(i)
(ii)
(iii)
C
Relay at C sees reverse fault until B opens
B
A
No infeed
Fault seen by A & B in zone 2
Fault
C
Z1A Z1B
A
B
= area where no zone 1 overlap exists
Z1A
Z1C
A
C
E03525
Figure 133: Problematic Fault Scenarios for PUR Scheme Application to Teed Feeders
● Scenario (i) shows a short tee connected to one nearby terminal and one distant terminal. In this case, Zone
1 elements set to 80% of the shortest connected feeder length don’t all overlap, resulting in a section not
covered by any Zone 1 element. Any fault in this section would rely on delayed Zone 2 tripping.
● Scenario (ii) shows an example where terminal C has no infeed. Distance elements at C may not operate for
faults close to the terminal. As the fault is outside the Zone 1 reaches of A and B, clearance will rely on
delayed Zone 2 tripping at A and B.
● Scenario (iii) shows an example where outfeed from terminal C feeds an internal fault via terminal B. In this
case, terminal C will not see the fault until the breaker at B has operated. The result would be sequential
(and hence delayed) tripping.
7.8.3
BLOCKING SCHEMES FOR TEED FEEDERS
With Blocking schemes, high speed operation can be achieved for circuits where there is no current infeed from
one or more terminals. This makes Blocking schemes particularly suitable for protection of teed feeders. The
scheme also has the advantage that the signalling requirement can be realised with one simplex channel.
Note:
Triangulated simplex channels could be used in place of a common simplex one if prefered.
As with Permissive Underreaching (PUR) schemes, a limitation of a Blocking scheme implementation is a scenario
where outfeed from one terminal feeds an internal fault via another terminal. The terminal with the outfeed sees a
P543i/P545i Chapter 8 - Carrier Aided Schemes
P54x1i-TM-EN-1 249
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