GE 845 Instruction Manual page 275

CHAPTER 4: SETPOINTS
NOTE:
845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
SLOPE 2
Range: 1% to 100% in steps of 1%
Default: 95%
The Slope 2 setting ensures stability during through fault conditions, resulting in CT
saturation and spurious high differential current. Slope 2 must be set high to cater for
the worst case scenario, where only CTs from one winding saturate, but the CTs from the
other winding(s) do not saturate. In such cases the differential/restraint ratio can go as
high as 95% to 98%.
Setting Slope 1 higher than Slope 2 must be avoided, as it is not practical, even though the
ranges for both slopes can allow for one to do so. Correct programming of the differential-
restraint characteristic is achieved when Slope 1 is used for sensitivity of operation during
internal fault in the range of 15% to 35%, and Slope 2 set usually from 80% to 98%, is used
to provide secure area for the diff./restr. trajectory during external faults and CT saturation.
PICKUP DELAY
Range: 0.000 to 10.000 in steps of 0.001 s
Default: 0.000 s
This setting defines the pickup time delay of the percent differential element.
INRUSH INHIBIT
Range: Disabled, 2nd Harm Block
Default: 2nd Harm Block
This setting enables or disables the inrush inhibit function. None of the settings for inrush
inhibit are active, when the function is set to "Disabled".
INRUSH INHIBIT LEVEL
Range: 1.0% to 40.0% in steps of 0.1%
Default: 20.0%
This setting specifies the ratio of the 2
fundamental frequency differential current for the selected mode of the 2
inhibit. The percent differential protection will be blocked from operation if the actual
ratio of the differential 2nd harmonic current to the fundamental frequency differential
current is above this threshold.
INRUSH INHIBIT MODE
Range: "Per Phase", "Average", "2-out-of-3", "1-out-of-3"
Default: Per Phase
This setting specifies the mode of blocking during transformer magnetizing (inrush)
conditions.
If set to "Per phase", the function performs inrush inhibit per each phase individually.
If set to "Average", the relay calculates the average 2nd harmonic level and compares
this level against the setting for inrush inhibit level. Averaging of the 2nd harmonics
follows an adaptive algorithm depending on per-phase magnitude of the fundamental
frequency differential current. If the differential current on any of the three phases goes
below the differential current cut-off level of 0.04 x CT, the 2nd harmonic current from
that phase is dropped (zeroed) from the equation for averaging, and the divider is
decreased from 3 to 2. The same happens if the magnitude of the differential current on
nd
harmonic differential current to the
PROTECTION
nd
harmonic
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