ABB Relion REG670, Technical Manual, Page: 131 / 1298

Section 1 : This is the most sensitive part on the characteristic. In section 1, normal
currents flow through the protected circuit and its current transformers, and risk for
higher false differential currents is relatively low. An un-compensated on-load tap-
changer is a 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 proportional to higher than normal currents through the
current transformers.
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 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.
1MRK502052-UEN B
Section 6
Differential protection
125
Technical manual