Experiences of Planning, Installation, and Operation of Battery Storage in DSO Grid
By Bence Bereczki, István Táczi, István Vokony, Sándor Kertész, István Vajk, Gábor Gabro, Bálint Hartmann
Energy storage is clearly a key technology both in the integration of renewable energy sources and aiming to reach the smart grids infrastructure. One of the key challenges besides the technical developments and market model issues, is the regulatory environment – a complex framework is required for fair and efficient use of those systems.
In the fall of 2016, the Hungarian Energy and Public Utility Regulatory Authority had granted a possibility for distribution system operators (DSO) to install and operate electric energy storage systems (up to 0.5 MW) as a part of distribution network in order to optimize the distribution activity, based on the least cost principle. This forward-thinking regulation approach triggered the Hungarian DSOs to deal with one of their great challenges: the integration of the rapidly growing photovoltaic (PV) generation to their system. Until the summer of 2019, approximately 1100 MW of PV was connected to the Hungarian grid (roughly 400 MW to the low voltage network), which has a peak load of 7100 MW. However, current projections estimate the installed power of PVs to grow up to 7000 MW until 2040. Therefore, DSOs will face great rise in their investment needs, and must find innovative alternatives to make their grid more flexible. As the first steps in the adaptation, two DSOs have already carried out pilot projects to control the voltage on long radial overhead lines, instead of building new transformer stations. There are other two storage systems in the MW range which provide frequency containment reserve in the market activity: the storage landscape in Hungary have been born in the recent years and the first results are promising. In this article, a voltage controlling energy storage system is introduced which operates on the low voltage level.
In November 2018 INFOWARE Ltd. installed Hungary’s second battery energy storage system (BESS) for voltage control on the distribution network of NKM Power Grid Ltd. Preliminary grid studies have been carried out by Budapest University of Technology and Economics. The installed system is placed in concrete housing with the DC (battery racks) and AC (inverter and auxiliary equipment) parts being separated. The BESS consists of two racks of Samsung SDI NMC battery modules – 9 modules of 7.6 kWh capacity per module – with special fire-proof covering developed by Intilion GmbH, a wholly owned subsidiary of Hoppecke Batterien GmbH. This means approximately 115 kWh usable capacity, taking into account the 85% DOD value tolerated by the battery cells. ABB’s 20 kW 4-quarter inverter, ESI-S is used for DC-AC and AC-DC conversion and connection to the grid. The main component of the system is the ‘MAB3 BSC’ controller. The primary task of this controller is to control the inverter within the limits set by the battery management system (BMS). The MAB3 BSC controller can work both as an intelligent electronic device (IED) and a gateway. As an IED, it receives digital and analogue inputs and outputs, and controls ancillary equipment such as air conditioning and heating. It acts as a gateway to communicate with top-level management.
Control is accomplished through the MAB3 BSC monitoring station, remote dispatcher central stations or local autonomous functions. Functions of the controller include but are not limited to: 3-phase manual control, 3-phase control with internal model based control, 3-phase control by hysteresis, 3-phase daily schedule program, 3-phase phase-voltage compensation (LV network only) and independent phase control (LV network only).
The voltage control module can use two control strategies: hysteresis-based or internal model control (IMC). For hysteresis-based voltage control, the controller does not use a network model. In IMC mode, the network voltage control task is modelling the network by its actual parameters. (Fig. 1.)
Fig. 1. Control loop of IMC-based (left) and hysteresis-based (right) voltage control
In the modelling stage, a control model is created for the voltage control task. The controlled value of the model is the voltage of the connection point, which changes due to the external unknown impedance (Z) disturbances. The required active power (P) output is determined by the controller, using the desired voltage value, measurements and, internal parameters. The power signal then passes through the power limiting unit, which is being reshaped by the battery protection’s power controller application. The ESI-S inverter is controlled by the power control module. IMC type voltage control uses a network model for best results. The network model is characterized by the droop parameter, which shows the supplied or consumed power’s effect on the voltage of the connection point. Droop values can be determined by empirical or experimental methods, or by generating a test signal. When configuring the network model, the parameters of the power limiting unit must also be specified. The power control module protects the batteries.
In IMC-based voltage control a comparison is made of the measured and the desired connection point voltage using the characteristic curve shown in Fig. 2.
Fig. 2. IMC (left) and hysteresis (right) characteristics
This control method is an integrative type, where choosing the right parameters does not cause the process to oscillate. The gap parameter is used to adjust how much to add or subtract from the breakpoint of the reference voltage in the event of a decrease or increase in the measured voltage. In hysteresis-based voltage control a comparison of the measured voltage and the desired connection point voltage is made using the characteristic curve shown in Fig. 2. This control mode is a proportional control, the parameters of which must be chosen very carefully so that the process does not oscillate.
For the hysteresis-based control, results of the first 6-month operational period show that the voltage values are within the set range. Theoretically, oscillations can occur, but none were observed during the examined period.
Fig. 3. Voltage measurements without control and with hysteresis-based control (from 18:30)
The BALANCED-IMC mode also keeps the voltage values between the set parameters. This mode could significantly reduce the voltage fluctuation.
Fig. 4. Voltage measurements for BALANCED-IMC-based control
The BESS fulfilled its purpose during the test run, which was necessary to carry out planned tests, based on which the required modifications and additions could be put into effect. It was also proved, that a remote monitoring workstation is essential for the operation. During the testing period, the system has demonstrated the capability to successfully control the voltage of the affected feeders.
Bence Bereczki is an MSc student in Electrical Engineering at the Budapest University of Technology and Economics. He received his bachelor’s degree in January 2019 in energy engineering. His main field of interest is the application of battery energy storage systems; he is the author of several conference papers on industry and DSO related BESS applications. He is employed by INFOWARE Zrt since September 2018, where he currently works as a project engineer. He is a graduate student member of IEEE (2019 - present).
Istvan Taczi received the BSc. degree in electrical engineering in 2013 and the MSc. degree in electrical engineering in 2018 from Budapest University of Technology and Economics, Budapest, Hungary. He is currently pursuing the Ph.D. degree in electrical engineering at The Doctoral School of Electrical Engineering at the Faculty of Electrical Engineering and Informatics. His research interest includes the renewable energy integration possibilities, energy storage applications and power system stability and synthetic inertia analysis.
Since 2017 he is a Research Assistant at the Integrated and Intelligent Technologies Research Centre of the Higher Education and Industry Cooperation at the Technical University of Budapest. Since 2017 he is a product implementation associate at the Grid Innovation Department of the E.ON Distribution System Operator in Hungary. Currently he is responsible for projects in the topic of grid energy storage, voltage control with power electronic devices.
Dr. István Vokony was born in 1983. He received M.S.c. degree in electrical engineering and obtained his Ph.D. degree from Budapest University of Technology and Economics in 2007 and 2012, respectively. He is working as an enterprise architect at E.ON Business Services Hungary at Department of Strategy and Architecture Business IT. He is a part-time senior lecturer and researcher at the Department of Electric Power Engineering, Budapest University of Technology and Economics. His fields of interest include power system stability analysis, renewable system integration, energy storage and smart grids.
Mr. Sándor Kertész is the CEO of INFOWARE Zrt. since 1997. INFOWARE became one of the leader private companies in Hungary on the Electrical Energy Market Segment during this more than 20 years. Under his technical and business control, INFOWARE performed several innovative developments and installations of high and medium voltage substations, power plants and renewable plants.
In the meantime since 2013, he teached the “Substation Automation Systems” course for the Electrical Faculty of Óbuda University. As an honour of his education activities, the Óbuda University of Budapest awarded him the Associate Professor status in 2017.
Vice president of the Hungarian Solar Association.
He is also the inventor of several patents connected to Battery Energy Storage System applications.
Dr. István Vajk has obtained his PhD degree in 1989 and his DSc degree in 2007. He is full time professor at the Department of Automation and Applied Informatics, Budapest University of Technology and Economics.
Gábor Gabro is a high-voltage electrical engineer, energy informatics. He started his career at ETV-Erőterv ZRt., where he worked on network design, power grid connection, and as a senior advisor at MAVIR ZRt. (Hungarian Independent Transmission Operator Company), he prepared various transmission network and innovation investments. He is currently one of the project leaders of the Network Innovation Department of NKM.
Bálint Hartmann, Dr. (M’2009) was born in 1984. He received M.Sc. degree in electrical engineering and obtained his Ph.D. degree from Budapest University of Technology and Economics in 2008 and 2013, respectively. He is associate professor with the Department of Electric Power Engineering, Budapest University of Technology and Economics. He is also a part-time research fellow with the Centre for Energy Research. His fields of interest include the role of energy storage in the power system, computer modelling and simulation of distribution networks and integration of variable renewable energy sources.