4 Protection Functions
D01908R02.12
B-PRO User Manual 8700
4-3
Detection of Fast CT Saturation
Rather than examination of individual line fault currents for the presence of
saturation, the detection method uses only IO and IR for its block/no-block de-
cision. A novel algorithm (patent pending) has been developed to detect the
fast CT saturation quickly, sensitively and accurately. The core of this algo-
rithm is to check the phase relationship of dIO/dt and dIR/dt. For an internal
fault, both IO and IR start to increase simultaneously and they are always kind
of in phase. For an external fault, the phase dIO/dt is always lagging dIR/dt.
Detection of Slow CT Saturation
Sometimes CT does not saturate in the first cycle after the fault occurs due to
the DC offset and the CT core remnant. Under these conditions, the algorithm
described in above "Detection of Fast CT Saturation" could miss the detection
of the changes of IR and IO, because of the limit of the sample window size in
the DFT calculation. A "block zone" under the differential characteristic curve
as shown in Figure 4.3 is defined to solve this.
Figure 4.3: Detection of Slow CT Saturation
IRs is the setting parameter, which is normally set to be greater than the max-
imum bus transfer load. The block level is preset to 2 times of IRs for safety
margin. The slope in the block zone is preset to 20% for safety margin to cover
the maximum CT mismatch. Whenever the IO, IR trajectory is entered into the
block zone, this algorithm for Slow CT Saturation Detection will set the block
flag to block the differential element from tripping.
The CT saturation detector is simple to use and can be enabled or disabled.
When enabled, you set a timer to control how long you want an external CT
saturation condition to block the differential protection. Normally you would
set this timer to be slightly longer than your maximum clearing time for an ex-
ternal fault.
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Summary of Contents for 8700/BUS
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