7SR11 & 7SR12 Description Of Operation
Chapter 1 Page 42 of 88 ©2017 Siemens Protection Devices Limited
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
which requires different hardware that must be specially ordered. SEF elements measure the fundamental
frequency RMS current.
3.4.1 DIRECTIONAL CONTROL OF SENSITIVE EARTH FAULT PROTECTION (67SEF) – 7SR12
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 forward directional zone of the relay is defined by the characteristic angle setting which is the angle by which
the polarising voltage differs from the operating current. The forward directional zone is symmetrical around the
characteristic angle with angular limits defined in the performance specification. 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.
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.
fwd
rev
67SEF
67SEF Min. Voltage
67SEF Charact. Angle
67SEF Fwd
67SEF Rev
V
0
V
L1
V
L2
V
L3
Sequence
Filters
I
4
(I
SEF
)
67SEF Compensated Network
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