ABB Relion Technical Manual page 107

1MRK 502 048-UUS A
6.1.3.7
Technical manual
typical reason for existence of the false differential currents in this section. The 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 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
a case of two-winding, YNd5 power transformer.
Section 6
Differential protection
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