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Protection and Automation Functions
5.25 67 Directional Overcurrent/Earth Fault
Negative sequence polarizing is used when a zero-sequence polarizing voltage is not available e.g. when a
phase to phase VT is installed, see Voltage Config (CT/VT CONFIG menu).
Parameter: 67N > Char Angle
•
Default setting: -15°
The characteristic angle is the phase angle by which the reference voltage must be adjusted such that the
directional detector gives maximum sensitivity in the forward operate zone i.e. when the operate current
is in phase with the polarizing voltage. A fault is determined to be in the selected direction if lies within
+/- 85° of the Characteristic Angle setting.
For fault current of -15° (I lagging V by 15°) a setting of -15° is required for maximum sensitivity OR
For fault current of -45° (I lagging V by 45°) a setting of -45° is required for maximum sensitivity.
The reverse operate zone is the mirror image of the forward zone.
Parameter: 67N > Minimum Voltage
•
Default setting: 0.33 V
Where the measured polarizing voltage is below this setting no directional output is given and operation
of protection elements set as directional will be inhibited. This prevents mal-operation under fuse
failure/MCB tripped conditions where noise voltages can be present.
Settings Example
Directional Protection
A number of studies have been made to determine the optimum MTA settings e.g. W.K Sonnemann's paper "A
Study of Directional Element Connections for Phase Relays". The following figure shows the most likely fault
angle for phase faults on overhead line and cable circuits.
[dw_7SR5_function67SettingExample1, 1, en_US]
Figure 5-40
Directional overcurrent elements allow greater fault selectivity than non-directional elements for intercon-
nected systems where fault current can flow in both directions through the relaying point. Consider the
network shown above. 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.
330
Phase Fault Angles
Reyrolle 7SR5, Overcurrent Protection Device, Device Manual
C53000-G7040-C014-1, Edition 11.2019
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