The Measuring Configuration; Recommendations For Current Transformers - ABB RELION REX640 Technical Manual

1MRS759142 C
4.3.5.7
4.3.5.8
REX640
Technical Manual
At internal fault, the secondary circuit voltage can easily exceed the isolation voltage
of the CTs, connection wires and protection relay. To limit this voltage, a voltage-
dependent resistor VDR is used as shown in
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.

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:
U
s
R
=
s
I
rs
GUID-DAD1D402-B078-45F6-9741-D20DBBF4FC40 V1 EN
R the resistance of the stabilizing resistor
s
U the stabilizing voltage of the protection relay
s
I the value of the Low operate value setting
rs
The stabilizing voltage is calculated with the formula:
I
k max
U ( R R )
= ⋅ +
s in m
n
GUID-C1BDCB31-CB0A-4B3D-A568-3CAAD4DDFBBC V1 EN
I the highest through-fault current
kmax
n the turns ratio of the CT
R the secondary internal resistance of the CT
in
R the resistance of the longest loop of secondary circuit
m
Additionally, it is required that the current transformers' knee-point voltages U
least twice the stabilizing voltage value U

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
Protection functions
Figure 375.
.
s
Section 4
(Equation 126)
(Equation 127)
are at
k
677