ABB RELION 620 Series Technical Manual page 569

1MRS757644 F
4.3.4.6
4.3.4.7
620 series
Technical Manual
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
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-00FCABE9-93E2-4BDD-83C6-EB1BE7FFE986 V1 EN
R the resistance of the stabilizing resistor
s
U the stabilizing voltage of the IED
s
Low operate value setting
I the value of the
rs
The stabilizing voltage is calculated with the formula:
I
k max
U ( R R )
= +
s in m
n
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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 288.
.
s
Section 4
(Equation 83)
(Equation 84)
are at
k
563