GE 845 Instruction Manual page 438

FLEXLOGIC
4–334
The voltage difference between calculated phase-phase voltage derived from Wye
connected phase VTs, and the directly measured phase-phase voltage from auxiliary VT
can be monitored by programming a FlexElement.
FlexElement settings:
• Input 1(+): J2 Vab RMS
• Input 2 (-): J2 Vaux RMS (input from VT connected between phases A and B)
• Operating Mode: Absolute
• Input Comparison Mode: Level
• Direction: Over
The analog input J2 Vab is phase-phase voltage computed by the relay based on three-
phase Wye voltages. As per the Phase VT setup, the primary RMS nominal voltage for J2
Vab input is 66.4 V *120 = 7.968kV.
The analog input J2 Vaux is directly measured phase-phase voltage and its primary RMS
nominal voltage is 115V *120 = 13.8kV
V = max (7.968kV, 13.8kV) = 13.8kV.
BASE
If we want to detect 2% voltage difference (2% @13.8kV = 276V) between the computed
phase to phase Vab voltage, and the measured Vaux voltage from a VT connected
between phases A and B, the pickup per-unit setting for the FlexElement can be set as
follows:
Pickup = 276V/13800V = 0.02 pu
If the voltage difference between the selected inputs becomes bigger than 276 Volts, the
FlexElement will pickup, and operate, which can be used to energize contact, or initiate
alarm, or trip.
Detecting current difference between Neutral and Ground currents:
In a balanced system, the computed neutral and the measured ground currents is 0 Amps.
However, during ground faults their values are not zero. More specifically if the phase and
ground CTs are located on the same transformer winding, such that the ground CT is
installed on the grounded neutral of the winding, their values supposed to be the same
during external fault, and would be different during internal fault. The FlexElement can be
used for detecting the differential signal between these quantities. For example the
following condition can be made:
I = max (2000A, 500A) = 2000A
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FlexElement settings:
• Input 1(+): J1
• InInput 2 (-): J1 Ig
• Operating Mode: Absolute
• Input Comparison Mode: Level
• Direction: Over
• Pickup = 200A/2000A: 0.1 pu
When no CT saturation conditions exist, if the difference between the neutral current and
the ground current becomes more than 200 Amps primary, this can be treated as an
indication of an internal ground fault, which should be cleared. With I
500A) = 2000A, the pickup can be set as follows: Pickup = 200A/2000A = 0.1 pu
Detecting Low 3-ph Apparent Power:
V = 7.968kV
BASE
I = 1000 A
BASE
P = V *I = 7968 V *2000A = 15.936MVA
BASE BASE BASE
The FlexElement can be set to detect under-power conditions and produce alarm, or trip if
the apparent power is less than 500kVA. In this case the pickup setting for the FlexElement
can be computed as follows:
845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 4: SETPOINTS
= max (2000A,
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