Preventing AC Drive Failures Due to Commutation Notches on a Drilling Rig

The operation of offshore drilling rigs and production platforms requires the extensive use of variable-speed drives. Historically, this involved dc drives almost exclusively, but more recently, ac drives have begun to find their way onto these facilities. Often, combinations of dc and ac drives are used. Silicon-controlled rectifier- or thyristor-bridge-based dc drives will distort the supply voltage by creating commutating notches. These notches can become quite deep if the source is weak, the dc drive load is heavy, and line reactors are not installed. Excessive voltage notching can cause operational problems and/or failures in other loads connected to the bus. When system resonance causes the ringing of these notches, the problem becomes even more severe. One such susceptible load is the ac drive. Wide-spectrum filters designed to reduce the harmonic currents being drawn by the ac drive can also be used to protect the drive from the harmful effects of commutation notching and other associated disturbances.