ABB Relion 670 series, Technical Manual

Page: 135 / 1432

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
M13039-226 v7
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.
1 2 3
2 2
_ 2 1 1 _ 1 1 0 1 _ 2
1 _ 2 1
_ 1 2 1 _ 1 1 1 0 _ 2
3 _ 1
3
_ 1 1 2 0 1 1
_ 1 _ 2
IDNS A INS W INS W
Vn W
IDNS B a INS W a INS W
Vn W
IDNS C
a INS W a INS W
- - -
= × - - × × + × × - × ×
- - -
× ×
é ù é ù
é ù é ù é ù
ê ú ê ú
ê ú ê ú ê ú
ê ú ê ú
ê ú ê ú ê ú
ê ú ê ú
ê ú ê ú ê ú
ë û ë û ë û
ë û ë û
EQUATION1560 V1 EN-US
(Equation 23)
where:
1. is the Negative Sequence Differential Currents
2. is the Negative Sequence current contribution from the W1 side
3. is the Negative Sequence current contribution from the W2 side
1MRK 502 066-UUS B Section 6
Differential protection
129
Technical manual