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

The multilevel converter, (MC) had been recommended for high- and medium-level power applications for decades. It is one of the most popular converters due to its ability to decrease the harmonic distortion in the output waveform without decreasing the converter power output. This paper presents the basic concept of four different types of multilevel converter: the neutral-point clamped, flying capacitor, cascaded H-bridge and modular. This paper also reviews five different modulation techniques for multilevel converters: pulse width modulation, space vector pulse width modulation, phase shifted carrier pulse width modulation, sinusoidal pulse width modulation and selective harmonic elimination pulse width modulation. In addition, recent publications regarding developments and improvements of fundamental concern to MCs and modulation techniques are reviewed in this paper. The implementation of MCs within renewable energy systems is also discussed here.

Reviews on multilevel converter and modulation techniques

This paper proposes a modular multilevel converter (MMC) based switched reluctance motor (SRM) drive with decentralized battery energy storage system for hybrid electric vehicle applications. In the proposed drive, a battery cell and a half-bridge converter is connected as a submodule (SM), and multiple SMs are connected together for the MMC. The modular full-bridge converter is employed to drive the motor. Flexible charging and discharging functions for each SM are obtained by controlling switches in SMs. Multiple working modes and functions are achieved. Compared to conventional and existing SRM drives, there are several advantages for the proposed topology. A lower dc-bus voltage can be flexibly achieved by selecting SM operation states, which can dramatically reduce the voltage stress on the switches. Multilevel phase voltage is obtained to improve the torque capability. Battery state-of-charge balance can be achieved by independently controlling each SM. Flexible fault-tolerance ability for battery cells is equipped. The battery can be flexibly charged under both running and standstill conditions. Furthermore, a completely modular structure is achieved by using standard half-bridge modules, which is beneficial for market mass production. Experiments carried out on a three-phase 12/8 SRM confirm the effectiveness of the proposed SRM drive.

MMC-Based SRM Drives With Decentralized Battery Energy Storage System for Hybrid Electric Vehicles

The modular multilevel converter (MMC) is currently one of the power converter topologies which has attracted more research and development worldwide. Its features, such as high quality of voltages and currents, high modularity and high voltage rating, have made the MMC a very good option for several applications including high-voltage dc (HVdc) transmission, static compensators (STATCOMs), and motor drives. However, its unique features such as the large number of submodules, floating capacitor voltages, and circulating currents require a dedicated control system able to manage the terminal variables, as well as the internal variables with high dynamical performance. In this paper, a review of the research and development achieved during the last years on MMCs is shown, focusing on the challenges and proposed solutions for this power converter still faces in terms of modeling, control, reliability, power topologies, and new applications.

Modular Multilevel Converters: Recent Achievements and Challenges

For modular multilevel converters (MMCs) applied to medium-voltage dc distribution grids, using the traditional nearest level modulation (NLM) as in HVdc systems can lead to severe current distortion due to significantly reduced module numbers. This paper proposes a hybrid modulation method combining NLM and pulse width modulation (PWM) where only one module per arm operates under PWM mode. The proposed nearest level PWM (NL-PWM) method not only significantly reduces the current distortion, but also avoids the complicated voltage balancing control in each module. The harmonic characteristics of NL-PWM are derived using double Fourier transform, which provides a theoretical basis for selecting the module number and switching frequency for medium-voltage applications in accordance with grid harmonic requirements. Finally, the harmonic characteristics and feasibility of the proposed modulation method are validated by simulation and experimental studies on an MMC with six modules per arm. The simulated and experimental results reveal that NL-PWM has better voltage and current harmonic characteristics over NLM and CPS-PWM, thereby suiting the application of MMC with few models.

/pdf/a-nearest-level-pwm-method-for-the-mmc-in-dc-distribution-2n4pnwyt3i.pdf

A Nearest Level PWM Method for the MMC in DC Distribution Grids

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

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

Modular multilevel converter (MMC) is very popular in medium-voltage applications, such as high-voltage direct current transmission, static Var generator, and motor drives. The arm capacitor voltage of MMC is usually constant and the output voltage is adjusted by regulating the pulse width modulation (PWM) index for switches in previous studies. In this paper, a novel flexible capacitor voltage control strategy for MMC as motor drives was proposed, with which the arm capacitor voltage was controlled flexibly according to the motor speed, by regulating the dc component and ac component of the PWM reference signal, and unsymmetrical control was used for the control of the upper arm and the lower arm in MMC. With the proposed control strategy, the circuit efficiency and the precision of the output voltage of the converter could be improved when the motor speed was low, and d i /d t and d v /d t of the motor windings could be decreased. These proposals were implemented on a laboratory prototype, and experiments were carried out. The validity of the control strategy under both steady and dynamic states was verified.

A Novel Flexible Capacitor Voltage Control Strategy for Variable-Speed Drives With Modular Multilevel Converters

Neutral-point-clamped (NPC) multilevel-converter (MLC) is very popular in high voltage converters with multilevel topology. However, the dc-link capacitor voltage unbalance of this topology is a key issue in the NPC applications, and which will cause distortion and asymmetry of the output voltages, and also increase the voltage stress of power switches. In this paper, the three-level (3L) NPC with unbalance capacitor voltages is analyzed. A capacitor voltage balancing control strategy based on fuzzy logic controller (FLC) for the 3L-NPC-MLC is studied and presented. A fuzzy membership function is ratiocinated and determined, and based of which a capacitor voltage control strategy to suppress the capacitor voltage imbalance is proposed. Finally, simulation results show that both the balancing strategy and the FLC are effective.

A voltage balancing controller with fuzzy logic strategy for neutral point clamped multilevel converter

Grid-connected solar photovoltaic (PV) system significantly contributes to worldwide renewable energy growth and penetration, specifically the integration of PV system into the grid has become an important issue in the last decade because of the huge expansion of new power plants. Modular multilevel converter (MMC) is an upcoming promising converter technology for medium-/high-voltage applications, which has unique features such as modular structure, enhanced energy harvesting capability, scalability and so on. This paper presents a combined system of static Var generator (SVG) and PV power generation based on MMC which is capable of the maximum power point tracking (MPPT) capability. The integrated system is proposed to achieve higher performance as SVG in a distorted and unbalanced medium-voltage large-current load situation. The main functions involve managing reactive power and unbalance compensation, voltage regulation and harmonic cancellation and also ensuring the source-end three-phase currents sinusoidal and balanced. A 20-kW, 1-kV PV-MMC-based SVG was modeled and simulated in the PSIM simulation platform. The feasibility and effectiveness of the proposed combined system was demonstrated by several test results.

Bingyong Tai

Papers

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

A Novel Flexible Capacitor Voltage Control Strategy for Variable-Speed Drives With Modular Multilevel Converters

A voltage balancing controller with fuzzy logic strategy for neutral point clamped multilevel converter

Combination system of var compensation and photovoltaic power generation based on modular multilevel converter