Grid-Connected Energy Storage Systems: State-of-the-Art and Emerging Technologies

High penetration of renewable energy resources in the power system results in various new challenges for power system operators. One of the promising solutions to sustain the quality and reliability of the power system is the integration of energy storage systems (ESSs). This article investigates the current and emerging trends and technologies for grid-connected ESSs. Different technologies of ESSs categorized as mechanical, electrical, electrochemical, chemical, and thermal are briefly explained. Especially, a detailed review of battery ESSs (BESSs) is provided as they are attracting much attention owing, in part, to the ongoing electrification of transportation. Then, the services that grid-connected ESSs provide to the grid are discussed. Grid connection of the BESSs requires power electronic converters. Therefore, a survey of popular power converter topologies, including transformer-based, transformerless with distributed or common dc-link, and hybrid systems, along with some discussions for implementing advanced grid support functionalities in the BESS control, is presented. Furthermore, the requirements of new standards and grid codes for grid-connected BESSs are reviewed for several countries around the globe. Finally, emerging technologies, including flexible power control of photovoltaic systems, hydrogen, and second-life batteries from electric vehicles, are discussed in this article.This work was supported in part by the Office of Naval Research Global under Grant N62909-19-1-2081, in part by the National Research Foundation of Singapore Investigatorship under Award NRFI2017-08, and in part by the I2001E0069 Industrial Alignment Funding. (Corresponding author: Josep Pou.

Reduced battery usage in a hybrid battery and photovoltaic stand-alone DC microgrid with flexible power point tracking

A hybrid control strategy for photovoltaic (PV) system and energy storage system (ESS) in a dc microgrid is proposed in this paper. The proposed control strategy minimizes the utilization of ESS in order to prolong the battery lifetime, reduce the total system power losses, and maintain the state-of-charge (SoC) of the battery within a desired range as long as the required power from PV is available. In contrast to the conventional control strategies that regulate the dc bus voltage with the ESS, the proposed control strategy exploits the use of both PV and ESS in regulating the dc bus voltage. It also employs an advanced flexible power point tracking algorithm for the PV system to further enhance the dynamic performance of the system by adaptively adjusting the PV output power according to the load profile.Ministry of Education (MOE)Submitted/Accepted versionThis work was supported by the Singapore Ministry of Education Academic Research Fund Tier 1 under Grant No: 2019-T1-001-168 (RG 80/19)

Battery fault tolerance of modular multilevel converter-based battery energy storage systems with redundant submodules

In a modular multilevel converter (MMC) based battery energy storage system (BESS), a fault tolerant design ensures uninterrupted operation of the MMC when a given number of submodules (SMs) have faulty batteries or no batteries. This paper quantitatively investigates the fault tolerance improvement in MMC-based BESSs with different numbers of redundant SMs, which is commonly practiced to improve the fault tolerance in modular converter topologies. Simulation results are obtained to verify the proposed analysis. The presented analysis provides guidelines for designers to choose an appropriate number of redundant SMs to achieve a desired fault tolerance.Accepted versionThis work was supported by the Office of Naval Research U.S. under DUNS Code: 595886219

Simplified hybrid control strategy for stand-alone DC microgrid with photovoltaic system to extend battery lifespan

Conventionally, a flexible photovoltaic (PV) power control strategy is used to reduce battery utilization in stand-alone dc microgrids (dcMGs). However, conventional control schemes deploy multiple voltage controllers to regulate the dc-link voltage, causing mode transitions during the transients and complicating the controller design. This paper proposes a simplified control strategy for the PV system and the battery energy storage system in a stand-alone dcMG with a single voltage controller to regulate the dc-link voltage. The proposed control strategy enhances the transient response of the system by eliminating the mode transitions between voltage controllers and simplifying the hybrid control scheme while reducing the battery utilization. Additionally, the proposed solution does not require the knowledge of the load, which facilitates its application. The effectiveness of the proposed control strategy is validated with simulation results.Ministry of Education (MOE)Accepted versionThis work was supported by the Singapore Ministry of Education Academic Research Fund Tier 1 under Grant No: 2019-T1-001-168

Comparative analysis of power ramp rate control strategies for photovoltaic systems

Intermittent changes in irradiance caused by passing clouds inevitably lead to fluctuations in photovoltaic (PV) power generation. To ensure the stability of a power grid with integrated solar PV generation, a battery energy storage system (BESS) is an intrinsic solution to effectively process the PV power before transmitting it into the grid. Alternatively, a PV software-based ramp rate (RR) control can be applied to mitigate the PV power fluctuations without any BESS, which is known to be cost-effective. The contribution of this paper is to compare various BESS-based and cost-effective software-based RR control strategies operating with real-time measured information, and analyze their effectiveness in regulating the PV power fluctuations. Furthermore, this paper studies the BESS utilization behavior among hardware-based control strategies, and also provides insights into which strategy is more appropriate for specific applications. The investigations are carried out through simulation case studies with a real-field one-day fluctuated irradiance profile.National Research Foundation (NRF)Submitted/Accepted versionThis research was supported by the Republic of Singapore’s National Research Foundation (NRF) through the "Distributed Energy Resource Management System for Energy Grid 2.0" project at the Energy Research Institute @ Nanyang Techno- logical University, Singapore

Decoupled discontinuous modulation for cascaded H-bridge StatCom with star configuration

This paper presents a novel zero-sequence voltage injection method for discontinuous modulation (DM) in star-connected cascaded H-bridge (CHB) static compensators (StatComs). Conventional DM methods in CHB StatComs suffer from control interactions during unbalanced grid conditions. This paper proposes a DM that eliminates this control interaction by inserting a clamping transition within a third of the fundamental period. The transition instant is decided by comparing a duty cycle, which is calculated to mitigate the fundamental-frequency in the zero-sequence voltage for discontinuous operation, with a triangular carrier signal with a frequency of three times the fundamental. The clamping intervals under the proposed DM are affected by the phase-shift of this carrier. Specifically, the paper analyses the effects on the harmonic content of the proposed zero-sequence voltage, switching loss, twice-fundamental-frequency capacitor voltage ripple, and current total harmonic distortion, for different phase-shift angles of the DM carrier signal. Besides, the feasibility of the proposed DM in dealing with a grid voltage sag is also discussed.Nanyang Technological UniversityNational Research Foundation (NRF)Submitted/Accepted versionThis work was supported by the Office of Naval Research U.S. under DUNS Code: 595886219 and in part by the National Research Foundation of Singapore under “Distributed Energy Resource Management Systems (DERMS) For Energy Grid 2.0” project. The authors wish to acknowledge the Interdisciplinary Graduate Programme (IGP) at Nanyang Technological University for providing the conference funding support

Minimizing energy storage utilization in a stand-alone dc microgrid using photovoltaic flexible power control

DC microgrids (dcMGs) are gaining popularity for photovoltaic (PV) applications as the demand for PV generation continues to grow exponentially. A hybrid control strategy for a PV and battery energy storage system (BESS) in a stand-alone dcMG is proposed in this paper. In contrast to the conventional control strategies that regulate the dc-link voltage only with the BESS, the proposed control strategy exploits both the PV system and the BESS to regulate the dc-link voltage. The PV acts as the primary dc voltage regulator allowing for the battery to remain standby as a secondary dc voltage regulating resource. As a result, the proposed control strategy minimizes the utilization of the BESS in order to prolong its lifetime while maintaining the state-of-charge (SoC) of the battery within a desired range. To achieve that, the flexible power point tracking (FPPT) concept is applied to the PV system to enhance the dynamic performance of the dcMG by adaptively adjusting the PV output power according to the load profile. The performance of the proposed control strategy is verified with experimental results. Furthermore, the effectiveness of the proposed control strategy on prolonging the lifetime of a lithium-ion battery and a lead-acid battery is investigated via a simulation case study with one-day load and irradiance curve profiles.Ministry of Education (MOE)Accepted versionThis work was supported by the Singapore Ministry of Education Academic Research Fund Tier 1 under Grant 2019-T1-001-168 (RG80/19). It was also supported by the Basque Country Government under the project ENSOL2 (Development of Advanced PV Technologies Grant No: KK-2020/00077)

Analysis of the inter-submodule active power disparity limits of modular multilevel converter-based battery energy storage systems

In operation of the modular multilevel converter (MMC)-based battery energy storage system (BESS), different submodules (SMs) may be required to provide a different amount of active power, which is defined as inter-SM active power disparity in this paper. This happens in various conditions, for example, when some SMs have faulty batteries and thus, cannot process any active power. This paper proposes a general analysis of the inter-SM active power disparity limits of the modular multilevel converter-based battery energy storage systems. An analytical method is provided to derive the maximum/minimum power that any subset of SMs in one arm can provide. Based on the derived results, the active power limits of SMs are nonlinear with the number of SMs. Both simulation and experimental results are provided, which verify the proposed analysis and method. The proposed analysis facilitates the design and operation of the MMC-based BESSs.Submitted/Accepted versionThis work was supported by the Office of Naval Research U.S. under DUNS Code: 595886219

A generalized voltage balancing algorithm for modular multilevel cascaded converters

The use of an inter-cell voltage balancing scheme to evenly distribute the total dc-link voltage among the multiple floating capacitors is crucial in modular multilevel cascaded converters (MMCs). This paper presents an intuitively simple and computationally efficient sorting-based modulation algorithm for capacitor voltage balancing applicable to any MMC. The proposed modulation stage is inherently decoupled from the upper-level control scheme and it is able to both generate the required converter output voltage and balance the capacitor voltages suitably. Furthermore, it maintains a constant harmonic performance even as the number of switching transitions is varied to achieve voltage balance under different loading conditions with minimum incurred switching losses. The effectiveness of the proposed algorithm is validated on an experimental seven-level 350-VA single-phase cascaded H-bridge working as a static compensator. In addition, a simulated 350-VA system is used to obtain complimentary results.Accepted versio