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The primary purpose of this paper is to obtain accurate analytical expressions of the dual active bridge (DAB) converter under the three-degree of freedom control technique. This technique, known as triple phase-shift (TPS) modulation, is utilized for the efficiency optimization of different operating zones and modes of the DAB. Three operating zones and modes have been retained to analyze the converter. Depending on the operating regions and due to the high nonlinearity of the obtained expressions, two optimization techniques have been used. The offline particle swarm optimization (PSO) method is utilized in local optimization (LO) and results in a numerical value. The Lagrange Multiplier (LM) is utilized in global optimization (GO) and results in a closed form expression. In the case of LO, the optimal duty cycles that minimize the power loss are not the optimal values for the minimum root-mean-square (RMS) current or peak current stress. Conversely, in the case of GO, the optimal duty cycles minimize the RMS, peak current and power loss at the same time for the entire power range. Detailed analyses and simulation results from MATLAB/Simulink are given to prove the effectiveness of the proposed method. 

With the growth in penetration number and power level of renewable energy resources, the need for a compact and high efficient solid state transformer becomes more important. The aim of this paper is to design a compact solid state transformer for microgrid application. The proposed transformer has four ports integrated on a single common core. Thus, it can integrate different renewable energy resources and energy storage systems. The transformer is operating at 50kHz switching frequency, and each port can handle 25kW rated power. In this paper, the ports are chosen to represent a realistic industrial microgrid model consisting of grid, energy storage system, photovoltaic system, and load. The grid port is designed to operate at 4160V AC, while the other three ports operate at 400V. Moreover, the grid, energy storage, and photovoltaic ports are active ports with dual active bridge topologies, while the load port is a passive port with full bridge rectifier one. In this paper, an extensive and complete design and modeling of the entire solid state transformer is presented. The proposed design is first validated with simulation results, and then the proposed transformer is implemented. Some preliminary experimental tests are also performed and the obtained results are reported. 

In this paper, design of a compact high frequency four-port transformer for a Solid-State Transformer (SST) arrangement is presented. Unlike other SSTs, the four-port system integrates three active sources and a load port with galvanic isolation via a single transformer core. In addition to this feature, one of the three source ports is designed to operate at Medium Voltage (MV) 7.2kV for direct connection to 4.16kV AC grid, while other ports nominal voltages are rated at 400V. The transformer is designed to operate at 50kHz and to supply 25kW/port. Thus, the proposed system connects the MV grid, Energy Storage System (ESS), PV, and DC load to each other on a single common transformer core. Based on the system power demand and availability of renewable energy resources, utility and energy storage ports can either supply or draw power, while PV port can only supply power, maintaining the required demand for the load. This work focuses mainly on the High Frequency Transformer (HFT) design. An extensive study is carried out to obtain the optimal, compact, cost effective, and high efficiency model. Modeling, mathematical, and simulation results are derived and presented to demonstrate the viability of this design.