GE 745 Instruction Manual page 155

CHAPTER 5: SETPOINTS
5.6.5.2 Time Overcurrent Curves
Note
NOTE
745 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
This section contains settings to configure the phase overcurrent elements. Included are
phase time overcurrents and two levels of phase instantaneous overcurrent for each phase
of each winding.
The inverse time overcurrent curves used by the time overcurrent elements are the ANSI,
IEC, and GE type IAC curve shapes. This allows for simplified coordination with
downstream devices. If however, none of these curve shapes is adequate, FlexCurves™
may be used to customize the inverse time curve characteristics. The Definite Time curve
is also an option that may be appropriate if only simple protection is required.
The following overcurrent curve types are available:
• ANSI curves: ANSI extremely inverse, ANSI very inverse, ANSI normally inverse, and
ANSI moderately inverse
• IEC curves: IEC curves A, B, and C (BS142); IEC short inverse
• GE type IAC curves: IAC extremely inverse, IAC very inverse, IAC inverse, and IAC short
inverse
• Other curves: FlexCurves™ A, B, and C; definite time curve
A time dial multiplier setting allows selection of a multiple of the base curve shape (where
the time dial multiplier = 1) with the curve shape (
electromechanical time dial equivalent, operate times are directly proportional to the time
multiplier (
TIME OC MULTIPLIER
are 10 times the multiplier 1 or base curve values. Setting the multiplier to zero results in an
instantaneous response to all current levels above pickup.
Graphs of standard time-current curves on 11" × 17" log-log graph paper are available
upon request from the GE Multilin literature department. The original files are also
available in PDF from the GE Multilin website at http://www.GEmultilin.com.
FlexCurves™:
The custom FlexCurve™ is described in FlexCurves™ on page 5–38. The curve shapes for
the FlexCurves™ are derived from the formula:
T M × FlexcurveTime at
=
where: T = operate time (in seconds)
M = multiplier setting,
I = input current, and
I = pickup current setpoint
pickup
Definite time curve:
The definite time curve shape operates as soon as the pickup level is exceeded for a
specified period of time. The base definite time curve delay is 0.1 seconds. The curve
multiplier makes this delay adjustable from 0.000 (instantaneous) to 10.000 seconds.
SHAPE ) setting. Unlike the
) setting value. For example, all times for a multiplier of 10
I
-------------- - when -------------- -
I I
pickup pickup
S4 ELEMENTS
I
≥ 1.00 (EQ 5.6)
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