Motors in Power System Dynamics Studies

John Undrill

NATF - Dallas - June 2015

Rapid changes of amplitude or phase of supply voltage produce significant transient variations of electrical torque

Phenomenon is common to all electrical machines:3 phase synchronous3 phase induction1 phase / capacitor induction

Transient torques are characterized by: unidirectional components components that oscillate at frequency of supply voltage

Amplitude of torque transients is strongly dependent on subtransient impedance of the machine and can exceed five times rated torque

Physics of motor behavior

Sudden phase retard

Transient torque has braking direction

Sudden phase advance

Transient torque has motor direction

Motor speed Motor speed

Electrical torque Electrical torque

Transients induced by sudden change of phase of supply voltagewith no change in amplitude

Point-On-Wave simulation of single phase air conditioner motor

Voltage dips instantaneously to 0.4 pu

At phase = 0 deg

Peak braking torque = 140 n-m

Voltage dips instantaneously to 0.4 pu

At phase = 90 deg

Peak braking torque = 90 n-m

Voltage ramps to 0.0 pu in 3 cycles

At phase = 0 deg

Peak braking torque = 30 n-m

Transients induced by sudden change of amplitude of supply voltagewith no change in phase

Point-On-Wave simulation of single phase air conditioner motor

speed speed speed

torque torque torque

Present understanding of motor behavior in power system transients:

Three phase motors: - stalling is an issue - is well understood on an individual motor basis

- reaccelerating after voltage depressions is a long standing concern of the industrial power sector

- most three phase motors are protected by relays and are tripped by overcurrent or undervoltage elements if they fail to reaccelerate

Air conditioner motors:- single phase - permanently connected capacitor

- inertia constant is 50 milliseconds or less - deceleration when voltage dips is very rapid - can stall within normal fault clearing time

- starting/restarting torque is seldom enough to overcome the breakout torque

of the compressor load- motors are not protected by relays - when stalled will draw ~5 time

rated current at very low power factor until tripped by thermal overcurrent

switches

Factors that affect stalling of single phase motors:

depth of voltage dip stalling threshold is in region of 60% when dip is initiated at unfavorable point

onthe voltage wave

phase of voltage when dip is initiated stalling is most likely when dip is initiated near voltage zero crossing is least likely when dip is initiated near voltage maximum

rate of change of voltage likelihood of stalling is reduced if voltage change occurs over 50 msec or longer

Will air conditioners stall or reaccelerate

In the foregoing examples:

Load is about 5.5 KW

Load torque is a triangular wave between 9 n-m and 29 n-m - average = 14.5n-m

Peaks of electrical torque transients are as high as 150 n-m - in either direction

If in braking direction, a large electrical torque transient can stop the motor very quickly

Thus - stalling is an electromagnetic matter

The time scale of air conditioner stalling is that of the point-on-wave timing of electrical events

Air conditioner motor modeling in fundamental frequency power system simulations

Fundamental frequency power system simulations (PSLF-PSS/E-PW) cannot represent the point-on-wave behavior of motors

Modeling of motor behavior is necessarily empirical

Stalling is not decided by modeling motor dynamics; it is declared on basis of a threshold voltage

P,Q are related to voltage by running curves until stall is declaredP,Q follow locked-rotor admittance characteristic after stall and until the motor is tripped

This modeling is imbedded in the cmpldw composite load model

Test data real power versus voltage Simulation real/reactive power versus voltage