On the Electrical Aging of the Insulation in PWM-Fed High-Speed Electric Machines: Analysis, Modelling, and Mitigation

This doctoral thesis represents the completion of a three-year research activity that has been carried out as part of the Ph.D. program in Electrical and Information Engineering, XXXII cycle, of the Polytechnic University of Bari, Italy. The work that is presented in this document addresses some specific aspects of the broader topic of reliability in high-frequency electric drives, which is the early failure of inverter-fed AC machine insulation systems due to the very high dv/dt’s of modern fast switching converters. A selection of the most important research works that have been published in the literature on this subject is reviewed here and can be found in the reference. These works have greatly contributed to the progression of the research and thesis. Hopefully, the content presented in this document will also add a modest contribution to this topic that is becoming more and more relevant in today's electric drive applications. The background of the thesis, the motivations, and ideas laying behind the research activity carried on the topic of insulation aging are presented. The role of electrification in many application areas and the benefits that the transition to cleaner energy forms can bring to the cause of environment and climate preservation are discussed. In this promising energy transition process, high-speed machines and high-frequency power electronics converters are key technologies to enable enormous energy-saving and to boost the penetration of more electrical systems in both industry and transportation sectors. The growing market diffusion of high-frequency electric drives is driven by the fast developments in the new generation of power switching devices, which are based on silicon carbide and gallium nitride composites, the so-called wide bandgap semiconductor materials. Thanks to these new materials, which present better properties than silicon with respect to breakdown electric field, electron mobility, and thermal stability, modern power switches can operate at higher commutation frequency, with lower losses and higher junction temperature than previous generations. Alongside the numerous advantages of high-frequency devices, new issues arise for their practical implementation because of their fast commutation transients at turn-on and turn-off. In fact, the very short rise and fall time of the applied voltage pulses, in the order of tens of nanoseconds, is source of increased electromagnetic interference that can negatively affect the surrounding low-power electronics, common-mode voltage can appear across the shaft of the machine leading to the circulation of currents along the bearings and subsequent mechanical deterioration, and increased stress on the motor turn insulation. The higher electrical stress on the insulation is the cause of partial discharges within the weak spots of the system, small sparks that delaminate the dielectric material, eventually leading to a turn short circuit and to the fault. The winding insulation system is a fundamental component of any electric machine, functionally required for the operation of the electromechanical conversion mechanism. The breakdown of the insulation inhibits the operation of the machine, which must be re-wound to be put in service again. Due to the high dv/dt’s in high-frequency power electronic converters, two phenomena occur that can threaten the life duration of motor insulation systems. Voltage waveform reflections along the cable that connects the inverter and the machine produce high overvoltage at the motor terminals when the rise time of the PWM voltage pulses is short. Furthermore, due to the parasitic capacitances of the winding insulation, the distribution of the voltage across the coil is such that, when the pulses are applied to the winding, the first turns are exposed to the higher share of the voltage. Consequently, within the defects of the insulation system, such as air pockets or small clearances, partial discharges are triggered by the electric field exceeding the dielectric strength of the air. Partial discharges are ionic discharge events that modify the physical and chemical properties of the material, making it weaker over time until it cannot sustain the normal field anymore. At this point, the irreversible fault occurs, and the machine must be put out of service. Many studies in the literature have investigated the role of partial discharge on shortening the life of winding insulation systems, and the main effects of PWM voltages on the partial discharge characteristics have been identified. The main approaches commonly adopted today for the mitigation of the electrical stress on the insulation are the use of inverter output filters, very effective but generally bulky and costly and that often reduce the dynamic performance of the drive, and the combined design of converter and machine to minimize the length of the cables, which has practical limitations in high-speed drives due to the different scaling of the two parts. Three are three main goals of the research activity presented in this doctoral thesis. The first objective is to achieve a better understanding of the effects of electrical aging on motor insulation, to develop a practical method for the estimation of the aging progression through the measure of electrical quantities such as voltage and current. The second goal is the development of an accurate and easy-to-tune model of the drive components in the high-frequency domain, to be used for both improving the drive design and performing advanced diagnosis features. The third objective is to investigate possible approaches for the mitigation of electrical aging in high-speed electrical machines. Two possibilities are taken into consideration to this purpose: the active regulation of the dv/dt to limit the overvoltage at the motor terminals, and the use of multilevel converters to reduce the voltage stress by increasing the number of voltage levels. To achieve a deep understanding of the aging phenomenon from a macroscopic point of view, i.e. by mean of easily measurable electrical quantities, a series of electrical aging test has been performed on a group of motors while their characteristics were cyclically measured. The data collected during this aging campaign, together with the experimental setup that has been developed, are presented in the thesis. The data are statistically used to build some indicators of the insulation condition that can be used as an estimation of the aging progression in the insulation system. The first indicator is obtained through the measure of the machine characteristic impedances at different aging steps, while the second and the third are calculated from the high-frequency response of the phase current when a voltage pulse is applied. Accurate models of the electric drive system in the high-frequency domain are required to both improve designs from a system point of view and to include advanced predictive diagnosis features in the drive control system. A series of high-frequency motor models are reviewed and analyzed in this thesis based on the findings from the electrical aging campaign performed. A first modeling approach is made using a lumped-parameter model of the machine and an automatic identification procedure. To improve both accuracy and model tuning time, a second model is proposed, which is based on the automatic identification of an analytical rational function with the measured data. The fitting algorithm that has been employed is described in the thesis, and results are shown about the achieved accuracy level. The experimental data collected during the electrical aging tests and the developed high-frequency models of the drive components are used to investigate some possible approaches for the mitigation of the electrical stress on the winding insulation system of electric machines. The first method that is considered is the idea of using the estimation of the winding insulation aging conditions as a feedback for an advanced control system for the active regulation of the dv/dt of the PWM pulses to the purpose of extending the life of the drive while optimizing efficiency and performance. Such a system requires different parts to properly function, an effective estimator, a controller based on an optimization algorithm, and a set of active gate drivers. In this thesis, the idea is explored by simulation with some simplifications. The second overvoltage mitigation approach is to have a multilevel voltage waveform applied to the motor instead of a train of PWM pulses. In this way, the level of the overvoltage applied to the motor is reduced, and so is the electrical stress on the insulation. To this purpose, in this thesis, a cascaded H-bridge multilevel converter is compared to a conventional 2-level inverter operating on the same load conditions and with the same overvoltage level at the motor terminals. A simulation model is used to compare the losses, the efficiency, and the thermal requirements in both cases.