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The ac and dc drives commonly used on land and offshore drilling rigs produce extremely high levels of harmonic distortion. With voltage distortion often exceeding 20%, equipment associated with the drilling operation can experience erratic operation and equipment damage. Repeated damage to one or several ac drives is common. One land rig in Northern Alberta was experiencing failures with its centrifuge equipment on a weekly basis. During one visit to the site, a drive in the centrifuge was found to have tripped off causing the centrifuge to plug up. After cleaning out the centrifuge and restoring the drive, it tripped off again shortly after drilling operations resumed. A power quality analyzer was connected, which revealed extremely high levels of voltage distortion during drilling operations. Deep notches, which are visible in the voltage waveform, were found to be the result of silicon-controlled rectifiers in the mud pump dc drives. Total harmonic voltage distortion (v_\mathrmTHD) reached 25%. A series-connected passive filter was installed ahead of the centrifuge equipment to reduce the voltage notching and lower voltage distortion. The filter reduced the notch depth by more than half and lowered overall voltage distortion at the centrifuge panel to $<$ 9% during the most severe drilling operations. With line-side voltage distortion levels remaining in the 20% range, the filter proved to be extremely effective in eliminating all centrifuge operational and premature failure issues. [ABSTRACT FROM AUTHOR]