GE 845 Instruction Manual page 276

PROTECTION
4–148
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
Range: Disabled, 5th Harmonic
Default: 5th Harmonic
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: 1.0% 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
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.
The 845 relay provides one instantaneous differential element per setpoint group.
The Instantaneous differential protection is not biased protection and operates similar to
the instantaneous overcurrent protection. Inputs to this protection are computed by the
relay per-phase differential currents. Operation occurs if any of the computed phase
differential current is above the instantaneous differential pickup setting. The
instantaneous differential protection is usually set to operate during high fault currents,
and if used, its pickup must be set according to the following criteria:
• The pickup setting (xCT) must be selected higher than the maximum inrush current
during transformer energization.
• The pickup setting must be set higher than the differential current caused CT
saturation during faults outside the zone of protection.
• The pickup setting must be selected lower than the maximum fault current during
internal faults.
harmonic differential protection blocking during an over
th
harmonic ratio for inhibiting the differential function
845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 4: SETPOINTS