GE Multilin B30 Bus Differential System 9-79 THEORY OF OPERATION 9.4 DIRECTIONAL PRINCIPLE99.4DIRECTIONAL PRINCIPLE 9.4.1 CURRENT DIRECTIONAL PROTECTIONFor better security, the B30 uses the current directional protection principle to dynamically supervise the main current differ-ential function. The directional principle is in effect permanently for low differential currents (Region 1 in Figure 0–1: TwoRegions of Differential Characteristic) and is switched on dynamically for large differential currents (Region 2 in the samefigure) by the saturation detector (see Section 9.5: Saturation Detector) upon detecting CT saturation.The directional principle responds to a relative direction of the fault currents. This means that a reference signal, such asbus voltage, is not required. The directional principle declares that• if all of the fault currents flow in one direction, the fault is internal, or• if at least one fault current flows in an opposite direction compared with the sum of the remaining currents, the fault isexternal.The directional principle is implemented in two stages.First, based on the magnitude of a given current, it is determined whether the current is a fault current. If so, its relativephase relation has to be considered. The angle check must not be initiated for the load currents as the direction will be outof the bus even during internal faults. The auxiliary comparator of this stage applies an adaptable threshold. The thresholdis a fraction of the restraining current. The current from a particular feeder is used for bus directional comparison if its mag-nitude is greater than K × I restraint or it is greater than 2 times its CT rating. For bus zones with two feeders, K = 0.2. For buszones with three to six feeders, K = 0.8 / (N – 1). For bus zones with more than six feeders, K = 0.16.Second, for – and only for – the selected fault currents, the phase angle between a given current and the sum of all theremaining currents is checked. The sum of all the remaining currents is the differential current less the current under con-sideration. Therefore, for each, say the pth, current to be considered, the angle between the and phasors is to bechecked.Ideally, during external faults, the said angle is close to 180° (see below); and during internal faults - close to 0 degrees.Figure 9–5: DIRECTIONAL PRINCIPLE OPERATION DURING EXTERNAL FAULTSFigure 9–6: DIRECTIONAL PRINCIPLE OPERATION DURING INTERNAL FAULTSThe B30 implementation calculates the maximum angle for the considered currents and compares it against a fixed thresh-old of 90°. The flag indicating whether the directional protection principle is satisfied is available as the FlexLogic operandBUS 1(2) DIR.I P I D I P–836726A2.CDRBLOCKOPERATEBLOCK⎟⎟⎠⎞⎜⎜⎝⎛− pDpIIIreal⎟⎟⎠⎞⎜⎜⎝⎛− pDpIIIimagIpID - IpExternal Fault ConditionsOPERATE836727A2.CDRBLOCKBLOCK⎟⎟⎠⎞⎜⎜⎝⎛− pDpIIIreal⎟⎟⎠⎞⎜⎜⎝⎛− pDpIIIimagIpID - IpInternal Fault ConditionsOPERATEOPERATE
