Section 3: Protection Functions; Current Protection: Phase Overcurrent (67, 51, 50); Directional Control Of Overcurrent Protection (67) - Siemens 7SR224 Description & Operation

(1) Description of Operation 7SR224

Section 3: Protection Functions

3.1 C P
URRENT
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)

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
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
to this to adjust the forward and reverse zones.
The centre of the forward zone is set by (V
with I Angle for maximum sensitivity i.e.
fault
For fault current of -60
sensitivity (i.e. due to quadrature connection 90
OR
For fault current of -45
sensitivity (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
reverse current flow, phases L1 and L3 will operate forwards, while phase L2 will be inhibited.
Page 18 of 66
: P
ROTECTION HASE
°
. Each phase current is compared to the voltage between the other two
I ~ V
L1 23
Angle + 67 Char Angle) and should be set to correspond
ref
°
(I lagging V by 60
°
(I lagging V by 45
and I are detected as forward flowing currents and I
L1 L3
O (67, 51, 50)
VERCURRENT
I ~ V I ~ V
L2 31 L3
) and the 67 Char Angle setting is added
ref
°
) a 67 Char Angle of +30
° ° °
- 60 = 30 ).
°
) a 67 Char Angle of +45
° ° °
- 45 = 45 ).
©2009 Siemens Protection Devices Limited
12
°
is required for maximum
°
is required for maximum
is detected as
L2