Current Protection: Measured Earth Fault (67G, 51G, 50G); Directional Control Of Measured Earth Fault Protection (67G) - Siemens 7SR224 Description & Operation

(1) Description of Operation 7SR224
3.3 C P
URRENT
The earth current is measured directly via a dedicated current analogue input.
All measured earth fault elements have a common setting to measure either fundamental frequency
RMS, True RMS current or the derived sum current:
True RMS current: 51/50 Measurement = RMS
Fundamental Frequency RMS current: 51/50 Measurement = Fundamental
Additionally these elements can be selected to operate on a Fundamental Frequency current which is
calculated from the sum of the phase currents, i.e. a Derived quantity. This option should only be used
when the fourth current input is required for connection of a Core Balance Current Transformer to
achieve very low earth fault sensitivity using the 50/51SEF elements.
51/50 Measurement = Calculated

3.3.1 Directional Control of Measured Earth Fault Protection (67G)

The directional element produces forward and reverse outputs for use with measured 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 measured directional 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 67G
Char Angle setting. The voltage is the reference phasor (V
added to this to adjust the forward and reverse zones.
The centre of the forward zone is set by (V
correspond with I Angle for maximum sensitivity e.g.
fault
For fault current of -15
sensitivity, OR
For fault current of -45
sensitivity.
Minimum Polarising Voltage
The 67G 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.
Page 24 of 66
: M
ROTECTION EASURED
°
(I lagging V by 15
°
(I lagging V by 45
E F (67G, 51G, 50G)
ARTH AULT
) and the 67G Char Angle setting is
ref
Angle + 67G Char Angle) and should be set to
ref
°
) a 67G Char Angle of -15
°
) a 67G Char Angle of -45
©2009 Siemens Protection Devices Limited
°
is required for maximum
°
is required for maximum