GE 489 Instruction Manual page 110

4.10 S9 THERMAL MODEL
b) CUSTOM OVERLOAD CURVE
If the induction generator starting current begins to infringe on the thermal damage curves, it may become necessary to use
a custom curve to tailor generator protection so successful starting may be possible without compromising protection. Fur-
thermore, the characteristics of the starting thermal (locked rotor and acceleration) and the running thermal damage curves
may not fit together very smoothly. In this instance, it may be necessary to use a custom curve to tailor protection to the
thermal limits to allow the generator to be started successfully and utilized to its full potential without compromising protec-
tion. The distinct parts of the thermal limit curves now become more critical. For these conditions, it is recommended that
the 489 custom curve thermal model be used. The custom overload curve allows users to program their own curves by
entering trip times for 30 pre-determined current levels.
The curves below show that if the running overload thermal limit curve were smoothed into one curve with the locked rotor
thermal limit curve, the induction generator could not be started at 80% voltage. A custom curve is required.
4
4-60
GE Multilin
10000
1
PROGRAMMED 469 CUSTOM CURVE
2
RUNNING SAFETIME (STATOR LIMIT)
3
ACCELERATION SAFETIME (ROTOR LIMIT)
4
MACHINE CURRENT @ 100% VOLTAGE
1000
5
MACHINE CURRENT @ 80% VOLTAGE
100
10
1.0
0.1
MULTIPLE OF FULL LOAD CURRENT SETPOINT
Figure 4–12: CUSTOM CURVE EXAMPLE
489 Generator Management Relay
489
TYPICAL CUSTOM CURVE
1
2
3
4
5
4 SETPOINTS
808825A3.CDR
GE Multilin