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Paper title “Dedicated Adjustable Speed Drive Switchboard”
Author:
Terence Hazel The use of adjustable speed drives (ASD) has many advantages in terms of energy efficiency, control precision and flexibility. On the other hand their use may cause problems in the power system due to the harmonic currents generated by power electronic devices. These harmonic currents will cause harmonic voltages which can have detrimental effects even on the ASDs themselves. For a large site several 690V ASDs were used in a particular production unit. It was decided to power all ASDs from one MCC rather than to install several ASDs in each MCC of the production unit. This allowed filtering the harmonic currents at one location in the plant only, rather than having harmonic currents generated at several locations and filtering at only one location. To reduce the amount of cabling work at site, all ASDs were installed within the MCC cubicles so that the motors were connected directly to the fully-withdrawable MCC. Using fully-withdrawable MCCs allows maintenance to be carried out without any shutdowns. The installation of the ASDs within the MCCs will be described. Filtering of the harmonic currents was done by means of active filters. These filters not only prevented harmonic currents from circulating in the power system but also ensured very clean voltage waveforms at the MCC busbars thus providing excellent operating conditions for the ASDs themselves. Active filters were selected rather than passive filters in order to avoid over-compensation when the plant production was low. The design of the active filters including provision for maintenance without any process shutdowns will be described.
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