(1) Description of Operation 7SR224
©2009 Siemens Protection Devices Limited Page 27 of 66
3.4 CURRENT PROTECTION: SENSITIVE EARTH FAULT (67SEF, 51SEF, 50SEF)
Current for the Sensitive Earth Fault (SEF) elements is measured directly via a dedicated current
analogue input. SEF elements measure the fundamental frequency RMS current.
3.4.1 Directional Control of Sensitive Earth Fault Protection (67SEF)
The directional element produces forward and reverse outputs for use with SEF 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 directional sensitive earth fault elements use zero phase sequence (ZPS) polarising.
Voltage polarisation is achieved for the earth-fault elements by comparison of the appropriate current
with its equivalent voltage:
I
0
~ V
0
The characteristic angle can be user programmed to any angle between -95° and +95° using the
67SEF Char Angle setting. The voltage is the reference phasor (V
ref
) and the 67SEF Char Angle
setting is added to this to adjust the forward and reverse zones.
The centre of the forward zone is set by (V
ref
Angle + 67SEF Char Angle) and should be set to
correspond with I
fault
Angle for maximum sensitivity i.e.
For fault current of -15
°
(I lagging V by 15
°
) a 67SEF Char Angle of -15
°
is required for
maximum sensitivity.
OR
For fault current of -45
°
(I lagging V by 45
°
) a 67SEF Char Angle of -45
°
is required for
maximum sensitivity.
Minimum Polarising Voltage
The 67SEF 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.
Figure 3.4-1 Logic Diagram: SEF Directional Element (67SEF)
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