The innovation towards sustainable solutions is involving all the industrial sectors. In this context, the marine world is going through a great transition. The integration of renewable energy sources and the adoption of flexible power systems, like DC microgrids, are some powerful solutions to develop advanced and sustainable marine grids. These concepts are not only adopted in the onboard shipboard distribution, but also when conceiving supplying platforms and marinas. In this paper a flexible DC microgrid is proposed for a yacht-marina, and a coordinated power control is presented. The ships at berth, renewable sources and energy storages are all connected through multiple power converters. A centralized controller is demanded in managing sources and loads. By properly coordinating the photovoltaic modules, landed/embarked storage and power flows to/from the main grid, the attained goal is the sustainable operation of the yacht-marina DC microgrid.
The use of wide bandgap semiconductor devices, such as SiC and GaN MOSFETs, in high-frequency converters introduces new challenges for the design of electric drives. The very fast switching transient of which these devices are capable, in fact, can become a serious threat for the reliability of the entire system. Electromagnetic interferences due to the high dv/dt and di/dt, voltage reflections along the cable that cause overvoltage and ringing at the motor terminals, and large common-mode voltages that produce current circulation in the motor bearings are recognized as the major phenomena leading to premature failure of the drive. It is therefore important for designers to approach the problem from a system point of view, having the possibility to accurately model the system in the high frequency domain to take appropriate measures to increase reliability. In this paper, an automated fitting procedure is proposed to identify one of the most accurate high-frequency models of an induction machine. The procedure relies on the experimental measurement of the differential, common-mode and parasitic impedance of the tested machine and adopts Exchange Market optimization algorithm to search for the best approximation of the three measured impedances. Implementation issues and advantages are presented and discussed.
Sensorless methods for the induction motor (IM) typically need to estimate the speed, fluxes, flux magnitude and field orientation angle of the motor. Observers that estimate the fluxes and speed simultaneously are available but suffer from accuracy issues. This paper presents a sequential estimation approach: the speed is estimated first; then, the speed estimate obtained is fed into a flux observer. Owing to a special gain design, this yields flux estimates that are in phase with the real fluxes (despite the input speed signal which may be inaccurate). The estimated aft fluxes have correct phases; however, they have incorrect magnitudes. Using these fluxes, the tan1 function yields the correct field orientation angle; however, the calculated flux magnitude is inaccurate. To obtain fluxes with the correct magnitude and improve the accuracy, the observer is augmented with a self-adaptation mechanism based on the gradient descent method: this adapts the input speed signal in order to minimize a cost function. The cost function is constructed using the observer’s internal variables.
Multilevel inverters can replace commonly used two-level inverters for three-phase electrical drives. In this low-voltage range new wide-bandgap power switches (SiC MOSFET, GaNFET) are available. The paper analyses three-level, four-level, five-level and seven-level neutral-point-clamped inverters, equipped with power semiconductors with blocking voltages between 200 and 600 V. For this purpose a simulation model of the complete drive system, including the 5.5 kW induction motor is developed. The greatest loss reduction (48% at 25°C, 43% at 150°C) at nominal motor operation point is achieved with the two-level inverter by replacing IGBTs with SiC FETs. By using the five-level inverter with GaN FETs, a further loss reduction of 15% at 25°C and 34 % at 150°C is possible. The theoretical inverter efficiency with five-level inverter at 20 kHz switching frequency achieves 99.0% at 25°C and 98.7% at 150°C in nominal operation point. Compared to the state of the art (two-level with IGBTs with 10 kHz), this is an improvement of 1.3 % at 25°C and 2.0 % at 150°C. The majority of inverter power losses are caused by the threshold voltage of the clamping diodes. The new idea is the replacement of the clamping diodes by active switches, since they only have low on-resistance in the forward direction. In this case freewheeling paths of the clamping diodes would have to be actively switched. However, the control signals for the clamping switches can be generated by logic operations from the control signals for the main switches. The use of active clamping switches has significant potential to reduce semiconductor losses, but requires the development of wide-bandgap power semiconductors with reverse blocking capability. At nominal motor operation point of the 5.5 kW induction motor a loss reduction of 56% between active clamping switches and clamping diodes is possible.
This paper presents a loss model for SiC MOSFET based power modules without external freewheeling diode. In current electric high perfomance drives for electric vehicles, a low material input, reduced costs, reduced construction volume with maximum efficiency and thus an increased power density play a crucial role. At the same time, the wide bandgap materials should be fully utilised with maximum switching gradients under the given boundary conditions of the power module, the commutation cell design and overall systemic aspects. Absolute maximum ratings and failure in time rates play a important design role in ensuring reliable power modules for the service life of electric vehicles. It is not only the technological leaps in assembly and connection technology that make a decisive contribution here, but also a wide variety of control methods, alternative converter topologies and of course novel semiconductor materials that have an influence on efficiency, thermal properties, volume and thus the use of materials. The maximum efficiency in the present application can be increased by a suitable gate control that leads to synchronous rectification. Due to the multitude of control concepts and the use of a fully variable switching frequency and the influence on the harmonic spectrum, harmonic losses are also analysed and evaluated using a high-speed numerical model. In addition, an analytical loss model is derived and subsequently validated and discussed on the test bench.
