Sung Hui Choi

The modular multilevel converter (MMC) is a promising candidate for voltage-sourced-converter-based high-voltage direct current (VSC-HVDC) transmission. The dc short-circuit fault management is a crucial issue especially for overhead line transmission, where the nonpermanent dc short-circuit faults occasionally occur. In this paper, a comprehensive dc short-circuit fault ride through (FRT) strategy is proposed for a hybrid MMC which combines half-bridge submodules and full-bridge submodules. By

DC grid technology is regarded as a promising solution for future electric networks integrating a great amount of renewable energies. It calls for high-efficiency dc–dc converters with high voltage step-up ratio to interconnect medium-voltage (MV) dc distribution grids and high-voltage (HV) dc transmission grids. This paper presents an isolated bidirectional soft-switching dc–dc converter combining two-level converters in parallel on the MV side and a modular multilevel converter (MMC) on the HV

Cell capacitor energy control of a Modular Multilevel Converter (MMC) is conventionally done by controlling leg current and modulation strategy. In most of literatures, leg current transient is analyzed under an assumption that the DC bus is a stiff DC voltage source. In a real MMC-based HVDC transmission system, however, there's no such virtual stiff DC voltage source and the conventional regulation method can lead to poor dynamics of cell capacitor energy control and even make system unstable.

When a single-line-to-ground (SLG) fault occurs on the ac side of the modular multilevel converter (MMC) in an high-voltage direct-current transmission (HVDC) system, it results in the ac-side voltage sag and leads to an instantaneous reduction of the MMC power capacity. Thus, it calls for the fault ride through (FRT) strategy to coordinate two MMC stations in the HVDC system to protect the MMCs against the submodule (SM) capacitor overvoltage in case of the SLG fault. In the meantime, the HVDC

This letter presents a modular multilevel converter (MMC) for bipolar medium-voltage direct-current (MVDC)) distribution systems. By employing a zigzag transformer as a grid interface transformer with a dedicated operation method, the operation of two dc poles of an MMC can be fully decoupled. Consequently, the MMC can provide full bipolar-operation capability. No additional component is required, and no penalty of increased power rating is imposed on the grid interface transformer. The scheme b

Lack of DC short circuit fault blocking and ride through capability is one of main issues in applications of Modular Multilevel Converter (MMC) to actual HVDC transmission system. Recently, several topologies have been proposed to provide DC short circuit fault blocking capability and/or DC fault ride through capability. In this paper, the operation principles, functionalities, and characteristics of several topologies are compared. And, it's revealed that the conventional leg capacitor energy b

An isolated bidirectional dc-dc converter, which combines parallel-connected two-level converters (TLCs) on the medium-voltage side and a modular multilevel converter (MMC) on the high-voltage side, namely a TLC-MMC converter, is a promising candidate for the interconnection of medium-voltage direct current (MVDC) and HVdc grids. Different from typical phase-shift controlled isolated bidirectional dc-dc converters, transformer currents of a TLC-MMC converter are regulated by the MMC in a closed-

An isolated bidirectional dc-dc converter, which combines multiple two-level converters (TLCs) in parallel on the medium-voltage side and a modular multilevel converter (MMC) on the high-voltage side, namely the TLC-MMC converter, is a promising candidate for the interconnection of MVdc and HVdc grids. In utility applications, the availability of power converters is of great importance, which makes the fault-tolerant operation (FTO) capability a required feature. In this paper, an FTO scheme of

The AC side Single Line to Ground (SLG) fault is one of the most frequent faults in power systems. And, in an HVDC system based on modular multilevel converter it calls for the fault ride through strategy to transmit maximum possible electricity during the fault to secure power system stability. It presents different characteristics of SLG faults at the voltage regulator side and the power dispatcher side. In this paper a comprehensive fault ride through strategy for AC side SLG fault occurred a

In the conventional control strategy of the VSC-HVDC system based on the MMC, direct modulation was employed and the terminal behavior of the MMC was similar to that of the two-level converter. The DC bus voltage of the power dispatcher side was regulated indirectly by controlling voltage regulator side DC bus voltage, and the transmission line current was determined passively by the power flow. Fluctuation of the transmission line voltage would occur during rapid power flow variation due to the

DC grid technology is regarded as a promising solution for future electric networks integrating a great amount of renewable energies. It calls for high efficiency dc-dc converters with high step-up ratio to interconnect medium voltage (MV) dc distribution grids and high voltage (HV) dc transmission grids. This paper presents an isolated soft-switching dc-dc converter combining two-level converters in parallel on MV side and a modular multilevel converter (MMC) on HV side. Moreover, a comprehensi

Natural capacitor voltage balancing of six arms of a direct modulated Modular Multilevel Converter (MMC) has been observed by several articles both by experiments and simulations. However, its principles have not been revealed clearly and its dynamics have not been analyzed analytically. In this paper it's shown that a DC component and a line frequency circulating current would be induced inside the converter inherently in case of arm capacitor voltage unbalance and the induced circulating curre

This paper presents a new wide input voltage range ZVS isolated bidirectional DC/DC converter for an ultra-capacitor module used as an auxiliary power supply in electric vehicles. Compared to the conventional bidirectional converters, this modified circuit minimizes device current stress over a wide input voltage range. A natural zero-voltage switching for the transformer-connected switches is achieved without additional device. Moreover, reverse-recovery process of the diode of the transformer-

DC grid technology is regarded as a promising solution for future electric networks integrating a great amount of renewable energies. It calls for high-efficiency bidirectional dc-dc converters with a high step-up ratio to interconnect mediumvoltage dc (MVDC) distribution grids and high-voltage dc (HVDC) transmission grids. This paper focuses on dynamic performance and fault-tolerant capability of a highly-efficient isolated bidirectional dc-dc converter for this application which combines two-l

This paper proposed a dual half bridge (DHB) DC-DC converter used for ultra-capacitor power management in electric vehicles. Different from conventional DHB converters, a PWM plus phase shift (PPS) control strategy is introduced here due to the variable voltage operation mandates of an ultra-capacitor. This novel combination will be discussed from following two aspects in this paper. In the theory level, a minute operation principle analysis of this PPS based DHB converter is presented, and so i