GE 845 Instruction Manual page 251

CHAPTER 4: SETPOINTS
845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
one of two remaining phases during this same energization drops below the cut-off
level. In this case the 2nd harmonic on this phase will be dropped from summation, and
the divider will be decreased to 1.
If set to "2-out-of-3", the relay estimates the 2nd harmonic differential current per-
phase, and blocks the differential protection for all three phases, if the 2nd harmonic
content from any two phases is higher than the 2nd harmonic value entered in the
setpoint "inrush inhibit level".
If set to "1-out-of-3", the relay estimates the 2nd harmonic differential current per-
phase, and blocks the differential protection for all three phases, if the 2nd harmonic
content from any phase is higher than the 2nd harmonic value entered in the setpoint
"inrush inhibit level". This method is also known as the "cross-blocking" method.
OVEREXCITATION INHIBIT
th
Range: Disabled, 5 Harmonic
Default: Disabled
th
This setting provides 5
excitation condition resulting from an increased V/Hz ratio.
An overexcitation condition resulting 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.
Instantaneous tripping of the transformer from the differential protection is not
desirable.
The relay uses a traditional 5
during overexcitation conditions.
OVEREXCITATION LEVEL
Range: 1% to 40.0% in steps of 0.1%
Default: 10%
This setting specifies the level of 5th harmonic during overexcitation (overfluxing)
transformer conditions. When the 5th harmonic level exceeds the specified
OVEREXCITATION LEVEL setting (5th harmonic ratio) the differential element is blocked.
The overexcitation inhibit works on a per-phase basis.
BLOCK
Range: Any FlexLogic operand
Default: Off
RELAYS
Range: Do Not Operate, Operate
Default: Do Not Operate
Any, or all of the output relays can be selected to operate, upon percent differential
operation.
INSTANTANEOUS DIFF FUNCTION
Range: Disabled, Trip, Alarm, Latched Alarm, Configurable
Default: Disabled
INST DIFF PICKUP
Range: 3.00 to 30.00 xCT in steps of 0.01 xCT
Default: 8.00 xCT
This setting defines the pickup level of the differential current required for operation.
INST DIFF BLOCK
Range: Any FlexLogic Operand
Default: Off
harmonic differential protection blocking during an over
th
harmonic ratio for inhibiting the differential function
PROTECTION
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