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CHAPTER 4: SETPOINTS
SYSTEM
889 GENERATOR PROTECTION SYSTEM – INSTRUCTION MANUAL
4–75
2.
The K1 CT bank is linked to Winding2 (which is the neutral side of the generator). The
measured currents from Winding 2 (K1 CT bank) are scaled with respect to the CT
ratings from Winding 1 (K2 CT bank) for CT mismatch compensation used by the
overall differential protection.
Transformer differential protection uses calculated quantities (per phase): fundamental,
2nd harmonic and 5th harmonic differential current phasors, and restraint current
phasors. This information is extracted from the current transformers (CTs) connected to the
relay by correcting the magnitude and phase relationships of the currents for each
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 889 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
Default: Wye
Enter the winding connection.
ANGLE WITH RESPECT TO W1 (only in Winding 2 menu)
Range: 0
°
, 30
°
Lag, 60
°
Lag, 90
°
Lag, 120
°
Lag, 150
°
Lag, 180
°
Lag, 210
°
Lag, 240
°
Lag, 270
°
Lag, 300
°
Lag, 330
°
Lag
Default: 0
°
Enter the angle by which the currents from Winding 2 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) 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