ABB Relion Technical Manual page 149

1MRK 502 048-UUS A
Technical manual
• If the two negative sequence currents, as seen by the differential relay, flow in the
same direction (that is with the CTs oriented as in figure 50), the fault is internal. If
the two negative sequence currents flow in opposite directions, the fault is external.
• Under external fault condition, the relative angle is theoretically equal to 180°. Under
internal fault condition, the angle is ideally 0°, but due to possible different negative-
sequence impedance angles on both sides of the internal fault, it may differ somewhat
from 0°.
The setting NegSeqROA, as shown in figure 55, represents the so called Relay Operate
Angle, which determines the boundary between the internal and external fault regions. It
can be selected 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 4 % of the bias current, in case of internal faults,
and by 8 % 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.
Section 6
Differential protection
143