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 multi-pulse 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.