A parallel hybrid modular multilevel converter for high voltage DC applications

Reliability and efficiency of power transmission has been at the forefront of research for some time and is currently being given critical consideration due to the increased dependence on electrical energy. With the increased demand for electricity, engineers are considering different methods of supply arrangement to improve the security of electricity supply. High Voltage Direct Current (HVDC) transmission is a technology that avails itself for distance power transmission, interconnection of asynchronous networks and cross sea or offshore power transmission. The main element of an HVDC system is the AC/DC or DC/AC power converter. Recently, a new breed of power converters suitable for HVDC transmission has been the subject of considerable research work. These converters are modular in structure with high efficiency and their operation results in higher power quality, with reduced filtering components when compared to the use of Line Commutated and two-level or three-level Voltage Source Converter (VSC) based transmission systems. One such modular circuit is the Parallel Hybrid Modular Multilevel Voltage Source Converter (PH-M2L-VSC). This research investigates the operation and control of the PH-M2L-VSC for HVDC applications. Control schemes supporting the operation of the converter as would be expected of an HVDC VSC are proposed, including operation with an unbalanced AC network. Simulation results from a medium voltage demonstrator and experimental results from a laboratory scale prototype are presented to validate the methods proposed and enable a performance comparison to be made with other topologies

A parallel hybrid modular multilevel converter for high voltage DC applications

Reliability and efficiency of power transmission has been at the forefront of research for some time and is currently being given critical consideration due to the increased dependence on electrical energy. With the increased demand for electricity, engineers are considering different methods of supply arrangement to improve the security of electricity supply. High Voltage Direct Current (HVDC) transmission is a technology that avails itself for distance power transmission, interconnection of asynchronous networks and cross sea or offshore power transmission. The main element of an HVDC system is the AC/DC or DC/AC power converter. Recently, a new breed of power converters suitable for HVDC transmission has been the subject of considerable research work. These converters are modular in structure with high efficiency and their operation results in higher power quality, with reduced filtering components when compared to the use of Line Commutated and two-level or three-level Voltage Source Converter (VSC) based transmission systems. One such modular circuit is the Parallel Hybrid Modular Multilevel Voltage Source Converter (PH-M2L-VSC). This research investigates the operation and control of the PH-M2L-VSC for HVDC applications. Control schemes supporting the operation of the converter as would be expected of an HVDC VSC are proposed, including operation with an unbalanced AC network. Simulation results from a medium voltage demonstrator and experimental results from a laboratory scale prototype are presented to validate the methods proposed and enable a performance comparison to be made with other topologies

Design optimization of a short-term duty electrical machine for extreme environment

This paper presents design optimisation of a short term duty electrical machine for extreme environments of high temperature and high altitudes. For such extreme environmental conditions of above 80⁰C and altitudes of 30km, thermal loading limits are a critical consideration in machines, especially if high power density and high efficiency are to be achieved. The influence of different material on the performance of such machines is investigated. Also the effect of different slot and pole combinations are studied for machines used for such extreme operating conditions but with short duty. In the research, A Non-dominated Sorting Genetic Algorithm (NSGAII) considering an analytical electromagnetic model, structural and thermal model together with Finite Element (FE) methods are used to optimise the design of the machine for such environments achieving high efficiencies and high power density with relatively minimal computational time. The adopted thermal model is then validated through experiments and then implemented within the Genetic Algorithm (GA). It is shown that, generally, the designs are thermally limited where the pole numbers are limited by volt-amps drawn from the converter. The design consisting of a high slot number allows for improving the current loading and thus, significant weight reduction can be achieved

Control of a hybrid modular multilevel converter during grid voltage unbalance

The recently proposed parallel hybrid modular multilevel converter is considered to be a low loss, low component count converter with soft switching capability of the ‘main’ bridge. The converter has similar advantages to other emerging modular multilevel converter circuits being considered for HVDC power transmission. However, during ac network unbalance the individual ‘chain-links’ exchange unequal amounts of power with the grid which requires appropriate remedial action. This paper presents research into the performance of the converter and proposes a suitable control method that enables the converter to operate during grid voltage unbalance. The proposed control concept involves the use of asymmetric third harmonic voltage generation in the ‘chain-links’ of the converter to redistribute the power exchanged between the individual ‘chain-links’ and the grid. Mathematical analysis and simulation modelling with results are presented to support the work described

Impact of soft magnetic material on design of high speed permanent magnet machines

This paper investigates the effect of two soft magnetic materials on a high speed machine design, namely 6.5% Silicon Steel and Cobalt-Iron alloy. The effect of design parameters on the machine performance as an aircraft starter-generator is analysed. The material properties which include B-H characteristics and the losses are obtained at different frequencies under an experiment and used to predict the machine performance accurately. In the investigation presented in this paper, it is shown that machines designed with 6.5% Silicon Steel at a high core flux density has lower weight and lower losses than the Cobalt-Iron alloy designs. This is mainly due to the extra weight contributed by the copper content especially in the end-windings. Due to the high operating frequencies, the core-losses in the Cobalt-Iron machine designs are found to outweigh the copper-losses incurred in the Silicon Steel machines. It is also shown that change in stack length/number of turns has a considerable effect on the copper losses at starting, however has no significant advantage on rated efficiency which happens to be in a field-weakening operating point. It is also shown that the performance of the machine designs depend significantly on material selection and the operating point of the core. The implications of the variation of design parameters on the machine performance is discussed and provides insight into the influence of parameters that effect overall power density

The star-switched MMC (SSMMC): a hybrid VSC for HVDC applications

This paper presents a new hybrid VSC topology (the Star-Switched MMC) – suitable for HVDC applications. The basic structure and operating principles of the topology are described. Control strategies that regulate the power exchanged between the VSC, the AC network and the DC network are presented. A modulation strategy ensuring appropriate switching of the individual chain-link sub-modules and a capacitor voltage balancing algorithm that ensures the capacitor voltages are maintained within the required tolerance are discussed. Results from a simulation model are presented to validate the expected performance of the converter and the proposed control schemes

Impact of slot/pole combination on inter-turn short-circuit current in fault-tolerant permanent magnet machines

This paper investigates the influence of the slot/pole (S/P) combination on inter-turn short-circuit (SC) current in fault-tolerant permanent magnet (FT-PM) machines. A 2-D sub-domain field computational model with multi-objective genetic algorithm is used for the design and performance prediction of the considered FT-PM machines. The electromagnetic losses of machines, including iron, magnet, and winding losses are systematically computed using analytical tools. During the postprocessing stage, a 1-D analysis is employed for turn-turn fault analysis. The method calculates self-and mutual inductances of both the faulty and healthy turns under an SC fault condition with respect to the fault locations, and thus SC fault current, considering its location. Eight FT-PM machines with different S/P combinations are analyzed. Both the performance of the machine during normal operation and induced currents during a turn-turn SC fault are investigated. To evaluate the thermal impact of each S/P combination under an inter-turn fault condition, a thermal analysis is performed using finite element computation. It is shown that low-rotor-pole-number machines have a better fault tolerance capability, while high-rotor-pole-number machines are lighter and provide higher efficiency. Results show that the influence of the S/P selection on inter-turn fault SC current needs to be considered during the design process to balance the efficiency and power density against fault-tolerant criteria of the application at hand

Operation of a hybrid modular multilevel converter during grid voltage unbalance

The recently proposed parallel hybrid modular multilevel converter is considered to be a low loss, low component count converter with soft switching capability of the ‘main’ H-bridge. The converter has similar advantages to other emerging modular multilevel converter circuits being considered for HVDC power transmission and can be made compact which is desirable for offshore application. However, during ac network unbalance the individual ‘chain-links’ exchange unequal amounts of power with the grid which requires appropriate remedial action. This paper presents research into the performance of the converter and proposes a suitable control method that enables the converter to operate during grid voltage unbalance. The proposed control concept involves the use of asymmetric third harmonic voltage generation in the ‘chain-links’ of the converter to redistribute the power exchanged between the individual ‘chain-links’ and the grid. Mathematical analysis and simulation modelling with results are presented to support the work described

The Series Bridge Converter (SBC): design of a compact modular multilevel converter for grid applications

This paper presents a novel hybrid modular multilevel voltage source converter suitable for grid applications. The proposed converter retains the advantages of other modular multilevel topologies and can be made more compact making it attractive for offshore stations and other footprint critical applications like city infeeds. In this paper, the basic operating principle and design criteria for the converter implementation are presented. The submodule capacitor requirements which have significant influence on the size of a converter station are also evaluated and compared to the MMC. The performance of the converter is supported by simulation results from a representative medium voltage scaled demonstrator

Movilidad ascendente de la inmigración en España : ¿asimilación o segmentación ocupacional?

Background of INCASI Project H2020-MSCA-RISE-2015 GA 691004. WP1: CompilationLa posibilidad de movilidad ascendente de la población inmigrante es un factor importante a la hora de explicar su integración social. Asimismo, a nivel individual, la expectativa de movilidad puede contribuir a aumentar o disminuir la incertidumbre socioeconómica. El objetivo de este artículo es analizar la movilidad ocupacional vertical de la inmigración. Tratamos de explicar cuales son las variables clave en el acceso de los inmigrantes a los salarios altos. Una regresión logística nos permite ver como interactúan las variables individuales, como la antigüedad y el nivel de estudios, con las variables estructurales, como el tamaño de la empresa y el sector de actividad. Además, un análisis factorial de correspondencias múltiples nos permite agrupar todas las variables y ofrecer una tipología que clasifica las trayectorias laborales con arreglo a la segmentación del mercado de trabajo.Possibilities for occupational mobility are an important factor in the explanation of the integration of immigrant workers into receiving societies. Moreover, at the individual level, the prospects for upward occupational mobility determine the uncertainty facing immigrant workers. In this paper, we examine the upward occupational mobility of immigrant workers in Spain. We attempt to explain the key variables determining immigrants' access to high wages. A multinomial logistic regression examines the interaction between individual variables such as age and education level, and structural variables such as company size and sector. In addition, a factor analysis allows us to group these variables into a typology that classifies career paths according to labour market segmentation