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Variable Frequency Drive Ride-Through On Sag and Outage
Copyright © 2001 Francis J. Martino

Power quality is a major concern with continuous processes which can not be allowed down time. Presented here are several techniques that are used in industry to allow a variable frequency drive to ride-through power problems. The solutions have different characteristics in terms of length of extended operating time during power sags, cost of implementation, and floor space requirements.


Maintaining Output Voltage


Upon power interruption or sag condition on a 460 volt system, the input section of the drive can not maintain the nominal 620 VDC level on the bus. Consequently, the bus capacitors discharge into the bus in an effort to maintain the proper bus voltage.

A drive operating under full load at full speed will trip on power outage within a nominal two millisecs. The drive manufacturer determines the trip point based on the characteristics of the internal drive control circuitry. When the power is low enough to cause loss of control, the drive must be safely shut down rather than allowed to risk uncontrolled operation of the driven motor.

To maintain operation of a PWM drive during power sag, a battery back-up, increased DC bus capacitance, or boost converter may be used to maintain a constant voltage level on the DC bus.

However, the VVI and CSI drives utilize SCR or Thyristor converters (AC to DC conversion) to vary the voltage level on the DC bus. The inverter section then varies the frequency output (DC to AC inversion). The converter and inverter sections therefore combine to vary both voltage and frequency in order to vary the speed of the driven motor. For that reason neither batteries nor boost converters may be utilized to maintain the DC bus voltage on the VVI or CSI units.

A UPS may be used on the input of PWM, VVI or CSI drives.


Re-powering After Power Outage

Upon a normal start-up, a PWM unit will have a pre-charge circuit or resistor to slow the charging of the bus capacitors. Limiting the capacitor charge inrush prevents the capacitors from being damaged or blowing the input line fuses. After start-up, the pre-charge circuitry is disabled.

Upon the return of power after a momentary outage, and with the pre-charge circuitry disabled, the PWM with a diode bridge rectifier on the input will have an inrush as the bus capacitors attempt to charge to maximum. For a quick restart, the drive must be designed to limit the inrush if the drive re-powers at a time when the driven motor is still spinning.

The VVI and CSI units will have a controlled bus current due to the action of the controlled SCR or Thyristor input rectifier, therefore, in-rush will be controlled by normal operation.


Inverter Designs for Power Ride-Through

a) The DC bus voltage will be monitored. Upon voltage sag the drive will reduce the frequency to the motor to set a speed slightly lower than the operating speed. The motor will then decelerate and, in so doing, will regenerate power into the drive which will contribute to maintaining DC bus volts.

The disadvantage is that the motor will operate at a lower speed during the sag. After the input power recovers, the motor will re-accelerate to its original speed.

b) A Variable Frequency Drive will be selected for twice the current capacity of the driven motor and must be capable of riding through a loss of one phase on the input. The internal software is programmed to inhibit the single-phase trip capability. The larger size drive allows greater input current on the two remaining phases which will be required to maintain motor horsepower output.

The drive must also be selected on the basis of a constant torque rating rather than variable torque. The constant torque rating will provide larger capacitors that will withstand the heating caused by the ripple effect of the single-phasing on the input of the drive.

c) A Variable Frequency Drive with an active transistor rectifier on the front end will boost DC bus amps on a PWM drive. In addition to allowing ride-through on sag, it will also generate less harmonic distortion than a diode bridge rectifier.


Techniques for Power Ride-Through

a) A typical UPS on the input of the drive.

b) Two separate power feeders that have a high probability that only one feeder will present an outage or sag at any one time. A transfer switch will sense the problem and switch from one feeder to another.

c) A twelve-pulse input rectifier will accept inputs from each of two separate feeders. Two transformers will be required, one with a delta secondary to feed one set of six input devices and a second transformer with a wye secondary to feed the other set of six. The delta and wye secondaries will provide the necessary thirty degrees of phase shift required between the inputs of the two rectifiers that is required for the rectifiers to maintain twelve pulses per cycle.

When one feeder losses power the other will supply all the power that is needed for the drive. Each of the two input rectifiers must be capable of allowing sufficient current to pass to handle full load requirements.

The two transformer operation will allow some harmonic cancellation. In addition, the harmonics of consequence generated by the drive will be of the eleventh, thirteenth, seventeenth and nineteenth orders. The amplitudes of those harmonics will be less than the amplitudes of the harmonics generated by a six-pulse inverter. In addition, those harmonics will be less troubling that the fifth, seventh, eleventh and thirteenth harmonics generated of the six-pulse inverter.

For the reason that the impedances of the delta and wye secondaries are different, the current passing through the two rectifier bridges will be unequal and harmonic cancellation will be minimal.

The typical twelve pulse inverter is normally supplied with one input transformer that will have two phase shifted outputs for the two diode bridges. That transformer will have higher harmonic cancellation.

d) A capacitor bank or a battery bank may be used to maintain the DC bus volts on a PWM drive. Either bank is very large and expensive.

e) Diesel Motor-Generator set to provide AC power to the drive. A time delay upon starting will eliminate nuisance starting of the diesel during very brief sags.

f) Electric Motor-Generator set to provide DC to the bus of the PWM drive. A flywheel will be placed between the motor and generator for energy storage. When the input power fails, the momentum of the flywheel will maintain rotation of the generator. The duration of ride-through will depend upon system inertia and degree of motor loading.

g) A superconducting magnet for energy storage. Upon power failure, the current circulating in the superconductors will be discharged to the DC bus.

h) Use a 460 VAC drive to power a 230 VAC motor. The drive must be over-sized to deliver full motor current. Program the drive for 230 VAC output at 60 HZ and for a constant Volts per Hertz ratio from 0 to 60 Hertz.

The PWM drive will continue to generate a peak voltage that will be the normal value for a 460 VAC motor, but the width of the pulses will be reduced to give an effective 230 VAC value to the motor. The motor must have the traditional 600 VAC insulation that is normally found in the standard 460 VAC motor.

i) A boost converter maintains the DC bus volts on a PWM drive. It will draw its power from the AC line. It will therefore be in parallel with the drive’s input rectifier. The converter will allow ride-through without the use of a battery or capacitor bank. The addition of a battery backup will assist during power outage.

Upon voltage sag, the output of the boost converter will be enabled, causing it to supply DC power directly to the drive’s DC bus. During a complete outage, however, the bus capacitors will quickly discharge into the bus and the drive will trip off.

j) A power supply is commercially available that will accept an input of 300 to 700 VDC directly off a DC bus. It will have an output of 24 VDC plus and minus 2%. The output can be used to power a 24 VDC control circuit within a drive. The power supply will then allow drive control circuitry to be maintained rather than necessitating a trip to prevent the drive from becoming uncontrolled. The drive will continue to operate at reduced input power but with reduced output torque and output speed.

k) A static compensator may be used to boost voltage on large portions of a facility distribution system.


References:

Richard A. Epperly, Frederick L. Hoadley and Richard W. Piefer, "Considerations When Applying ASD’s in Continuous Processes," IEEE Transactions on Industry Applications, Vol. 33, pp 389-396, March/April 1997.

Annabelle van Zyl, Rene Spee, Alex Faveluke and Shibashis Bhowmik,
“Voltage Sag Ride-Through for Adjustable-Speed Drives With Active Rectifiers,” IEEE Transactions on Industry Applications, Vol. 34, pp 1270-1277, November/December 1998.

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