System harmonic interaction between DC and AC adjustable speed drives and cost effective mitigation

In industrial applications where both DC (direct current) and AC (alternating current) adjustable speed drives (ASD) are used, high levels of harmonic voltage and current distortions are produced. For instance, in oil and gas industry, including land and offshore drilling rigs, ships, deep notches in the voltage waveform exist due to SCR´s (silicon controlled rectifier or thyristor) operation in the mud pump DC drives. With voltage distortion often exceeding 20%, equipment associated with the drilling operation can experience erratic operation and equipment damage. Even though the notch phenomenon is well understood, the failure mode of AC ASDs has not been analyzed in published literatures. This paper provides the background of the typical power distribution installation in such an application, analyzes the voltage notches in AC/DC converters and the problems that they create, more significantly, presents the AC ASD components failure mechanisms and proposes a cost effective solution without introducing expensive harmonic filters. The theoretical foundation and analytical derivation that are used to calculate notch depth, width, and frequency resonance excitation interacting with AC ASD´s electromagnetic interference (EMI) filter will be discussed. Variation factors influencing the harmonic characteristics including SCR firing angle, generator and cable impedance, ASD´s input AC inductors or DC chokes are quantitatively evaluated. It will be demonstrated that ASDs with DC chokes are less sensitive to cause resonances through system harmonics interaction, possibly eliminating the need to install expensive and large passive filters. Theoretical and simulation models are validated through an experimental setup of a 15kW, 480V ASD system.