Yongjoong Lee

A Power Conversion System for AC Furnace With Enhanced Arc Stability

This paper proposes a criterion for ac arc stability and an optimal converter topology for a non-ferrous metal ac arc furnace. The ac arc furnace rated for 36kA/224V/21MW has been investigated. The key parameters of modified Cassie-Mayr arc model have been calculated by the electromagnetic and flow simulations for electric arc. The used arc simulation model has been validated by a laboratory free-burning arc setup. It has been shown that the steepness of arc current at zero-crossing determines the arc stability, and the associated peak arc resistance can be used as its quantitative measure. Among the evaluated three power converter systems, the forced commutated inverter topology with a six-step modulation realizes the steepest current at zero-crossing resulting in the best arc stability. The proposed converter topology having a high degree of arc stability is expected to bring about reliable operation of ac arc furnace and enhanced productivity of the total furnace system.

Arc stability criteria in AC arc furnace and optimal converter topologies

In this paper, the fundamental features of the arc stability in nonferrous metal AC arc furnace have been investigated, from the converter point of view. Due to the limited access to the measured arc data from the arc furnaces, the advanced arc simulations of magnetohydrodynamics (MHD) and the well known Cassie-Mayr arc model have been extensively used. The MHD based arc simulation has been validated in the subcomponent level, for the free burning arc set up in the laboratory. The arc simulation predicted the arc voltage for different currents with the accuracy which satisfies engineering requirements. It has been shown that the arc current steepness at current zero determines the arc stability, and the associated peak arc resistance can be used as its quantitative measure. Based on the presented insight into the AC arc stability, a converter topology solution which realizes an optimal arc stability has been proposed. The main results presented in this paper provide a design guideline for the future AC arc furnace converter topology developments.

A Comparative Study of Medium-Voltage Power Converter Topologies for Plasma Torch Under Dynamic Operating Conditions

This paper compares several medium-voltage power conversion systems for plasma torch application. The thermal plasma torch rated for 3 MW, 2 kA with the physical size of 1 m long is selected. The dynamic characteristics of the dc arc in a plasma torch are investigated using advanced 3-D magnetohydrodynamics simulation. The arc voltage noise of plusmn10% due to the periodic pressure wave is modeled as a sawtooth waveform in arc power loss term. The parameters of the Cassie-Mayr arc model are calculated based on 3-D simulation. A 12-pulse thyristor rectifier and two-phase staggered three-level step-down dc-dc converter having 12-pulse front-end diode rectifier are compared under the same dynamic operating condition. The three-level dc-dc converter has superior dynamic performance over the 12-pulse thyristor rectifier under the existence of arc disturbance noise. This power converter topology provides higher ignition voltage around 5 kV during ignition phase and higher arc stability.

A study on medium voltage power conversion system for plasma torch

This paper investigates the medium voltage power conversion system for plasma torch application. The plasma torch rated for 3MW, 2kA and having the physical size of 1 m long is selected. The dynamic characteristics of the DC arc in plasma torch are investigated using advanced 3D magneto-hydrodynamics simulation. The parameters of Cassie-Mayr arc model are calculated based on 3D simulation. Two-phase staggered series connected 3-level dc/dc converters is proposed to be a suitable power converter topology. Simulation shows that the ripple of arc current can be maintained within plusmn11.5% of its rated value, 460A under the existence of load disturbance of plusmn10% in arc voltage. This power converter topology provides high ignition voltage around 5kV during ignition phase and high arc stability under the existence of intrinsic arc disturbance noise.

A power conversion system for ac furnace with enhanced arc stability

This paper proposes a criterion for ac arc stability and an optimal converter topology for a non-ferrous metal ac arc furnace. The ac arc furnace rated for 36kA/224V/21MW has been investigated. The key parameters of modified Cassie-Mayr arc model have been calculated by the electromagnetic and flow simulations for electric arc. The used arc simulation model has been validated by a laboratory free-burning arc setup. It has been shown that the steepness of arc current at zero-crossing determines the arc stability, and the associated peak arc resistance can be used as its quantitative measure. Among the evaluated three power converter systems, the forced commutated inverter topology with a six-step modulation realizes the steepest current at zero-crossing resulting in the best arc stability. The proposed converter topology having a high degree of arc stability is expected to bring about reliable operation of ac arc furnace and enhanced productivity of the total furnace system.

Control algorithm of high power rectifier system in DC arc furnace for improved arc stability

Fundamental features of the arc stability in DC arc furnace of 720V/100kA/72MW have been investigated. Cassie-Mayr arc model has been employed and applied for the target DC arc furnace. In order to characterize the parameters of Cassie-Mayr arc model and the behavior of unstable arc dynamics, the advanced arc simulations of magneto-hydrodynamics (MHD) has been performed. From the results of MHD simulation, DC arc dynamic resistance is proposed to be an effective arc stability function reflecting the instability of dynamic arc behavior. A control strategy of high power rectifier system to regulate the arc stability function is also proposed in this paper. The simulation and experimental result confirm the usefulness of proposed dynamic arc resistance as arc stability function along with the active control strategy. The proposed arc stability function can be regarded as an effective criterion for the overall power conversion system to maintain highly stable arcing operation leading to better productivity and reliability in a DC arc furnace.

Arc stability control of high power thyristor rectifier system in DC arc furnace

Fundamental features of the arc stability in DC arc furnace of 720V/100kA/72MW have been investigated. Cassie-Mayr arc model has been employed and applied for the target dc arc furnace. In order to characterize the parameters of Cassie-Mayr arc model and the behavior of unstable arc dynamics, the advanced arc simulations of magneto-hydrodynamics (MHD) has been performed. From the results of MHD simulation, dc arc dynamic resistance is proposed to be an effective arc stability function reflecting the instability of dynamic arc behavior. A control strategy to regulate the arc stability function is also proposed in this paper. The simulation and experimental result confirm the usefulness of proposed dynamic arc resistance as arc stability function along with the active control strategy. The proposed arc stability function can be regarded as an effective criterion for the overall power conversion system to maintain highly stable arcing operation leading to better productivity and reliability in a dc arc furnace.