Ioan Serban

Bio: Ioan Serban is an academic researcher from Transilvania University of Brașov. The author has contributed to research in topics: Microgrid & Automatic frequency control. The author has an hindex of 17, co-authored 57 publications receiving 1002 citations.

Control Strategy of Three-Phase Battery Energy Storage Systems for Frequency Support in Microgrids and with Uninterrupted Supply of Local Loads

Abstract: Frequency control in autonomous microgrids (MG) with high penetration of renewable energy sources represents a great concern to ensure the system stability. In this regard, this paper presents an enhanced control method for battery energy storage systems (BESS) to support the frequency of MG and with the ability of disconnecting from the MG to supplying in the island mode a local consumer. A frequency controller, combining a conventional droop control with an inertia emulation function, governs the BESS active power transfer during the primary frequency control level. The BESS may also provide voltage support in the point of common coupling with the MG. Moreover, the proposed BESS may compensate, partially or totally, the power absorbed by the local loads in order to improve the MG frequency response. When the MG power quality worsens below a certain level, in terms of voltage and frequency, the BESS detaches from the MG and continues to operate islanded. The reconnection is accomplished following a smoothly resynchronization of the local voltage with the MG, without disturbing the local loads supply. Additionally, this paper also discusses about the aspects related to the BESS management and its integration within the proposed system. The simulation and experimental results assess the feasibility of the proposed control solutions.

Power Decoupling Method for Single-Phase H-Bridge Inverters With No Additional Power Electronics

Abstract: An active method for double-frequency power ripple decoupling in single-phase inverters is presented in this paper, exhibiting the main advantage of not using additional power semiconductors besides the H-bridge. The proposed method requires only two capacitors placed between the midpoint and one end of each inverter leg. An original control solution of the inverter ensures the power ripple transfer toward the two decoupling capacitors without affecting the inverter output voltage. The simple design makes the proposed solution easy to adapt for single-phase inverters in H-bridge configuration. This paper focuses on the autonomous operation mode of the inverter, detailing its operating principle and the control analysis. The system performances, including the impact of the decoupling circuit on the inverter efficiency, are assessed by means of experimental results.

Battery energy storage system for frequency support in microgrids and with enhanced control features for uninterruptible supply of local loads

Abstract: This paper proposes a battery energy storage system (BESS) to support the frequency control process within microgrids (MG) with high penetration of renewable energy sources (RES). The solution includes features that enhance the system’s stability and security of supply. The BESS can operate connected to MG or islanded and the transition between the two states is seamlessly coordinated by an original method. The BESS active power response is governed by an improved frequency controller on two layers, namely primary and secondary. It responds to frequency deviations by combining a conventional droop control method with a virtual inertia function to improve the system’s stability. The proposed BESS may also compensate the power of the local loads, so that the MG frequency transients can be reduced and, depending on the remaining inverter capacity, voltage support in the point of common coupling with the MG may be provided. If the MG power quality degrades in terms of the voltage and frequency, the BESS and the local load are disconnected from the MG and continue operating islanded. The BESS is reconnected to the MG after a smoothly resynchronization of the local voltage with the MG, without disturbing the local loads supply. Simulation and experimental results assesses the proposed control solutions.

Microgrid control based on a grid-forming inverter operating as virtual synchronous generator with enhanced dynamic response capability

Abstract: This paper focuses on improving the dynamic response of autonomous microgrids (MGs) by proposing a grid-forming inverter controlled as a virtual synchronous generator (VSG), in combination with a supercapacitor (SC)-based energy storage system (ESS). By this arrangement, the MG-forming VSG is designed to react only in transitory regimes, the steady-state load being distributed to other MG-supporting inverters spread within the MG. In this way, the MG-forming VSG can maintain its full power reserve capacity for dynamic response. The paper details the control solution for the MG-forming inverter, including the VSG and SC-ESS control. The control method for the MG-supporting inverters that allow achieving the proposed control approach is also described. To prove the concept, the paper includes simulation results and experiments accomplished on a complex laboratory MG system based on three parallel inverters, one being controlled as MG-forming VSG, while the others operating as MG-supporting inverters.

Energy storage systems impact on the short-term frequency stability of distributed autonomous microgrids, an analysis using aggregate models

Abstract: This study analyses the integration impact of battery energy storage systems (BESSs) on the short-term frequency control in autonomous microgrids (MGs). Short-term frequency stability relates with the primary or speed control level, as defined in the regulations of the classical grids. The focus is on autonomous MGs that dynamically behave similarly to the classical power systems. This is the systems case with classical distributed generators (DGs), but which can also contain renewable energy sources (RESs) in a certain penetration level. During MG islanded operation, the local generators take over most of the frequency control process, by means of their automatic generation control, which include inertia response and primary control. However, RES-based DGs are rarely able to provide grid frequency support, as they lack controllability and usually the power conversion chain does not have the possibility of storing and releasing energy when required by the system. Therefore the need of boosting the MG power reserves by adding energy storage systems is often a requirement. The study highlights the improvement in the MG short-term frequency stability brought by an original BESS control structure enhanced with both inertial response and an adaptive droop characteristic during battery state-of-charge limitations. The conducted analysis is accomplished by adopting aggregated models for the involved control mechanisms. The developed model is analysed in frequency domain, whereas an experimental test bench including a real-time digital simulator with BESS controller in a hardware-in-the-loop structure is used for assessing the system performances.

Virtual synchronous generators: A survey and new perspectives

Abstract: In comparison of the conventional bulk power plants, in which the synchronous machines dominate, the distributed generator (DG) units have either very small or no rotating mass and damping property. With growing the penetration level of DGs, the impact of low inertia and damping effect on the grid stability and dynamic performance increases. A solution towards stability improvement of such a grid is to provide virtual inertia by virtual synchronous generators (VSGs) that can be established by using short term energy storage together with a power inverter and a proper control mechanism. The present paper reviews the fundamentals and main concept of VSGs, and their role to support the power grid control. Then, a VSG-based frequency control scheme is addressed, and the paper is focused on the poetical role of VSGs in the grid frequency regulation task. The most important VSG topologies with a survey on the recent works/achievements are presented. Finally, the relevant key issues, main technical challenges, further research needs and new perspectives are emphasized.

Three-Phase PLLs: A Review of Recent Advances

Abstract: A phase-locked loop (PLL) is a nonlinear negative-feedback control system that synchronizes its output in frequency as well as in phase with its input PLLs are now widely used for the synchronization of power-electronics-based converters and also for monitoring and control purposes in different engineering fields In recent years, there have been many attempts to design more advanced PLLs for three-phase applications The aim of this paper is to provide overviews of these attempts, which can be very useful for engineers and academic researchers

Optimization in microgrids with hybrid energy systems – A review

Abstract: Fast depleting fossil fuels and the growing awareness for environmental protection have led us to the energy crisis. Hence, efforts are being made by researchers to investigate new ways to extract energy from renewable sources. ‘Microgrids’ with Distributed Generators (DG) are being implemented with renewable energy systems. Optimization methods justify the cost of investment of a microgrid by enabling economic and reliable utilization of the resources. This paper strives to bring to light the concept of Hybrid Renewable Energy Systems (HRES) and state of art application of optimization tools and techniques to microgrids, integrating renewable energies. With an extensive literature survey on HRES, a framework of diverse objectives has been outlined for which optimization approaches were applied to empower the microgrid. A review of modelling and applications of renewable energy generation and storage sources is also presented.