7SG164 Ohmega 400 Series Impedance Functions
Figure 10 Polarised Directional Characteristic
As can be seen, the characteristic moves behind the origin for forward faults, and forward of it for
reverse faults. In addition, as the SIR level increases (
ZS
increases relative to
Z
) the characteristic
moves further from the origin. This ensures operation for close-up forward faults and stability for
reverse faults.
The Reactance Characteristic is shown in Figure 11, and consists of a straight line which cuts the
reactive axis at a value
XF
. This requires a replica impedance
ZF
of magnitude
F X X
cos
Φ
and angle
Φ
X
. If the reactive component of the fault impedance is less than this value, the comparator operates.
The angle
Φ
X
is normally set at about -3° to the horizontal, so that the characteristic slopes in order to
ensure that with increasing resistance, the relay will not overreach beyond setting. This angle is
referred to as the reactive drop angle.
The vector equation for the reactance characteristic is
IZF
−
V
≡
IZF
which becomes
ZF
−
Z
≡
ZF
If the angle between
ZF
and
ZF
−
Z
is less than 90°, the comparator will operate.
The Resistive Characteristic is shown below, and is identical in nature to the reactance characteristic,
except for the choice of replica impedance. This gives a characteristic which is inclined at the line
angle, but crosses the resistive axis at a value
RF
, giving increased resistive coverage over the entire
line length. The vector equation is again
IZF
−
V
≡
IZ F
which becomes
ZF
−
Z
≡
Z F
©2010 Siemens Protection Devices Limited
Chapter 2 Page 11 of 21
