RFL Electronics RFL 9300 - Very Short Lines; Figure 3-3. High Source-To-Line Impedance Ratio

3.3 VERY SHORT LINES
Figure 3-3 shows a condition where the ratio between the source impedance and the line impedance is very
high. This condition may occur on very short lines, or longer lines connected to a weak source.
Zs
LOCAL
REMOTE
VR
WEAK SOURCE
Source impedance (large)
SHORT LINE
Protected line impedance (small)
ZL
F1
F
2
Figure 3-3. High source-to-line impedance ratio
If Z
S
is much greater than Z
L
, V
R
is very small for both faults (F1 and F2). This makes it difficult for a distance
relay at the local station to distinguish between a fault at F1 and one at F2. Current-based relays compare the
currents (or charges) at the local station to those at the remote station. This makes it easy to tell whether the
fault is internal or external to the protected line.
RFL 9300 RFL Electronics Inc.
August 25, 2000 3 - 3 (973) 334-3100