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

Real-Time Machine Learning Based Open Switch Fault Detection and Isolation for Multilevel Multiphase Drives

Due to the rapid proliferation interest of the multiphase machines and their combination with multilevel inverters technology, the demand for high reliability and resilient in the multiphase multilevel drives is increased. High reliability can be achieved by deploying systematic preventive real-time monitoring, robust control, and efficient fault diagnosis strategies. Fault diagnosis, as an indispensable methodology to preserve the seamless post-fault operation, is carried out in consecutive steps; monitoring the observable signals to generate the residuals, evaluating the observations to make a binary decision if any abnormality has occurred, and identifying the characteristics of the abnormalities to locate and isolate the failed components. It is followed by applying an appropriate reconfiguration strategy to ensure that the system can tolerate the failure. The primary focus of presented dissertation was to address employing computational and machine learning techniques to construct a proficient fault diagnosis scheme in multilevel multiphase drives. First, the data-driven nonlinear model identification/prediction methods are used to form a hybrid fault detection framework, which combines module-level and system-level methods in power converters, to enhance the performance and obtain a rapid real-time detection. Applying suggested nonlinear model predictors along with different systems (conventional two-level inverter and three-level neutral point clamped inverter) result in reducing the detection time to 1% of stator current fundamental period without deploying component-level monitoring equipment. Further, two methods using semi-supervised learning and analytical data mining concepts are presented to isolate the failed component. The semi-supervised fuzzy algorithm is engaged in building the clustering model because the deficient labeled datasets (prior knowledge of the system) leads to degraded performance in supervised clustering. Also, an analytical data mining procedure is presented based on data interpretability that yields two criteria to isolate the failure. A key part of this work also dealt with the discrimination between the post-fault characteristics, which are supposed to carry the data reflecting the fault influence, and the output responses, which are compensated by controllers under closed-loop control strategy. The performance of all designed schemes is evaluated through experiments

EFFICIENCY AND RELIABILITY ENHANCEMENT OF MULTIPHASE SYNCHRONOUS MOTOR DRIVES

Multiphase electric machines are attractive in comparison with three-phase ones due to advantages such as fault-tolerant nature, smaller rating per phase and lower torque ripple. More specifically, the machines with multiple three-phase windings are particularly convenient, because they are suitable for standard off-the-shelf three-phase dc/ac converter modules. For instance, they are becoming a serious option for applications such as electric vehicles and wind turbines. On the other hand, in these applications, operation at low power is often required for long time intervals; hence, improving the efficiency under such conditions is highly desired and could save a significant amount of energy in the long term. This dissertation proposes a method to enhance the efficiency of electric drives based on multiple three-phase windings at light load. The number of active legs is selected depending on the required torque at each instant. To ensure that the overall efficiency is effectively optimized, not only the converter losses, but also the stator copper losses, are taken into account. Experimental results verify the theoretical outcomes. Surface-mounted permanent-magnet synchronous motors (SPMSMs) require a position measurement to ensure a high-performance control. To avoid the cost and maintenance associated to position sensors, sensorless methods are often preferred. The approaches based on high-frequency signal injection are currently a well-established solution to obtain an accurate position estimation in SPMSMs. These techniques can be roughly divided into two groups: those based on sinusoidal or on square-wave high-frequency signals. The main drawback of the former is the limitation on the response speed, due to the presence of several low-pass filters (LPFs). On the other hand, the latter methods are sensitive to deadtime effects, and high-frequency closed-loop current control is required to overcome it. This dissertation proposes to improve the sensorless strategies based on sinusoidal high-frequency injection by simplifying the scheme employed to extract the information about the position error. Namely, two LPFs and several multiplications are removed. Such simplification does not only reduce the computational complexity, but also permits to obtain a faster response to the changes in the angle/speed, and hence, a faster closed-loop control. Experimental results based on a SPMSM prove the enhanced functionality of the proposed method with respect to the previous ones based on high-frequency sinusoidal signal injection

Modelling and indirect field‐oriented control for pole phase modulation induction motor drives

In the recent days, for the traction and electric vehicle (EV) applications, multiphase machines with pole phase modulation (PPM) technique have been proposed. The smoother operation during pole changeovers as well as steady-state operations is a significant constraint while adopting the PPM-based multiphase induction motor (PPMIM) drives for EV and traction applications. So, in this paper, the PPMIM dynamic model and associated vector control are proposed for attaining a smoother operation of the machine. The machine modelling equations and transformation matrices are implemented in an arbitrary reference frame by considering the different pole phase combinations. Based on the modelling equations, the indirect field-oriented control (IFOC) is proposed for PPMIM drives by reflecting the associated changes in parameters for different pole phase modes. In the IFOC, for regulating the d-axis and q-axis current components, single PI control loops have been implemented for all pole-phase combinations. The proposed IFOC scheme is robust and applicable for adopting any type of pulse width modulation. The experimental, as well as simulation results, are given to illustrate the potentiality of the proposed dynamic model and IFOC. The PPMIM machine performance during the steady state as well as pole changeovers in different pole phase modes are analyzed and associated. Simulation and experimental results are presented

Predictive current control in electrical drives: an illustrated review with case examples using a five-phase induction motor drive with distributed windings

The industrial application of electric machines in variable-speed drives has grown in the last decades thanks to the development of microprocessors and power converters. Although three-phase machines constitute the most common case, the interest of the research community has been recently focused on machines with more than three phases, known as multiphase machines. The principal reason lies in the exploitation of their advantages like reliability, better current distribution among phases or lower current harmonic production in the power converter than conventional three-phase ones, to name a few. Nevertheless, multiphase drives applications require the development of complex controllers to regulate the torque (or speed) and flux of the machine. In this regard, predictive current controllers have recently appeared as a viable alternative due to an easy formulation and a high flexibility to incorporate different control objectives. It is found however that these controllers face some peculiarities and limitations in their use that require attention. This work attempts to tackle the predictive current control technique as a viable alternative for the regulation of multiphase drives, paying special attention to the development of the control technique and the discussion of the benefits and limitations. Case examples with experimental results in a symmetrical five-phase induction machine with distributed windings in motoring mode of operation are used to this end

Fault Tolerant Multilevel Inverter Topologies with Energy Balancing Capability: Photovoltaic Application

The continuous increase in energy demand and depletion of conventional resources motivates the research towards the environment friendly renewable energy sources like solar and wind energy. These sources are best suitable for rural, urban and offshore locations, because of easy installation, less running cost and ample resources (sun light and wind). The remote locations are mostly islanded in nature and far away from technical expertise in case of troubleshooting. This motivates the research on development of fault tolerant converters. These fault tolerant converters increases the reliability, which provides the continuous power supply to critical loads. From the last few decades, the integration of multilevel inverters with renewable energy systems is also increasing because of advantages like, improved power quality, total harmonic distortion (THD) and reduced output filter size requirement. Employing conventional multilevel inverters for increasing the number of voltage levels increases the device count and isolated DC sources. As a result probability of semiconductor switch failure is more and energy balancing issue between sources, which in-turn degrades the reliability and performance of the inverter. The majority of conventional multilevel inverter topologies cannot address energy balancing issues between multiple photovoltaic (PV) sources, which may need because of partial shading, hotspots, uneven charging and discharging of associated batteries etc. If energy sharing not addressed effectively, the batteries which are connected to the shaded or faulty PV system will discharge faster which may cause total system shutdown and leads to under-utilization of healthier part of the system. To address these issues, fault tolerant multilevel inverter topologies with energy balancing capability are presented in this thesis. The major contributions of the proposed work are Single phase and three phase fault tolerant multilevel inverter topologies. viii Energy balancing between sources and dc off set minimization (or batteries) due to uneven charging and discharging of batteries for five-level inverter. Extending the fault tolerance and energy balancing for higher number of voltage levels. The first work of this thesis is focused to develop fault tolerant single phase and three phase multilevel inverter topologies for grid independent photovoltaic systems. The topologies are formed by using three-level and two-level half bridge inverters. The topology fed with multiple voltage sources formed by separate PV strings with MPPT charge controllers and associated batteries. Here the topologies are analyzed for different switch open circuit and/or source failures. The switching redundancy of the proposed inverters is utilized during fault condition for supplying power with lower voltage level so that critical loads are not affected. In general, the power generation in the individual PV systems may not be same at all the times, because of partial shading, local hotspots, wrong maximum power point tracking, dirt accumulation, aging etc. To address this issue energy balancing between individual sources is taken care with the help of redundant switching combinations of proposed five-level inverter carried out in second work. Because of partial shading the associated batteries with these panels will charge and discharge unevenly, which results voltage difference between terminal voltages of sources because of SOC difference. The energy balance between batteries is achieved for all operating conditions by selecting appropriate switching combination. For example during partial shading the associated battery with low SOC is discharged at slower rate than the battery with more SOC until both SOC’s are equal. This also helps in minimization of DC offset into the ac side output voltage. The mathematical analysis is presented for possible percentage of energy shared to load by both the sources during each voltage level. The third work provides single phase multilevel inverter with improved fault tolerance in terms of switch open circuit failures and energy balancing between sources. Generally multilevel inverters for photovoltaic (PV) applications are fed ix with multiple voltage sources. For majority of the multilevel inverters the load shared to individual voltage sources is not equal due to inverter structure and switching combination. This leads to under-utilization of the voltage sources. To address this issue optimal PV module distribution for multilevel inverters is proposed. Mathematical analysis is carried out for optimal sharing of PV resources for each voltage source. The proposed source distribution strategy ensures better utilization of each voltage source, as well as minimizes the control complexity for energy balancing issues. This topology requires four isolated DC-sources with a voltage magnitude of Vdc/4 (where Vdc is the voltage requirement for the conventional NPC multilevel inverter). These isolated DC voltage sources are realized with multiple PV strings. The operation of proposed multilevel single phase inverter is analyzed for different switch open-circuit failures. All the presented topologies are simulated using MATLAB/Simulink and the results are verified with laboratory prototyp

Power quality improvement utilizing photovoltaic generation connected to a weak grid

Microgrid research and development in the past decades have been one of the most popular topics. Similarly, the photovoltaic generation has been surging among renewable generation in the past few years, thanks to the availability, affordability, technology maturity of the PV panels and the PV inverter in the general market. Unfortunately, quite often, the PV installations are connected to weak grids and may have been considered as the culprit of poor power quality affecting other loads in particular sensitive loads connected to the same point of common coupling (PCC). This paper is intended to demystify the renewable generation, and turns the negative perception into positive revelation of the superiority of PV generation to the power quality improvement in a microgrid system. The main objective of this work is to develop a control method for the PV inverter so that the power quality at the PCC will be improved under various disturbances. The method is to control the reactive current based on utilizing the grid current to counteract the negative impact of the disturbances. The proposed control method is verified in PSIM platform. Promising results have been obtaine

Magnetic Material Modelling of Electrical Machines

The need for electromechanical energy conversion that takes place in electric motors, generators, and actuators is an important aspect associated with current development. The efficiency and effectiveness of the conversion process depends on both the design of the devices and the materials used in those devices. In this context, this book addresses important aspects of electrical machines, namely their materials, design, and optimization. It is essential for the design process of electrical machines to be carried out through extensive numerical field computations. Thus, the reprint also focuses on the accuracy of these computations, as well as the quality of the material models that are adopted. Another aspect of interest is the modeling of properties such as hysteresis, alternating and rotating losses and demagnetization. In addition, the characterization of materials and their dependence on mechanical quantities such as stresses and temperature are also considered. The reprint also addresses another aspect that needs to be considered for the development of the optimal global system in some applications, which is the case of drives that are associated with electrical machines