Harmonic Filter Analysis and Redesign for a Modern Steel Facility with Two Melt Furnaces Using Dedicated Capacitor Banks

This modern steel making facility relies on reactive compensation at the substation level in the form of harmonic filtering to support voltage for casting operations, and at the melt furnace level in the form of capacitor banks to support LMF operations. A history of harmonic filter failures at the substation, and intermittent capacitor unit failures at the melt furnace banks, culminated in a recent catastrophic failure of the filter reactor. This paper describes the analytical methods used to redesign the harmonic filter to achieve the desired level of compensation yet avoid future failure of the filter components. Field measurements showed the harmonic filter provided a low impedance path for harmonic currents produced during arcing of either or both melt furnace transformer resulting in overload of the filter reactor. Preliminary harmonic analysis enabled temporary detuning of the harmonic filter in the field to avoid this damaging resonance condition permitting steel making operations to continue while redesigning the filter. Detailed harmonic analysis and filter design considered a wide range of normal and emergency system conditions, especially the effects of extended line outages by the utility for reconductoring. Recommendations included specifications for the redesigned harmonic filter as well as changes to improve performance of the melt furnace capacitor banks