•
If the bias current is lower than 1.25 times
IBase.
•
Under external fault conditions.
•
If
NegSeqDiffEn = Off (the default is On )
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
M12541-201 v4
The set value is the ratio of the 2
nd
harmonic component of the differential current to the
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
nd
harmonic in the differential current.
The default value of 15% is a reliable value to detect power transformer inrush currents.
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.
6.3.3.4
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
. 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 505 343-UEN B
Differential protection
128
Application manual
Содержание Relion 670 series
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