Solid-state Transformers for Microgrid Applications

The use of power electronics is increasing in the industry for both low-scale applications like chargers for batteries and LED drivers and high-scale applications like photovoltaic (PV) systems and electric vehicles.² Normally, the electrical power system has three parts: power generation plant, transmission strains, and distribution system.³ Traditionally, low-frequency transformers are used for two purposes, electrical isolation and voltage matching, but the 50-/60-Hz transformers are large in size and weight.⁴ Power converters are used to make the old and new power system compatible, which has utilized the concept of a solid-state transformer (SST). It has a power converter with high or medium frequency, which reduces the size of the transformer, and a high-power density compared with the old transformers.

The advancement in the magnetic materials with high flux density, high power and frequency, and low power losses has helped the researchers to develop SST with high power density and efficiency.^5–7 Mostly, the research is focused on the traditional two-winding transformer. Increase in distributed generations and the development of smart and microgrid has triggered the concept of a multi-port solid-state transformer (MPSST).

At each port of the converter, dual active bridge (DAB) converters are used, which uses the leakage inductor of the transformer as converter inductor. This reduces the size, as no additional inductor is required, and also reduces losses. The leakage inductance depends upon the winding placement, core geometry, and coupling factor, which make the design of the transformer more complicated.¹ Phase shift is used for power flow from one port to the other in DAB converters, but in MPSSTs, phase shift in one port affects the power flow in the other ports. So control complexity increases with the increase in number of ports. Therefore, MPSST focuses on a three-port system.

This article will focus on the design of solid-state transformers for microgrid applications. The transformer has four ports integrated on a single core.¹ The transformer is operating at 50 kHz and each port can handle 25-kW rated power.¹ The ports are chosen in such a way to represent a realistic microgrid model consisting of grid, energy storage system, PV system, and load, with the grid port operating at 4,160 VAC while the other three ports operate at 400 V.¹

Designing a transformer

Table 1 shows different materials with their pros and cons, which are commonly used to manufacture the transformer core. The idea is to select a material that can support 25 kW/port at 50-kHz frequency. Commercial generally used transformer core materials are silicon steel, amorphous, ferrite, and nanocrystalline. The targeted application dictates the use of most desirable cloth for a 25-kW/port four-port transformer working at 50-kHz switching frequency. By analyzing the table, we can select nanocrystalline and ferrite. Nanocrystalline has a disadvantage of power losses at switching frequency more than 20 kHz, so ferrite is finalized as the core material of the transformer.

The design of the transformer core is also important because it has an impact on the compactness, power density, and volume, but most importantly, it affects the leakage inductance of the transformer. For a 330-kW 50-Hz two-port transformer, the core shape and shell type have been compared and proven that shell type provides lower leakage inductance and a smooth power flow.⁸ So a shell-type configuration will be used in which all four windings will be on top of each other in the middle leg of the transformer, which improves the coupling coefficient.¹

The shell-type core has dimensions of 186 × 152 × 30 mm, and the dimensions of ferrite 3C94 are 4×U93 × 76 × 0 mm.⁹ Litz wire is used for winding and for multi-voltage (MV) ports; the rated value of current is 3.42 A and 62.5 A. For LV ports, 16 AWG and 4 AWG wires are used. The coupling effect can also be improved by wrapping the LV wires together.

Simulations

After finalizing the design of the proposed MV MPSST, Maxwell-3D/Simplorer simulations are carried out. For MV grids, the port voltage is 7.2 kVDC and 400 VDC for storage, load ports, and PV systems.¹ The simulations are carried out at full load to provide 25 kW at the load port and at 50-kHz frequency, duty cycle is 50% and power control is obtained by shifting the phase between the converters. The results are shown in the table. Different models are shown with different attributes like the core shape, cross-sectional area, loss volume, etc. The table 2 shows that Model 7 shows lower leakage inductance and higher efficiency.

Experimental setup

One layer of core is produced from 4 U cores. The core is comprised of three layers with winding on it. Three low-voltage port windings are wrapped together.¹ DAB converters are designed for testing the proposed transformer. SiC MOSFETs are used to design converters. For MV ports, a rectifier bridge is designed from the SiC diodes and it is also loaded with the resistor bank to support 7.2 kV.¹

Conclusion

This article focuses on the design of a four-port MV MPSST transformer, which enables the connectivity of four different loads or sources for microgrid applications. One port of the transformer is an MV port that supports 4.16-kV AC. Different models and materials for transformers are reviewed. In addition to designing the transformer, the testing setup is also designed for both the MV port and the LV ports. The efficiency obtained is 99%.

References
¹ Design and Implementation of a Medium Voltage, High Power, High Frequency Four-Port Transformer Ahmad El Shafei, Saban Ozdemir, Necmi Altin, Garry Jean-Pierre, and Adel Nasiri.Center for Sustainable Electrical Energy Systems, University of Wisconsin-Milwaukee, Milwaukee, USA; Department of Electricity and Energy, Technical Science Vocational School, Gazi University, Ankara, Turkey; Electrical-Electronics Engineering Department, Faculty of Technology, Gazi University, Ankara, Turkey.

²Y. Wei, Q. Luo, Z. Wang, L. Wang, J. Wang and J. Chen, “Design of LLC resonant converter 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 transformer in the future smart electrical system,”2013 IEEE Power & Energy Society General Meeting, Vancouver, BC, 2013, pp. 1–5.

⁴N. Kimura and T. Morizane, “Middle Frequency Transformer Investigation for Solid-State Transformer,” 2018 International Conference on Smart Grid (icSmartGrid), Nagasaki, Japan, 2018, pp. 107–112.

⁵W. Shen, F. Wang, D. Boroyevich and C. W. Tipton, “Loss Characterization and Calculation of Nanocrystalline Cores for High-Frequency Magnetics Applications,” IEEE Transactions on Power Electronics, Vol. 23, No. 1, pp. 475–484, Jan. 2008.

⁶W. A. Reass et al., “High-frequency multimegawatt polyphase resonant power conditioning,” IEEE Transactions on Plasma Science, Vol. 33, No.

⁷M. K. Das et al., “10 kV, 120 A SiC half H-bridge power MOSFET modules suitable for high frequency, medium voltage applications,” IEEE Energy Conv. Congress and Expo., Phoenix, AZ, 2011, pp. 2,689–2,692.

⁸Designed by Akacia System www.akacia.com.tw, “Cores & 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. El Shafei, S. Ozdemir, N. Altin, G. Jean-Pierre, and A. Nasiri, “A High power high frequency transformer design for solid state transformer applications”, International Conference on Renewable Energy Research and Applications (ICRERA 2019), Brasov, Romania, 2019, pp. 1–6.