Siemens 7SR10 Argus Series, Manual

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7SR10 Description of Operation
©2018 Siemens Protection Devices Limited Chapter 1 Page 30 of 77
3.2 Current Protection: Derived Earth Fault (67N, 51N, 50N)
The earth current is derived by calculating the sum of the measured line currents. The elements measure the
fundamental frequency RMS current.
3.2.1 Directional Control of Derived Earth Fault Protection (67N)
The directional element produces forward and reverse outputs for use with derived earth fault elements. These
outputs can be mapped as controls to each shaped and instantaneous element.
If a protection element is set as non-directional then it will operate independently of the output of the directional
detector. However, if a protection element is programmed for forward directional mode then operation will occur
only for a fault lying within the forward operate zone. Conversely, if a protection element is programmed for
reverse directional mode then operation will occur only for a fault lying within the reverse operate zone. Typically
the forward direction is defined as being ‘away’ from the busbar or towards the protected zone.
The Characteristic angle is the phase angle by which the polarising voltage must be adjusted such that the
directional detector gives maximum sensitivity in the forward operate zone when the current is in phase with it.
The reverse operate zone is the mirror image of the forward zone.
The derived directional earth fault elements can use either zero phase sequence (ZPS) or negative phase
sequence (NPS) polarising. This is selected using the 67N Polarising Quantity setting. Whenever a zero-
sequence voltage is available (a five-limb VT that can provide a zero sequence path or an open-delta VT
connection) the earth-fault element can use zero-sequence voltage and current for polarisation. If zero-sequence
polarising voltage is not available e.g. when a two phase (phase to phase) connected VT is installed, then
negative-sequence voltage and negative-sequence currents must be used. The type of VT connection is specified
by Voltage Config (CT/VT CONFIG menu). Settings advice is given in the Applications Guide.
Voltage polarisation is achieved for the earth-fault elements by comparison of the appropriate current with its
equivalent voltage:
67N Polarising Quantity : ZPS I
0
~ V
0
67N Polarising Quantity : NPS I
2
~ V
2
The characteristic angle can be user programmed to any angle between -95 ° and +95 ° using the 67N Char Angle
setting. The voltage is the reference phasor (Vref) and the 67N Char Angle setting is added to this to adjust the
forward and reverse zones.
The centre of the forward zone is set by (Vref Angle +
67N Char Angle ) and should be set to correspond with
Ifault Angle for maximum sensitivity e.g.
For fault current of -15° (I lagging V by 15°) a 67N Char Angle of -15° is required for maximum
sensitivity.
OR
For fault current of -45° (I lagging V by 45°) a
67N Char Angle
of
-45°
is required for maximum
sensitivity.
Minimum Polarising Voltage
The 67N Minimum Voltage setting defines the minimum polarising voltage level. Where the measured polarising
voltage is below this level 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.
V
0
/ V
2
fwd
rev
67N
67N Min. Voltage
67N Charact. Angle
67N Fwd.
67N Rev.
67N Polarizing Quantity
VL1
VL2
VL3
Sequence
Filters
IL1
IL2
IL3
Sequence
Filters
I
0
/ I
2
Figure 3-5 Logic Diagram: Derived Directional Earth Fault Element