GE 889 Instruction Manual page 256

PROTECTION
4–126
PICKUP
Range: 0.05 to 1.00 x CT in steps of 0.01
Default: 0.20 x CT
This setting defines the minimum differential current required for operation. The setting
is selected based on the amount of differential current that might be seen under normal
conditions. This differential current is a result of the transformer magnetizing current, CT
inaccuracies, and transformer ratio that would change by the transformer on-load tap
changer. When the tap changer feature is set properly, the 889 compensates the
transformation ratio which impacts the differential protection automatically.
SLOPE 1
Range: 1 to 100% in steps of 1
Default: 25%
This setting is applicable for restraint currents (through currents) of zero to kneepoint 1,
and defines the ratio of differential/restraint currents, above which the element will
operate. This slope is set to ensure sensitivity on internal faults at normal transformer
loading. The value of slope 1 must be above the maximum differential currents which
may be seen for through currents with magnitudes less than break 1 setting. The
differential currents can be caused by winding CT inaccuracies, the constant
transformer magnetizing current, and the leakage current from in-zone grounding
transformers. Usually a margin of 2% to 5% is added to this setting.
BREAK 1
Range: 0.50 to 2.00 x CT in steps of 0.01 x CT
Default: 1.50 x CT
The setting for Break 1 defines the limit of linear operation of the magnitude reference
CT, accounting for up to 80% residual flux, that effectively reduces the capability of the
CT by a factor of 5. Break 1 must be set below the current that can cause CT saturation
due to DC components and/or residual magnetism. Very often the Break 1 setting is
based on the transformer winding 1 nominal current (100% transformer loading). In such
cases, one must check whether or not this setting complies with the above
recommendation regarding the CT linear performance. This breakpoint marks the end of
slope 1, where no CT saturation is expected for restraining (through) currents smaller
than that breakpoint.
BREAK 2
Range: 2.00 to 30.00 x CT in steps of 0.01 x CT
Default: 4.00 x CT
Break 2 must be set below the fault current that is most likely to saturate any of the
transformer CTs due to an AC component alone. The setting is expressed in times CT,
where the CT rating is the magnitude reference CT either user selected, or selected
automatically.
SLOPE 2
Range: 1% to 100% in steps of 1%
Default: 95%
The Slope 2 setting ensures stability during through fault conditions, resulting in CT
saturation and spurious high differential current. Slope 2 must be set high to cater for
the worst case scenario, where only CTs from one winding saturate, but the CTs from the
other winding(s) do not saturate. In such cases the differential/restraint ratio can go as
high as 95% to 98%.
889 GENERATOR PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 4: SETPOINTS