ABB Relion 670 series, Technical Reference Manual

Page: 136 / 1232

The second analysis is the 2
nd
and 5
th
harmonic analysis on the differential current.
Occurrence of these harmonics over a level that is set separately for each one will
block tripping action from the biased slope evaluation.
The third analysis is the negative sequence current analysis. Effectively this is a fault
discriminator that distinguishes between internal and external faults. It works such that
the phase angle of the negative sequence current from the local end is compared with
the phase angle of the sum of the negative sequence currents from the remote ends. The
characteristic for this fault discriminator is shown in figure
53 , 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
Figure 53: Operating characteristic of the internal/external fault discriminator
Reference direction of currents is considered to be towards the line. Thus, when both
currents to be compared have this direction, the phase difference between them will
ideally be zero. In the opposite case, when one current is entering and the other is
leaving the protected object, the phase difference will ideally be 180 degree. In case
either the local or the sum of the remote negative sequence currents or both is below
the set level, the fault discriminator will not make any fault classification and the value
120 degree is set. This value is then an indication that negative sequence directional
comparison has not been possible to make, and it does not mean classification as
external fault.
Section 5
1MRK505222-UUS C
Differential protection
130
Technical reference manual