7SR224 Argus Description of Operation
3.4
Current Protection: Sensitive EF (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.
For application of 67SEF protection to networks with compensation (Peterson) coils fitted, an additional setting,
67SEF Compensated Networks
is available which when set to Enabled will increase the directional boundary to
closer to 90º to suit this application. This setting should be set to Disabled for application on networks with other
earthing arrangements.
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)
©2010 Siemens Protection Devices Limited
Chapter 1 Page 26 of 70