Teed Feeder Applications; Figure 101: Apparent Impedances Seen By Distance Protection On A Teed Feeder - GE MiCOM P40 Technical Manual

Chapter 8 - Carrier Aided Schemes P446SV
Current Reversal Guard
Current reversals during fault clearances on adjacent parallel lines need to be treated with care.To prevent
maloperation, a current reversal guard timer must be set.
The recommended setting (tReversal Guard) is the maximum signalling channel reset time + 35 ms.
7.8

TEED FEEDER APPLICATIONS

Distance protection can be applied to protect three terminal lines (teed feeders). Interconnecting three terminals,
however, affects the apparent impedances seen by the distance elements and creates certain problems.
Consider, as an example, the following figure which represents a teed feeder with terminals A, B, and C, with a fault
applied near to terminal B:
A
B
I
I
a
b
Z
at
Z
bt
I
c
Z
ct
V
a
V = I Z + I Z =
C
Impedance seen by relay A
a a at b bt
I
a
I = I + I
a a c
E03524

Figure 101: Apparent Impedances seen by Distance Protection on a Teed Feeder

The impedance seen by the distance elements at terminal A is given by:
Za = Zat + Zbt + [Zbt.(Ic/Ia)]
For faults beyond the Tee point, with infeed from terminals A and C, the distance elements at A (and C) will
underreach. If terminal C is a relatively strong source, the underreaching effect at A can be substantial. If Zone 2
was set to a typical value of 120% of line AB, the element may fail to operate for internal faults. To compensate,
the Zone 2 element must be set to further overreach by a factor which takes into account the effect of the infeed
from the tee-point.
So, if infeed is present on a teed circuit, all Zone 2 elements should be set to overreach both of their remote
terminals by a factor which takes into account the effect of the infeed from the tee-point.
Like overreaching of Zone 2 elements, underreaching of Zone 1 elements must also be assured. Zone 1 elements
at each terminal must be set to underreach the true impedance to their nearest terminal (limiting case = no infeed
to the tee-point - hence no overreach contribution).
Changing the reach requirements to match the infeed expectations is possible using the alternative setting group
feature. Tailoring setting group contents to the different conditions, coupled with appropriate setting group
switching, enables the changing reach requirements to be met.
204 P446SV-TM-EN-1