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2.2.1
Operation of Current Differential Protection
Current differential protection compares the currents flowing into and out of the protected line.
The difference of the currents, that is, the differential current, is almost zero when a fault is
external or there is no fault, and is equal to the fault current when the fault is internal. The
differential protection operates when the difference of the currents exceeds a set value.
The GRL150 relay installed at each line terminal samples the local currents and transmits the
current data to the remote terminal via pilot wire or direct fibre optic communication. The
GRL150 performs master/master type current differential protection using the current data from
all terminals.
The GRL150 utilises the individual three phase currents to perform segregated-phase current
differential protection. The segregated-phase differential protection transmits the three phase
currents to the remote terminal, calculates the individual differential currents and detects both
phase-to-phase and phase-to-earth faults on a per phase basis.
Figure 2.2.1.1 shows the scheme logic of the segregated-phase current differential protection.
Output signals of differential elements DIF-A, -B and -C perform instantaneous three-phase
tripping. (See Figure 2.12.1.) The output signals of DIF-A, -B and -C are blocked when a
communication circuit failure is detected by the data error check, sampling synchronism check or
interruption of the received signals. For DIF-A_FS, -B_FS and -C_FS signals, see Section 2.2.3.
ICD is the inrush current detector ICD, which detects second harmonic inrush current during
transformer energisation, and can block the DIF element if activated by the scheme switch
[DIF-ICD]. If the inrush current detection signal COM4-R1_UF is received from the remote
terminal, the DIF is also blocked. (See Section 2.10.) The logic sequence is configured by the
PLC.
The DIF protection can be disabled by the scheme switch [DIFEN] or by the PLC command
DIF_BLOCK.
Note: For the symbols used in the scheme logic, see Appendix K.
DIF-A
&
DIF-B
&
DIF-C
&
Communication
1
failure
1
DIF_BLOCK
1
1553
373 ICD
≥1
1099 COM4-R1_UF
By PLC
264 DIFFS_OP
Figure 2.2.1.1 Scheme Logic of Segregated-phase Current Differential Protection
48
49
50
63
RELAY_BLOCK
[DIFEN]
(+)
"
"
ON
DIF-A_IC_BLK
1680
DIF-B_IC_BLK
1681
DIF-C_IC_BLK
1682
[DIF-ICD]
(+)
"
"
BLK
DIF-A_FS
1584
DIF-B_FS
1585
DIF-C_FS
1586
By PLC
&
&
&
1
&
1
&
1
&
⎯ 11 ⎯
6 F 2 S 0 8 2 8
82
257
&
83
258
&
84
259
&
256
≥1
DIF_TRIP
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