GE 845 Instruction Manual page 194

SYSTEM
4–90
winding, so as to obtain zero (or near zero) differential currents under normal operating
conditions. Traditionally, these corrections were accomplished by interposing CTs and
tapped relay windings with some combination of CT connections.
The 845 simplifies these configuration issues. All CTs at the transformer can be connected
wye (polarity markings pointing away from the transformer). User-entered settings in the
relay characterizes the transformer being protected and allows the relay to automatically
perform all necessary magnitude, phase angle, and zero-sequence compensation.
The settings specific to each winding are shown as follows.
WINDING SETTINGS
RATED MVA
Range: 0.001 to 2000.000 MVA in steps of 0.001
Default: 5.000 MVA
Enter the self-cooled (100%) load rating for the power transformer.
NOMINAL PH-PH VOLTAGE
Range: 0.001 to 1000.000 KV in steps of 0.001
Default: 13.800 kV
Enter the nominal phase to phase voltage rating winding 1 of the transformer.
CONNECTION
Range: Wye, Delta, Zig-Zag
Default: Wye
Enter the winding connection.
ANGLE WITH RESPECT TO W1 (only in Winding 2 and Winding 3 menu)
° ° °
Range: 0 , 30 Lag, 60 Lag, 90
° °
Lag, 300 Lag, 330 Lag
°
Default: 0
Enter the angle by which the currents from Winding 2 (Winding 3) are lagging the
currents from Winding 1.
GROUNDING
Range: Not within zone, Within zone
Default: Not within zone
Select "Within Zone", if there is a grounding path at the winding 1(2, 3) side of the
transformer such as grounded neutral, grounding transformer, or grounded corner of a
delta winding. Select "Not Within Zone", if there is no grounding path for the winding in
the zone.
The Delta CT connection has the effect of removing the zero sequence components of
the phase currents. If there were a grounding bank on the Delta winding of the power
transformer within the zone of protection, a ground fault would result in differential (zero
sequence) current and false trips. In such case, it would be necessary to insert a zero
sequence current trap with the wye connected CTs on the Delta winding of the
transformer.
In general, zero sequence removal is necessary if zero sequence can flow into and out of
one transformer winding but not the other winding. Transformer windings that are
grounded inside zone protection allow zero sequence current flow in that winding, and
therefore it is from these windings that zero sequence removal is necessary.
WNDG RESISTANCE (3-Ph)
Range: 0.0001 to 100.0000 ohms in steps of 0.0001
Default: 10.0000 ohms
Enter the three-phase winding resistance from the transformer nameplate.
° ° °
Lag, 120 Lag, 150 Lag, 180
845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 4: SETPOINTS
° ° °
Lag, 210 Lag, 240 Lag, 270
°