ABB RED650, Technical Manual

Page: 103 / 926

Harmonics blocking based on 2
nd
and 5
th
harmonics is used only used if one of the
following conditions are fulfilled:
• When a power transformer is included in the protected zone.
• When the bias current is less than 125 % Ibase .
• When an external fault has been detected by the negative sequence internal/
external fault discriminator and 200 ms after that.
The third analysis is the negative sequence current analysis. Effectively this is a
fault discriminator that distinguishes between internal and external faults. The
directional test is made so that the end with the highest negative sequence current is
found. Then, the sum of the negative sequence currents at all other circuit ends is
calculated. Finally, the relative phase angle between these two negative sequence
currents is determined. The characteristic for this fault discriminator is shown in
Figure
35 , where the directional characteristic is defined by the two setting
parameters IminNegSeq and NegSeqRoa .
0 deg
180 deg
90 deg
270 deg
120 deg
IMinNegSeq
If one or the
other of
currents is too
low, then no
measurement
is done, and
120 degrees
is mapped
External
fault
region Internal
fault
region
Internal/external
fault boundary
NegSeqROA
(Relay
Operate
Angle)
en05000188-3-en.vsd
IEC05000188 V3 EN-US
Figure 35: Operate characteristic of the internal/external fault discriminator
The reference direction (that is, the positive direction) of currents in a power line is
considered to be towards the line. Thus, when both negative sequence currents to
be compared have this direction, the phase difference between them is ideally zero.
In reality, the angle will usually be greater than zero, and this because of the
possible different negative sequence impedance angles on both sides as seen from
the fault location.
1MRK 505 394-UEN A
Section 6
Differential protection
Line differential protection RED650 2.2 IEC 97
Technical manual