Figure 69: Apparent Impedances Seen By Distance Protection On A Teed Feeder - GE MiCOM P40 Technical Manual

P446SV Chapter 7 - Distance Protection
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 69: 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.
Carrier aided schemes can also be used in conjunction with distance elements to protect teed feeders. Although
Permissive Overreaching and Permissive Underreaching schemes may be used, they suffer some limitations.
Blocking schemes are generally considered to be the most suitable.
For a full explanation of Teed Feeders applications in carrier aided schemes, please refer to the Carrier Aided
Schemes chapter.
P446SV-TM-EN-1 163