Harmonic Analysis and Filter Bank Design for a New Rectifier for a Cold Roll Mill

The trend in industry to replace motor-generator sets with static rectifiers often results in soft voltage and harmonic distortion issues which must be corrected after the fact that the distribution system not only experiences increased reactive load with the added rectifier transformers but also generally experiences a loss in reactive power when the synchronous motor is removed. In the cold roll mill of this large steel-processing facility, these concerns were addressed proactively early in the planning stages with the design of a stepped harmonic filter to be installed with a new static rectifier. This paper describes the harmonic analysis of the new rectifier and the design of the harmonic filter to correct harmonics and support voltage. The harmonic analysis identified a unique parallel resonance below the filter tuning and quantified the likelihood of the rectifier exciting this resonance. The filter bank design incorporated a small modification to the filter to mitigate this concern. Additionally, this paper presents the analysis of the possible interaction of the harmonic filter with an electrically nearby advanced reactive compensator.