7SR11 & 7SR12 Applications Guide
Page 20 of 48 ©2017 Siemens Protection Devices Limited
V
Transformer Feeders
(Cable Circuits
)
- 45
0
MTA
Current lagging
Voltage
Plain Feeders
(
Overhead Lines)
- 30
0
MTA
I
V
I
Figure 2.7-2 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. 2.6-3.
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 7SR12 relays may be programmed with forward, reverse
and non-directional elements simultaneously when required by the protection scheme.
Figure 2.7-3 Application of Directional Overcurrent Protection
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