Modular Multilevel Converter based Power Electronic Transformers for Grid Integration of Renewables and Motor Drives

University of Minnesota Ph.D. dissertation. September 2016. Major: Electrical/Computer Engineering. Advisor: Ned Mohan. 1 computer file (PDF); xii, 162 pages.At high voltage and power levels in utility-scale applications, transformers are needed for integrating renewables and motors. At present, 60-Hz transformers are used that are bulky and have significant weight. A novel interface using modular multilevel converter (MMC) based power electronic transformer (PET) is proposed which operates at much higher frequencies than 60-Hz transformers and thus can be significantly more compact and energy-efficient. Due to its modular structure, the high voltage side can be easily scaled resulting in higher reliability and easy maintenance. Such PETs also offer a “smart” solution for improving reliability in future power systems and interfacing of auxiliaries, such as storage batteries and STATCOMs. The first part of the thesis describes the modular multilevel converter operation. By using an array of series connected submodules, this converter can generate high number of voltage levels resulting in a near sinusoidal output voltage waveform. This eliminates the need for lossy snubbers required otherwise for connecting devices in series to meet the high voltage stress. A new submodule of the MMC is proposed which requires lesser number of these submodules to result in smaller system footprint and lower losses. A hybrid modulation scheme with voltage balancing algorithm is proposed to balance the floating capacitors in the new MMC. An intelligent commutation technique results in 2/3rd of the switching transitions to be soft switched in the proposed submodule. The proposed power electronic transformers using MMCs generate sinusoidal voltages and currents through the high frequency transformer (HFT) resulting in significant reduction in transformer magnetic losses. Also natural commutation of leakage energy is obtained. Two variants of the low voltage side renewable or motor connected power converter are presented using either a back-to-back connected voltage source converter or single-stage matrix converter. Control of the output voltage requirement by the rotating machine is met by controlling the output voltage of the MMC on the high voltage side of the HFT, to result in reduced voltage stress and losses in the transformer, machine interfaced converter and the machine. A multi-winding transformer architecture to integrate multiple renewable energy sources is also presented. The thesis presents the analysis and operating principle of such PETs for use in future power distribution systems and addresses the challenges for commercialization of such PETs. Simulations with experimental results on a scaled down laboratory prototype verify proof of concept