Application of Back-to-Back Hybrid Filter to a Hot Strip Mill with Cycloconverters

In steel mills that employ the hot strip mill process, cycloconverters with nominal power ratings in the megawatt range are commonly used to drive synchronous motors. However, these cycloconverters draw highly distorted currents from the power grid, causing significant voltage distortion at the point of common coupling (PCC) and leading to numerous power quality (PQ) issues. Multi-stage passive filters are widely used to mitigate harmonics in this context. However, this approach can lead to harmonic resonance, exacerbating distortion and overloading the passive filtering system. This study presents a novel integration of a back-to-back hybrid filter, designed specifically for hot strip mills with cycloconverters at a steel mill located in the Metropolitan Area of Vitória, ES, Brazil. The proposed method combines active and passive filtering, where the active filter works in tandem with existing passive elements to compensate for harmonic components while damping resonances across a broad frequency range. Simulations are conducted to evaluate the hybrid filter’s efficacy in harmonic compensation and resonance damping, particularly during load expansion scenarios for the hot strip mill. Results indicate that the back-to-back hybrid filter significantly improves PQ by reducing harmonic overloads on pre-existing passive filter branches, thereby enhancing the reliability of the entire power system. This improvement is achieved with active filters of relatively low-rated capacity compared to the hot strip mill load, making it a cost-effective and scalable solution.