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2.1.2.2 Directional Characteristics
Figure 2.1.31 illustrates the directional characteristic, with the forward operate zone shaded. The
reverse zone is simply a mirror image of the forward zone. The forward operate zone or reverse
operate zone is selectable by the scheme switch [DOC-DIR], [DEF-DIR], [DSE-DIR] and
[DNC-DIR]. As shown in Figure 2.1.32, the each directional characteristic is composed of
forward directional characteristic, reverse directional characteristic and overcurrent thresholds.
CA + 90
CA - 180
CA - 180
Reverse
Operate
Zone
CA - 90
CA: Characteristic angle
(a) Characteristic of DOC, DEF and DNOC
D∗∗R
Polarising signals of directional elements are shown in Figure 2.1.33. Polarisation for DOC is
achieved by the 90° quadrature method, whereby each current's phase angle is compared with the
phase to phase voltage between the other two phases. Since the voltage inputs to the relay will
normally be connected phase to neutral, the polarising phase to phase voltages are derived
internally. The polarizing negative sequence voltage is also derived internally. The polarizing zero
sequence voltage is derived from a residual voltage or internally depending on the model.
Direction is determined in each case by measuring the phase angle of the current with respect to a
suitable polarising quantity. Table 2.1.2 summarises the current inputs and their respective
polarising signals. For details of the relationship between directional earth fault protection and
power system earthing, see Appendix B.
Boundary of Operation
(leading)
CA + 60
CA + 30
10 x Is
5 x Is
CA
Forward
Operate
CA - 30
Zone
CA - 60
Boundary of Operation
(lagging)
Figure 2.1.31
Directional Operate Characteristic
Stage
4
3
2
1
+θ: lead angle
−θ: lag angle
0
θ
Vpol
D∗∗F
I
θ: Characteristic angle
Figure 2.1.32
⎯
⎯
40
CA + 90
CA - 180
Reverse
Operate
Zone
CA - 90
CA: Characteristic angle
(b) Characteristic of DSEF
Directional (Forward)
(0 ≤ θ setting ≤ -95)
Directional (Reverse)
(95 ≥ θ setting > 0)
Overcurrent (1-4 stage)
(Note) DNOC provides stage 1 and 2 only.
Directional element
6 F 2 S 0 8 4 2
Boundary of Operation
°
+87.5
(leading)
CA + 60
CA + 30
10 x Is
5 x Is
CA
Forward
Operate
CA - 30
Zone
CA - 60
Boundary of Operation
°
- 87.5
(lagging)
D∗∗F1-4
&
D∗∗R1-4
&
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