Suppression of Time-Varying Interharmonics Produced by Medium-Frequency Induction Melting Furnaces by a HAPF System

This paper describes the design and field implementation of a hybrid active power filter (HAPF) system to suppress time-varying interharmonics injected into the grid by medium-frequency coreless induction melting furnaces (IMFs). In a sample steel melt shop, variable-frequency load-resonant inverters of works coils are supplied from the medium-voltage grid via 12- and 24-pulse thyristor rectifiers. The cross-modulation phenomenon in ac-dc-ac link of the medium-frequency coreless IMF installations produces interharmonics and characteristic and uncharacteristic harmonics in the grid-side line current waveforms. Furthermore, frequencies of dominant interharmonics are migrating in time inside a frequency window as the operating frequency of the load-resonant inverter varies in wide range during a melting cycle. The HAPF system developed for this application consists of nine HAPF units operating in parallel in current control mode and is connected to the grid via a coupling transformer. Current control is achieved by extracting high-order fixed frequency characteristic harmonic components from the reference current signal and defining a fixed hysteresis band for each HAPF unit. The input LC filter of the HAPF system is optimized in the design stage by taking into account the frequency range of prominent interharmonics. Theoretical findings are verified in a sample steel melt shop by extensive field measurements. Field test results have shown that the developed HAPF system suppresses successfully the dominant time-varying interharmonics and harmonics in the frequency range from 250 to 650 Hz