ABB Relion 670 series, Technical Manual

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in the range ±30° to ±90°, with a step of 1°. The default value is ±60°. The default setting, ±60°,
favors somewhat security in comparison to dependability.
Magnitudes of both negative-sequence currents which are to be compared as to their phase
positions in the complex plane must be high enough so that one can be sure that they are due to a
fault. The limit value IMinNegSeq is settable in the range [0.02 – 0.20] of the protected generator
rated current. Adaptability is introduced if the bias current is higher than 150% rated current.
Adaptability is introduced 10 ms after this limit of 150 % rated current has been crossed so that
the internal/external discriminator is given the time to detect correctly a fault before an eventual
CT saturation sets in. The threshold
IMinNegSeq is dynamically increased by 5% of the bias
current, in case of internal faults, and by 10% of the bias current in case of external faults. Only if
magnitudes of both currents are above the limit
IMinNegSeq, the angle between the two currents
is calculated. If any of the two currents is too small, no decision is taken regarding the relative
position of the fault, and this feature then remains inactive rather than to produce a wrong
decision. The relative angle is then assigned the value of 120° (2.094 radians). If this value persists,
then this is an indication that no directional comparison has been made. Neither internal, nor
external fault (disturbance) is declared in this case.
Internal fault
region
0 deg
180 deg
90 deg
270 deg
120 deg
Angle could not be
measured. One or both
currents too small
NegSeqROA
(Relay Operate Angle)
IminNegSeq
Internal /
external fault
boundary.
Default ± 60 deg
External fault
region
The characteristic is defined by the
settings:
IMinNegSeq and NegSeqROA
en06000433-2.vsd
IEC06000433 V2 EN-US
Figure 56: NegSeqROA determines the boundary between the internal and external fault
regions
Unrestrained negative sequence differential protection
SEMOD155649-76 v2
If one or more pickup signals have been set by the restrained differential protection algorithm,
because one or more of the fundamental frequency differential currents entered the “operate”
region of the restrained differential protection, then the internal/external fault discriminator can
enhance the final, common, trip command by the differential protection. If a fault is classified as
internal, then any eventual block signals by the harmonic criterion are ignored, and the differential
protection operates immediately without any further delay. This makes the overall generator
differential protection very fast. Operation of this protection is signaled on the output of GENPDIF
(87G) as TRNSUNRE.
1MRK 502 066-UUS B Section 6
Differential protection
159
Technical manual