Towards a partial discharge free insulation system for the More Electrical Transportation

Currently, electric motor manufacturers install a ground and turn insulation capable of withstanding normal operating voltages without exceeding the dielectric strength of the air. If the electric field generated during operation exceeds this characteristic quantity, there are pre-discharge phenomena, which accelerate the degradation of the insulation and therefore lead to a shortening of the useful life of the machine. The main objective of this thesis was to develop a model capable of determining the so-called Partial Discharge Inception Voltage for random-wound machines and to facilitate the design of their electrical insulation system. In addition to this, an in-depth study was carried out on how to improve the qualification process of electric motors and actuators intended for use in the world of More Electric Transportation. First, it was considered how to effectively test the insulation systems of random-wound electrical machines to prove that they are PD-free. Subsequently, the impact of thermo-mechanical stress due to frequent speed and torque variations was studied. Finally, the impact of partial discharges on insulation at different altitudes was investigated to better understand the potential risk these can have on the correct operation of the machine, since it is easier to trigger them at pressures above sea level. Ultimately, this thesis seeks to provide various tools for a complete study of the reliability of the insulation system of an electric random-wound motor (or actuator) applied to the transport sector, with particular regard to preventing the partial discharge triggering. Nevertheless, a general approach has been adopted that can be extended to the study of other low-voltage machines, such as dry-type high-frequency transformers or hairpin motors.Attualmente, i produttori di motori elettrici installano isolamenti di terra e di spira in grado di sopportare le normali tensioni di funzionamento senza superare la rigidità dielettrica dell'aria. Se il campo elettrico generato durante il funzionamento supera questa quantità caratteristica, si verificano fenomeni di pre-scarica che accelerano l'invecchiamento del materiale e quindi portano ad un accorciamento della vita utile della macchina. L'obiettivo principale di questa tesi è quello di sviluppare un modello in grado di determinare la cosiddetta Tensione di Innesco delle Scariche Parziali (PDIV) per macchine a filo avvolto e di facilitare la progettazione del loro sistema di isolamento elettrico. Oltre a questo, è stato condotto uno studio approfondito su come migliorare il processo di qualificazione dei motori elettrici e degli attuatori destinati ad essere utilizzati nel mondo del More Electric Transportation. In primo luogo, si è considerato come testare efficacemente i sistemi di isolamento delle macchine elettriche a filo avvolto per dimostrare che sono privi di scariche parziali. Successivamente, è stato studiato l'impatto delle sollecitazioni termo-meccaniche dovute alle frequenti variazioni di velocità e di coppia. Infine, si è analizzato l'impatto delle scariche parziali sull'isolamento a diverse altitudini per comprendere meglio il potenziale rischio che queste possono avere sul corretto funzionamento della macchina, poiché è più facile innescarle a pressioni sopra il livello del mare. In definitiva, questa tesi cerca di fornire vari strumenti per uno studio completo dell'affidabilità del sistema di isolamento di un motore (o attuatore) elettrico a filo avvolto applicato al settore dei trasporti, con particolare riguardo alla prevenzione dell'innesco delle scariche parziali. Tuttavia, è stato adottato un approccio generale che può essere esteso allo studio di altre macchine a bassa tensione, come i trasformatori a secco ad alta frequenza o i motori hairpin

Assessment Techniques to Ensure Reliable Electrical Insulation for More Electric Transportation

Partial discharge testing of complete, low-voltage induction machines and dry-type convertor transformers for the More Electric Transportation is discussed. The current standard (i.e. IEC 60034-18-41, 60317-0-1 and 61378\u20131) offers very few answers to the new challenges posed to the insulating systems reliability of these increasingly electrified electrical machines. Based on tests performed on different kind of specimens and the literature, the most promising assessment technique to ensure a reliable insulation is based on optical sensors. In addition, this work seeks to give some precise, but simple, indications on how to take into account the effect of altitude and high frequency in the qualification of an insulation system for More Electric Transportation and the limits of the different standards involved are highlighted

Partial Discharge Phenomena in Electrical Machines for the More Electrical Aircraft. Part II: Impact of Reduced Pressures and Wide Bandgap Devices

This paper focuses on the inception of partial discharges within the insulation system of electrical actuators used for the More Electrical Aircraft (MEA). Since these machines should operate in the absence of Partial Discharges (PDs), the dependence of the PD Inception Voltage (PDIV) on voltage impulses typical of wide bandgap (SiC) devices is investigated at both 1 bar, reduced pressures close to those typical of aircraft cruising altitudes (150 mbar – 200 mbar) or lower (down to 5 mbar). Propagation issues are not dealt with here as results were obtained working on insulation models consisting of couples of wires twisted together (twisted pairs), thus knowing exactly the potential differences between all points of the insulation model. The results show that the rise times and the switching frequencies associated with wide bandgap devices have little impact on the PDIV. A model able to predict the PDIV of the turn/turn insulation of random wound motors (the most vulnerable part of the insulation) at different pressures is proposed. The model is also able to deal with temperature changes, with limitations that depend on the type on insulation systems

Insulation Design of Low Voltage Electrical Motors Fed by PWM Inverters

This paper proposes a model to determine the partial discharge inception voltage of magnet wires, including the effect of elevated temperatures, and shows its applicability to the complete range of wire geometries considered in IEC Standard 60317-13

Thermal overload and insulation aging of short duty cycle, aerospace motors

Electrical machines for transportation applications need to be highly reliable, particularly if they drive safety-critical systems. At the same time, another main requirement is represented by the significant torque density, especially for aerospace, where weight constraints are extremely stringent. For achieving high peak torque, an effective strategy consists in supplying the windings with a current greater than the rated value; thus, thermally overloading the machine for limited time periods. However, if the insulation is overheated, the machine lifetime is shortened and reliability issues can arise. This paper experimentally investigates the influence of short-time thermal overload on the insulation lifetime for low voltage, random wound electrical machines. The analysis is performed on round enamelled magnet wire coils, which are aged by accelerated thermal cycles. The obtained results are statistically processed through a two parameter Weibull distribution. According to the findings of the experimental data post processing, a lifetime prediction model is built. This model is employed for predicting the lifetime consumption of a motor embedded into an electromechanical actuator for aerospace application

A model to determine the probability distribution of partial discharge inception voltage as a function of the voltage waveform and of the test procedures

Measuring PDIV with 50 Hz sinusoidal waveforms to find what happens using wide bandgap devices having slew rates of x 100 V/ns can give misleading results. Since PDIV tests using wide bandgap devices offers contradicting results, a probabilistic model is proposed to figure the probability density function of PDIV results. This is an intermediate step developing a statistical procedure enabling a more correct estimation of the actual PDIV

Modelling Partial Discharge Inception in magnet wires at different altitudes

The design of the insulation for inverter-fed induction motors is chiefly empirical. In general, a commercial conductor is selected based on experience. Qualification of motor insulation is carried out according to the IEC Std. 60034-18-41. However, qualification might be time consuming. Therefore, an approach based on a preliminary screening of alternative conductors would be useful. Goal of this screening would be to determine which geometries ensure that partial discharges are not incepted under operation. Therefore, we investigate the feasibility of using streamer inception criteria in combination with FEM modelling to determine PDIV. Emphasis is given to the possibility of extrapolating results at different altitudes, to deal with aircraft actuators. PDIV measurement were carried out on twisted pairs. Pressures were in the range from 100 kPa to 20kPa, i.e. that typical of aircraft. Frequencies from 50Hz to 10 kHz were investigated. The results were compared to those provided by streamer inception criteria. A slight dependence on supply frequency was observed, although it could not be explained in terms of the models employed (based on the electrical field lines and the ionization characteristics of air). This dependence seems to be related to phenomena not explicitly considered in the models (e.g., secondary emission from the cathode, although covered by a dielectric, partial discharges occurring below detection sensitivity threshold). Streamer inception criteria failed to predict correctly these phenomena, especially al low pressures, and need further research work to become applicable

Acute respiratory failure as presentation of late-onset Pompe disease complicating the diagnostic process as a labyrinth: a case report

Abstract Background Acute respiratory failure can be triggered by several causes, either of pulmonary or extra-pulmonary origin. Pompe disease, or type II glycogen storage disease, is a serious and often fatal disorder, due to a pathological accumulation of glycogen caused by a defective activiy of acid α-glucosidase (acid maltase), a lysosomal enzyme involved in glycogen degradation. The prevalence of the disease is estimated between 1 in 40,000 to 1 in 300,000 subjects. Case presentation This case report describes a difficult diagnosis of late-onset Pompe disease (LOPD) in a 52 year old Caucasian woman with acute respiratory failure requiring orotracheal intubation and subsequent tracheostomy for long-term mechanical ventilation 24 h/day. Despite a complex diagnostic process including several blood tests, bronchoscopy with BAL, chest CT, brain NMR, electromyographies, only a muscle biopsy allowed to reach the correct diagnosis. Discussion The most frequent presentation of myopathies, including LOPD, is proximal limb muscle weakness. Respiratory related symptoms (dyspnea on effort, reduced physical capacity, recurrent infections, etc.) and respiratory failure are often evident in the later stages of the diseases, but they have been rarely described as the onset symptoms in LOPD. In our case, a third stage LOPD, the cooperation between pulmonologists and neurologists was crucial in reaching a correct diagnosis despite a very complex clinical scenario due to different confounding co-morbidities as potential causes of respiratory failure and an atypical presentation. In this patient, enzyme replacement therapy with infusion of alglucosidase alfa was associated with progressive reduction of ventilatory support to night hours, and recovery of autonomous walking