GE B90 UR Series, Instruction Manual

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GE Multilin B90 Low Impedance Bus Differential System 9-5
9 THEORY OF OPERATION 9.3 DIFFERENTIAL PRINCIPLE
9
9.3.3 ENHANCED SECURITY
In order to enhance the performance of the B90, the differential characteristic is divided into two regions having diverse
operating modes as shown in following diagram.
The first region applies to comparatively low differential currents and has been introduced to deal with CT saturation on low-
current external faults. Certain distant external faults may cause CT saturation due to extremely long time constants of the
DC component or multiple autoreclosure shots. The saturation, however, is difficult to detect in such cases. Additional secu-
rity via the “directional check” is permanently applied to this region without regard to the saturation detector.
Figure 9–3: TWO REGIONS OF DIFFERENTIAL CHARACTERISTIC
The second region includes the remaining portion of the differential characteristic and applies to comparatively high differ-
ential 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 B90 operates in the 2-out-of-2 mode in the first region of the differential characteristic. Both differential and directional
principles (see Sections 9.3 Differential Principle and 9.4 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 Section 9.5 Saturation Detector) 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.
differ
ential
restraining
Region 1
(low differential
currents)
Region 2
(high differential
currents)
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