ABB REG650 Technical Manual page 115

1MRK 502 034-UEN -
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 47), 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 52, 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 5
Differential protection
109