Section 3: Protection Functions; Current Protection: Phase Overcurrent (67, 51, 50); Directional Control Of Overcurrent Protection (67) - 7Sr22 - Siemens 7SR21 Manual

Section 3: Protection Functions

3.1

Current Protection: Phase Overcurrent (67, 51, 50)

All phase overcurrent elements have a common setting to measure either fundamental frequency RMS or True
RMS current:
True RMS current: 51/50 Measurement = RMS
Fundamental Frequency RMS current: 51/50 Measurement = Fundamental
3.1.1 Directional Control of Overcurrent Protection (67) – 7SR22
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 (V
forward and reverse zones.
The centre of the forward zone is set by (V
Angle for maximum sensitivity i.e.
For fault current of -60
(i.e. due to quadrature connection 90
OR
For fault current of -45
(i.e. due to quadrature connection 90
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
flow, phases L1 and L3 will operate forwards, while phase L2 will be inhibited.
©2010 Siemens Protection Devices Limited
I ~ V
L1 23
) and the 67 Char Angle setting is added to this to adjust the
ref
Angle + 67 Char Angle) and should be set to correspond with I
ref
°
(I lagging V by 60
°
- 60
°
(I lagging V by 45
°
- 45
and I are detected as forward flowing currents and I
L1 L3
7SR21 & 7SR22 Argus Description of Operation
I ~ V I ~ V
L2 31 L3
°
) a 67 Char Angle of +30
° °
= 30 ).
°
) a 67 Char Angle of +45
° °
= 45 ).
12
°
is required for maximum sensitivity
°
is required for maximum sensitivity
is detected as reverse current
L2
Chapter 1 Page 22 of 71
fault