GE SR345 Instruction Manual page 182

S3 PROTECTION
6–70
Courtesy of NationalSwitchgear.com
INRUSH INHBT LEVEL
Range: 0.1 to 40.0% in steps of 0.1
Default: 10%
This setting specifies the ratio of 2nd harmonic differential current to the fundamental
frequency differential current for the selected mode of 2nd harmonic 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 INHBT MODE
Range: "Per Phase", "2-out-of-3", "Average"
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 for each phase individually.
If set to "2-out-of-3," the relay checks the 2nd harmonic level onthe per-phase
differential current, and blocks the differential protection for all three phases if the 2nd
harmonic on any two of these three phases is higher than the setting for the inrush
inhibit level.
If set to "Average," the relay calculates the average 2nd harmonic level and compares
this level against the setting for the inrush inhibit level. Averaging of the 2nd harmonics
is adaptive and depends on the magnitude of the differential current per-phase. 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 gets dropped (zeroed) from
the equation for averaging, and the dividing number (divisor) is decreased from 3 to 2.
The same happens if the magnitude of the differential current on one of the two
remaining phases drops below the cutoff level during energization. In this case the 2nd
harmonic on this phase will be dropped from summation, and the dividing number
(divisor) will be decreased to 1.
OVEREXCIT INHIBIT
Range: Disabled, 5th Harm
Default: Disabled
This setting provides 5th harmonic differential protection blocking during an
overexcitation condition that results from an increased V/Hz ratio.
Such an overexcitation condition from an increased V/Hz ratio, poses a danger to the
protected transformer. A given transformer can tolerate an overfluxing condition for a
limited time, as the danger is associated with thermal processes in the core. So
instantaneous tripping of the transformer by the differential protection is not desirable.
The relay uses a traditional 5th harmonic ratio for inhibiting the differential function
during overexcitation conditions.
OVEREX INHT LEVEL
Range: 1.0 to 40.0% in steps of 0.1
Default: 10.0%
This setting specifies the level of 5th harmonic during overexcitation (overfluxing)
transformer conditions. When the 5th harmonic level exceeds the specified OVEREX
INHT LEVEL setting (5th harmonic ratio) the differential element is blocked. The
overexcitation inhibit works on a per-phase basis.
345 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 6: SETPOINTS