VAMP 265 - Voltage Limitation; Actual Operating Sensitivity

Technical description
7 Applications
7.3 Calculating the stabilizing
resistance RS, VDR value and actual
sensitivity
VM265.EN011
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7.3.2. Voltage limitation
During heavy inside faults the voltage in the secondary circuit
may rise to several kilovolts depending on the fault currents,
CT properties and the stabilizing resistor R
S
. If the secondary
voltage would exceed 2 kV it should be limited using a voltage
dependent resistor (VDR).
The peak voltage according a linear CT model is:
Equation 7.3.2-1
SWCT
PRIM
SEC
MAXFp
RRB
CT
CT
IV 
I
MAXF
=
Maximum fault current when the fault is inside the protected
zone
CT
SEC
=
Nominal secondary current of the CT
CT
PRI
=
Nominal primary current of the CT
R
CT
=
Resistance of CT secondary.
R
W
=
Total resistance of wiring, connections, relay input etc.
R
S
=
Stabilizing resistor according Equation 7.3.1-2.
The peak voltage of a saturating CT can be approximated using
P. Mathews' formula:
Equation 7.3.2-2
KPPKPsp
VVVV  22
V
KP
=
Knee point voltage of the CT. The secondary voltage at which
a 50 % increase of primary current is needed to increase the
secondary voltage by 10%.
V
P
=
Peak voltage according linear model of a CT
This approximating formula does not hold for an open circuit
condition and is inaccurate for very high burden resistances.
7.3.3. Actual operating sensitivity
The differential scheme will multiply the fault current by two
thus increasing the sensitivity from the actual setting. The
quiescent current of the possible VDR will decrease the
sensitivity from the actual setting value.