Generators: Standby and Co-generation
Copyright © 2003 Francis J. Martino
Due to the high impedance and low short-circuit capacity characteristics of generators,
problems do indeed occur. Here are some simple solutions to problems that occur after
the application is in place:
Problem 1: A facility co-generation system has a generator that has the capacity to
run a large motor but does not have the capacity to start that motor.
Solution: Use a reduced voltage starter that is modified to act as an automatic transfer
switch. The starter will start the motor on utility power and, once the motor is up to speed,
transfer the motor to generator power.
Problem 2: THD generated by a six pulse VFD does not comply with IEEE Std 519
Problem 3: A six pulse VFD operates well on utility power but, when operating on generator
power, is found to resonate with the facility lighting system.
Solution to Problems 2 and 3: Attenuate the harmonic currents that are generated by the
VFD by adding a line reactor and a 5th and 7th passive harmonic filter to the input of the
drive. Also add a load reactor to the drive output.
Problem 4: A large UPS and a generator are found to be incompatible. The converter
capacitors of the UPS will draw current with a high leading phase angle upon application of
generator power. As a result, the iron core of the generator regulator becomes saturated
and the generator output then rises without regulation. The generator will shut down on
Solution: Add a contactor that will remove the UPS capacitors when the generator is starting.
Problem 5: Due to a motor being too large for a genertor to handle, it is required that
a reduced voltage starter be utilized that limits current inrush to 150% of full load
A solid-state reduced voltage starter can indeed be adjusted for 150% maximum inrush
current. However, it is a rare application in which a motor will start on reduced voltage with
less than 300% full load amps.
Solution: The combination of reduced voltage and reduced frequency that is available with
a variable frequency drive will allow the motor to develop full starting torque with a limit of
150% of full load amps. The disadvantage: You may have created Problem 6.
Problem 6: To start a large motor on generator power, a VFD is used to soft-start a motor
and then run the motor continuously at full speed. The harmonic generation of the VFD will
add a nominal 5% current to the generator load. In addition, EMI from the VFD will also be
introduced into the generator system and spread to other parts of the facility.
Solution: Use a VFD with a closed-transition by-pass contactor. The contactor will allow the
motor to run directly off the generator when the motor achieves full speed. Thus, both
harmonic and EMI generation is eliminated during the run cycle. Both motor and generator
will run cooler and more efficiently.
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