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Chapter 1) 7SR17 Rho Description Of Operation
3.4
C
P
URRENT
The earth current is measured directly via a dedicated current analogue input, I
All measured earth fault elements have a common setting to measure either fundamental frequency RMS or True
RMS current:
True RMS current: 50 Measurement = RMS, 51 Measurement = RMS
Fundamental Frequency RMS current: 50 Measurement = Fundamental, 51 Measurement =
Fundamental
3.4.1
Directional Control of Measured Earth Fault Protection (67G) – 7SR1705/6
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 (motor).
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 ZPS operate current with the ZPS polarising 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
the forward and reverse zones.
The centre of the forward zone is set by (V
Angle for maximum sensitivity e.g.
For fault current of -15° (I lagging V by 15°) set 67G Char Angle = -15° for maximum sensitivity,
For fault current of -45° (I lagging V by 45°) set 67G Char Angle = -45° for maximum 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.
Figure 3-13
Page 42 of 78
: M
ROTECTION
EASURED
Angle + 67G Char Angle) and should be set to correspond with I
ref
Logic Diagram: Measured Directional Earth Fault Protection
E
F
(50/51G)
ARTH
AULT
) and the 67G Char Angle setting is added to this to adjust
ref
©2014 Siemens Protection Devices Limited
.
G
fault
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