Rockwell Automation 865 Differential Protection Relay - Manual (Page 146)

8-4 Applications
865-UM001A-EN-P – July 2009
R
W
= Total resistance of wiring, connections, relay input
etc.
R
S
= Stabilizing resistor according to Equation 8.2.
The peak voltage of a saturating CT can be approximated using P.
Mathews' formula:
Equation 8.4
KP P KP sp
V V V V − = 2 2
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.
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.
Example:
CT = 2000/1 V
KP
= 100 V
I
MAXT
= 16 kA = 8 x I
N
I
REF
= 5 % = 50 mA Setting value scaled to secondary level
R
CT
= 6 Ω
R
W
= 0.4 Ω
I
MAXF
= 25 kA
Maximum secondary voltage during a through fault (Equation 8.1):
V 2 . 51 4 . 0 6
2000
1
16000 − + =
S
V
Conclusion: The knee point voltage of 100 V is acceptable being
about twice the V
S
..
Serial resistance for the relay input (Equation 8.2):
R
S
= = ≈
512
0 05
1024 1000
.
.
Ω Ω
Maximum peak voltage during inside fault using a linear model for
CT (Equation 8.3):
kV 6 . 12 ) 1000 4 . 0 6 (
2000
1
25000 = + + =
p
V