Model Predictive Control for Distributed Microgrid Battery Energy Storage Systems

© 2017 IEEE. This brief proposes a new convex model predictive control (MPC) strategy for dynamic optimal power flow between battery energy storage (ES) systems distributed in an ac microgrid. The proposed control strategy uses a new problem formulation, based on a linear d-q reference frame voltage-current model and linearized power flow approximations. This allows the optimal power flows to be solved as a convex optimization problem, for which fast and robust solvers exist. The proposed method does not assume that real and reactive power flows are decoupled, allowing line losses, voltage constraints, and converter current constraints to be addressed. In addition, nonlinear variations in the charge and discharge efficiencies of lithium ion batteries are analyzed and included in the control strategy. Real-time digital simulations were carried out for an islanded microgrid based on the IEEE 13 bus prototypical feeder, with distributed battery ES systems and intermittent photovoltaic generation. It is shown that the proposed control strategy approaches the performance of a strategy based on nonconvex optimization, while reducing the required computation time by a factor of 1000, making it suitable for a real-time MPC implementation

A New Method for Evaluating the Carbon Isotope Characteristics of Carbonate Formed Under Cryogenic Conditions Analogous to Mars

The two upcoming robotic missions to Mars, Phoenix and MSL, will both have the capability of measuring the carbon isotopic composition of CO2 in the martian atmosphere, as well as possible CO2 trapped in carbonate minerals in the Martian soil. Results from orbital and landed missions now clearly indicate that no large scale deposits of carbonate materials exist at the surface. However, some results from orbital remote sensing have been interpreted to indicate that carbonate minerals are present as fine particles interspersed at low concentrations (approx. 2%) in the martian dust. One likely mechanism for the production of these carbonates is during the freezing of transient water near the surface. Large deposits of near surface ice and photographic evidence for flowing water on the surface suggest that transient melting and refreezing of H2O is an active process on Mars. Any exposure of these fluids to the CO2 rich atmosphere should al-low the production of HCO3- solutions. Carbonates are likely precipitates from these solutions during freezing as extensive CO2 degassing, driven by the fluid s decreasing volume, drives CO2 out. This rapid CO2 degassing increases the pH of the solution and drives carbonate precipitation. It has been shown in previous studies that this rapid CO2 degassing also results in a kinetic isotopic fractionation where the CO2 gas has a much lighter isotopic composition causing a large isotope enrichment of C-13 in the precipitated carbonate. This kinetic isotope enrichment may be very common in the current martian environment, and may be a very important factor in understanding the very high deltaC-13 values of carbonates found in the martian meteorites. However, while previous studies have succeeded in generally quantifying the magnitude of this effect, detailed studies of the consistency of this effect, and the freezing rates needed to produce it are needed to understand any carbon isotope analyses from carbonate minerals in the martian soil or dust. This study demonstrates an innovative new method for measuring the isotopic composition of gas evolved from the freezing of carbonate solutions in real time, which allows for a much clearer view of the chemical processes involved. This method now sets the stage for detailed analysis of the chemical and isotopic mechanisms that produce cryogenic carbonates

Delta-connected cascaded H-bridge multilevel photovoltaic converters

© 2015 IEEE. Multilevel cascaded H-bridge converters are becoming popular for next generation large-scale photovoltaic power converters. However, the power generation levels in the three phases can be significantly unequal, especially in a large plant, owing to the non-uniform irradiance levels and/or ambient temperatures. This paper proposes the delta-connected cascaded H-bridge converter for large-scale photovoltaic farms. Compared to the existing star connection, the delta connection reduces the converter overrating required. Experimental results obtained from a 430 V, 10 kW, three-phase, seven-level, delta connected cascaded H-bridge converter prototype are provided to demonstrate the superiority of the delta connection

Confirmation of Soluble Sulfate at the Phoenix Landing Site: Implications for Martian Geochemistry and Habitability

Over the past several decades, elemental sulfur in martian soils and rocks has been detected by a number of missions using X-ray spectroscopy [1-3]. Optical spectroscopy has also provided evidence for widespread sulfates on Mars [4,5]. The ubiquitous presence of sulfur in soils has been interpreted as a widely distributed sulfate mineralogy [6]. However, direct confirmation as to the identity and solubility of the sulfur species in martian soil has never been obtained. One goal of the Wet Chemistry Laboratory (WCL) [7] on board the 2007 Phoenix Mars Lander [8] was to determine soluble sulfate in the martian soil. The WCL received three primary samples. Each sample was added to 25 mL of leaching solution and analysed for solvated ionic species, pH, and conductivity [9,10]. The analysis also showed a discrepancy between charge balance, ionic strength, and conductivity, suggesting unidentified anionic species

August Šenoas historische Romane

In der Entwicklung der kroatischen Literatur des 19. Jahrhunderts nimmt August Šenoa eine Schlüsselposition ein. Die Bedeutung Šenoas beruht in erster Linie auf seiner Prosa - Erzählungen und historischen Romanen. Absicht der vorliegenden Arbeit ist es, die Geschichtskonzeption Šenoas die in seiner Geschichtsdarstellung zum Ausdruck kommt, am Beispiel seiner drei wichtigsten Romane, sichtbar zu machen

Network topology independent multi-agent dynamic optimal power flow for microgrids with distributed energy storage systems

This paper proposes a multi-agent dynamic optimal power flow (DOPF) strategy for microgrids with distributed energy storage systems. The proposed control strategy uses a convex formulation of the ac DOPF problem developed from a d-q reference frame voltage-current model and linear power flow approximations. The convex DOPF problem is divided between autonomous agents and solved based on local information and neighbour-to-neighbour communication over a sparse communication network, using a distributed primal subgradient algorithm. Each agent is only required to solve convex quadratic sub-problems, for which robust and efficient solvers exist, making the control strategy suitable for receding horizon model predictive control. Also, the agent sub-problems require limited power network information and include only a subset of the centralised optimisation problem decision variables and constraints, providing scalability and data privacy. Unlike existing distributed optimal power flow methods, such as alternating direction method of multipliers, under the proposed control strategy the information required by each agent is independent of the communication network topology, providing increased flexibility and robustness. The performance of the proposed control strategy was verified for an ac microgrid with distributed lead-acid batteries and intermittent photovoltaic generation, using an RTDS Technologies real-time digital simulator

Control strategies for microgrids with distributed energy storage systems: An overview

This paper presents an overview of the state of the art control strategies specifically designed to coordinate distributed energy storage (ES) systems in microgrids. Power networks are undergoing a transition from the traditional model of centralised generation towards a smart decentralised network of renewable sources and ES systems, organised into autonomous microgrids. ES systems can provide a range of services, particularly when distributed throughout the power network. The introduction of distributed ES represents a fundamental change for power networks, increasing the network control problem dimensionality and adding long time-scale dynamics associated with the storage systems' state of charge levels. Managing microgrids with many small distributed ES systems requires new scalable control strategies that are robust to power network and communication network disturbances. This paper reviews the range of services distributed ES systems can provide, and the control challenges they introduce. The focus of this paper is a presentation of the latest decentralised, centralised and distributed multi-agent control strategies designed to coordinate distributed microgrid ES systems. Finally, multi-agent control with agents satisfying Wooldridge's definition of intelligence is proposed as a promising direction for future research

2013 IEEE ECCE Asia Downunder - 5th IEEE Annual International Energy Conversion Congress and Exhibition, IEEE ECCE Asia 2013

A low complexity control system for a conventional hybrid battery-ultracapacitor power source with two individual DC/DC converters is proposed in this paper. The advantages of the proposed control system are that it (i) allows to allocate the high frequency current demands to the ultracapacitor, (ii) allows to specify the current limits for both the battery and the ultracapacitor, (iii) maintains operation of the battery within its state of charge limits, (iv) maintains the ultracapacitor voltage at a predefined value and (v) charges the ultracapacitor from the battery rather than from the common DC bus. Presented simulation results verify operation of the proposed system. © 2013 IEEE

A Low Complexity Control System for a Hybrid DC Power Source Based on Ultracapacitor–Lead–Acid Battery Configuration

A dc hybrid power source based on the combination of ultracapacitor and lead-acid battery is considered in this paper. The various control systems for such hybrid power source reported in the technical literature thus far are rather complex. A low complexity control system for such hybrid power source is proposed in this paper. The key feature of the proposed control system is its capability to maintain operation of the hybrid power source within all important operational limits. The proposed control system allows one to allocate the high-frequency current demands to the ultracapacitor and specify the current limits for both the battery and the ultracapacitor. It also maintains operation of the battery within its state of charge limits and the ultracapacitor voltage at a predefined value while charging the ultracapacitor from the battery rather than from the common dc bus. Presented experimental results verify the satisfactory operation of the power source utilizing the proposed control system

Control and mechanical component reduction in an EV direct wheel drive

This paper looks at control and mechanical component reduction in an EV direct wheel driv