Fundamental Frequency Negative Sequence Differential Currents - ABB REG650 Technical Manual

1MRK 502 043-UUS B
6.1.3.7
Technical Manual
changer is a typical reason for existence of the false differential currents in this section.
Slope in section 1 is always zero percent.
Section 2: In section 2, a certain minor slope is introduced which is supposed to cope
with false differential currents due to higher than normal currents through the current
transformers, such as during a transformer overloading situation.
Section 3: The more pronounced slope in section 3 is designed to result in a higher
tolerance to substantial current transformer saturation at high through-fault currents,
which may be expected in this section.
The operate - restrain characteristic should be designed so that it can be expected that:
• for internal faults, the operate (differential) currents are always safely, that is, with
a good margin, above the operate - restrain characteristic
• for external faults, the false (spurious) operate currents are safely, that is, with a
good margin, below the operate - restrain characteristic

Fundamental frequency negative sequence differential currents

Existence of relatively high negative sequence currents is in itself a proof of a
disturbance on the power system, possibly a fault in the protected power transformer.
The negative-sequence currents are measurable indications of abnormal conditions,
similar to the zero sequence currents. One of the several advantages of the negative
sequence currents compared to the zero sequence currents is however that they provide
coverage for phase-to-phase and power transformer turn-to-turn faults as well, not only
for ground-faults. Theoretically the negative sequence currents do not exist during
symmetrical three-phase faults, however they do appear during initial stage of such
faults (due to the DC offset) for long enough time (in most cases) for the IED to make
proper decision. Further, the negative sequence currents are not stopped at a power
transformer of the Yd, or Dy connection type. The negative sequence currents are
always properly transformed to the other side of any power transformer for any
external disturbance. Finally, the negative sequence currents are not affected by
symmetrical through-load currents.
For power transformer differential protection application, the negative sequence based
differential currents are calculated by using exactly the same matrix equations, which
are used to calculate the traditional phase-wise fundamental frequency differential
currents. However, the same equation shall be fed by the negative sequence currents
from the two power transformer sides instead of individual phase currents, as shown in
matrix equation 23 for a case of two-winding, YNd5 power transformer.
Section 6
Differential protection
93