The increasing penetration of renewable energy sources (RES) poses a major challenge to the operation of the electricity grid owing to the intermittent nature of their power output. The ability of utility-scale battery energy storage systems (BESS) to provide grid support and smooth the output of RES in combination with their decrease in cost has fueled research interest in this technology over the last couple of years. Power electronics (PE) is the key enabling technology for connecting utility-scale BESS to the medium-voltage grid. PE ensure energy is delivered while complying with grid codes and dispatch orders. Simultaneously, the PE must regulate the operating point of the batteries, thus for instance preventing overcharge of batteries. This paper presents a comprehensive review of PE topologies for utility BESS that have been proposed either within industry or the academic literature. Moreover, a comparison of the presently most commercially viable topologies is conducted in terms of estimated power conversion efficiency and relative cost.

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A Review of Power Electronics for Grid Connection of Utility-Scale Battery Energy Storage Systems

Solid-state transformer (SST) is an emerging technology integrating with a transformer power electronics converters and control circuitry. This paper comprehensively reviews the SST topologies suitable for different voltage levels and with varied stages, their control operation, and different trends in applications. The paper discusses various SST configurations with their design and characteristics to convert the input to output under unipolar and bipolar operation. A comparison between the topologies, control operation and applications are included. Different control models and schemes are explained. Potential benefits of SST in many applications in terms of controllability and the synergy of AC and DC systems are highlighted to appreciate the importance of SST technologies. This review highlights many factors including existing issues and challenges and provides recommendations for the improvement of future SST configuration and development.

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State of the art of solid-state transformers: Advanced topologies, implementation issues, recent progress and improvements

The transient response of the voltage source converter (VSC) interfaced dc system is significantly different from the ac counterpart. The rapid discharge current from the dc-link capacitors and the vulnerability of the freewheeling diodes during the short circuit in dc grid demand that the transient detection algorithm executes within a few milliseconds. The rapidly rising fault current in the dc grid is expected to have high-frequency components which might be an effective indicator of the transient condition. This paper presents quantitative investigation of the high-frequency components utilizing short-time Fourier transform (STFT) during transient conditions. Detailed operating principles with various factors affecting the STFT operation such as ripple content of the input dc signal and window type and length have been thoroughly investigated. STFT algorithm is able to detect low-impedance faults within 1 ms and high-impedance faults in 2 ms. Moreover, it is able to distinguish between short-circuit fault and less severe transient conditions such as sudden load change. The STFT algorithm is evaluated analytically and subsequently applied to a MATLAB/Simulink based dc test system. It is further validated and substantiated with the real fault current data obtained from a scaled-down experimental testbed. Sensitivity analysis and comparison with the existing frequency-domain-based fault-detection method are done to support the efficacy of the proposed method.

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Short-Time Fourier Transform Based Transient Analysis of VSC Interfaced Point-to-Point DC System

The electric power industry is facing unprecedented transformations and challenges with the implementation of the smart grids. This new grid paradigm has arisen to build a flexible electric power system that better coordinates energy resources and loads aiming at efficiently delivering sustainable, economic and secure electricity supplies. As a part of the smart grids (SGs), microgrids (MGs) have been developed to exploit the full benefits from the integration of distributed energy resources, especially distributed renewable generation based on variable and intermittent sources, such as wind and solar. Nevertheless, meeting all these goal requires the implementation of innovative energy storage technologies integrated with high efficiency and very fast response electronic power conditioning systems to interface with the electrical grid. Power electronics systems play a key role in regulating the raw energy from energy storage systems (ESSs) and connecting to the electrical grid. Hence, this paper performs a comprehensive analysis of major technologies in electrical energy storage systems and their electronic interface for applications in smart grids. The work provides a complete study of the technology profile of both energy storage and power electronics suitable for applications in the evolving grid.

Energy Storage and Power Electronics Technologies: A Strong Combination to Empower the Transformation to the Smart Grid

Energy storage (ES) is a pivotal technology for dealing with the challenges caused by the integration of renewable energy sources. It is expected that a decrease in the capital cost of storage will eventually spur the deployment of large amounts of ES. These devices will provide transmission services, such as spatiotemporal energy arbitrage, i.e., storing surplus energy from intermittent renewable sources for later use by loads while reducing the congestion in the transmission network. This paper proposes a bilevel program that determines the optimal location and size of storage devices to perform this spatiotemporal energy arbitrage. This method aims to simultaneously reduce the system-wide operating cost and the cost of investments in ES while ensuring that merchant storage devices collect sufficient profits to fully recover their investment cost. The usefulness of the proposed method is illustrated using a representative case study of the ISO New England system with a prospective wind generation portfolio.

Ensuring Profitability of Energy Storage

Power electronic conversion units will serve as a key enabling technology for assisting in the continued growth of grid-scale energy storage. This paper presents existing and future power electronic conversion systems and components that aid the interconnection of grid-scale energy storage or utilize storage to minimize grid disruption at all voltage classes including transmission, distribution, and future grid architectures such as the microgrid. New R&D solutions to aid the interconnection process including efforts in bidirectional charger design and potentially solid-state transformers (SSTs) are emphasized. The role of energy storage to support microgrid research and growth, while highlighting power electronic behavior within this environment, is considered. Last, an example that bridges the microgrid and energy storage theme is given through the design and operation of a direct current (dc) electric vehicle (battery) charging station. When appropriate, manufacturer solutions and success stories of utilizing latest battery technologies interfaced via power electronic solutions at the utility scale are provided.

Power Electronics for Grid-Scale Energy Storage

The potential to harness offshore wind power and transmit that energy to shore is of widespread interest in the electric power and energy industry sector, as well as within U.S. government agencies such as the Department of Energy. Studies are currently being conducted to find optimal placement of offshore wind turbines along the perimeter of the United States coast lines, and evaluations of current technology solutions for transmitting the electrical energy onshore are being investigated. Transmitting high capacities of energy from sea to shore presents a significant challenge due to the need for a very efficient, robust, and reliable technical solution that must be cost effective. High Voltage AC (HVAC) and two forms of High Voltage DC (HVDC), Voltage Source Converter (VSC) based and Line Commutated-Converter (LCC) based, are possible transmission solutions to these challenges. Therefore, we present herein the applicability of these topologies for offshore wind power transmission and present the necessary procedures to select the most cost effective solution for a given application.

Comparison of HVAC and HVDC solutions for offshore wind farms with a procedure for system economic evaluation

This paper addresses the protection of a high voltage direct current (HVDC) transmission system, utilizing the modular multilevel converter (MMC) topology in addition to incorporating a hybrid transmission corridor (transmission line including overhead line and cable sections). A solution is proposed for identifying the section within which a dc fault is located for the purpose of maintaining power delivery. A detailed model of the MMC-HVDC system is simulated using PSCAD and an in depth fault analysis is performed. A characteristic signal is discovered and then implemented into a novel solution. The end result is a fault section identification protection algorithm, implementing protective relay coordination to protect the system from false circuit breaker reclose as well as enabling fast system restart for nonpermanent faults. This restart protection algorithm is implemented without the use of a communications channel between converter stations, introducing novelty and quick restart response.

Fault Section Identification Protection Algorithm for Modular Multilevel Converter-Based High Voltage DC With a Hybrid Transmission Corridor

Disclosed herein are methods, systems, and devices for identifying a location of a fault in a hybrid high voltage direct current (HVDC) transmission system having a ground or underground cable section and an overhead line section. Some methods comprise determining whether a characteristic voltage oscillation is present at a cable side terminal of the HVDC transmission system, and in response, determining that the fault is located in the overhead line section. Some methods comprise determining whether a characteristic current oscillation or a terminal current exceeding a peak current threshold is present at an overhead line side terminal, and in response, determining that the fault is located in the overhead line section. The methods can be communication-less, wherein the actions are based on local measurements and/or are performed locally to where the fault is located or detected.

Communication-less fault section identification for hybrid hvdc transmission systems

With the increasingly high demand for well-trained power systems engineers, the University of Pittsburgh has partnered with Eaton in the creation of the new Electric Power Systems Laboratory at the Swanson School of Engineering. This laboratory demands the capability to perform a wide variety of experimental procedures while providing students and researchers with a seamless working environment. To accomplish that goal, a novel lab workbench was developed. This lab workbench integrates the load banks necessary for laboratory procedures with advanced metering, controls and safety features, and contains them within a work station ideally suited for the laboratory space. This paper details the development of those lab workbenches, from concept, to design, to prototype construction, to testing. Additionally, this work describes planned classroom experiments utilizing the developed bench. Ultimately the realized workbench demonstrates its capability to be reconfigured to suit the demands of both education as well as new research projects for undergraduate and graduate studies.

Patrick T. Lewis

Papers

Power Electronics for Grid-Scale Energy Storage

Comparison of HVAC and HVDC solutions for offshore wind farms with a procedure for system economic evaluation

Fault Section Identification Protection Algorithm for Modular Multilevel Converter-Based High Voltage DC With a Hybrid Transmission Corridor

Communication-less fault section identification for hybrid hvdc transmission systems

Design and Realization of an Innovative Workbench for Electric Power Systems Laboratories