Power Factor Detection & Indication - Overhauling Load - GE SPM Series Instruction Manual

3 SYNCHRONOUS MOTOR APPLICATIONS 3.2 COLLECTOR-RING MOTORS
3.2.9 POWER FACTOR DETECTION & INDICATION – OVERHAULING LOAD
Many synchronous motor applications require that the motor operate with an overhauling load (in generating
mode). The power factor protection must be able to provide pull-out protection during overhauling load condi-
tions.
The SPM provides pull-out protection for the synchronous motor operating in both the generating and the
motoring mode. However, conventional power factor detection and indication for motors and generators are
opposite. Simply, the convention is that a motor has a leading power factor when it is overexcited (producing
reactive power). A generator, by convention, is leading power factor when it is underexcited (consuming reac-
tive power). In order to understand this difference, it is necessary to recognize that the definition of a motor
voltage reference phasor is 180° displaced from its corresponding conventional generator voltage reference
phasor.
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Figure 3–12: POWER FACTOR SENSING – MOTOR MODE VS. GENERATOR MODE
Therefore, a given line current that leads the conventional generator voltage phasor will lag the corresponding
conventional motor phasor. The diagram above shows that IA is lagging VAB (conventional motor phasor)
while it is leading VBA (conventional generator phasor).
This confusion can be eliminated by defining one terminal voltage phasor for both generating and motoring
modes. Simply, if VAB is used as the reference phasor, then the leading power factor is always when the syn-
chronous machine is producing reactive power and lagging power factor when it is consuming reactive power.
This is the approach that is taken with the PF display for the SPM. When the motor/generator is producing
reactive kVA the sign of the power factor is displayed plus (+), indicating leading power factor, regardless of the
operating mode. When it is consuming reactive kVA the power factor is negative (–).
Therefore, whether the machine is motoring or generating, pull-out protection is provided by limiting the degree
of lagging power factor (under excitation) as detected by the SPM.
The power factor regulation option also performs to force the field in advance of a pull-out condition regardless
of whether the machine is operating as a motor or a generator.
GE Power Management SPM Synchronous Motor Protection & Control 3-
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