3-PHASE 5-LEVEL E-TYPE CONVERTER TOPOLOGIES FOR INDUSTRIAL POWER SUPPLY APPLICATIONS

Abstract

The target of this dissertation is to investigate, to analyze and implement an innovative multi-level converter configuration topology solution to obtain high efficiency and high-power density to be used in industrial power supply applications. To this purpose, a wide analysis of the existing multi-level converter topologies has been carried out. Accordingly, it has been found out that the multi-level T-Type topology can be considered as an important topology to be used for high speed generation (i.e. generation units composed by gas turbines, aerospace, etc.) and UPS applications. After that, the investigation focused on the three-phase unidirectional AC/DC multi-level rectifier (3Φ5L E-Type Rectifier). The theoretical study and preliminary simulation has been addressed with reference to 3Φ5L E-Type Rectifier. Afterwards, the laboratory prototype 3Φ5L E-Type Rectifier has been built in order to validate theoretical analysis. As a result of the amazing performances of the rectifier in terms of weight, volume and efficiency, the final delivery project on the AC/AC double conversion system has been realized with reference to industrial applications like Uninterruptible Power Supply (UPS). The AC/AC converter is composed of two multi-level topologies: the AC/DC multi-level rectifier (5L E-Type Rectifier) and DC/AC multi-level inverter (5L E-Type Inverter). This connection is called Back to Back E-Type (BTB) multi-level converter. A 20 kVA BTB E-Type multi-level Converter has been analyzed in order to ensure the following targets: • AC/AC peak efficiency equal to 98.25% including filter, driver, control and fan, with a resistive load; • a power density greater than 3.5 kW/L including filter; • input current total harmonic distortion (THDi) less than 3%; • output voltage total harmonic distortion (THDv) less than 1%; • a short circuit capability equal to 3 times nominal current for 60 milliseconds. The multi-level rectifier topology can be easily extended, from the hardware point of view, to the double conversion configuration, which is typical of UPS applications. The main difference between drive system and UPS multi-level double conversion is the operating point. On one hand, a drive system usually works with a frequency which depends on the rotational speed of the electric drive. On the other hand, a double conversion UPS works at almost constant voltage and fixed frequency. In both the considered applications, all the power semiconductors need to be selected to optimize the overall performance of the multi-level converter topology. Consequently, the analytical approach to calculate the conduction and switching losses in the BTB E-Type Converter has been presented. The power losses are used to investigate converter performance and efficiency as a function both of the converter output power and of the switching frequency. Finally, the control strategy for BTB E-Type Converter has been presented and discussed in this dissertation. The control algorithm has been implemented in LabVIEW and the resulting program is made up of two different targets: FPGA and Real-time, which are run on dedicated hardware. The circuit of the BTB E-Type Converter has been built in NI Multisim environment. Afterwards, the control loop algorithm has been tested using the co-simulation capabilities between Multisim and LabVIEW environments in order to evaluate its performances.