Resumen de Contributions to the design methodology of three-phase active rectifiers to comply with avionic standards: input voltage distortion and phase loss
In the ever-growing market of the civil aircraft, there has been a constant need for improvement in many fields. Due to rapid advancements in the field of electrical engineering the trend called More Electric Aircraft (MEA) has emerged as a response. Its goal is to reduce CO2 emissions by implementing new technologies in the aircraft. The natural way of reducing the emissions is by reducing the total weight of the aircraft which will in turn reduce operating costs and total fuel consumption. The MEA trend is predominantly reflected in replacing heavy and maintenance costly hydraulic, pneumatic and mechanical parts of the aircraft with electrical equivalents.
This work is focused on providing contributions in the field of three-phase active rectifiers that could be employed in the future aircraft, while complying with requirements from aircraft standards such as DO-160G. The AC/DC conversion is mainly done by passive three-phase multi-pulse rectifiers that are extremely reliable, but present significant drawbacks in the weight, volume, efficiency and lack of controllability. The active rectifiers can overcome all these challenges, keeping in mind that reliability must not be significantly impaired.
In this thesis a review of the state-of-the-art three-phase AC/DC converters is done in Chapter 2, followed by first main contribution regarding low-frequency current emissions under 10 % three-phase voltage Total Harmonic Distortion (THD) from DO-160G in Chapter 3. The design methodology is proposed that translates the individual harmonic requirements to an admittance profile that the connected three-phase load must consider in order to be fully compliant with this requirement. The design methodology is chosen to be carried out in synchronous reference frame or dq frame. The proposed admittance limits are then applied to the three-phase buck-type rectifier. It is shown that in this case, the bandwidth of the current controller used for input admittance shaping is not a very demanding parameter. On the other hand, design of input Electromagnetic Interference (EMI) differential filter is demonstrated to be critical due to its rather low characteristic impedance. The design is afterwards extensively analyzed and verified by simulation results.
The same design methodology is then applied on the three-phase boost-type rectifier, namely the VIENNA rectifier. It is shown that in this case there exists a trade-off between input inductor size and current controller bandwidth, and that, for the approximately same size of magnetic components, the boost-type rectifier requires higher current controller bandwidth than its buck-type counterpart. It is also demonstrated that boost-type topologies are rather insensitive to the EMI filter design due to rather small values of the input capacitors compared to the buck-type case. Finally, the results are verified by simulation and conducted on a 10 kW SiC prototype.
In Chapter 4, the second main contribution of this work is reflected and it aims to provide a robust control strategy for a three-phase three-wire six-switch boost-type rectifier against arbitrary input phase failure scenario, in order to cope with the single-phase loss requirement of DO-160G. The fundamental idea lies in control of positive and negative sequence components of the three-phase system after the failure occurrence. Therefore, the applied rectifier closed-loop current control consists of four identical PI controller, two for each sequence d and q components. Since each grid fault case generates unique values of positive and negative sequence voltage vectors, a mathematical derivation of d and q components of each rotating sequence is presented. The precise mathematical extraction of necessary positive and negative sequence voltage and current components needed to cope with any grid fault scenario is proposed. The total number of analyzed failure cases is nine, where three are related to phase-to-phase short-circuit, three to open-phase case, and three to phase-to-neutral fault. Moreover, a mathematical link between instantaneous amplitudes of each individual phase and positive and negative sequence component values is also provided. The derived link utilized on input three-phase voltages and currents provides a simple way of detecting the nine grid failure cases so that adequate current controller references can be provided which guarantee optimal power flow. Finally, the proposed analysis is backed up by simulation and experimental results, conducted on a full SiC 3.45 kW prototype.
In the last Chapter, a summary with highlighted contributions and conclusions from this work is addressed and a vision of possible future work is provided.
