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 proposes a method for power flow control between utility and microgrid through back-to-back converters, which facilitates desired real and reactive power flow between utility and microgrid. In the proposed control strategy, the system can run in two different modes depending on the power requirement in the microgrid. In mode-1, specified amount of real and reactive power are shared between the utility and the microgrid through the back-to-back converters. Mode-2 is invoked when the power that can be supplied by the DGs in the microgrid reaches its maximum limit. In such a case, the rest of the power demand of the microgrid has to be supplied by the utility. An arrangement between DGs in the microgrid is proposed to achieve load sharing in both grid connected and islanded modes. The back-to-back converters also provide total frequency isolation between the utility and the microgrid. It is shown that the voltage or frequency fluctuation in the utility side has no impact on voltage or power in microgrid side. Proper relay-breaker operation coordination is proposed during fault along with the blocking of the back-to-back converters for seamless resynchronization. Both impedance and motor type loads are considered to verify the system stability. The impact of dc side voltage fluctuation of the DGs and DG tripping on power sharing is also investigated. The efficacy of the proposed control ar-rangement has been validated through simulation for various operating conditions. The model of the microgrid power system is simulated in PSCAD.

Power management and power flow control with back-to-back converters in a utility connected microgrid

Reliability engineering dates back to reliability studies in the 20th century; since then, various models have been defined and used. Software engineering plays a key role from several viewpoints, but the main concern is that we're moving toward a more connected world, including enterprises and mobile devices. The three articles in this special issue illustrate current trends in this domain.

Reliability Engineering

The increased penetration of Distributed Energy Resources (DER) is challenging the entire architecture of conventional electrical power system. Microgrid paradigm, featuring higher flexibility and reliability, becomes an attractive candidate for the future power grid. In this paper, an overview of microgrid configurations is given. Then, possible structure options and control methods of DER units are presented, which is followed by the descriptions of system controls and power management strategies for AC microgrids. Finally, future trends of microgrids are discussed pointing out how this concept can be a key to achieve a more intelligent and flexible power system.

A Review of Power Electronics Based Microgrids

Renewable energy resources are becoming the dominating element in power systems. Along with de-carbonization, they transform power systems into a more distributed, autonomous, bottom-up style one. We speak of Smart Grid and Microgrid when distributed energy resources take over. While being a means to improve technical and financial efficiency, planning, operations, and carbon footprint, it is these new technologies that also introduce new challenges. Reliability is one of them, deserving a new way of describing and assessing system and component reliability. This paper introduces a new reliability framework that covers these new elements in modern power systems. It can be seen that reliability assessment of modern power systems also requires introducing local reliability concepts as well as incorporating different electro-magnetic/mechanical stability issues.

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An Overview on the Reliability of Modern Power Electronic Based Power Systems

Reliability prediction in power electronic converters is of paramount importance for converter manufacturers and operators. Conventional approaches employ generic data provided in handbooks for random chance failure probability prediction within useful lifetime. However, the wear-out failures affect the long-term performance of the converters. Therefore, this article proposes a comprehensive approach for estimating the converter reliability within useful lifetime and wear-out period. Moreover, this article proposes a wear-out failure prediction approach based on a structural reliability concept. The proposed approach can quickly predict the converter wear-out behavior unlike conventional Monte Carlo-based techniques. Hence, it facilitates reliability modeling and evaluation in large-scale power electronic-based power systems with huge number of components. The proposed comprehensive failure function over the useful lifetime and wear-out phase can be used for optimal design and manufacturing by identifying the failure prone components and end-of-life prediction. Moreover, the proposed reliability model can be used for optimal decision-making in design, planning, operation, and maintenance of modern power electronic-based power systems. The proposed methodology is exemplified for a photovoltaic inverter by predicting its failure characteristics.

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A Guideline for Reliability Prediction in Power Electronic Converters

This article aims to incorporate the reliability model of power electronic converters into power system reliability analysis. The converter reliability has widely been explored in device- and converter-levels according to physics of failure analysis. However, optimal decision-making for design, planning, operation, and maintenance of power electronic converters require system-level reliability modeling of power electronic-based power systems. Therefore, this article proposes a procedure to evaluate the reliability of power electronic-based power systems from the device-level up to the system-level. Furthermore, the impact of converter failure rates including random chance and wear-out failures on power system performance in different applications such as wind turbine and electronic transmission lines is illustrated. Moreover, because of a high calculation burden raised by the physics of failure analysis for large-scale power electronic systems, this article explores the required accuracy for reliability modeling of converters in different applications. Numerical case studies are provided employing modified versions of the Roy Billinton Test System (RBTS). The analysis shows that the converter failures may affect the overall system performance depending on its application. Therefore, an accurate converter reliability model is, in some cases, required for reliability assessment and management in modern power systems.

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Incorporating Power Electronic Converters Reliability Into Modern Power System Reliability Analysis

Power converters are one of the failure sources in modern power systems, and hence driver of maintenance and downtime costs, which should be reduced by reliable design, control, and operation of converters. This paper proposes a power sharing control strategy for evenly distributing the thermal stresses among dc converters in dc microgrids, and consequently enhancing the overall system reliability. The aim of this paper is to extend the aging process of failure prone converters by adjusting their loadings. The proposed approach employs the prior experienced thermal damages on the converter's fragile components in order to adjust its contribution on demand supply. According to the proposed strategy, the higher the thermal stress on a converter is, the lower the power it will supply. As a result, the overall system reliability will be improved. A numerical case study on a dc microgrid is presented to illustrate the effectiveness of the proposed power sharing strategy. Moreover, experimental tests are provided to demonstrate the applicability of the reliability-oriented power sharing method.

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System-Level Reliability-Oriented Power Sharing Strategy for DC Power Systems

Mission profiles such as environmental and operational conditions together with the system structure including energy resources, grid and converter topologies induce stress on different converters and thereby play a significant role on power electronic systems reliability. Temperature swing and maximum temperature are two of the critical stressors on the most failure-prone components of converters, i.e., capacitors and power semiconductors. Temperature-related stressors generate electrothermal stress on these components ultimately triggering high potential failure mechanisms. Failure of any component may cause converter outage and system shutdown. This paper explores the reliability performance of different converters operating in a power system and indicates the failure-prone converters from wear out perspective. It provides a system-level reliability insight for design, control, and operation of multiconverter system by extending the mission-profile-based reliability estimation approach. The analysis is provided for a dc microgrid due to the increasing interest that dc systems have been gaining in recent years; however, it can be applied for reliability studies in any multiconverter system. The outcomes can be worthwhile for maintenance and risk management as well as security assessment in modern power systems.

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Saeed Peyghami

Papers

An Overview on the Reliability of Modern Power Electronic Based Power Systems

A Guideline for Reliability Prediction in Power Electronic Converters

Incorporating Power Electronic Converters Reliability Into Modern Power System Reliability Analysis

System-Level Reliability-Oriented Power Sharing Strategy for DC Power Systems

Mission-Profile-Based System-Level Reliability Analysis in DC Microgrids