GE SPM Series, Instruction Manual

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GE Power Management SPM Synchronous Motor Protection & Control 3-
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3 SYNCHRONOUS MOTOR APPLICATIONS 3.2 COLLECTOR-RING MOTORS
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3.2.8 EFFECT OF VOLTAGE DIPS ON MOTOR POWER FACTOR
Solid-state excitation systems have an effect on the way motor power factor responds to line voltage dips. The
effect may be to cause a power-factor relay to operate inadvertently. This causes the motor to trip on lagging
power factor caused by a transient condition which is not an actual pull-out condition.
A solid-state exciter differs from a rotating exciter in the way it responds to voltage dips. The rotating inertia of
a Motor-Generator set may maintain excitation voltage relatively constant for several seconds, but a solid-state
exciter has practically no built-in delay in the way it responds to line voltage. Therefore, any delay in change of
motor-rotor flux following an excitation voltage change is determined by the time constant of the rotor field
poles themselves. This is usually 0.5 to 1.0 seconds.
The sequence of events transpiring during a voltage dip with a solid-state exciter is shown in Figure 3–11:
POWER FACTOR RESPONSE TO LINE VARIANCE on page 3–14.
Assuming the condition of a line voltage decrease of 15% with the motor initially at unity power factor, the
power factor will swing leading momentarily because the generated EMF does not change until the rotor flux
decreases (determined by field time constant). The motor will tend to maintain constant horsepower by slightly
increasing line current. As the field flux decreases, generated EMF also decreases, and the power factor will
move back towards unity, and there will be a load angle increase to permit motor torque to be restored to that
required to drive the load. During both of these sequences the motor power factor has not become significantly
lagging, so the power-factor relay does not operate.
Finally, when line voltage comes back to normal, the power factor will momentarily swing over to lagging and
the power factor protection relay will trip because the rotor flux does not respond as rapidly to change as the
stator. The generated EMF is low relative to line volts for a time period long enough to operate the relay.
A power-factor device with a 1.0 second built-in time delay should remain unaffected by these changes.