S9 Thermal Model; Thermal Model - GE Multilin 489 Instruction Manual

5.10 S9 Thermal Model

5.10.1 489 Thermal Model

5–70
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The thermal model of the 489 is primarily intended for induction generators, especially
those that start on the system bus in the same manner as induction motors. However,
some of the thermal model features may be used to model the heating that occurs in
synchronous generators during overload conditions.
One of the principle enemies of generator life is heat. Generator thermal limits are dictated
by the design of both the stator and the rotor. Induction generators that start on the
system bus have three modes of operation: locked rotor or stall (when the rotor is not
turning), acceleration (when the rotor is coming up to speed), and generating (when the
rotor turns at super-synchronous speed). Heating occurs in the generator during each of
these conditions in very distinct ways. Typically, during the generator starting, locked rotor
and acceleration conditions, the generator will be rotor limited. That is to say that the rotor
will approach its thermal limit before the stator. Under locked rotor conditions, voltage is
induced in the rotor at line frequency, 50 or 60 Hz. This voltage causes a current to flow in
the rotor, also at line frequency, and the heat generated (I
rotor resistance. At 50 or 60 Hz, the reactance of the rotor cage causes the current to flow
at the outer edges of the rotor bars. The effective resistance of the rotor is therefore at a
maximum during a locked rotor condition as is rotor heating. When the generator is
running at above rated speed, the voltage induced in the rotor is at a low frequency
(approximately 1 Hz) and therefore, the effective resistance of the rotor is reduced quite
dramatically. During overloads, the generator thermal limit is typically dictated by stator
parameters. Some special generators might be all stator or all rotor limited. During
acceleration, the dynamic nature of the generator slip dictates that rotor impedance is
also dynamic, and a third thermal limit characteristic is necessary.
The figure below illustrates typical thermal limit curves for induction motors. The starting
characteristic is shown for a high inertia load at 80% voltage. If the machine started
quicker, the distinct characteristics of the thermal limit curves would not be required and
the running overload curve would be joined with locked rotor safe stall times to produce a
single overload curve.
The generator manufacturer should provide a safe stall time or thermal limit curves for any
generator that is started as an induction motor. These thermal limits are intended to be
used as guidelines and their definition is not always precise. When operation of the
generator exceeds the thermal limit, the generator insulation does not immediately melt,
rather, the rate of insulation degradation reaches a point where continued operation will
significantly reduce generator life.
CHAPTER 5: SETPOINTS
2
R) is a function of the effective
489 GENERATOR MANAGEMENT RELAY – INSTRUCTION MANUAL