Islanding and Seamless Reconnection ofMultiple Solid-State Transformers Based On Droop Contro

Solid state transformer (SST) is an emerging power electronics-based technology that can replace traditional distribution transformers and actively manage renewable energy resources, energy storage devices, and residential loads. The SST has been focused on due to its low volume and weight which is achieved by using high-frequency AC-link transformer instead of 50=60Hz.Moreover, SST itself has features of reactive power compensation, harmonic current filtering, and fault ride through capability while the conventional distribution system needs additional equipments such as static synchronous compensator (STATCOM), unified power flow controller (UPFC), unified power quality conditioner (UPQC), etc.;The SST is a core component of a smart distribution system, Future Renewable Electric Energy Delivery andManagement (FREEDM) system. By taking advantage of DC bus that SST provides, DC microgrid can be easily integrated to the SST-based distribution system. The renewable energy sources or energy storage devices of the DC microgrid can be used for saving energy and more reliable operation of the distribution system, for example, islanding operation.Wireless communication and power management system of the FREEDM also improves the stability and controllability of the distribution system.;Among many functionalities of the FREEDM system, the autonomous islanding is investigated in this thesis to attain fault ride-through capability when the system is islanded fromthe utility.Moreover, a method is developed to achieve smooth reconnection to the grid after a fault is cleared. The main component of the proposed control strategy is the control of the high-voltage side converter of the SST, which is based on a combination of droop control and an LCL filter. The controllers of the other stages, such as the Dual Active Bridge (DAB), Distribued Energy Storage Device (DESD), and the Distributed Renewable Energy Resources (DRER) are also developed and presented in detail. The harmonic compensation scheme of the SST is also presented including a novel hybrid harmonic compensation (HHC) method. The novel virtual impedance is also developed to cope with the wide range of the grid impedance. A low-voltage scaled SST system is introduced, and the controllers of the converters within the system are described. The proposed control strategy has been tested in simulation and experimentally on a low-voltage scaled testbed.;The stability of the system that consists of the parallel connected voltage source inverters is analyzed and a framework for the stability analysis is provided. The impedance based models of the voltage source inverters are derived from large signal models. Solving the impedance based circuit diagram of the overall power system, the current of the inverter can be expressed by the sum of the transfer functions. The stability of the system can be determined by analyzing the poles and zeros of the transfer functions. It is shown that the grid impedance and the line impedance between SSTs can degrade the damping ratio of the system and even make the system unstable if the controller bandwidth of the inverter and the damping resistance are not selected propely with enough design margin. The impedance based model and the criterion to determine the stability of the system are verified by simulation.