ABB REG650 Technical Manual page 131

1MRK 502 043-UUS B
Technical Manual
sequence system, faults can be distinguished as internal or external, even for three-
phase faults.
The internal or external fault discriminator responds to the relative phase angles of the
negative sequence fault currents at both ends of the stator winding. Observe that the
source of the negative sequence currents at unsymmetrical faults is at the fault point.
• 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 46), 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 51, 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
125