fault, is calculated assuming that one of the parallel
connected CT is fully saturated. The stabilizing voltage can be calculated with the
formula
Protection functions
1MRS759142 F
764 REX640
Technical Manual