Siemens 7PG21 Solkor Rf - Figure 4 Through Fault, zero ohm pilots, Positive half cycle; Figure 5 Through Fault, zero ohm pilots, Negative half cycle

7PG21 Solkor Rf Description of Operation
D3
R
a
D4
XY
B
C
R
p
+
-
A
D2
D
-
+
W Z
XYW Z
D
C
B
A
Figure 4 Through Fault, zero ohm pilots, Positive half cycle.
D4
R
a
D3
YX
C
B
R
p
+
-
D
D1
A
-
+
XW
YXZW
A
B
C
D
Figure 5 Through Fault, zero ohm pilots, Negative half cycle.
Figure 4 & Figure 5 above represents the operations of Solkor R protection with zero ohm pilots so that the loop
resistance is represented entirely by the 500 ohm padding resistor in each relay and the 1000ohm sum in the pilot
circuit is in one leg of the pilot circuit as shown, R
P
.
Resistors R
a
are of greater resistance than the pilot loop resistance R
p
and this causes the point of zero potential
to occur within the resistors R
a
, as shown in Figure 5. The voltage across each relaying point (B-X and C-Y)
throughout the cycle is now always negative. This voltage bias must be overcome before operation can take
place; consequently the effect is to enhance the stability of the protection against through faults.
©2011 Siemens Protection Devices Limited Chapter 1 Page 7 of 13