Internal/External Fault Discriminator - ABB RELION 650 Series Applications Manual

Section 6
Differential protection
• If the bias current is lower than 1.25 times
• Under external fault conditions.
NegSeqDiffEn = Off (the default is On )
• If
When the harmonic content is above the set level, the restrained differential operation is
blocked. However, if a fault has been classified as internal by the negative sequence fault
discriminator, any harmonic restraint is overridden.
I2/I1Ratio
The set value is the ratio of the 2
fundamental frequency component of the differential current. To obtain this information, the
instantaneous differential current must be analyzed.
Transformer inrush currents cause high degrees of the 2
The default value of 15% is a reliable value to detect power transformer inrush currents.
CT saturation causes 2
contributes to the stabilization of the relay at through fault conditions. It is strongly
recommended to maintain a sensitive setting of the
is not included in the protected zone.
I5/I1Ratio
The set value is the ratio of the 5
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
default setting of 25% will be suitable in most cases to detect the phenomenon. As CT
saturation also causes 5
this setting at 25% or lower even if no power transformer is included in the protected zone.
6.2.3.4

Internal/external fault discriminator

NegSeqDiffEn
The negative sequence fault discriminator can be set
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 (
NegSeqROA
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 42. 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
and dependability of the differential protection. The value
emphasizes security against dependability. Tests have proven that 60 degrees is a good
choice.
86
nd
harmonic component of the differential current to the
nd
harmonics of considerable value on the CT secondary side, which
th
harmonic of the differential current to the fundamental
th
harmonics on the secondary side, it is recommended to maintain
NegSeqROA is a compromise between the security
IBase .
nd
harmonic in the differential current.
I2/I1Ratio also when a power transformer
th
On / Off . It is an important complement
On ).
NegSeqROA = 60 degrees
1MRK 505 363-UEN A
M12541-201 v4
M12541-206 v4
harmonic, and the
M12541-210 v2
M12541-213 v4
M12541-218 v4
Application manual