ABB REF615 L - The Measuring Configuration

At internal fault, the secondary circuit voltage can easily exceed the isolation
voltage of the CTs, connection wires and IED. To limit this voltage, a voltage-
dependent resistor VDR is used as shown in Figure 234.
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.
4.3.4.6 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
=
GUID-00FCABE9-93E2-4BDD-83C6-EB1BE7FFE986 V1 EN (Equation 58)
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
( )
GUID-6A4C58E7-3D26-40C9-A070-0D99BA209B1A V1 EN (Equation 59)
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
.
Section 4 1YHT530004D05 D
Protection functions
436 615 series
Technical Manual