The Highly Efficient And Reliable smart Transformer (HEART)

STATE-OF-THE-ART

After more than 125 years since the first use of a transformer in the electrification of a city, the current project will provide the modern electric distribution system a new durable heart: the Smart Transformer (ST). The project will develop a new understanding of how energy flows are managed inside the complex structure of the ST with the twofold goal of complying with high efficiency and reliability constraints while achieving new “managerial” functionalities absent in the “passive” transformer of the past.
In fact the increasing penetration of distributed energy resources (solar PVs, small wind turbines and CHPs etc.) and new sizeable loads (electric vehicles, heat pumps etc.) have posed many technical and operational challenges in the electric distribution grids [1]. Encouraged by attractive tariffs and promotion policies, the end-consumers in the local grids are not only consumers of electricity but in many cases also producers and even becoming distributed energy storage [2]. This could result in many instances of short and long-term expectancy and emergency loadings and reverse power flows in local grids (Fig. 1a). Hence the existing electric distribution grid capacities lead to critical bottlenecks in the form of over/under voltages, network congestions and power quality issues that affect the reliability and stability of the network, limiting the hosting capacity of distributed energy resources and new sizeable loads [3].
Power electronics play a significant role in this scenario. In fact, most of the actors, either sources or loads, are connected to the electrical distribution grid through power converters, as successfully investigated in the last 10 years by the principal investigator of this proposal [4]. Moreover many of the solutions proposed to improve the reliability and stability of the electric distribution grid are still based on power electronics, such as active filter, HVDC, FACTS, solid-state transformer and electronic breaker [5]. In fact, it is now possible to handle power conversion characterized by high voltage and high current with low losses due to the last generation of available power semiconductor devices based on Silicon (Si). These possibilities are expected to be further enhanced, in terms of higher efficiency and power density, by forthcoming power semiconductor devices based on compound materials like Silicon Carbide (SiC) or Gallium Nitride (GaN) [6]. 



State-of-the-art   

Figure 1. Three electric distribution scenarios: today, tomorrow and tomorrow with Smart Transformer (ST).

The latest control technologies allow, with the help of these power electronics solutions, controlling the electric power flow in a wide range of electrical conditions and they make it possible for the implementation of the widely discussed concept of smart grids, where information and communication technology (ICT) is applied in the planning and operation of the distribution grids [7]. Such intelligent grids could facilitate predictability and adaptiveness in the system thus utilizing the flexibility and controllability from individual units, equipped with power electronics, to support quality operation of the grid, together with other system level solutions such as electronics-based transformers and switches.
Among many projects that have investigated these possibilities, two large ones, UNIFLEX in Europe and FREEDM in the USA, better define the landscape for the current proposal. UNIFLEX, an FP6 EU project, has successfully demonstrated the possibility of a power-electronics-based interconnected transmission network [8]. FREEDM is focused on plug and play of any energy resource or storage device, anywhere and anytime using solid-state transformers in each of them and electronic breakers at the higher system level [9]. It is very important to model and measure the reliability of such solutions, as preliminary investigated in UNIFLEX, to understand the robustness of the future smart grid scenario and how failures in the system can be handled effectively [10].
The current research trend in smart grids is towards an extreme-decentralization scenario as a counter-effect to the past history of extreme-centralization management of the power grid. However one of the main problems of a completely decentralized solution is the large number of data inputs, actors, control and decision-making options resulting in higher complexities and interdependencies in the architecture of these future distribution grids (Fig. 1b).
A different research approach could be to identify the infrastructure enabling a seamlessly linking between the past power system (centralized) and the new one (de-centralized). A good candidate could be the distribution transformer and the short term solution is to use its stepping voltage functionality to integrate automation and monitoring elements in order to make it ‘smart’ [11]. However the short-term solution has its own techno-economic limits since it is not possible to regulate the frequency and the harmonic behavior, both of paramount importance, in a scenario that will be progressively dominated by power converter-based solutions.
In this regard, it is quite natural to consider an automated transformer based on the latest power electronics and communication technologies. We will call this the Smart Transformer and it is an excellent solution to implement a semi-decentralized control of the electric grid, where smart meters provide node information, and distributed sources and new sizeable loads are managed by a local controller embedded in the ST (Fig. 1c). This solution offers better management of the large amount of data presented in the smart grid scenario. In fact the ST is characterized by embedded communication and monitoring capabilities and it could even act as an interface node for electric vehicle charging, storages, solar PV systems etc. The ST decouples the local grid from the main grid enabling a self-resilient grid that provides more system stability. The ST can provide fast and real-time voltage regulation in secondary distribution grids, interface demand response, demand side management, islanding, etc., to optimize the distribution grid performance and its reliability. Further, the fast active and reactive power control capabilities of the ST can be used in strengthening the grid congestion management, loss reduction and power quality of the future local grids.
The Smart Transformer is the component that could solve the system level challenges and it is a challenge itself [12]-[13]. In fact, while the use of power electronics based transformers is becoming a reality in traction applications [14], the possibility of using it in distribution systems as a “Smart Transformer” (ST) is still considered futuristic. The ST has to compete in terms of low cost, high efficiency and high reliability with a well proven technology, the traditional transformer, while offering the wide range of new functionalities, which create working conditions very different with respect to those of a standard transformer and make it even more difficult to cope with efficiency and reliability requirements [15]. In particular, inside the ST there is an important challenge: controlling the temperature or thermal loading, that has a significant impact on the efficiency and reliability of power electronics In fact, the temperature influences the losses of power semiconductor devices and together with the thermal excursion is the main failure mechanism inside power semiconductors [16].

This project will take this challenge with a paradigm shift in how to approach it and use a new set of methodologies. The breakthrough results of this research will be obtained taking the following high-risk high-gain bet: on-line management of efficiency and reliability of the Smart Transformer through new hardware and software architectures. The ST will be modeled in terms of energy flows managed by units, formed by power electronics modules, and connected by passive elements. A new understanding of how the energy flows are managed by the ST, with respect to the requirements of the power system, will lead to new hardware architectures for the ST allowing different routes for the energy flows to be chosen on the basis of efficiency and reliability considerations. Graph theory [17] will be used to find optimal paths for the energy flows with the goal of maximizing efficiency and minimizing the ageing of power modules, also using active thermal control [18]-[19] based on advanced sensing of the chip temperature [20]. The energy flows will be managed akin to Internet switched packet data, thus relying on information enclosed in packet headers. The new software architectures for the ST will switch “packets of energy” by taking into account the information traveling in the communication system and coming from the electric distribution system sensors (requirements) and from the power module sensors (constraints).
To summarize, the challenges of designing the proposed Smart Transformer are:
-    at the system level, to obtain the advanced functionalities allowing the ST to fulfill its “managerial” role in the electric distribution system, while complying with its physical constraints;
-    at the component level, the Smart Transformer, to find the hardware and software architectures for the ST allowing several paths for the energy flows and to find the one that maximizes efficiency and reliability;
-    inside the component, to implement the routing of energy flows through sensors, communication technology and advanced drivers for semiconductor devices.

References
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[2]    W. Kempton, J. Tomić, ”Vehicle-to-grid power implementation: From stabilizing the grid to supporting large-scale renewable energy”, Journal of Power Sources, Vol. 144, Issue 1, June 2005, pp. 280-294.
[3]    G. Pepermans, J. Driesen, D. Haeseldonckx, R. Belmans and W. D. Haesler, “Distributed generation: definition, benefits and issues,” Energy Policy, Vol. 33, 2005, pp. 787-798.
[4]    M. Liserre, T. Sauter, J. Y. Hung, “Integrating Renewable Energy Sources into the Smart Power Grid Through Industrial Electronics” IEEE Industrial Electronics Magazine, vol. 4, Issue 1, March 2010, pp. 18-37.
[5]    Vijay K. Sood, “HVDC and FACTS Controllers: Applications of Static Converters in Power Systems”, Springer, 2004.
[6]    J. Rabkowski, D. Peftitsis, H. Nee, "Silicon Carbide Power Transistors: A New Era in Power Electronics Is Initiated," IEEE Industrial Electronics Magazine, vol.6, no.2, June 2012, pp.17-26.
[7]    F. A. Robert and R. C. Dugan, “Distribution System Analysis and the Future Smart Grid” IEEE Transactions Industrial Applications, Vol. 47, No. 6, Nov. 2011, pp. 2343 – 2350.
[8]    S. Bifaretti, P. Zanchetta, A. Watson, L. Tarisciotti, J. C. Clare, "Advanced Power Electronic Conversion and Control System for Universal and Flexible Power Management," IEEE Transactions on Smart Grid, vol.2, no.2, June 2011, pp.231-243.
[9]    J. Wang, A. Huang, W. Sung, Y. Liu and B.J. Baliga: "Smart grid technologies," IEEE Industrial Electronics Magazine, vol.3, no.2, June 2009, pp.16-23.
[10]    M. C. Magro, S. Savio, "Reliability and availability performances of a universal and flexible power management system," 2010 IEEE International Symposium on Industrial Electronics (ISIE), 4-7 July 2010, pp. 2461-2468.
[11]    L. Xiaohu, A. Aichhorn, L. Liming, L. H. Li, "Coordinated Control of Distributed Energy Storage System With Tap Changer Transformers for Voltage Rise Mitigation Under High Photovoltaic Penetration,", IEEE Transactions on Smart Grid, vol.3, no.2, June 2012, pp. 897-906.
[12]    E.R. Ronan, S.D. Sudhoff, S.F. Glover, D.L. Galloway, "A power electronic-based distribution transformer," IEEE Transactions on Power Delivery, vol.17, no.2,Apr 2002, pp. 537-543.
[13]    L. Heinemann, G. Mauthe, "The universal power electronics based distribution transformer, an unified approach," Proceedings of the 32nd IEEE Annual Power Electronics Specialists Conference (PESC 2001), vol.2, 2001, pp. 504-509.
[14]    D. Dujic, A. Mester, T. Chaudhuri, A. Coccia, F. Canales, J.K. Steinke, "Laboratory scale prototype of a power electronic transformer for traction applications," Proceedings of the 2011-14th European Conference on Power Electronics and Applications (EPE 2011), Aug. 30 2011-Sept. 1 2011.
[15]    S. Xu, R. Burgos, W. Gangyao, W. Fei, A. Q. Huang, "Review of solid state transformer in the distribution system: From components to field application," 2012 IEEE Energy Conversion Congress and Exposition (ECCE), 15-20 Sept. 2012, pp. 4077-4084.
[16]    E. Wolfgang, L. Amigues, N. Seliger and G. Lugert, “Building-in Reliability into Power Electronics Systems”. The World of Electronic Packaging and System Integration, 2005, pp. 246-252.
[17]    A. Bondy, U. S. R. Murty, “Graph Theory”, Springer, August 14, 2008
[18]    K. Ma, M. Liserre, F. Blaabjerg, “Reactive Power Influence on the Thermal Cycling of Multi-MW Wind Power Inverter”, IEEE Transactions on Industry Applications, 2013 (to appear).
[19]    D. A. Murdock, J. Torres, J. J. Connors,  R.D. Lorenz, “Active Thermal Control of Power Electronic Modules,” IEEE Trans. on Industry Applications, vol. 42, no. 2, 2006, pp. 552-558.
[20]    Y. Avenas, L. Dupont, “Temperature Measurement of Power Semiconductor Devices by Thermo-Sensitive Electrical Parameters—A Review”, IEEE Trans. on Power Electronics, vol. 27, no. 6, 2012 pp. 3081-3092.

Chair of Power Electronics

  • Chair of Power Electronics, CAU Kiel