Rockwell Automation 865 Differential Protection Relay - Manual (Page 145)

Applications 8-3
865-UM001A-EN-P – July 2009
Value of stabilizing resistor R S
The voltage V
S
(Figure 8.1) is:
Equation 8.1
W CT
PRIM
SEC
MAXT S
R R
CT
CT
I V + =
I
MAXT
= Maximum through fault current not to cause an REF trip
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 etc.
The CT should be of class X (see “CT Requirements” on page 8-2)
and the knee point voltage should be twice the calculated V
S
.
Selecting a low value helps to achieve more sensitivity and helps to avoid the usage of
a voltage limiting VDR. An unselective earth fault pick-up/trip is not always a problem if
a fast overcurrent stage will clear the fault anyway.
The stabilizing resistor R
S
is calculated as:
Equation 8.2
R
V
I S
S
Set
=
I
Set
= Setting value of the relay as secondary value.
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 8.3
S W CT
PRIM
SEC
MAXF p
R R B
CT
CT
I V + + =
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.
Calculating the Stabilizing
Resistance R
S , VDR Value
and Actual Sensitivity