Siemens 7SR10 Argus Series, Manual

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7SR10, 7SR11, 7SR12 Applications Guide
© 2018 Siemens Protection Devices Limited Chapter 7 Page 16 of 41
V
Transformer Feeders
(Cable Circuits)
- 45
0
MTA Current lagging
Voltage
Plain Feeders
(Overhead Lines)
- 30
0
MTA
I
V
I
Figure 2-9 Phase Fault Angles
Directional overcurrent elements allow greater fault selectivity than non-directional elements for interconnected
systems where fault current can flow in both directions through the relaying point. Consider the network shown in
Fig. 2.10 .
The Circuit breakers at A, B, E and G have directional overcurrent relays fitted since fault current can flow in both
directions at these points. The forward direction is defined as being away from the busbar and against the
direction of normal load current flow. These forward looking IDMTL elements can have sensitive settings applied
i.e. low current and time multiplier settings. Note that Directional overcurrent relays may be programmed with
forward, reverse and non-directional elements simultaneously when required by the protection scheme.
Load
A
C
E
B
D
G
Figure 2-10 Application of Directional Overcurrent Protection