GE 845 Instruction Manual page 230

SYSTEM
NOTE:
FASTPATH:
4–102
Path:
Setpoints > System > Transformer > Xfmr Status Detection
SENSING BY BREAKERS OPEN
Range: Disabled, Enabled
Default: Disabled
This method of sensing indicates that when all transformer breakers are detected open,
the transformer must be de-energized. For a more secure detection, the breakers open
detection can be supervised by the detection of the minimum current, when the Current
Supervision setpoint is enabled.
ALL BREAKERS OPEN
Range: Off, Any FlexLogic operand
Default: Off
Select the FlexLogic operand from the list representing to assert a flag when all
transformer breakers are open. This can be "52b" contacts from the transformer
breakers wired in series before connected to the relay, or another FlexLogic operand
(Virtual Output) becoming high upon the open status of all breakers.
SENSING BY VOLTAGE
Range: Disabled, Enabled
Default: Disabled
Select "Enabled", to detect the transformer status by measuring voltage. In order for the
relay to detect the transformer status accurately, the VTs must be placed within the
differential zone of protection between the Breaker and the winding. If the VTs are
placed outside the zone of protection, the transformer status will not be correct. Even
when the breaker is open, the relay can show the Transformer Energized status.
The transformer status detection will work even if only one voltage input is available from
the power system. In such cases, the voltage input must be connected to the appropriate
phase voltage terminals on the relay. Since the other two voltages per the logic diagram
will be zero, the transformer de-energized status will be detected when the magnitude of
the connected single voltage input is below the minimum voltage setting.
MINIMUM VOLTAGE
Range: 0.50 to 0.99 x VT in steps of 0.01 x VT
Default: 0.85 x VT
Select the minimum voltage level required for the energized/de-energized transformer
status. Note that during faults the voltage tends to decrease. If the voltage stays below
the minimum voltage for more than 50 ms, the transformer will show the transformer
de-energized status. This situation can be avoided if Current Supervision is set to
"Enabled". In this case, even if all the voltages collapse below the minimum voltage level,
the transformer will still show energized, because during fault conditions at least one
phase will measure substantial current.
The 845 relay calculates delta voltages, and compares them against the specified
minimum voltage threshold given in times VT. If Wye is programmed for phase VT
connection, the VT unit is obtained by multiplying the programmed secondary voltage
through the VT ratio and SQRT(3). If Delta is programmed for phase VT connection,
multiplication of SQRT(3) is not necessary.
CURRENT SUPERVISION
Range: Disabled, Enabled
Default: Disabled
Current supervision can be used to add another element into one or another sensing
mechanism for detecting the Transformer Energized, or Transformer De-Energized
status. When "Enabled" the relay scans all the winding currents and compares the
measurements against the programmed setpoint "Minimum Current" level.
845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 4: SETPOINTS