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I L (=I F )
LOCAL
DIFF RELAY GRL100
(LOCAL)
CBDS-A,B,C
I R (Current)
CBDS-A
CBDS-B
CBDS-C
I R
I L
If DS or CB signals (CBDS-A, B, C) changes to "0", remote current data
(I R ) is cancelled to zero (0). Therefore, differential current (Id) equals to
local current (I L ).
2.2.10 Application to Three-terminal Lines
When current differential protection is applied to a three-terminal line, special attention must be
paid to the fault current flowing out of the line in the case of an internal fault and CT saturation at
the outflowing terminal in case of an external fault.
Fault current outflow in case of internal fault
In case of a two-terminal line, fault current never flows out from the terminals for an internal
fault. But in case of a three-terminal line with an outer loop circuit, a partial fault current can
flow out of one terminal and flow into another terminal depending on the fault location and
magnitude of the power source behind each terminal.
Case 1 in Figure 2.2.10.1 shows a fault current outflow in a single circuit three-terminal line with
outer loop circuit. J and F in the figure indicate the junction point and fault point. A part of the
fault current flowing in from terminal A flows out once from terminal C and flows in again from
terminal B through the outer loop.
Case 2 shows the outflow in a double-circuit three-terminal line. The outer loop is generated
when one terminal is open in the parallel line. A part of the fault current flowing in from terminal
A flows out from the fault line to the parallel line at terminal C and flows in again at terminal B
through the parallel line.
LINE
DIFF RELAY GRL100
(REMOTE)
Comm. Link
CBDS-A,B,C
(Remote terminal closed: "0" logic)
≧1
1
(Cancel circuit of remote terminal current I R )
1
&
Σ
Figure 2.2.9.1 Blind Zone Protection
26
I R (=I F )
DS
89L1
52A
52B
52C
≧1
I R (Current)
Remote terminal "OPEN"
Differential Current (Id)
6 F 2 S 0 8 5 0
Blind Zone
REMOTE
CB
FAULT
BUSBAR
PROT.
CB CLOSE COMMAND
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