ABB RED650, Applications Manual

Page: 98 / 454

CT saturation causes 2
nd
harmonics of considerable value on the CT secondary
side, which contributes to the stabilization of the relay at through fault conditions.
It is strongly recommended to maintain a sensitive setting of the
I2/I1Ratio also
when a power transformer is not included in the protected zone.
I5/I1Ratio
M12541-206 v4
The set value is the ratio of the 5
th
harmonic of the differential current to the
fundamental frequency of the differential current. To obtain this information, the
instantaneous differential current must be analyzed.
A 20...30% over-excitation of a transformer can cause an increase in the excitation
current of 10...100 times the normal value. This excitation current is a true
differential current if the transformer is inside the protected zone. It has a high
degree of the 5
th
harmonic, and the default setting of 25% will be suitable in most
cases to detect the phenomenon. As CT saturation also causes 5
th
harmonics on the
secondary side, it is recommended to maintain this setting at 25% or lower even if
no power transformer is included in the protected zone.
6.1.3.4 Internal/external fault discriminator
M12541-210 v2
NegSeqDiffEn
M12541-213 v4
The negative sequence fault discriminator can be set On / Off . It is an important
complement to the percentage restrained differential function. As it is directional, it
can distinguish between external and internal faults also in difficult conditions,
such as CT saturation, and so on. It is strongly recommended that it is always
activated (
On ).
NegSeqROA
M12541-218 v4
This is the setting of the relay operate angle of the negative sequence current based
internal/external fault discriminator. The directional test is made so that the end
with the highest negative sequence current is found. Then, the sum of the negative
sequence currents at all other circuit ends is calculated. Finally, the relative phase
angle between these two negative sequence currents is determined. See figure
40 .
Ideally the angle is zero degrees for internal faults and 180 degrees for external
faults. However, measuring errors caused by, for example, CT saturation as well as
different phase angles of the negative sequence impedances, require a safety
margin, expressed as the ROA (Relay Operate Angle). The default value 60
degrees is recommended in most cases. The setting
NegSeqROA is a compromise
between the security and dependability of the differential protection. The value
NegSeqROA = 60 degrees emphasizes security against dependability. Tests have
proven that 60 degrees is a good choice.
Section 6
1MRK 505 393-UEN B
Differential protection
92 Line differential protection RED650 2.2 IEC
Application manual