Multilevel Converters: An Enabling Technology for High-Power Applications

| Multilevel converters are considered today as the state-of-the-art power-conversion systems for high-power and power-quality demanding applications. This paper presents a tutorial on this technology, covering the operating principle and the different power circuit topologies, modulation methods, technical issues and industry applications. Special attention is given to established technology already found in industry with more in-depth and self-contained information, while recent advances and state-of-the-art contributions are addressed with useful references. This paper serves as an introduction to the subject for the not-familiarized reader, as well as an update or reference for academics and practicing engineers working in the field of industrial and power electronics.Ministerio de Ciencia y Tecnología DPI2001-3089Ministerio de Eduación y Ciencia d TEC2006-0386

Open-circuit fault diagnosis and maintenance in multi-pulse parallel and series TRU topologies

©2020 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.Transformer Rectifier Units (TRUs) are a reliable way for DC generation in several electric applications. These units are formed by multiple three-phase uncontrolled bridge rectifiers connected according to two main topologies (parallel and series), and fed by a phase-shifting transformer, which can have different configurations. Fault diagnosis of the uncontrolled bridge rectifier diodes is one of the most important concerns on the electronic devices, nonetheless, rectifier units are inherently not protected in front of Open-Circuit (O/C) faults, which cause malfunction and performance deterioration. In order to solve this drawback, the proposed fault diagnosis method is based on the O/C fault signature observed in the DC-link output voltage of TRUs rectifier. It allows detecting the O/C diodes of parallel and series TRUs with different phase-shifting transformer configurations and for the most usual fault scenarios. Moreover, it also helps the prediction of diodes that could be exposed to failure after the fault, which provides corrective maintenance for the TRU development. The proposed method is illustrated from MATLABTM numerical simulations of a 12-pulse TRU, and is validated with experimental tests.This work supported in part by the Research Project Estabilidad de Redes MVdc Integrando Tecnologias de Energias Renovables, Almacenamiento de Energia y Convertidores de Fuente de Impedancia, RTI2018-095720-B-C33, in part by the Ministerio de Ciencia, Innovación y Universidades, and in part by the European Union.Peer ReviewedPostprint (author's final draft

An On-line Diagnostic Method for Open-circuit Switch Faults in NPC Multilevel Converters

On-line condition monitoring is of paramount importance for multilevel converters used in safety-critical applications. A novel on-line diagnostic method for detecting open-circuit switch faults in neutral-point-clamped (NPC) multilevel converters is introduced in this paper. The principle of this method is based on monitoring the abnormal variation of the dc-bus neutral-point current in combination with the existing information on instantaneous switching states and phase currents. Advantages of this method include simpler implementation and faster detection speed compared to other existing diagnostic methods in the literature. In this method, only one additional current sensor is required for measuring the dc-bus neutral-point current, therefore the implementation cost is low. Simulation and experimental results based on a lab-scale 50 kVA adjustable speed drive (ASD) with a three-level NPC inverter validate the efficacy of this novel diagnostic method

Fault analysis and protection for wind power generation systems

Wind power is growing rapidly around the world as a means of dealing with the world energy shortage and associated environmental problems. Ambitious plans concerning renewable energy applications around European countries require a reliable yet economic system to generate, collect and transmit electrical power from renewable resources. In populous Europe, collective offshore large-scale wind farms are efficient and have the potential to reach this sustainable goal. This means that an even more reliable collection and transmission system is sought. However, this relatively new area of offshore wind power generation lacks systematic fault transient analysis and operational experience to enhance further development. At the same time, appropriate fault protection schemes are required. This thesis focuses on the analysis of fault conditions and investigates effective fault ride-through and protection schemes in the electrical systems of wind farms, for both small-scale land and large-scale offshore systems. Two variable-speed generation systems are considered: doubly-fed induction generators (DFIGs) and permanent magnet synchronous generators (PMSGs) because of their popularity nowadays for wind turbines scaling to several-MW systems. The main content of the thesis is as follows. The protection issues of DFIGs are discussed, with a novel protection scheme proposed. Then the analysis of protection scheme options for the fully rated converter, direct-driven PMSGs are examined and performed with simulation comparisons. Further, the protection schemes for wind farm collection and transmission systems are studied in terms of voltage level, collection level wind farm collection grids and high-voltage transmission systems for multi-terminal DC connected transmission systems, the so-called “Supergrid”. Throughout the thesis, theoretical analyses of fault transient performances are detailed with PSCAD/EMTDC simulation results for verification. Finally, the economic aspect for possible redundant design of wind farm electrical systems is investigated based on operational and economic statistics from an example wind farm project

Electric Vehicle Powertrain Integrated Charging

Batterieelektrische Fahrzeuge benötigen ein im Fahrzeug eingebautes Ladegerät, um die Energie aus dem Wechselstromnetz für die Gleichstrom- Batterie aufzubereiten. Integriertes Laden ist eine Methode der Integration von Ladefunktionalität in die Antriebsstrangkomponenten, welche während des Parkens außer Betrieb sind, mit dem Ziel, Kosten, Gewicht und Volumen des Ladegerät zu sparen. Das Laden ohne die Sicherheitsmaßnahme einer galvanischen Trennung im Ladegerät ist möglich mit zusätzlichen Maßnahmen gegen elektrischen Schlag, z.B. mit einer Fehlerstromerkennung und entsprechenden Trenneinrichtung. Im Stand der Technik wurden 33 integrierte Ladekonzepte gefunden und bezüglich Antriebsstrangnutzung, benötigte Komponenten, Drehmoment der elektrischen Maschine und Wirkungsgrad verglichen. Im Rahmen dieser Arbeit wird ein neues galvanisch getrenntes integriertes Ladekonzept beschrieben, mit dem Ziel, die Effizienz zu verbessern und gleichzeitig auftretendes Drehmoment in der Maschine zu vermeiden. Der Antriebsstrang wird als DC/DC-Wandler mit der elektrischen Maschine als Transformator im Stillstand genutzt. Berechnungen zeigen eine maximale Effizienz von 88%. Ansätze zur Verbesserung des Wirkungsgrads und zur Integration des Energieflusses im Bordnetz werden in dieser Arbeit vorgeschlagen und diskutiert. Allerdings muss der Rotorkäfig geöffnet werden, um ein Drehmoment während des Laden zu vermeiden. Dies stellt einen ähnlichen Aufwand dar wie die Darstellung eines separaten Ladegeräts. Somit ist dieses Konzept aus heutiger Sicht wegen niedriger Effizienz und hoher Kosten gegenüber einem separaten Ladegerät nicht konkurrenzfähig. Zwei Ladekonzepte ohne galvanische Trennung, die eine sechsphasige elektrische Maschine als in Serie geschaltete Hoch- und Tiefsetzsteller nutzen, werden im Rahmen der Arbeit vorgestellt und bezüglich der benötigten Komponenten, der Effizienz und des Drehmoments des Maschine ausgearbeitet. Die Antriebsstrangverluste werden für die Ladebedingungen mit Gleichströmen analysiert, basierend auf neuen Materialcharakterisierungen für die angewendete Belastung. Es wurden Wirkungsgrade bis zu 93% demonstriert und auch in theoretischen Berechnungen mit einer maximalen Abweichung von ±1% zum experimentellen Befund bestätigt. Zum Schutz gegen elektrischen Schlag bei nicht isolierten Ladekonzepten werden drei Konzepte für eine Fehlerstrommessung präsentiert und anhand von Messergebnissen analysiert. Siliziumkarbid-Inverter-Technologien zeigen in Kombination mit diesen Ladekonzepten Wirkungsgrade, die vergleichbar zu herkömmlichen separaten Ladegeräten sind, und weisen dabei deutlich geringere Kosten auf

Fault signal propagation through the PMSM motor drive systems

This paper describes how a mechanical disturbance on the shaft of a variable speed permanent magnet motor (PMSM) is propagated to the supply input side of the drive system, and therefore may be detected by monitoring specific frequency components in the rectifier input current. The propagation of the disturbance from the torque disturbance, to the motor current, then to the dc link current and finally to the rectifier input current is derived as a series of transfer functions so that both the frequency and the amplitude of the disturbance component in the rectifier input current can be predicted for a specific mechanical disturbance. The limitations to detect the mechanical fault by monitoring only the supply currents are also addressed. Simulation and experimental results are presented to demonstrate the accuracy of the quantitative analysis, and the potential for fault detection using the rectifier input currents

Average value of the DC-link output voltage in multi-phase uncontrolled bridge rectifiers under supply voltage balance and unbalance conditions

Average value of the DC-link output voltage is a variable of interest in multi-phase uncontrolled bridge rectifiers. The aim of this paper is to present a new, effort-saving procedure capable of providing an accurate value of this variable, a value which can be later corrected considering the usually omitted voltage drops. The proposed method, based on the Cauchy’s formula (1841), allows the limitations of the existing methods to be overcome and can be used under supply voltage balance and unbalance conditions. Time-domain simulations and experimental tests were conducted to show the usefulness of the method and validate its accuracy. Under supply voltage balance conditions, the new method allows results as accurate as those provided by analytical expressions available in the literature or time-domain simulations performed by any software to be obtained. Moreover, under supply voltage unbalance conditions, this method outperforms analytical expressions available in the literature and at least equals time-domain simulations performed by any software in terms of accuracy of the obtained results. Therefore, under supply voltage balance and unbalance conditions, the proposed method makes the mathematical effort required to elaborate analytical expressions or the computational effort required to perform time-domain simulations unnecessary. In addition, the new method provides suitable estimates of values experimentally determined.This work was supported in part by the Ministerio de Ciencia, Innovación y Universidades under Grant RTI2018-095720-B-C33.Peer ReviewedPostprint (author's final draft

Grounding and Charging Strategy for Ships during Cold Ironing Operation

In order to minimize the pollution that ships generate at ports, ships can be connected to the utility grid during charging, also known as shore-to-ship connection or cold ironing operation. The pollution can also be remarkably reduced if the ships are full-electric or hybrid. With the utilization of a common DC bus, several ships can be charged simultaneously. However, due to the common DC bus, the ships are not galvanic isolated from each other such that leakage current can occur among the ships and the quay when the current leaks to the ground during a fault. Hence, this paper proposes a complete charging and grounding strategy, which will provide galvanic isolation between the ships and the quay. The charging and grounding strategy are verified through simulations in the Matlab/Simulink environment. An isolated and ideal PSFB DC-DC converter with a rated power of 400 kW was proposed to obtain galvanic isolation. The proposed converter obtained a stable output during nominal and half load from the simulation results. In addition, two grounding systems on the shore-side and the ship-side were proposed. On the shore-side, a double grounding TN-C grounding system with a NGR resistor was designed such that the leakage current can easily be detected when a ground fault occurs. On the ship-side, an IT system with HRMG resistors was designed to reduce the leakage current such that the risk of corrosion was reduced and provided safety for personnel. As a result, a fault on the shore-side did not affect the ship-side grounding system and opposite. Faults that can appear on the charging system were found through research and simulated with the complete charging and grounding system to verify that the grounding system was optimally designed. The results during a fault on the system showed that the shore-side grounding system was not optimally designed because the NGR did not reduce the fault current to a lower value than 25 A. The common DC bus was created from an uncontrolled rectifier that suffered a substantial power dissipation. As a result, the output of the PSFB DC-DC converter was unstable. Therefore, a resistor was added to the TN-C grounding configuration during simulations of the charging system to achieve a stable output of the DC-DC converter. The IT grounding configuration on the ship-side reduced the fault current to 6 mA during a LG fault, and personnel safety was kept at a safe level when a person touched one of the DC lines. However, it was shown that the personnel safety was not obtained when a person touched the energized chassis due to a dangerous voltage potential

Converter fault diagnosis and post-fault operation of a doubly-fed induction generator for a wind turbine

Wind energy has become one of the most important alternative energy resources because of the global warming crisis. Wind turbines are often erected off-shore because of favourable wind conditions, requiring lower towers than on-shore. The doubly-fed induction generator is one of the most widely used generators with wind turbines. In such a wind turbine the power converters are less robust than the generator and other mechanical parts. If any switch failure occurs in the converters, the wind turbine may be seriously damaged and have to stop. Therefore, converter health monitoring and fault diagnosis are important to improve system reliability. Moreover, to avoid shutting down the wind turbine, converter fault diagnosis may permit a change in control strategy and/or reconfigure the power converters to permit post-fault operation. This research focuses on switch fault diagnosis and post-fault operation for the converters of the doubly-fed induction generator. The effects of an open-switch fault and a short-circuit switch fault are analysed. Several existing open-switch fault diagnosis methods are examined but are found to be unsuitable for the doubly-fed induction generator. The causes of false alarms with these methods are investigated. A proposed diagnosis method, with false alarm suppression, has the fault detection capability equivalent to the best of the existing methods, but improves system reliability. After any open-switch fault is detected, reconfiguration to a four-switch topology is activated to avoid shutting down the system. Short-circuit switch faults are also investigated. Possible methods to deal with this fault are discussed and demonstrated in simulation. Operating the doubly-fed induction generator as a squirrel cage generator with aerodynamic power control of turbine blades is suggested if this fault occurs in the machine-side converter, while constant dc voltage control is suitable for a short-circuit switch fault in the grid-side converter.Wind energy has become one of the most important alternative energy resources because of the global warming crisis. Wind turbines are often erected off-shore because of favourable wind conditions, requiring lower towers than on-shore. The doubly-fed induction generator is one of the most widely used generators with wind turbines. In such a wind turbine the power converters are less robust than the generator and other mechanical parts. If any switch failure occurs in the converters, the wind turbine may be seriously damaged and have to stop. Therefore, converter health monitoring and fault diagnosis are important to improve system reliability. Moreover, to avoid shutting down the wind turbine, converter fault diagnosis may permit a change in control strategy and/or reconfigure the power converters to permit post-fault operation. This research focuses on switch fault diagnosis and post-fault operation for the converters of the doubly-fed induction generator. The effects of an open-switch fault and a short-circuit switch fault are analysed. Several existing open-switch fault diagnosis methods are examined but are found to be unsuitable for the doubly-fed induction generator. The causes of false alarms with these methods are investigated. A proposed diagnosis method, with false alarm suppression, has the fault detection capability equivalent to the best of the existing methods, but improves system reliability. After any open-switch fault is detected, reconfiguration to a four-switch topology is activated to avoid shutting down the system. Short-circuit switch faults are also investigated. Possible methods to deal with this fault are discussed and demonstrated in simulation. Operating the doubly-fed induction generator as a squirrel cage generator with aerodynamic power control of turbine blades is suggested if this fault occurs in the machine-side converter, while constant dc voltage control is suitable for a short-circuit switch fault in the grid-side converter

A New MMC Topology Which Decreases the Sub Module Voltage Fluctuations at Lower Switching Frequencies and Improves Converter Efficiency

Modular Multi-level inverters (MMCs) are becoming more common because of their suitability for applications in smart grids and multi-terminal HVDC transmission networks. The comparative study between the two classic topologies of MMC (AC side cascaded and DC side cascaded topologies) indicates some disadvantages which can affect their performance. The sub module voltage ripple and switching losses are one of the main issues and the reason for the appearance of the circulating current is sub module capacitor voltage ripple. Hence, the sub module capacitor needs to be large enough to constrain the voltage ripple when operating at lower switching frequencies. However, this is prohibitively uneconomical for the high voltage applications. There is always a trade off in MMC design between the switching frequency and sub module voltage ripple