Design and Implementation of a Hybrid System for the Mitigation of PQ Problems of Medium-Frequency Induction Steel-Melting Furnaces

This paper describes a hybrid system for the mitigation of power quality (PQ) problems of medium-frequency coreless induction steel-melting furnaces. The hybrid system is composed of passive shunt-detuned filters and a hybrid active power filter (HAPF). Since the single-phase load-resonant thyristor inverter of the work coil operates at variable frequency and supplied from a multipulse thyristor rectifier, interharmonic current components, due to cross-modulation phenomenon across ac-dc-ac link, appear in supply lines. The frequencies of these interharmonics are migrating in time over a wide range, and they interact with passive shunt LC filters and other power-system components resulting in harmful effects on the overall system. Reactive power compensation of medium-frequency coreless induction steel-melting furnace is achieved by passive shunt-detuned filter banks, thus avoiding the amplification risk of interharmonic current components. Interharmonics are satisfactorily suppressed by a HAPF with K control strategy. The performance of controller is improved by modifying the conventional K control strategy and adding feedforward control loops for dominant characteristic harmonics. The resulting hybrid system is operating in a steel-melt shop successfully since March 2013.