Novel Ultra-High-Frequency AC Link Based Distributed Energy Systems and Microgrids

The majority of the existing U.S. electricity grid infrastructure was built in the early 1930s. This aging and overburdened critical infrastructure is under great pressure to meet the rising demand of domestic electricity consumption. To address these challenges, Microgrids have emerged as a relatively new and promising solution to restructuring the current electricity grid infrastructure and ensuring the reliability of energy supply. Microgrids consist of a variety of components including distributed generators (DGs), distributed energy storage devices (DESDs), and controllable loads. The two major forms of Microgrids are AC Microgrids and DC Microgrids. The inherent issues in DC Microgrids are the high costs for short distance transmission; complicated short circuit protection; and challenging realization of circuit breaker. Thus we placed our focus on AC Microgrids, which can be further categorized into 60 Hz line-frequency-AC (LFAC) Microgrids and high-frequency-AC (HFAC) Microgrids. Based on the state-of-the-art, the HFAC bus frequency ranges from several hundreds of Hz to tens of kHz. The HFAC buses are used to link the distributed sources and loads with the utility grid. Compared with the traditional LFAC Microgrids, HFAC Microgrids have the advantages of less bulky transformers and lower profile of passive components. However, several power conversion stages are needed to realize sources HFAC parallel operation. A central controller is needed as well to synchronize the amplitude, frequency and phase of the HFAC voltage.;Our motivation for a new solution is to achieve that all the distributed sources or loads can access the HFAC bus through single-stage power electronics interface, enable soft switching in power electronics interfaces, and realize distributed control. Therefore, a framework for novel ultra-high-frequency-AC (UHFAC) link based Microgrids is proposed in this work. Unlike traditional LFAC Microgrids or DC Microgrids, the bus frequency of the UHFAC Microgrids is pushed to 54 kHz, as well as the power transmission frequency. The bus voltage waveform is quasi-square (trapezoidal) and the peak value is 400 V. As a result, 60 Hz transformers will be eliminated from the system no matter the energy sources or the kind of loads interfaced. DC-arcing-flash will not exist as well with the utilization of UHFAC bus.;For the proposed system architecture, all the power electronics interfaces are single-stage converters, which increases the system efficiency and reduces the system complexity significantly. Two ZVZCS modulation strategies corresponding to two switch structures of the single-stage converter are proposed and analyzed in detail. Simulation and experiment are carried out to validate the converter functionality and effectiveness of the ZVZCS modulation strategies. System simulation for the proposed UHFAC Microgrids is also conducted to verify the functionality of the whole system architecture. A hardware platform mimicking the scenario of DC source powering LFAC load and DC load is also developed and tested. The experiment results are presented to demonstrate the system operation.