Fundamental Frequency Negative Sequence Differential Currents - ABB RET650 Technical Manual

Section 6
Differential protection
6.1.4.7
96
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 with a good
margin above the operate - restrain characteristic
• for external faults, the false (spurious) operate currents are 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 a measurable indication of an abnormal condition,
similar to the zero sequence current. One of the several advantages of the negative
sequence currents compared to the zero sequence currents is that they provide
coverage for phase-to-phase and power transformer turn-to-turn 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 a
long enough time (in most cases) for the IED to make the proper decision. Further, the
negative sequence currents are not stopped at a power transformer by 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 applications, 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. 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.
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Technical manual