Optimization of PWM for Overmodulation Region of Two-level Inverters

Three optimized PWM techniques for the overmodulation region of two-level inverter-fed ac drives are introduced and investigated from harmonic loss minimization point of view. The optimization is elaborated for the lowest loss-factor, which is proportional to the square of rms value of current harmonics. The loss-factors are computed for different switching numbers as the function of the motor fundamental voltage. It is shown that, respect to the motor heating and torque ripples, the acceptable drive condition can be guaranteed by relatively low value of inverter switching frequency up to 96-97% of maximal possible motor voltage. Furthermore, it is shown that, the so-called Three vector methods have considerably better performance in the lower part of the overmodulation region than the so-called Two vector method for the same number of switching. The performance of the techniques is compared with other existing PWM techniques. The paper discusses the implementation details of the proposed optimal PWM techniques. The theoretical results are verified by experimental and simulation tests

Digital Control of Power Converters and Drives for Hybrid Traction and Wireless Charging

In the last years environmental issues and constant increase of fuel and energy cost have been incentivizing the development of low emission and high efficiency systems, either in traction field or in distributed generation systems from renewable energy sources. In the automotive industry, alternative solutions to the standard internal combustion engine (ICE) adopted in the conventional vehicles have been developed, i.e. fuel cell electric vehicles (FCEVs), hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEV) or pure electric vehicles (EVs), also referred as battery powered electric vehicles (BEV). Both academic and industry researchers all over the world are still facing several technical development areas concerning HEV components, system topologies, power converters and control strategies. Efficiency, lifetime, stability and volume issues have moved the attention on a number of bidirectional conversion solutions, both for the energy transfer to/from the storage element and to/from the electric machine side. Moreover, along with the fast growing interest in EVs and PHEVs, wireless charging, as a new way of charging batteries, has drawn the attention of researchers, car manufacturers, and customers recently. Compared to conductive power transfer (usually plug-in), wireless power transfer (WPT) is more convenient, weather proof, and electric shock protected. However, there is still more research work needs to be done to optimize efficiency, cost, increase misalignment tolerance, and reduce size of the WPT chargers. The proposed dissertation describes the work from 2012 to 2014, during the PhD course at the Electric Drives Laboratory of the University of Udine and during my six months visiting scholarship at the University of Michigan in Dearborn. The topics studied are related to power conversion and digital control of converters and drives suitable for hybrid/electric traction, generation from renewable energy sources and wireless charging applications. From the theoretical point of view, multilevel and multiphase DC/AC and DC/DC converters are discussed here, focusing on design issues, optimization (especially from the efficiency point-of-view) and advantages. Some novel modulation algorithms for the neutral-point clamped three-level inverter are presented here as well as a new multiphase proposal for a three-level buck converter. In addition, a new active torque damping technique in order to reduce torque oscillations in internal combustion engines is proposed here. Mainly, two practical implementations are considered in this dissertation, i.e. an original two-stage bi-directional converter for mild hybrid traction and a wireless charger for electric vehicles fast charge

Sensorless position control of induction machines using high frequency signal injection

The aim of this research project was to develop a position controlled induction machine vector drive operating without a speed or position sensor but having a dynamic performance comparable to that of a sensored position vector drive. The methodology relies on the detection of a rotor saliency in the machine by persistent high-frequency voltage injection. The rotor position is then estimated from the resulting stator current harmonics that are modulated by the spatial rotor saliency. This can be a built-in rotor saliency (a designed asymmetry) or the natural saliency due to rotor slotting. This project investigates the demodulation of the extracted high-frequency current spectrum and different topologies for the estimation of rotor position. The tracking of rotor position through rotor saliencies helps to overcome the limitations of model-based approaches that are restricted to speeds above 30rpm on a 4-pole machine and are sensitive to parameter mismatches. The project addresses the difficult problem of separating the modulation effects due to the rotor saliency from distorting modulations due to the saturation saliency and inverter effects. In previous research it had been found that the saturation saliency causes a deterioration of the position estimate that can result in a loss of position and eventually causes the drive to fail. The application of filters to remove the interfering saturation harmonics is not possible. In this research a new approach was developed that compensates online for the saturation effect using pre-commissioned information about the machine. This harmonic compensation scheme was utilized for a 30kW, 4-pole induction machine with asymmetric rotor and enabled the operation from zero to full load and from standstill up to about ±150rpm (±5Hz). The steady-state performance and accuracy of the resulting sensorless drive has been found to operate similarly to a sensored drive fitted with a medium resolution encoder of 600ppr. The project involved studies of the inverter switching deadtime and its distorting effect on the position estimation. A second compensation strategy was therefore developed that is better suited if a large interfering modulation due to the inverter deadtime is present in the machine. The new compensation method was implemented for a second 30kW machine that utilizes the rotor slotting saliency. Good tracking results were obtained with a mean error of less than ±0.5° mechanical under steady-state. The derivation of the position signal for higher speeds introduces an additional speed-dependent error of about 4° mechanical at 170rpm. Sensorless position control was realized for operation from zero to full load for the fully fluxed machine. The performance allowed low and zero speed operation including position transients reaching a speed of 50rpm. The high-frequency modulation introduced by the fundamental currents during transient operation was examined and identified as the main factor limiting the dynamics of the sensorless drive. Two rigs were used for the research. The first rig is build around a network of Transputers, the second rig uses state-of-the-art TMS320C40 and TMS320F240 digital signal processors for the control and was designed and constructed as part of the research

Investigations of LC Filter Unbalance in an Inverter-Fed Permanent Magnet Synchronous Motor Drives

Permanent magnet synchronous machines (PMSMs) are usually controlled using two-level inverters. The output voltage of the inverter is in the form of the switching pulses between the positive DC-bus voltage and the negative DC-bus voltage. Such voltage waveforms have several adverse effects on the motor. These include, higher stress on winding insulation, higher eddy current losses and acoustic noise. Thus, to overcome these problems, different types of filters, typically LC-filters are used between the inverter and motor terminals to smooth the pulse width modulation (PWM) output voltages of the motor drives. Theoretically, the inductance and capacitance used for the filters are considered identical in each phase. However, in a practical scenario, it is difficult to have identical filter elements for all three phases. This non-ideal condition of filter elements amongst the three phases is considered as filter unbalance. This thesis investigates the impacts of filter unbalance on the PMSM drive system. Specifically, a comprehensive model of the motor drive system considering filter unbalance is proposed and developed at first. With the developed model, conventional field oriented control (FOC) is implemented to investigate the impact of this filter unbalance. A range of filter parameter variation and the corresponding impact on the motor drive including the motor current, torque and speed ripples is then studied in detail. Thereafter, the results obtained from the proposed model are validated through both circuit simulations and experimental tests. Based on the investigation results, this thesis will discuss the allowable parameter variation in the LC filters to limit the motor performance deterioration within the required bounds, which will be beneficial to engineering practice in motor drive area. In addition, this investigation shows that a conventional FOC with proportional integral (PI) controller might not be capable of mitigating the negative impact on the motor due to filter unbalance, for example, the negative sequence current. Therefore, this thesis implemented an adaptive proportional resonant (PR) controller to address negative sequence current and the corresponding impacts. A detailed mathematical framework to develop this proposed controller will also be presented in the thesis. Finally, the proposed adaptive PR controller is extensively evaluated on a laboratory PMSM drive system under different operating conditions

Power Converters in Power Electronics

In recent years, power converters have played an important role in power electronics technology for different applications, such as renewable energy systems, electric vehicles, pulsed power generation, and biomedical sciences. Power converters, in the realm of power electronics, are becoming essential for generating electrical power energy in various ways. This Special Issue focuses on the development of novel power converter topologies in power electronics. The topics of interest include, but are not limited to: Z-source converters; multilevel power converter topologies; switched-capacitor-based power converters; power converters for battery management systems; power converters in wireless power transfer techniques; the reliability of power conversion systems; and modulation techniques for advanced power converters

Control of Static Converters for Grid-Side and Machine-Side Applications

The research activities summarized in this Ph.D. thesis are mainly referred to the power electronics field, with some extensions related to electric machines and electrical drives. The first chapter focuses on the analysis and control of an unconventional static converter able to extend a common 1-phase mains feeder into a standard 3-phase power supply featuring either a 3-wires or a 4-wires output, the latter including neutral. Such converter exhibits a complete power reversibility and permits to achieve a good power quality level both at the input and the output side. It is proposed as an attractive alternative to conventional solutions possibly available in the market, such as converters for drives supplied by 1-phase mains yet using 3-phase motors, thanks to the following benefits: greater simplicity, lower cost, inherent active-filter-like operation at supply side, low harmonic distortion at load side. Such converter might be then successfully applied in any application requiring a 3-phase standard supply when a 1-phase mains feeder is available. A theoretical analysis of the converter is presented as well as a semi-ideal simulation model implemented referring to different control strategies. Several simulation results are finally reported and commented, confirming the effectiveness of the proposed solution. The second chapter focuses on the real-time control of 3-phase single-dc-bus shunt active filters employed for the parallel compensation of harmonics, reactive components and unbalancing in the currents drawn by a power supply when generic 3-phase non-linear, non-resistive and unbalanced loads are connected. In particular, the specific issues related to applications featuring a high fundamental frequency, such as in aerospace ambit, were addressed, investigating an innovative improved dead beat digital control algorithm. Such solution was proposed and get ready mainly aiming to achieve a rather high bandwidth of the current control loop and a good reference tracking even when the number of commutations per fundamental period that can be used is rather low. In order to probe the performances of the proposed control strategy, a simulation model was first developed and a prototype system was finally get ready. The results obtained from several virtual and experimental tests are reported and commented referring both to standard industrial and much more demanding aerospace operative conditions, thus proving the validity of the proposed solution. The third chapter focuses on the real-time control of 3-phase multilevel shunt active filters employing a multilevel cascaded H-bridges structure, again mainly referring to applications featuring a high fundamental frequency such as in aerospace ambit. In fact, such power structures may permit to improve the equivalent converter performances while keeping at relatively low values the actual switching frequency of the power devices. In particular, the combined application of an innovative modulation technique and of a dead-beat strategy analogous to the one described in the previous chapter was investigated. A theoretical analysis of the proposed control strategy is reported, as well as several experimental results obtained from a prototype system purposely get ready and tested at both industrial and aerospace frequency, highlighting the potential of the proposed solution especially for the latter applications. The experimental activities related to chapters 2 and 3 were developed during a study period spent at the University of Nottingham, UK. The fourth chapter deals with the modeling and control of an innovative rotary-linear brushless machine. In particular, after its ideal analytical modeling and operation principle, its basic control strategy inspired to sinusoidal brushless machines is presented, reporting some simulation results. A more detailed simulation model based on the equivalent magnetic circuit approach is then presented, permitting to approximately take into account several secondary aspects neglected by the simpler basic sinusoidal model while remaining not much computationally intensive as a finite element model would be. Simulation results obtained by such model are reported and commented, highlighting its potential usefulness for both preliminary machine design purposes and for analyzing the operation of a complete drive. Finally, the fifth chapter presents the application of the same intermediate-level modeling approach described in chapter 4 to a consequent-pole brushless machine featuring an unconventional magnet-pole angular width ratio. After some considerations on the specific arrangement examined, which was conceived to achieve a better exploitation of the active materials, a simulation model of the machine is presented and numerical results are reported and commented, highlighting the usefulness of the proposed intermediate-level modeling approach

Advanced power converters for railway traction systems

This thesis presents a new traction drive suitable for fuel-cell powered light rail vehicles based on a multilevel cascade converter with full-bridge cells. The converter provides dc-ac power conversion in a single stage, while compensating for the variation of fuel cell terminal voltage with load power. The proposed converter can replace the conventional combination of dc-dc converter, as it benefits from having a multilevel ac voltage waveform and much smaller power inductors, compared to conventional solutions. The converter numerical and analytical models are derived showing that the converter can be modelled as a cascaded boost converter and 3-phase inverter. The design methodology for the energy storage capacitors and power inductors is presented, showing that inductance is reduced at a quadratic rate with the addition of more sub-modules, while total converter capacitance remains constant. A simulation of a full-scale traction drive in a fuel cell tram demonstrates that the proposed converter is a viable solution for light rail applications. The concept of a boost modular cascaded converter is fully validated through a bespoke laboratory prototype driving a small induction machine. The experimental inverter achieves operation from standstill, with full motor torque, to field weakening with constant power, boosting a 50V dc supply to 200V peak line-to-line voltage