Yongsug Suh

Parallel Connection of Integrated Gate Commutated Thyristors (IGCTs) and Diodes

This paper describes the parallel connection of 4.5 kV integrated gate commutated thyristors (IGCTs) and diodes. The impact of varying device characteristics on the stationary current distribution of parallel connected semiconductors is investigated. Possible solutions to improve the current sharing at steady state are presented. A thermal stabilization effect of parallel connected IGCTs is discussed. Furthermore, the behavior of parallel connected devices during switching transients is investigated experimentally in a 1.5-kV, 5-kA buck converter. Especially, the impact of asymmetrical circuit layouts, magnetic couplings, different turn-on and turn-off delays, and junction temperatures are considered. It is shown that a substantial current derating is necessary to enable a reliable operation of parallel IGCTs and diodes.

A Comparative Study on Control Algorithm for Active Front-End Rectifier of Large Motor Drives Under Unbalanced Input

This paper investigates unbalance compensating control algorithms for the active front-end rectifier employed in high-power motor drive systems. The generic voltage-source-converter topology of the three-level neutral-point clamped rectifier is selected for the comparative study. Three different control algorithms have been formulated based on the symmetrical components and dq synchronous frames under generalized unbalanced operating conditions. Control algorithms have been investigated with respect to major design factors in large drive systems: fault ride-through capability, harmonic distortions, and efficiency. The control algorithm having zero amplitude of negative-sequence input current shows the most optimized and cost-effective performance regarding the fault ride-through capability and efficiency aspects. The control algorithm nullifying the oscillating components of the instantaneous active power generates the least harmonic distortions. A combination of these two control algorithms depending on system requirements and operating modes provides improved performance factors of fault ride-through capability, low harmonic distortions, and efficiency under generalized unbalanced operating conditions.

Application of IGCT in High Power Rectifiers

This paper investigates the possibility of applying IGCT (Integrated Gate Commutated Thyristor) into high current high power rectifier systems. The optimal power converter topology that fully utilizes the functionality of IGCT is proposed. The proposed power converter consists of a front-end diode rectifier and 3-level step-down DC/DC converter. As compared to 2-level operation, half the input DC voltage and twice the effective switching frequency of 3-level operation can provide a wider operation range of continuous conduction mode resulting in a higher average output current of less ripple size than 2-level case. A laboratory proto-type of 5 kA, 3.1 MW rectifier has been simulated and tested to confirm the feasibility of IGCT in the proposed power converter topology. The proposed solution generates a relatively smooth output DC current with less contents of high frequency harmonics, thereby avoiding the high frequency harmonic related problems found in IGBT based high current rectifiers. IGCT would play a significant role to improve the cost-effective performance and reliability in low and medium ranged high current high power rectifier systems.

Efficiency comparison of voltage source and current source drive system for medium voltage applications

This paper calculates and compares the efficiency and loss distribution in three most popular state-of-the-art types of medium voltage drive systems; current source drive with active front-end rectifier, current source drive with 18-pulse thyristor rectifier, and three-level voltage source drive with 12-pulse/24-pulse diode rectifier. Based on the proposed simple and accurate method of calculating switching losses, snubber and filter losses, three-level voltage source drive system has been found to produce the highest efficiency of 98.77% under the rated load of 1.6 MW

Application of IGCT in High-Power Rectifiers

This paper investigates the possibility of applying an integrated gate commutated thyristor (IGCT) into high-current high-power rectifier systems. The optimal power converter topology that fully utilizes the functionality of the IGCT is proposed. The proposed power converter consists of a front-end diode rectifier and three-level step-down dc/dc converter. As compared to two-level operation, half the input dc voltage and twice the effective switching frequency of three-level operation can provide a wider operation range of continuous conduction mode resulting in a higher average output current of less ripple size than two-level case. A laboratory prototype of a 5-kA 3.1-MW rectifier has been simulated and tested to confirm the feasibility of the IGCT in the proposed power converter topology. The proposed solution generates a relatively smooth output dc current with less contents of high-frequency harmonics, thereby avoiding the high-frequency harmonic-related problems found in insulated-gate-bipolar-transistors-based high-current rectifiers. The IGCT would play a significant role to improve the cost-effective performance and reliability in low- and medium-ranged high-current high-power rectifier systems.

Fault detection and tolerant control of 3-phase NPC active rectifier

Compared to conventional 2-level AC/DC PWM converter, the 3-phase NPC active rectifier has a high number of power semi-conductors. Hence the possibility of the open-switch faults is much higher. Due to the many number of active elements, the detection and the tolerant control of the open-switch faults are more complicated than the conventional 2-level converter. Furthermore, the open-switch faults give rise to imbalance of the AC input currents and the fluctuation DC-link voltage. This paper proposes the open-switch fault detection method and the tolerant control method. In particular, open-switch fault case of one switch among inner six switches of NPC active rectifier is analyzed. The usefulness of this paper is verified through the computer simulation and experimentations, respectively.

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.

Fault Response of a DFIG-based Offshore Wind Power Plant Taking into Account the Wake Effect

In order to meet the low voltage ride-through requirement in a grid code, a wind power plant (WPP) has to stay connected to a grid, supporting the voltage recovery for a grid fault. To do this, a plant-level controller as well as a wind generator (WG) controller is essential. The dynamic response of a WPP should be analyzed in order to design a plant-level controller. The dynamic response of a WPP for a grid fault is the collective response of all WGs, which depends on the wind speed approaching the WG. Thus, the dynamic response of a WPP should be analyzed by taking the wake effect into consideration, because different wind speeds at WGs will result in different responses of the WPP. This paper analyzes the response of a doubly fed induction generator (DFIG)-based offshore WPP with a grid fault taking into account the wake effect. To obtain the approaching wind speed of a WG in a WPP, we considered the cumulative impact of multiple shadowing and the effect of the wind direction. The voltage, reactive power, and active power at the point of common coupling of a 100 MW DFIG-based offshore WPP were analyzed during and after a grid fault under various wind and fault conditions using an EMTP-RV simulator. The results clearly demonstrate that not considering the wake effect leads to significantly different results, particularly for the reactive power and active power, which could potentially lead to incorrect conclusions and / or control schemes for a WPP.

A comparative study on control algorithm for active front-end rectifier of large motor drives under unbalance input

This paper investigates unbalance compensating control algorithms for the active front-end rectifier employed in high-power motor drive systems. The generic voltage source converter topology of three-level neutral-point clamped rectifier is selected for the comparative study. Three different control algorithms have been formulated based on the symmetrical components and dq synchronous frames under generalized unbalanced operating conditions. Control algorithms have been investigated with respect to major design factors in large drive systems; fault ride-through capability, harmonic distortions, and efficiency. The control algorithm having zero amplitude of negative sequence input current shows the most optimized and cost-effective performance regarding fault ride-through capability and efficiency aspects. The control algorithm nullifying the oscillating components of the instantaneous active power generates least harmonic distortions. Combination of these two control algorithms depending on system requirements and operating modes provides improved performance factors of fault ride-through capability, low harmonic distortions, and efficiency under the generalized unbalanced operating conditions.

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.