GE 869 Instruction Manual page 245

CHAPTER 4: SETPOINTS
869 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL
under normal operating conditions. A setting of 0.1 to 0.3 pu is generally recommended.
The CT base of the setting is the reference CT nominal (the CT with the maximum
primary current rating) for the internal summation method, and the core balance CT
nominal for the core balance method.
SLOPE 1
Range: 1 to 100% in steps of 1
Default: 25%
The setting is applicable for restraint currents from zero to BREAK 1, and defines the
ratio of differential to restraint current above which the element operates. The slope is
set to ensure sensitivity to internal faults at normal operating current levels. The criterion
for setting this slope is to allow for maximum expected CT mismatch error when
operating at the maximum permitted current. This maximum error is generally in the
range of 5 to 10% of CT rating.
BREAK 1
Range: 0.50 to 2.00 x CT in steps of 0.01
Default: 1.50
The setting defines the end of the Slope 1 region and the start of the transition region. It
should be set just above the maximum normal operating current level of the machine.
BREAK 2
Range: 2.00 to 30.00 x CT in steps of 0.01
Default: 4.00
The setting defines the end of the transition region and the start of the SLOPE 2 region. It
must be set to the level at which any of the protection CTs are expected to begin to
saturate. To avoid discontinuity in the cubic spline calculation, the relay applies a
minimum margin of 0.3 CT between BREAK 1 and BREAK 2.
SLOPE 2
Range: 1 to 100% in steps of 1
Default: 95%
The setting is applicable for restraint currents above the BREAK 2 setting. The slope is set
to ensure stability under heavy external fault conditions that could lead to high
differential currents as a result of CT saturation. A setting of 80 to 100% is
recommended. The transition region (as shown on the percent differential characteristic)
is a cubic spline, automatically calculated by the relay to result in a smooth transition
between SLOPE 1 and SLOPE 2 with no discontinuities.
RESTRAINT FACTOR STARTING
Range: 0.000 to 1.000 in steps of 0.001
Default: 0.100
This setting defines the restraint factor during machine starting for the core balance
method applications only. A portion (defined by this setting) of the current from the
terminal CT is subtracted from the core balance CT differential current to avoid possible
faulty operation caused by inrush current during motor starting. It must be noted that
current from the terminal CT and current from the core balance CT are in per unit based
on their own CT nominal with the application of this restraint factor in the relay.
PICKUP DELAY
Range: 0.000 to 10.000 in steps of 0.001
Default: 0.000
This setting defines the pickup time delay of the percent differential element.
BLOCK
Range: Off, Any FlexLogic operand
Default: Off
PROTECTION
4–113