GE B30 Instruction Manual page 548

DIRECTIONAL PRINCIPLE CHAPTER 9: THEORY OF OPERATION
First, based on the magnitude of a given current, it is determined whether the current is a fault current. If so, its relative
phase relation has to be considered. The angle check must not be initiated for the load currents as the direction is out of
the bus even during internal faults. The auxiliary comparator of this stage applies an adaptable threshold. The threshold is
a fraction of the restraining current. The current from a particular feeder is used for bus directional comparison if its
magnitude is greater than K × I or it is greater than 2 times its CT rating. For bus zones with two feeders, K = 0.2. For
restraint
bus zones with three to six feeders, K = 0.8 / (N – 1). For bus zones with more than six feeders, K = 0.16.
Second, for the selected fault currents, the phase angle between a given current and the sum of all the remaining currents
is checked. The sum of all the remaining currents is the differential current less the current under consideration. Therefore,
for each, say the pth, current to be considered, the angle between the I and I - I phasors is to be checked.
p D p
Ideally, during external faults, the said angle is close to 180° (as shown); and close to 0 degrees during internal faults.
Figure 9-5: Directional principle operation during external faults
Figure 9-6: Directional principle operation during internal faults
9
The B30 implementation calculates the maximum angle for the considered currents and compares it against a fixed
threshold of 90°. The flag indicating whether the directional protection principle is satisfied is available as the FlexLogic
operand
BUS 1(2) DIR A/B/C.
9-8 B30 BUS DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL