Development of Grid-Connected and Front-End Converters for Renewable Energy Systems and Electric Mobility

The spread of renewable energy sources and electric vehicles is increasing thanks to the greater awareness of the climate problems due to the large and long-lasting use of the non-renewable energy sources. The integration of renewable energy sources to the power grid, however, poses significant technical challenges, since it drastically changes its topology and nature. In fact, while the traditional power generation system is centralized, the renewable energy is distributed and intermittent. In this scenario, power converters play a central role. Power converters are the technology that enables the interconnection of different players to the electric power system. In this work, a control system for grid-connected converters has been developed. The main focus is on the current control. The most renowned current controllers, such resonant and repetitive regulators, have been studied and tested in laboratory in order to compare the performance in terms of harmonic compensation and burden of the processor. The problem of the saturation of a multi-frequency current controller has been investigated and different saturation algorithms have been proposed. The power converters have, however, wide use and the same of the method, developed for grid-connected converters can be applied to electrical motor drives with open-end windings. If a floating capacitor bridge is connected to the secondary side of the open-end stator windings, it can supply the reactive power needed by the motor and completely exploit its current capability of the power source. This feature allows the drive to obtain higher torque at higher speed, increasing therefore the output power over all the flux-weakening speed range. The floating bridge, operating as harmonic compensator, allows the inverter connected to the primary energy source to work in overmodulation and even six-step modulation, in order to further boost the performance of the drive, without compromising the quality of the phase current

Modulation and control strategies for multilevel five-phase open-end winding drives

Industrial and automotive trends clearly demonstrate an increased interest in medium- and high-power variable speed drives. Despite constant progress in the technology, the semiconductor characteristics are still the bottleneck in drive designs, due to their limitations to block high voltage (several kilovolts) and conduct high current (several hundreds of amperes per-phase). For this reason and numerous other advantages, solutions based on multilevel inverters and multiphase machines are considered in recent years. The open-end winding drives are an alternative approach for drives construction. This thesis investigates carrier based pulse width modulation schemes for five-phase open-end winding drives. Two drive topologies, with isolated dc-links of two inverters, are considered. The first one consists of two two-level inverters and a five-phase machine. The second topology utilises one three- and one two-level five-phase inverter. It is shown that the same drive structure can produce a different number of phase voltage levels, when different dc-link voltages of two inverters are in use. Hence, dc-link voltage ratio is considered as an additional degree of freedom. An open-end winding structure that comprises of two two-level inverters offers harmonic performance equivalent to three- and four-level single-sided supply. The second drive structure under analysis is able to produce four, five and six voltage levels, depending on utilised dc-link voltage ratio. Modulation schemes are classified in two categories. So-called coupled modulation schemes are developed under the assumption that open-end winding drives are equivalent to certain single-sided multilevel solutions. This enables the application of slightly modified modulation methods for multilevel inverters, to the open-end winding configurations. As a consequence, number of utilised voltage levels can be higher than the sum of two inverters’ number of levels. However, this boost in number of levels relies on simultaneous switching in two inverters’ legs connected to the same drive phase,which causes so-called dead-time spikes. The second group, referred to in this thesis as decoupled modulation schemes, rely on the separate modulation of two inverters, using voltage references obtained by splitting the overall phase voltage reference, proportionally to inverters’ dc-link voltages. Hence, this kind of modulation offers somewhat worse harmonic performance, when compared to coupled modulation schemes. Special attention is paid to the stability of dc-link voltage levels, which is one of the most important figures of merits of quality for multilevel drives. Using a novel analysis approach, it is demonstrated that utilisation of optimal harmonic performance offered by coupled modulation methods leads to unstable dc-link voltages, but only in the cases where dc-link voltage ratio is used for increment of available number of voltage levels. Decoupled modulation methods offer stable dc-link voltages, regardless of drive configuration. One of the drawbacks of the analysed open-end winding drives is the need for two isolated dc sources, which form dc-link voltages of two inverters. For that reason, a possibility to use only one dc-source in open-end winding drives with isolated inverters is considered. Analysis shows that both drive topologies can be operated using so-called bulk and conditioning inverter control, where bulk inverter is supplied from an active dc source, but operates in staircase mode, while conditioning inverter performs high-frequency pulse width modulation, in order to suppress low-order harmonic content. This kind of operation is investigated in details for two specific configurations in which two inverters never operate at the same time in PWM mode, when coupled modulation methods are used. Comparison of the results shows that topology which comprises from one three- and one two-level inverter is more suitable for this kind of control. Together with previously analysed configurations and modulation strategies, dynamic performance of this novel drive is tested under the closed-loop speed control. Experimental results show that open-end winding drives are suitable for a wide range of applications