ADJUSTABLE SPEED AC DRIVES BEHAVIOR DUE TO VOLTAGE SAG
Many adjustable-speed drives are equally sensitive to voltage sags as process control equipment discussed in the previous section. Tripping of adjustable-speed drives can occur due to several phenomena:
• The drive controller or protection will detect the sudden change in operating conditions and trip the drive to prevent damage to the power electronic components.
• The drop in de bus voltage that results from the sag will cause mal-operation or tripping of the drive controller or of the PWM inverter.
• The increased ac currents during the sag or the post-sag over currents charging the de capacitor will cause an overcurrent trip or blowing of fuses protecting the power electronics components.
• The process driven by the motor will not be able to tolerate the drop in speed or the torque variations due to the sag.
After a trip some drives restart immediately when the voltage comes back; some restart after a certain delay time and others only after a manual restart. The various automatic restart options are only relevant when the process tolerates a certain level of speed and torque variations. The effect of the voltage sag on the de bus voltage is the main cause of equipment tripping.
Abstract – Voltage sags are normally described by magnitude variation and duration. In addition to these quantities, sags are also characterized by unbalance (asymmetry), non- sinusoidal waveshapes, and phase angle shift (phase jump). These factors are important for determining the behavior of ac motor drives during sags. Voltage unbalance and phase angle shifts cause large unbalanced source currents to, excessive voltage ripple in the dc-link, and reduced dc-link average voltage. The response of the motor and drive to these varies considerably. Experimental results clearly show the load- dependent behavior of a typical drive. The ability of the drive to ride-through a voltage sag is dependent upon the energy storage capacity of the dc-link capacitor, the speed and inertia of the load, the power consumed by the load, and the trip point settings of the drive. The control system of the drive has a great impact on the behavior of the drive during the sag and after recovery. The trip point settings of many drives can by field-adjusted and greatly improve many nuisance trips resulting from minor voltage sags.
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