72-hour Field Test of a 500kVA Electronic Power Transformer
By Chengxiong Mao, Dan Wang, Jie Tian, and Jiming Lu
An electronic power transformer (EPT) is one kind of novel power electronics-based power transformer with great potential in smart grid development. It can handle both alternating current and direct current to make the grid more controllable, flexible, and robust. A three-phase 10-kV/400-V 500-kVA EPT industrial prototype has been designed and developed by researchers at Huazhong University of Science and Technology in China and has completed its 72-hour field test in a steel mill with rapid load variations.
Improving the controllability of the power systems is critical for the smart grid to enhance self-healing capability, improve efficiency, and co-optimization between load and source for better performance. An electronic power transformer (EPT) provides a competitive edge to achieve these goals.
EPTs are also called solid state transformer (SST), power electronics transformer (PET), or smart transformer. It had been called one of the 10 most emerging technologies of 2010 by the “MIT Technology Review” in 2011. The core idea of EPT is to develop a new electricity transferring and converting device to enhance the flexibility and robustness of power systems. Generally, it is realized by the integration of electromagnetic induction-based medium-/high- frequency transformation and power electronics conversion technologies. Besides acting like traditional line frequency transformers, EPT is a new class of electrical transformer that can handle both alternating current and direct current power, and control the flow of electricity easily and flexibly. It is expected as an effective approach to improve the controllability of the power systems and play an important role in future smart grid development.
EPT can seamlessly integrate various renewable generation resources and manage them to meet the continually changing load demand for real and reactive power at minimum cost and with minimum impact. For example, EPT can optimize wind generators, pumped storage power generators, and hydropower generators in variable frequency conditions for maximum benefits. In transmission systems, the capability of fast and flexible power control enables EPT to dampen the system oscillation, realize economic dispatch, and provide voltage support at critical situations. In the distribution systems, EPT can be used to improve power quality and block harmonic propagation because of its strong waveform controllability. It will also play an important role in various kinds of new distribution grids, e.g. DC distribution networks, AC/DC and/or DC/AC interfaces, and multi-terminal AC-DC input/output interfaces.
On September 14, 2015, one 10-kV/400-V 500-kVA electronic power transformer developed by the research group at Huazhong University of Science and Technology (HUST), Wuhan, China, and in cooperation with Wuhan Iron and Steel (Group) Corp. (WISCO), Wuhan, China, successfully completed its 72-hour industrial field trial operation in the 10 kV distribution power system of the transport station of WISCO. This is further progress of the application of large power electronics to power systems. In the transport station, the EPT is used to provide constant voltage and constant frequency electricity for the nonlinear impact loads, and to prevent load harmonics from injecting into the utility grid.
The newly developed three-phase EPT industrial prototype is realized by 18 identical AC-DC-AC modules, which provide advantages with respect to the implementation, maintenance, and scalability. Each AC-DC-AC module consists of a high voltage power cell (HVPC) based on back-to-back HV-IGBT H-bridge converter, low voltage power cell (LVPC) consisting of a diode rectifier and an IGBT-based H-bridge inverter, and medium frequency isolation transformer (MFIT) with iron based amorphous alloy power core. A hierarchical and distributed controller with multi-DSPs is employed to meet the demand of the main circuit of the EPT prototype. An intelligent control and protection strategy is developed for efficient and secure operation of the unit.
The laboratory and field test results demonstrated that EPT can provide strong power control capability such as inrush current limitation, power factor compensation, constant frequency and constant voltage output, and harmonic blocking.
By introducing EPT into the power systems, it is possible to seamlessly integrate various generation facilities, to realize flexible power flow control in the transmission system, and to improve power quality and control custom power supply in the distribution system. With the development of power electronic technologies, such as wide band-gap power device and advanced converter control strategy, the performances of the EPT will enhance while the overall system cost will go down.
The research group at HUST has focused on this area for over 10 years, and has published one monograph "Electronic Power Transformer" (in Chinese) supported by National Foundation of Academic Publication of Science and Technology (NFAPST) of China in 2010.
This work was supported by the National Nature Science Foundation of China (51277083).
Chengxiong Mao is a professor at Huazhong University of Science and Technology (HUST). He was a visiting scholar at the University of Calgary, Canada from January 1989 to January 1990, and at Queen's University of Belfast from December 1994 to December 1995, respectively. He was a researcher at Technische Universitaet in Berlin from April 1996 to April 1997 under the support of the Humboldt Foundation. His fields of interest include power system operation and control, the excitation control of synchronous generators and the application of high power electronic technology to power systems. He received his B.S., M.S. and Ph.D. degrees in electrical engineering from HUST, Hubei, China, in 1984, 1987 and 1991, respectively.
Dan Wang is an associate professor at Huazhong University of Science and Technology (HUST). He was a postdoctoral researcher from 2006 to 2008, sponsored by China’s Postdoctoral Science Foundation in the Department of Control Science & Engineering at HUST. From 2008 to 2009, he was a visiting research associate in the Department of Electrical and Computer Engineering at Michigan State University. In 2008, he joined HUST. His research interests include power system operations and control and power conditioning and grid connection of alternative energy sources. He received B.S., M.S. and Ph. D. degrees in electrical engineering from HUST in 1999, 2002 and 2006, respectively.
Jie Tian received a B.S. degree in electrical engineering from HUST in 2010 and is currently working toward a Ph.D. His main research interest is the application of power electronic technology to power systems.
Jiming Lu is a professor in electrical engineering at Huazhong University of Science and Technology (HUST), where he has worked since 1984. His research focuses on excitation control based on microcomputers. He received a B.S. in electrical engineering from Shanghai Jiaotong University and an M.S. in electrical engineering from HUST.