GE 845, Instruction Manual

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CHAPTER 4: SETPOINTS SYSTEM
845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL 4–105
TC WINDING CURRENTS
Range: None, J1, K1, K2
Default: None
Enter the CT bank associated with the winding with tap changer. Enter 'None' if the
transformer is not equipped with a tap changer. Enter 'None' if automatic magnitude
compensation based on tap position used for the differential protection is not needed.
The setting “None” can be selected in cases where either the transformer is not
equipped with Tap Changer, or it is equipped with Tap Changer, but it is not desired to
automatically compensate winding currents forming the differential current in the
transformer differential protection, or in cases where the differential protection is not
used at all.
All transformer windings from 845 are associated with CT bank inputs. Depending on
845 ordering, the following CT banks represent the winding currents:
1. 845 orders for two-winding transformers without voltage inputs:
–
K1 CT bank is used for Winding 1
– K2 CT bank is used for Winding 2
2. 845 orders for two-winding transformers with voltage inputs:
–
J1 CT bank is used for Winding 1
– K1 CT bank is used for Winding 2
3. 845 orders for three-winding transformers (voltage inputs always included):
–
J1 CT bank is used for Winding 1
– K1 CT bank is used for Winding 2
– K2 CT bank is used for Winding 3
NOTE
NOTE:
If the Tap Changer setpoint TC Winding Currents is set to J1, K1, or K2, the computed tap
position voltage will be used for automatic magnitude mismatch compensation for the
winding represented by the same CT bank currents. Otherwise, if the setpoint TC Winding
Currents is set to “None”, the mismatch magnitude computation is not automatically
updated to include the new transformation ratio. In such cases, the relay may display
differential current, where this current will be higher for taps far away from the tap
representing the nominal winding voltage, and it will be lower for taps closer to the
nominal voltage tap. It is recommended to increase the minimum PKP setting of the
Percent Differential characteristic to be above the maximum differential current that can
be seen at either Maximum tap, or Minimum tap.
TAP DETECTION INPUT
Range: BCD, Ohms In 1, Analog lp 1
Default: BCD
Select either BCD(Binary Coded Decimals), or Ohms In 1 or Analog Ip 1 input for
detection of the tap positions. The Ohms input provides maximum total resistance of up
to 5.1 k
Ω . For example, the maximum resistance increment per tap for a 33-position tap
changer should not exceed 151 Ω .If Analog In 1 is selected, the relay measures DC mA
current, and detects the tap position according to the selection for “DCmA at Min Tap”,
and “DCmA at Max Tap” setpoints.
IMPORTANT:
If Analog Input 1 is selected as the tap detection input, then Analog Input 1 will not be
available for use in any other application. The Analog Input 1 is excluded from the list
of analog inputs, and its range is forced to 0.000 – 20.000 mA. The Tap changer
monitors the measured µA. When the total number of taps per the tap changer setup is
computed, the tap position is detected based on minimum tap corresponding to the
measured DCmA at MIN TAP, maximum tap corresponding to the measured DCmA at
MAX TAP, and the µA step obtained by dividing the total µA of DCmA at MIN TAP and
DCmA at MAX TAP by the total number of taps.