GE b30, Instruction Manual

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CHAPTER 9: THEORY OF OPERATION DIRECTIONAL PRINCIPLE
B30 BUS DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL 9-7
9
Figure 9-4: Two regions of differential character
The second region includes the remaining portion of the differential characteristic and applies to comparatively high
differential currents. If, during an external fault, the spurious differential current is high enough so that the differential-
restraining current trajectory enters the second region, then saturation is guaranteed to be detected by the saturation
detector.
The B30 operates in the 2-out-of-2 mode in the first region of the differential characteristic. Both differential and
directional principles (Differential Principle and Directional Principle) must confirm an internal fault in order for the biased
differential element to operate.
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 B30 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.