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1 Introduction.
21
Ia
23
25
Ib
27
Ic
Figure 1. Connection of residual current.
i
2
D
EF Direction and Characteristic Angle.
The direction element can be set to operate in the forward or reverse direction. With the DEF Direction set to
forward, relay operation will occur for fault current occurring within the forward operating zone. With the relay set
to reverse
operation will occur for fault current occurring within the reverse operating zone. This can be seen in
fi
gure 2.
The Characteristic Angle sets the maximum torque angle of the
the range +-85º of this setting, as shown in the diagram below:
85°
85°
Reverse
DEF
©2010 Siemens Protection Devices Ltd
22
In
24
26
28
Forward DEF
DEF Char Angle
85°
85°
7SG163 Ohmega 300 Series SingleDEF
High resistance earth faults present difficulties to
impedance
measuring
elements
resistive coverage required can often extend
beyond the apparent impedance presented by
maximum load conditions. In those cases a
directional earth fault element can be used to
supplement the basic distance protection.
The design of the directional earth fault element
is based on similar techniques as used for
impedance
measurement.
voltage polarised deriving this by summating the
three phase to neutral voltages available in the
relay. The zero sequence current providing the
other input is derived from the fourth C.T. input
which must be connected as indicated in Figure
1.
directional relay. Operation will occur for angles in
The DEF element is normally used
signalling channel, operating as a simple high
speed
directional
comparison
conjunction with the remote end DEF. This is
referred to as a
DEF POR (Permissive
O
verreach Scheme).
If the signalling channel fails, however, a the
relay will carry out a time delayed backup trip if
the residual current
is detected, and not cleared
af
ter a time delay.
The element can also be used with no signalling
as a time graded directional IDMTL relay. T
m
ode is referred to as DEF DIRECT TRIP.
F
igure 2. DEF Operating Zones
Chapter 12 Page 3 of 5
since
the
The
element
is
with a
scheme
in
his
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