Current Protection: Sensitive Earth Fault (67Sef, 51Sef, 50Sef); Directional Control Of Sensitive Earth Fault Protection (67Sef) - 7Sr22 - Siemens Argus 7SR21 Technical Manual

7SR210 & 7SR220 Description of Operation
3.5 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.
3.5.1
Directional Control of Sensitive Earth Fault Protection (67SEF) – 7SR22
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 ~ V
0 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
adjust the forward and reverse zones.
The centre of the forward zone is set by (V
I Angle for maximum sensitivity i.e.
fault
For fault current of -15
sensitivity
OR
For fault current of -45
sensitivity.
For resonant grounded systems where compensation (Petersen) coils are fitted, earth fault current is deliberately
reduced to zero and therefore is difficult to measure for protection purposes. However, the wattmetric component
in the capacitive charging currents, which are close to the directional zone boundary, can be used to indicate fault
location. It is advantageous to increase the directional limits towards ±90º so that the directional boundary can be
used to discriminate between faulted and healthy circuits. A 67SEF Compensated Network Enable user setting
is provided to provide this feature for use with compensated networks only.
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.
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) and the 67SEF Char Angle setting is added to this to
ref
Angle + 67SEF Char Angle) and should be set to correspond with
ref
° (I lagging V by 15 ° ) a 67SEF Char Angle of -15
° (I lagging V by 45 ° ) a 67SEF Char Angle of -45
° is required for maximum
° is required for maximum
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