How to design and use the solid state transformer in microgrid applications


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Power electronics are increasingly used in industry for both low-range applications such as battery chargers and LED drivers, and high-scale applications such as photovoltaic (PV) power systems and electric vehicles. 2 Typically, an electric power system consists of three parts: power generation plant, transmission strains, and distribution system. 3 Traditionally, low frequency transformers are used for two purposes, dielectric and voltage matching, but 50- / 60Hz transformers are large in size and weight. 4 Transducers are used to make the old and new power system compatible, which used the concept of Solid State Transformer (SST). It contains high or medium frequency transducer, which reduces transformer size, and high power density compared to old transformers. Advances in magnetic materials with high flux density, high power, frequency and low power loss have helped researchers to develop SST with high energy density and efficiency. 5-7 Mostly, research focuses on conventional two-winding transformers. The increase in distributed generations and the development of the smart grid and the small grid led to the launch of the concept of the multi-port solid state switch (MPSST). In each of the switch ports, double active bridge (DAB) transformers are used, which use the dropout inductor of the transformer as the transformer inductance. This reduces volume, as no additional inductor is required, and also reduces losses. The inductance of the leakage depends on the position of the winding, the basic geometry, and the coupling factor, which makes the design of the transformer more complex. 1 A phase offset is used for power flow from one port to another in DAB converters, but in MPSSTs, a phase shift in one port affects the power flow at other ports. So the complexity of control increases with the increase in the number of ports. Therefore, MPSST focuses on a three-port system. This article will focus on designing solid state inverters for micro-network applications. The switch has four ports built into one core. 1 The transformer operates at 50 kHz and each port can handle the rated power of 25 kW. 1 The ports are selected in a way that represents a realistic micro-network model consisting of network, power, storage system, PV system, and load, with the network port operating at 4,160 VAC while the other three ports operate at 400 V. 1 four SST ports transformer design Table illustrates 1 Various materials with their pros and cons, which are commonly used for manufacturing transformer core. The idea is to choose a material that can support 25 kW / 50 kHz outlet. The commercial base materials generally used are silicon steels, amorphous, ferrous, and nanocrystals. The target application forces the desired fabric to be used for a 25 kW / outlet four-port switch operating at a 50 kHz switching frequency. By analyzing the table, we can determine nanocrystalline and ferrite. Nanocrystalline has the disadvantage of power loss when switching frequency by more than 20kHz, so the ferrite is finished as the core material of the transformer. Different core materials and their properties The design of the transformer core is also important as it affects the compressibility, power density, and volume, but more importantly, it affects the leakage inductance of the transformer. For a two-port transformer with a power of 330 kW 50Hz, the basic shape and the sheath type have been compared and the proof that the sheath type provides less leakage induction and smooth power flow.8 So a casing type configuration in which all will be used will be four windings on top of each other in the middle part Of the converter, which improves the coupling coefficient. 1 Shell type core has dimensions 186 x 152 x 30mm, and 3C94 ferrite dimensions are 4xU93 x 76 x 0mm 9 Litz wire is used for winding and for multi-voltage (MV) outlets; The rated value of current is 3.42 A and 62.5 A. For low-voltage outlets, 16 AWG and 4 AWG wires are used. The coupling effect can also be improved by winding the low voltage wires together. Simulation After completion of the proposed MV MPSST design, Maxwell-3D / Simplorer simulations are carried out. For MV networks the port voltage is 7.2 kVDC and 400 VDC for storage, load ports, and PV systems. 1 The simulations are performed at full load to provide 25 kW at the load port and at a frequency of 50 kHz, the duty cycle is 50% and the power control is obtained by switching the phase between the transformers. The results are shown in the table. Various models are shown with different attributes such as basic shape, cross-sectional area, loss size, etc. The table shows that Model 7 shows less leakage induction and higher efficiency. Models and simulation results Experimental setup A single layer of nucleus is produced from 4 U nuclei. The core is made of three layers with winding on them. Three windings of LV outlets are coiled together, and the DAB transformers are designed to test the proposed transformer. SiC MOSFETs are used to design the transformer. For MV outlets, the rectifier bridge is designed from SiC diodes and is also loaded to a resistor bank to support 7.2 kV. Prototype conclusion This article focuses on designing a four-port MV MPSST switch, which enables four different loads or sources to be connected for small network applications. One of the ports of the switch is an MV port that supports 4.16kV AC. Different transformer models and materials are reviewed. In addition to the transformer design, the test setup is also designed for both MV and LV outlets. The efficiency obtained is 99%. 1 Design and implementation of a four-port transformer for medium voltage, high power and high frequency, Ahmed El Shafei.[1] Saban Ozdemir,[2] My stars are Altin[3] Gary Jean-Pierre,[1] And Adel Nasiri.[1] [1]Center for Sustainable Electric Power Systems, University of Wisconsin – Milwaukee, Milwaukee, USA; [2]Department of Electricity and Energy, Vocational School of Technical Sciences, Gazi University, Ankara, Turkey; [3]Department of Electrical and Electronic Engineering, Faculty of Technology, Gazi University, Ankara, Turkey 2S. Wei, Q. Luo, Z. Wang, L. Wang, J. Wang and J. Chen, “Designing a LLC Resonance Transformer with Magnetic Control for LEV Application”, 2019 IEEE 10th International Symposium on Power Electronics for Distributed Generation Systems (PEDG), Xi’an China, 2019, pp. 857-862. 3x. She and A. Huang, “Solid State Transformers in the Intelligent Electrical System of the Future”, IEEE Power & Energy 2013 General Meeting, Vancouver, British Columbia, 2013, pp. 1-5. 4N. Morizan Kimura and T. Morizan, “Realization of Solid State Transformer Medium Frequency Transformer”, 2018 International Conference on Smart Grid (icSmartGrid), Nagasaki, Japan, 2018, pp. 107-112. 5 watts. Shen, F. Wang, D. Boroyevich and CW Tipton, “Loss characterization and computation of nanocrystalline cores for applications of high-frequency magnetism,” IEEE coefficients on power electronics, Vol. 23, No. 1, pp. 475-484, Jan 2008.6W. A. Reass et al., “High-Frequency Multiphase Multiphase Resonance Energy Conditioning,” IEEE Coefficients in Plasma Science, Vol. 33, No. 7 m. K. Das et al., “10 kV, 120 A SiC half H-bridge power MOSFET Modules suitable for high-frequency and medium-voltage applications,” IEEE Energy Conv. Congress and Gallery, Phoenix, Arizona, 2011, pp. 2689-2692. 8 Designed by Akacia System www.akacia.com.tw, ​​”Cores and Accessories”, Ferroxcube. https://www.ferroxcube.com/englobal/products_ferroxcube/stepTwo/shape_cores_accessories? s_sel = 161 & series_sel = 2658 & material_sel = 3C94 & material = & part =. Accessed July 24, 2019. 9A. Al-Shafei, S. Ozdemir, N. Altin, G. Jean-Pierre, and A. Nasiri, “Designing a high-frequency high-power transformer for solid-state transformer applications”, International Conference on Renewable Energy Research and Applications (ICRERA 2019), Brasov, Romania, 2019, pages 1-6. .


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