The hybrid storage is coupled to the medium-voltage grid by means of a new modular inverter design. During a one-year field test on the North Sea island of Borkum, the researchers are testing and comparing different control approaches in the energy management system.
As part of the EU project NETfficient, the electricity distribution network is equipped with a high proportion of renewable energies and various storage technologies. The developed solutions for energy autonomy will be tested under real conditions in order to transfer them later on to other regions. The spatially distributed storage facilities and generators are integrated into a smart grid and controlled by an intelligent energy and network management system. 40 home storage units, five commercial storage units, a thermal storage unit and a hybrid energy storage unit are integrated into the medium-voltage network. “In addition to the 500 kWh lithium-ion battery, one of the most important components in the system is the battery inverter developed at Fraunhofer ISE. It has a total output of one megawatt and consists of highly compact and particularly dynamic subunits with an output of 125 kW each. This allows us to realize all system sizes down to the multi-megawatt range,” explains Olivier Stalter, head of the Power Electronics, Networks and Intelligent Systems division at Fraunhofer ISE. In addition to the lithium-ion battery, a supercapacitor is integrated as short-term storage. This absorbs power peaks and thus extends the service life of the battery.
Due to its increased switching frequency, the inverter can react more quickly to fluctuations in the mains supply than commercially available devices. As a result, the system is suitable as a very fast primary reserve that can reduce peak loads and for self-consumption solutions on an industrial scale. Implemented in a 19-inch rack with a height of 125 cm, the inverter is smaller by a factor of 2 to 4 than comparable devices on the market. This has been made possible by the use of modern silicon carbide semiconductors and by optimizing the design of circuit boards, filter elements and cooling methods. In order to be able to realize the extremely fast switching speeds and to limit the resulting overvoltages at the semiconductors, a thick copper conductor board with only film capacitors was developed which has been optimized for this purpose. Cooling is provided by a liquid cooler. In order to make the chokes both compact and low-loss, a high-quality powder core material in tablet form was used.
The control of the power electronics is based on a new model-based predictive scheme. By measuring all relevant currents and voltages in the system and through model-based prediction of future conditions, it is possible to achieve significant performance gains compared with available current regulators.
In addition to power electronics, Fraunhofer ISE researchers also developed the energy management for the hybrid energy storage system, using the “OpenMUC” energy management software framework developed at the institute. The algorithms of the energy management system divide the power between battery and super capacitor in the field test. Two different approaches with different parameters and reloading strategies are examined. Medium-term resource planning is ensured by the project partners by means of a generation and consumption forecast and integration into the platform for controlling the distributed systems. Other developments from Fraunhofer ISE that will be included in the project are new business models for storage systems, such as solar self-supply, peak shaving – i. e. the reduction of the power price – as well as for control energy markets and the control of power gradients and reactive power supply in the low-voltage grid.