RAISE European Project, coordinated by the group of Electrical Machines and Drives – MeltingLab of University of Modena and Reggio Emilia (UniMoRe), aims at offering solutions to improve the reliability of power converters, of machines and of the entire electric drive, without damaging their performance and efficiency. Benefits and ameliorations that are likely to exert positive repercussions in both the short and the long term, besides being exploited by manufacturers to face the competitive Asian market.

In recent years, the need of implementing increasingly powerful, light and efficient electric and electronic devices has become a priority target in all industrial sectors. For this reason, the release on the market of new wide bandgap based power devices, made of silicon carbide (SiC) and gallium nitride (GaN) semiconductor materials, has aroused lively interest in scientific and industrial communities.

Time trend of power devices’ power densities
Time trend of power devices’ power densities

Automotive and aerospace industries rank among those that can most benefit from these devices.
In this context, according to recent development trends focused on “More Electric Vehicle” (MEV) and “More Electric Aircraft” (MEA), the replacement of hydraulic/mechanical actuators with electric drives implies a neat improvement of efficiency and power density, which means also weight reduction, minor fuel consumption and lower emissions of noise and polluting substances.

The added-value of wide bandgap based power devices

The converters based on wide bandgap power devices feature superior characteristics than conventional devices based on silicon (Si). Such devices offer faster switchovers, minor power losses and they can operate at higher voltages and temperatures than their silicon counterparts, so allowing also a further reduction of volume and weight in converters’ cooling system.

Simplified scheme of an electric converter-cable-motor system.
Simplified scheme of an electric converter-cable-motor system

«It is then clear – explains Dr Stefano Nuzzo, member of the research team of UniMore, University of Modena and Reggio Emilia, “Electrical Converters, Machines and Drives”, together with Professor Davide Barater, Professor Giovanni Franceschini and Mr Marco Pastura – that the new devices based on SiC and GaN semiconductors materials perfectly match the above-mentioned targets».

Simplified scheme of an electric converter-cable-motor system
Simplified scheme of an electric converter-cable-motor system

However, the electric stress combined with the high voltage gradients they can reach, in addition to the combined action of other environmental stresses (like temperature, pressure and humidity) can shorten the life time of the insulation system of electric motors powered by converters that use SiC and GaN semiconductors.
Therefore, reliability problems can emerge in this type of electric drives. Moreover, the physical length of the power supply cables that connect the power converter to the motor can be comparable, or even exceed, the so-called “critical length”. «In this case – Dr Nuzzo underlines– the high rising and falling voltage pulses make cables behave like transmission lines, with waves that travel forward and backward along cables themselves due to the reflection phenomenon. These high-frequency phenomena can cause dangerous overvoltages at motor terminals, up to twice the direct-current bus voltage, the worst case, which can seriously damage the electric insulation of motor windings».

Prototype of the SiC converter used, with possibility of setting different levels of voltage gradients
Prototype of the SiC converter used, with possibility of setting different levels of voltage gradients.

In particular, if the values of the dielectric rigidity of insulations are exceeded because of voltage levels at electric motor terminals, conductive phenomena through a portion of the insulation material occur, leading to the so-called partial discharge phenomenon. Partial discharges occur on the surface of the electric insulation, causing a progressive erosion up to the complete breakdown. The critical length is proportional to the switchover time, therefore the very short switching times of SiC and GaN devices have made the cable use problematic even for an extension of few metres. Besides, electric power transmission networks currently proposed for aircrafts are addressing the use of higher and higher voltage levels on bus in direct current, to minimize conduction losses.

“Motorettes” implemented to study the effects of high voltage gradients on the voltage distribution among windings
“Motorettes” implemented to study the effects of high voltage gradients on the voltage distribution among windings

However, the voltage rise on bus in direct current causes a further stress on the stator winding insulation, thus worsening the issues connected with the use of wide bandgap based devices.

From the state-of-the-art to the progress beyond it

The project RAISE – “Reliable Aircraft electrical Insulation sElection” is framed in this context. The project, started in March 2018, is funded by Clean Sky consortium, the primary research programme on European scale, included in Horizon 2020 funding system, which aims at the development of innovative solutions for the reduction of CO2, of greenhouse gases and of the acoustic pollution. The group of Electrical Machines and Drives – MeltingLab of University of Modena and Reggio Emilia (UniMoRe), led by Professor Giovanni Franceschini, is coordinator of RAISE project and Professor Davide Barater is its Principal Investigator. Other two members of the MeltingLab assist their work in the project: Dr Stefano Nuzzo himself and Mr Marco Pastura. «MeltingLab – Dr Nuzzo specifies – collaborates with the main groups of the Department of Engineering “Enzo Ferrari”, of which it too is part, and cooperates with the most renowned area companies operating in the sectors of automotive, electric drives, industrial electronics and electrical machines».

Complete experimental bench: converter-cable-motor
Complete experimental bench: converter-cable-motor

In the last years, the group’s research has focused on electric mobility, mainly working on the design of electric drives for the control of high-performance electric motors through advanced prototypes of power converters with SiC modules, and on the design of electric machines featuring high efficiency and reliability.
Considering all criticalities previously highlighted, it is worth first of all assessing whether the insulating materials currently in use in electrical machines and in electronic power devices are suitable for applications that exploit higher and higher power supply voltages and switching gradients, such as in the case of the wide bandgap based semiconductors previously mentioned. Therefore, the first phase of RAISE project was committed to the analysis of the state-ofthe-art and to the experimental evaluation of insulation materials and systems used in in current aerospace applications. Afterwards, to better understand the impact of high voltage gradients, they have developed some models to quantitatively assess the values observed in a typical system composed by power converter, cable and electric machine, considering the typical operating conditions of the aerospace environment and the dependence on the alternating voltage (three-phase at 115 Vrms versus 230 Vrms).

Typical voltage distribution in the windings of an electric machine
Typical voltage distribution in the windings of an electric machine

The limit for the direct-current bus voltage was fixed at 1 kV, while the maximum voltage gradient considered is 20 kV/µs. Nevertheless, for these studies, they have considered also higher voltage and gradient levels, which might be reached in the next future. «These models – Dr Nuzzo specifies– can assess also the voltage distribution inside the windings of the electric machine and its dependence on the key parameters of the machine».

RAISE PROJECT (RELIABLE AIRCRAFT ELECTRICAL INSULATION SYSTEM SELECTION)
RAISE (Reliable aircraft electrical insulation system selection) is a 30-month project, receiving funding from the Clean Sky 2 Joint Undertaking under the European Union’s Horizon 2020 research and innovation programme. The consortium brings together two of the most prestigious Italian Universities: University of Modena and Reggio
(UniMoRe) and University of Bologna (UniBo). UniMore leads the consortium as project coordinator whereas the company LiebherrAerospace, multinational operating in the aerospace sector, acts as Topic Manager for the project. RAISE aims at investigating and assessing partial discharges and breakdowns of electric insulation in electrical machines’ windings driven by wide bandgap converters of silicon carbide (SiC) and gallium nitride (GaN). Goal of the project is offering solutions to improve the reliability of power converters, of machines and of the whole electric drive, without damaging performance and efficiency. Consequently, the aircraft of the future, besides being “more electric”, will be more reliable, more compact and lighter, and fuel consumptions and the environmental impact, mainly measured on CO2 emissions, will be drastically reduced. It will be possible to transfer conceptually and practically the results of the project to the automotive industry, where the same electrification trend is taking place.
To demonstrate the validity and the accuracy of these models and to customize simulation parameters, they have built some “motorettes” representing senting a typical stator slot, suitably wound. Afterwards, they have also built some samples of electric motors, on which they have carried out some specific experimental tests. The experimental bench represents a typical converter-cable-motor system, with which it is possible to set up different levels of direct-current bus voltage and of voltage gradients, eventually exceeding, where required, the maximum pre-established of 20 kV/µs.

Innovative methodologies to reduce high voltage gradients

The project subjected to in-depth study is also aimed at verifying when the partial discharge phenomenon on insulation systems starts. «In this second phase, – Dr Nuzzo highlights – we developed some models based on the estimation of components’ life time and we carried out some experimental tests aimed at studying the impact of the voltage variation speed on partial discharges.

Research team of UniMore, MeltingLab of University of Modena and Reggio Emilia, “Electrical Converters, Machines and Drives”, (from the left): Professor Giovanni Franceschini, Mr Marco Pastura, Professor Barater, Dr Stefano Nuzzo
Research team of UniMore, MeltingLab of University of Modena and Reggio Emilia, “Electrical Converters, Machines and Drives”, (from the left): Professor Giovanni Franceschini, Mr Marco Pastura, Professor Barater, Dr Stefano Nuzzo

The investigation was executed considering also the variations of environmental conditions in terms of temperature, humidity and pressure».
The final goal consists in making the developed models represent some guidelines for the detail design of components “free” from partial discharges, through the estimation of the service life of insulation systems. The capability of designing insulation systems free from partial discharges will allow then granting the reliability of all the components of the electric system at stake and consequently of the entire aircraft.

AUTO-MEA PROJECT (AUTOmated Manufactoring of wound components for next generation Electrical Machines)
The final project phase, whose activities will be performed during 2020 and will go on until the end of the project planned by October 2020, will consist in the study and in the development of innovative methodologies for the reduction of the high voltage gradients iat the machine input terminals, without damaging the demanded efficiency and power density requirements.The results of this project – Dr Nuzzo specifies– will allow using power converters based on wide bandgap SiC and GaN semiconductor materials on board of aircrafts. Such converters will be characterized by high power densities and, as direct consequence, we will achieve a substantial reduction of the weight of the aircraft itself, of the fuel consumption and of its environmental impact». The outcomes of the project itself will be used by designers of motors and electric drives to test solutions and to adopt precautions before the implementation of physical prototypes.Since November 2019, UniMoRe has been also the coordinator of the European project AUTO-MEA (AUTOmated Manufacturing of wound components for next generation Electrical machines). The target of the initiative, included in Clean Sky 2 European Research Programme, is the development of new methodologies for the manufacturing of wound components of electric motors to be used in the aerospace field. AUTO-MEA consortium, in addition to UniMoRe, is composed by CopperING srl, reference company in the production of manufacturing lines for electrical machines’ stators in automotive fi ld, and by the University of Nottingham, which acts also as project Topic Manager. The replacement of pneumatic and hydraulic actuators with electric drives standing out for compactness and reliability represents one of the most promising trends nowadays but it needs innovative solutions for the design of electrical machines. AUTO-MEA will focus on the typology of preformed “hairpin” windings that, even providing numerous advantages compared to “random” windings at low frequencies (such as higher reliability and power density), reduce electrical machines’ efficiency at high frequencies. The project is aimed at pushing the boundaries of hairpin windings’ operational frequencies. The results by AUTO-MEA will have relevant repercussions also in other sectors, especially in the automotive industry where the Emilia territory is protagonist on a world scale.
This will improve the reliability of the electric drive and of the whole aircraft, which has always been one of the limiting factors in MEA context. «We expect – Dr Nuzzo concludes– such improvements will exert positive repercussions in both the short and long term and they can be exploited by European electric drive manufacturers to face the Asian market and then to be more competitive on a world scale».
(by Gianandrea Mazzola)

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