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 gives an overview of medium-voltage (MV) multilevel converters with a focus on achieving minimum harmonic distortion and high efficiency at low switching frequency operation. Increasing the power rating by minimizing switching frequency while still maintaining reasonable power quality is an important requirement and a persistent challenge for the industry. Existing solutions are discussed and analyzed based on their topologies, limitations, and control techniques. As a preferred option for future research and application, an inverter configuration based on three-level building blocks to generate five-level voltage waveforms is suggested. This paper shows that such an inverter may be operated at a very low switching frequency to achieve minimum on-state and dynamic device losses for highly efficient MV drive applications while maintaining low harmonic distortion.

Medium-Voltage Multilevel Converters—State of the Art, Challenges, and Requirements in Industrial Applications

Multilevel inverters have been widely accepted for high-power high-voltage applications. Their performance is highly superior to that of conventional two-level inverters due to reduced harmonic distortion, lower electromagnetic interference, and higher dc link voltages. However, it has some disadvantages such as increased number of components, complex pulsewidth modulation control method, and voltage-balancing problem. In this paper, a new topology with a reversing-voltage component is proposed to improve the multilevel performance by compensating the disadvantages mentioned. This topology requires fewer components compared to existing inverters (particularly in higher levels) and requires fewer carrier signals and gate drives. Therefore, the overall cost and complexity are greatly reduced particularly for higher output voltage levels. Finally, a prototype of the seven-level proposed topology is built and tested to show the performance of the inverter by experimental results.

Design and Implementation of a New Multilevel Inverter Topology

In this paper, authors propose a new power multilevel converter topology that is very competitive compared to the existing ones. It consists of packed U cells (PUC). Each U cell consists of an arrangement of two power switches and one capacitor. It offers high-energy conversion quality using a small number of capacitors and power devices and consequently, has a very low production cost. An averaged model of the topology is detailed. The operating principle of the transformerless seven-level inverter is analyzed and detailed. The multilevel sinusoidal modulation has been adapted for use with the PUC-based structure. The control strategy has been designed to reduce the harmonic contents of the load voltage. With such converters, filters' rating is considerably reduced. A comparative study is performed to highlight the advantages of the new packed U cells topology. The operation of the proposed converter topology has been verified through simulation. Experimental validation was performed using DS1103 DSP of dSpace.

Packed U Cells Multilevel Converter Topology: Theoretical Study and Experimental Validation

Multilevel inverters are in state-of-the-art power conversion systems due to their improved voltage and current waveforms. Cascaded H-bridge (CHB) multilevel inverters have been considered as an alternative in the medium-voltage converter market and experimental electric vehicles. Their variant, the asymmetrical CHB (ACHB) inverter, optimizes the number of voltage levels by using dc supplies with different voltages. However, the CHB and ACHB inverters require a large number of bidirectional and isolated dc supplies that must be balanced, and as any multilevel inverter, they reduce the power quality with the voltage amplitude. This paper presents a solution to improve the already mentioned drawbacks of ACHB inverters by using a high-frequency link using only one dc power source. This single power source can be selected according to the application (regenerative, nonregenerative, and with variable or permanent voltage amplitude). This paper shows the experimental results of a 27-level ACHB inverter with a variable and single dc source, but the strategy can be applied to any ACHB inverter with any single dc source. As a result, the reduction of active semiconductors, transformers, and total harmonic distortion was achieved using only one dc power source.

High-Frequency Link: A Solution for Using Only One DC Source in Asymmetric Cascaded Multilevel Inverters

This paper proposes an isolated cascaded multilevel inverter employing low-frequency three-phase transformers and a single DC input power source. The proposed circuit configuration can reduce a number of transformers compared with traditional three-phase multilevel inverters using single-phase transformers. It controls switching phase angles to obtain an optimal switching pattern identified with the fundamental frequency of the output voltage. Owing to this control strategy, harmonic components of the output voltage and switching losses can be diminished considerably. To verify the performance of the proposed approach, we implemented computer-aided simulations and experiments using a prototype.

Cascaded Multilevel Inverter Employing Three-Phase Transformers and Single DC Input

For 3-phase applications, an isolated multilevel inverter employing 3-phase transformers can reduce the number of transformers compared with that of using single- phase transformers. However it needs a large number of switches. To solve this problem, we propose a modified 3-phase isolated multilevel inverter adopting common-arm scheme. Phase angle control based on equal-area method is presented to obtain optimal switching functions identified with fundamental frequency of output voltage. Thanks to this control strategy, harmonic components of output voltage and switching losses can be diminished considerably. To verify performance of the proposed multilevel inverter, we implemented computer-aided simulations.

The isolated multilevel inverter using common-arm scheme for 3-phase system

Small-scale and self-sustainable grids known as microgrid (MG) are a key element for future power systems with a high penetration of renewable generators. The uncertainty of renewable power generation in systems such as photovoltaic (PV) and wind turbine needs to be compensated with balancing devices. The balancing devices can be an Energy Storage System (ESS) or conventional thermal fossil-fuel generators with enhanced flexibility. The uncertainty of renewable generators can be statistically analyzed for cost-effective assessment and operation of those compensating devices. Multiple uncertainty factors can be investigated and modeled as a joint probability distribution function (PDF) considering the temporal correlation among themselves. Several uncertainty factors with different marginal distributions and scales can be integrated as multivariate probability distribution by transforming them into normal distribution using rank correlation. As the number of uncertainty factors considered in a microgrid increases, it leads to much more complexity to in defining the conditional probability distribution generated from a joint PDF. In this paper, a method to model the distribution of net-load forecast error is proposed considering the correlation among uncertainty factors. A data-driven Gaussian process regression is introduced to train and validate conditional PDF among uncertainty factors, which are transformed into normal distribution without losing intrinsic marginal distribution. The conditional density function based on the proposed method has better suitability to estimate distribution of netload error. The conditional density function based on the proposed method shows better suitability for estimation of net load error distribution.

Forecast Error Modeling for Microgrid Operation Considering Correlation among Distributed Generators Using Gaussian Process Regression

Multiple battery charging systems can be configured with various combinations of ac-dc and dc-dc converters. This paper reviews the reliability and economy of three representative configurations of multi-battery charging systems. Existing PSA (part stress analysis) can analyse the failure rate considering the environmental factors for the type of parts, the number of parts, the connection state of parts and the voltage and current stress of parts. However, since it does not reflect the system's operating characteristics, the Fault-tree analysis (FTA) method based on the fault tree is used. Still, it cannot be analysed considering the operational risk of the converter according to various load conditions. Therefore, in this study, by defining a failure according to the minimum and maximum load of the multiple battery charging systems and designing a separate fault tree, the converter operation characteristics were considered by reflecting the number of battery charges, the characteristics according to SOC (state of charge) and the redundancy effect that appears when cyclic charging is applied. Through the new enhanced FTA method, the multiple battery charging systems (Type 1) to which sequential charging is applied are quantitatively proven advantageous in terms of reliability and economy. 

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1049/els2.12082

Sung Geun Song

Papers

Cascaded Multilevel Inverter Employing Three-Phase Transformers and Single DC Input

The isolated multilevel inverter using common-arm scheme for 3-phase system

Forecast Error Modeling for Microgrid Operation Considering Correlation among Distributed Generators Using Gaussian Process Regression

Comparison of reliability and economic feasibility for the three configurations of multiple battery charging systems

Regression model-based adaptive receding horizon control of soft open points for loss minimization in distribution networks