GE 850 Instruction Manual page 600

FLEXLOGIC
9–22
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
BASE
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:
Pickup = 0.5MVA / 15.936 MVA = 0.0313 pu
FlexElement settings:
• Input 1(+): Pwr1 Apparent
• Input 2(-): Off
• Operating Mode: Absolute
• Input Comparison Mode: Level
• Direction: Under
• Pickup: 0.0313 pu
Power Factor Cap Bank Switch-In Example
PF = 1.00
BASE
FlexElement can be programmed to switch-in cap bank, if for example the measured 3Ph
Power Factor has negative value(lag), and drops below the pickup of -0.7 pu. Programming
the Hysteresis setpoint to the desired percentage can define the PF value at which the cap
bank can be switched off. For example, if the cap bank is required to be switched off at PF
value of -0.9, than the percent hysteresis is computed as:
CHAPTER 9: FLEXLOGIC AND OTHER SETPOINTS
850 FEEDER PROTECTION SYSTEM – INSTRUCTION MANUAL
= max (2000A,
BASE