Nam-Joon Ku

A Modularized Equalization Method Based on Magnetizing Energy for a Series-Connected Lithium-Ion Battery String

A single charge equalizer using the multi-winding transformer (SCEMT) is useful for the precise and fast equalization. Since this type of equalizer requires voltage sensing circuits for each battery cell, the size and cost of the overall equalizer system increase. In this paper, a novel switching method, which does not require voltage sensing circuits, is proposed for the SCEMT. However, due to the problems related to the implementation of multiwinding in a single transformer, the SCEMT is definitely difficult to apply to long series-connected lithium-ion battery string. In order to overcome these drawbacks, a novel module charge equalizer based on the configuration of the SCEMT is developed. Unlike the conventional module charge equalizer using additional components, the proposed novel module charge equalizer utilizes the magnetizing energy of the multiwinding transformer for the equalization among modules. Therefore, proposed module charge equalizer does not suffer from the size, cost, and loss related to the modularization. The validity of the proposed module charge equalizer is verified through experimental results.

A simple control method for neutral-point voltage oscillation reduction of three-level Neutral-Point-Clamped inverter

Three-level Neutral-Point-Clamped inverter has inherent problem which is the neutral-point voltage fluctuation caused by the neutral-point current that flows into or out from the neutral-point. Even though the neutral point voltage is balanced with certain method, the oscillation of neutral point voltage is still remains due to the output phase current. If total capacitance of DC-link capacitor is smaller, the amplitude of oscillation is larger. In this paper, three-level neutral-point-clamped inverter system is analysed by the small-signal modeling and the control method reducing the oscillation of the neutral-point voltage is proposed. By the proposed method, the amplitude of the neutral-point voltage oscillation is reduced to almost zero. The validity of the proposed method is verified by the simulation and the experiment.

A novel switching loss minimized PWM method for a high switching frequency three-level inverter with a SiC clamp diode

The previous papers introduce the inverters replacing Si diode with SiC diode to reduce the switching loss. In the NPC inverter, the switching loss is also reduced by replacing device in the clamp diode. However, the switching loss in IGBT is large and the reduced switching loss cannot be still neglected. This paper presents the reverse recovery characteristic according to the change of switching states when Si diode and SiC diode are used as clamp diode and proposes a method to minimize the switching loss containing the reverse recovery loss in NPC inverter at low modulation index. It is expected that the reverse recovery loss can be almost eliminated and the switching loss can be considerably reduced by the proposed method. Therefore, it is also possible to operate the inverter at the higher frequency with the better system efficiency. The effectiveness of the proposed method is verified by numerical analysis.

A Simplified Space-Vector PWM Scheme for N-Level NPC Inverter Based on Two-Level Space-Vector PWM

This paper proposes a general dwell-time equations for N-level inverter that are derived without the complex calculation. Contrary to the conventional method which has two dwell-time equations due to types, the proposed dwell-time equation is defined as one equation without the separated types. This method proceeds following step. After the voltage reference in abc-coordinate are transformed to αβ - coordinate, the dwell-times of each vector are derived from the equation of time balance and voltage-time characteristic equation. Next, the space-vector diagram of N-level inverter is simplified to the space-vector diagram of two level inverter by moving the α -axis component and β -axis component respectively. Finally, only one general equation is derived. Availability of the proposed general equation is verified by the simulation results.

The Control Algorithm of Three-Level NPC Inverter under Unbalanced Input Voltage Conditions

This paper performs the control algorithm of three-level Neutral-point-clamped (3L-NPC) inverter system including the neutral point voltage balance control under unbalanced voltage conditions because the 3L-NPC inverter as compared with a conventional two-level inverter has an inherent problem of neutral point potential variation. The control of 3L-NPC inverter system under unbalanced input supply conditions is as follows. First, the reference input currents are selected to eliminate the constant component of reactive power and the second-order harmonics of active power, and these are induced by the dc-link voltage control. Second, to operate the inverter system under unbalanced conditions, both positive-and negative sequence component of grid currents should be controlled simultaneously. Finally, to neutral point voltage control, the simplified neutral point voltage balancing method is used to control the neutral point voltage. This method is achieved by the calculated off-set voltage including the neutral point balancing factor. So, the overall control algorithm is composed with a dc-link voltage control, a current control, and neutral point voltage control. The validity of overall control algorithm with the neutral point voltage balancing control method has been proven through the simulation results.

A Design Methodology of Digital Controller Considering Time Delay Effect for a Modular Multilevel Converter VSC HVDC System

A modular multilevel converter is widely adapted for a high-voltage direct current power transmission system. This study proposes a design methodology for a novel digital control that mitigates the negative effects caused by time delay, including communication transport delay for a modular multilevel converter. The modeling and negative effect of time delay are analyzed theoretically in a frequency domain, and its compensation methodology based on an inverse model is described fully with practical considerations. The proposed methodology is verified through several simulation results using a modular 21-level converter system.

A novel switching loss minimization method for single-phase flying-capacitor multilevel inverter

This paper proposes a novel method to reduce the switching loss in a single-phase flying-capacitor multilevel inverter. The proposed method satisfies the regulation of a flying-capacitors voltage and also reduces the switching loss. The proposed method is based on the redundant state selection (RSS) method, reported as the solution for a control of the flying-capacitors voltage. The modified RSS criterion considers the switching state transient for minimizing the switching loss. The theoretical analysis and detail implementation of the proposed approach are presented. Through simulation results, the effectiveness of the proposed RSS method is verified.

Partial O-state Clamping PWM Method for Three-Level NPC Inverter with a SiC Clamp Diode

This paper presents the reverse recovery characteristic according to the change of switching states when Si diode and SiC diode are used as clamp diode and proposes a method to minimize the switching loss containing the reverse recovery loss in the neutral-point-clamped inverter at low modulation index. The previous papers introduce many multiple circuits replacing Si diode with SiC diode to reduce the switching loss. In the neutral-point-clamped inverter, the switching loss can be also reduced by replacing device in the clamp diode. However, the switching loss in IGBT is large and the reduced switching loss cannot be still neglected. It is expected that the reverse recovery effect can be infrequent and the switching loss can be considerably reduced by the proposed method. Therefore, it is also possible to operate the inverter at the higher frequency with the better system efficiency and reduce the volume, weight and cost of filters and heatsink. The effectiveness of the proposed method is verified by numerical analysis and experiment results.

A Two-Phase Interleaved Bidirectional DC-DC Converter with Zero-Voltage-Transition

The two-phase interleaved bidirectional DC-DC converter (TIBDC) is a very attractive solution to problems related to battery energy storage systems. However, the hard-switching TIBDC increases the switching loss and electromagnetic interference noise when the switching frequency increases. Hence, a soft-switching technique is required to overcome these disadvantages. In this study, a novel TIBDC with zero-voltage transition (TIBDC-ZVT) is proposed. Soft switching in the boost and buck main switches is achieved through a resonant cell that consists of a single resonant inductor and four auxiliary switches. Given its single resonant inductor, the proposed TIBDC-ZVT has a reduced size and can easily be implemented. The validity of the proposed TIBDC-ZVT is verified through experimental results.

Small-signal modeling and control of three-phase grid-connected three-level neutral-point-clamped inverter with a LCL filter

The propose of this paper is to present the systematic processes of constructing the averaged small-signal model of the three-phase grid-connected three-level neutral-point-clamped inverter with a LCL filter and to design the control system for the grid current by using the model. The systematic processes are composed of the averaging, perturbing and linearizing step about the circuit equations. In addition, this paper presents a frequency-domain analysis to design the control systems and study the system stability and the control scheme of the grid current based on the averaged small-signal model. The validity of the model is verified by the simulation and the experiments.