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.

 For a downloadable copy of the January 2020 eNewsletter which includes this article, please visit the IEEE Smart Grid Resource Center.