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Section 3: Protection Functions
3.1
Current Protection: Phase Overcurrent (67, 51, 50)
All phase overcurrent elements have a common setting for the 50 elements and 51 elements 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
3.1.1
Directional Control of Overcurrent Protection (67) – 7SR12
The directional element produces forward and reverse outputs for use with overcurrent elements. These outputs
can then be mapped as controls to each shaped and instantaneous over-current 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.
Voltage polarisation is achieved for the phase-fault elements using the quadrature voltage i.e. at unity power
factor I leads V by 90°. Each phase current is compared to the voltage between the other two phases:
The characteristic angle can be user programmed to any angle between -95° and +95° using the 67 Char Angle
setting. The voltage is the reference phasor (Vref) and the 67 Char Angle setting is added to this to adjust the
forward and reverse zones.
The centre of the forward zone is set by (Vref Angle + 67 Char Angle) and should be set to correspond with Ifault
Angle for maximum sensitivity i.e.
For fault current of -60° (I lagging V by 60°) a 67 Char Angle of +30° is required for maximum sensitivity
(i.e. due to quadrature connection 90° - 60° = 30°).
OR
For fault current of -45° (I lagging V by 45°) a 67 Char Angle of +45° is required for maximum sensitivity
(i.e. due to quadrature connection 90° - 45° = 45°).
Two-out-of-three Gate
When the 67 2-Out-Of-3 Logic setting is set to Enabled, the directional elements will only operate for the
majority direction, e.g. if I
and I
L1
flow, phases L
and L
will operate forwards, while phase L
1
3
Minimum Polarising Voltage
The 67 Minimum Voltage setting defines the minimum polarising voltage level. Where the measured polarising
voltage is below this level no directional control signal 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.
©2012 Siemens Protection Devices Limited
I
~ V
I
~ V
L1
23
L2
31
are detected as forward flowing currents and I
L3
will be inhibited.
2
7SR11 & 7SR12 Description Of Operation
I
~ V
L3
12
is detected as reverse current
L2
Chapter 1 Page 26 of 76
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