9-6 B90 LOW IMPEDANCE BUS DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL
DIRECTIONAL PRINCIPLE CHAPTER 9: THEORY OF OPERATION
9
The relay operates in the dynamic 1-out-of-2 / 2-out-of-2 mode in the second region of the differential characteristic. If the
saturation detector (see the Saturation Detector section) does not detect CT saturation, the differential protection principle
alone is capable of operating the biased differential element. If CT saturation is detected, both differential and directional
principles must confirm an internal fault in order for the biased differential element to operate.
Because of diverse operating modes in the first and second regions of the differential characteristic, the user gains double
control over the dependability and security issues. The first level includes slopes and breakpoints of the characteristic with
regard to the amount of the bias. The second level includes control over the split between the first and second regions of
the characteristic.
9.4 Directional principle
9.4.1 Current directional protection
For better security, the B90 uses the current directional protection principle to dynamically supervise the main current
differential function. The directional principle is in effect permanently for low differential currents (region 1 in the Two
Regions of Differential Characteristic figure) and is switched on dynamically for large differential currents (region 2 in the
same figure) by the saturation detector (see the Saturation Detector section) upon detecting CT saturation.
The directional principle responds to a relative direction of the fault currents. This means that a reference signal, such as
bus 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 is
external
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 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
restraint
or it is greater than 2 times its CT rating. For bus zones with two feeders, K = 0.2. For
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
p
and I
D
- I
p
phasors is to be checked.
Ideally, during external faults, the said angle is close to 180° (as shown); and close to 0 degrees during internal faults.
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