The authors discuss high-voltage power conversion. Conventional series connection and three-level voltage source inverter techniques are reviewed and compared. A novel versatile multilevel commutation cell is introduced: it is shown that this topology is safer and more simple to control, and delivers purer output waveforms. The authors show how this technique can be applied to either choppers or voltage-source inverters and generalized to any number of switches.<<ETX>>

Multi-level conversion : high voltage choppers and voltage-source inverters

This paper introduces a new configuration of bidirectional multilevel converter in electric vehicle (EV) applications It has multilevel dc–dc converter with a dc link capacitor voltage balance feature The multilevel dc–dc converter operates in a bidirectional manner, which is a fundamental requirement in EVs Compared to the conventional configurations, the proposed one only implements two extra power switches and a capacitor to balance the voltage of the T-type multilevel inverter (MLI) capacitor over a complete drive cycle or at fault conditions Therefore, no extra isolated sensor, control loops, and/or special switching pattern are required Moreover, the proposed configuration due to the high-frequency cycle-by-cycle voltage balance between ${C_N}$ and ${C_P}$ , the bulky electrolytic capacitors used in T-type MLI, are replaced with more reliable longer life film capacitors This will result in a size and weight reduction of the converter by 20% This allows more real estate for the EV battery in the chassis’ space envelope to increase its capacity The proposed configuration is tested and validated using a matlab /Simulink simulation model A laboratory prototype of 1 kW is built to provide the proof of concept results as well

A Novel Bidirectional T-Type Multilevel Inverter for Electric Vehicle Applications

In Part I of this paper, the self-voltage balancing nature of modular multilevel converters (MMCs) has been proven mathematically. To utilize this merit of MMCs, a novel modulation, namely Y-matrix modulation (YMM), is proposed to transform the math analysis of Part I into modulation practice. With the proposed YMM, MMCs have secured self-voltage balancing. Conventionally, either a complicated voltage balancing control, or extra components are embedded in MMCs to balance the capacitor voltage. Compared to conventional MMC capacitor voltage balancing strategies, YMM features simple algorithms and good reachability to high-level MMCs while maintaining the original half-bridge submodule topology. To simplify the analysis, YMM is introduced to two-level and three-level MMCs as examples. Then, the generalized YMM is derived, which is suitable for high-level MMCs. Several YMM based MMC case studies are provided for verification purposes.

A Modular Multilevel Converter With Self-Voltage Balancing Part II: Y-Matrix Modulation

During the lifespan of a polymer electrolyte membrane fuel cell (PEMFC) system, some heterogeneities between the cells constituting the stack can appear. The voltage of one particular cell in a stack may decrease because of specific aging or local malfunctioning such as drying. As a result, more heat is generated in this cell leading to an increase in its temperature and, thus, an additional voltage loss. This snowball effect can result in the failure of the cell. Therefore, the lifetime of a PEMFC stack can be increased by applying energy management to its cells. Note that the output voltage of a cell is lower than a stack. Hence, a high conversion ratio converter is necessary to implement such energy management. An efficient way to increase the output voltage is to connect the output capacitors of the converters such as the boosts in series. Ensuring the converters’ controllability is a key point to implement energy management. In this paper, an equalizer system is proposed to ensure the controllability of the boost converters. The balancing speed and the low number of switches are the main advantages of this system. The validity of the proposed system is verified through simulation and experiments.

/pdf/design-and-modeling-of-an-equalizer-for-fuel-cell-energy-3601f9i43q.pdf

Design and Modeling of an Equalizer for Fuel Cell Energy Management Systems

The modular feature of the modular multilevel converter (MMC) distinguishes itself from other topologies for medium-/high-voltage applications. However, as the count of submodules increases, the control complexity of voltage balancing for each submodule sharply increases. Conventionally, the MMC is deemed to have no inherit voltage balancing property without voltage monitoring and control. This paper mathematically proves that MMC capacitor voltage is self-balanced by nature if considering certain submodule patterns. This implies that MMC could achieve the submodule capacitor voltage balancing without any monitoring or control. The mathematical proof starts from observing the two- and three-level MMCs. Mathematically, two- and three-level MMCs are proved to be self-balanced. Then, a similar observation is extended to $N$ -level MMC. And $N$ -level MMC is proven to be self-balanced mathematically. The modulation which utilizes this MMC merit and secures the self-voltage balancing for MMC will be fully explained in Part II.

A Modular Multilevel Converter With Self-Voltage Balancing Part I: Mathematical Proof

Multilevel converters have become very attractive for high-voltage-level power conversion in renewable power generation applications. The converter topology is an important issue in the studies of multilevel converter. Many multilevel topologies have been developed, but few of them are qualified with capacitor voltage self-balancing capability. This paper proposes a novel diode-clamped modular multilevel converter with simplified capacitor voltage-balancing control. In this topology, low-power rating diodes are used to clamp the capacitor voltages of the converter. Only the top submodule in each arm of the converter requires capacitor voltage control. Consequently, very few voltage sensors are needed for voltage control and the control computation burden is reduced greatly when the quantity of the submodules is high. A simple voltage-balancing control method with carrier phase-shifted modulation strategy is developed for this topology. Experiments based on a laboratory prototype were carried out and the results validated the capacitor-balancing performance of the proposed topology.

/pdf/a-novel-diode-clamped-modular-multilevel-converter-with-4oh2p1s619.pdf

Zhigang Gao

Papers

A Novel Diode-Clamped Modular Multilevel Converter With Simplified Capacitor Voltage-Balancing Control