Tulga Ersal

Impact of controlled plug-in EVs on microgrids: A military microgrid example

Increasing concerns about energy security and reliability are intensifying the interest in microgrid and vehicle-to-grid (V2G) technologies. Although the role of V2G technology within the context of optimal scheduling for larger grids has received much attention in the literature, its role within the regulation of microgrids has not yet been studied extensively. In this paper, we focus on the voltage and frequency regulation problem. We develop a microgrid model that is representative of the microgrid architecture considered in the SPIDERS (Smart Power Infrastructure Demonstration for Energy Reliability and Security) project of the Department of Defense. The model is parameterized to reflect the characteristics of Camp Smith, HI, the targeted installation of the SPIDERS project, and the long term Army goals regarding renewable energy penetration and reduction in fuel consumption. The model is augmented by power, frequency, and voltage control algorithms for the inverters that connect microsources to the microgrid. It also incorporates charging/discharging control algorithms for plug-in electric vehicles (PEVs) to take advantage of their capacity as both controllable loads and sources. Using this model, we study the impact of PEVs on the microgrid at different penetration levels and for different control parameters, with the aim of identifying the conditions needed for the vehicle-to-grid technology to have a positive impact on microgrid performance.

Statistical Transparency Analysis in Internet-Distributed Hardware-in-the-Loop Simulation

Internet-distributed hardware-in-the-loop simulation (ID-HILS) is emerging as a critical enabler for geographically dispersed concurrent systems engineering. This paper is concerned with transparency in ID-HILS, which is a measure of fidelity with respect to the nondistributed alternative of integration. Specifically, recognizing the need for a transparency analysis method for stochastic and nonlinear ID-HILS systems in general, the paper first proposes a statistical transparency analysis method. Next, this method is applied to a novel ID-HILS system. This application helps draw two important general conclusions: 1) Distributing the simulation can in and of itself be an important source of transparency degradation and can even dominate the adverse effects of the Internets delay, jitter, and loss when delay is relatively small; and 2) transparency is not an independent property of the system, but is a system property that needs to be defined with respect to an output, as different output signals in the same system can experience different levels of transparency. These conclusions are important for guiding future efforts to improve transparency in a given ID-HILS system, and the proposed method enables such transparency analysis in other stochastic nonlinear ID-HILS systems, as well.

Coupling Between Component Sizing and Regulation Capability in Microgrids

Increasing energy security and reliability concerns are intensifying the interest in microgrids. In this setting, design optimization is vital to achieve a reliable infrastructure without overbuilding. This paper considers the impact of frequency and voltage regulation on the optimal design of a conceptual, autonomous military microgrid. This microgrid comprises a solar panel and vehicles as power sources, with each vehicle incorporating a battery and generator. The power output and terminal voltage of these inverter-based sources must be regulated. The paper investigates the effects of battery DC voltage variations on a decentralized regulation scheme, and the resulting influence on optimal component sizing. To this end, controllers are first designed based on the typical assumption that the voltage on the DC side of each inverter is constant. The battery internal resistance is then considered and its impact on regulation performance is investigated. The results show that the battery internal resistance can affect the performance of both frequency and voltage regulation, and consequently must be taken into account in component sizing decisions. Thus, the paper identifies an important coupling between regulation and component sizing problems through battery characteristics, and highlights the need for a combined sizing and regulation framework for microgrid design.

An Iterative Learning Control Approach to Improving Fidelity in Internet-Distributed Hardware-in-the-Loop Simulation

Abstract : One of the main challenges of co-simulating hardware-in-the-loop systems in real-time over the Internet is the fidelity of the simulation. The dynamics of the Internet may significantly distort the dynamics of the network-integrated system. This paper presents the development of an iterative learning control based approach to improve fidelity of such networked system integration. Towards this end, a new metric for characterizing fidelity is proposed first, which, unlike some existing metrics, does not require knowledge about the reference dynamics (i.e., dynamics that would be observed, if the system was physically connected). Next, using this metric, the problem of improving fidelity is formulated as an iterative learning control problem. Finally, the proposed approach is illustrated on a purely simulation-based case study. The conclusion is that the proposed approach holds significant potential for achieving high fidelity levels.

Theoretical and experimental indicators of falls during pregnancy as assessed by postural perturbations

Throughout pregnancy, women experience physical, physiological, and hormonal alterations that are often accompanied by decreased postural control. According to one study, nearly 27% of pregnant women fell while pregnant. This study had two objectives: (1) to characterize the postural responses of pregnant fallers, nonfallers, and controls to surface perturbations, and (2) to develop a mathematical model to gain insights into the postural control strategies of each group. This retrospective analysis used experimental data obtained from 15 women with a fall history during pregnancy, 14 women without a fall history during pregnancy, and 40 nonpregnant controls. Small, medium, and large translational support surface perturbations in the anterior and posterior directions were performed during the pregnant participants second and third trimesters. A two-segmented mathematical model of bipedal stance was developed and parameterized, and optimization tools were used to identify ankle and hip stiffness, viscosity, and the feedback time delay by searching for the best fits to experimental COP data. The peak differences between the center of pressure and center of gravity (COP-COG) values were significantly smaller for the pregnant fallers compared with the pregnant nonfallers and controls (p<0.01). Perturbation magnitude was a significant factor (p<0.01), but perturbation direction was not (p=0.24). Model fits were obtained with a mean goodness of fit value of R(2)=0.92. Theoretical results indicated that pregnant nonfallers had higher ankle stiffness compared with the pregnant fallers and the controls, which suggests that ankle stiffness itself may be the dominant reason for the different dynamic response characteristics (e.g., peak COP-COG) observed. We conclude that increasing ankle stiffness could be an important strategy to prevent falling by pregnant women.

Effect of coupling point selection on distortion in internet-distributed hardware-in-the-loop simulation

The degree to which an Internet-Distributed Hardware-In-the-Loop (ID-HIL) simulation loses fidelity relative to the single-location alternative is referred to as distortion. This paper shows that, besides delay, the choice of coupling point, i.e., the port at which the system model is integrated across the Internet, also affects distortion. To quantify distortion, a frequency-domain metric is proposed using a linear systems framework. This metric is then used to analyse how the choice of coupling point affects distortion, leading to guidelines for selecting a coupling point that gives minimal distortion. The theory is demonstrated on a quarter-car model.

Noninvasive Battery-Health Diagnostics Using Retrospective-Cost Identification of Inaccessible Subsystems

Health management of Li-ion batteries depends on knowledge of certain battery internal dynamics (e.g., lithium consumption and film growth at the solid-electrolyte interface) whose inputs and outputs are not directly measurable with noninvasive methods. This presents a problem of identification of inaccessible subsystems. To address this problem, we apply the retrospective-cost subsystem identification (RCSI) method. As a first step, this paper presents a simulation-based study that assumes as the truth model of the battery an electrochemistrybased battery charge/discharge model of Doyle, Fuller, and Newman, and later augmented with a battery-health model by Ramadass. First, this truth model is used to generate the data needed for the identification study. Next, the film-growth component of the battery-health model is assumed to be unknown, and the identification of this inaccessible subsystem is performed using RCSI. The results show that the subsystem identification method can identify the film growth quite accurately when the chemical reactions leading to film growth are consequential.

Realization-Preserving Structure and Order Reduction of Nonlinear Energetic System Models Using Energy Trajectory Correlations

Previous work by the authors developed algorithms for simplifying the structure of a lumped dynamic system model and reducing its order. This paper extends this previous work to enable simultaneous model structure and order reduction. Specifically, it introduces a new energy-based metric to evaluate the relative importance of energetic connections in a model. This metric (1) accounts for correlations between energy flow patterns in a model using the Karhunen-Loeve expansion; (2) examines all energetic connections in a model, thereby assessing the relative importance of both energetic components and their interactions, and enabling both order and structural reduction; and (3) is realization preserving, in the sense of not requiring a state transformation. A reduction scheme based on this metric is presented and illustrated using a simple example. The example shows that the proposed method can successfully concurrently reduce model order and structure without requiring a realization change, and that it can provide an improved assessment of the importance of various model components due to its correlation-based nature.

On the effect of DC source voltage on inverter-based frequency and voltage regulation in a military microgrid

Increasing concerns about energy security and reliability are intensifying the militarys interest in the microgrid technology. This paper focuses on the frequency and voltage regulation part of a case study that seeks to design a conceptual military microgrid that has to operate completely autonomously. This microgrid comprises a solar panel and vehicles as power sources, where the vehicles contain a battery and generator, and managing their power setpoints is the control problem of interest. The goal is to investigate the validity of assuming the battery as a constant voltage source during the control design. To this end, the control design is first carried out assuming the battery as a constant voltage source. It is then illustrated how critical this assumption can be depending on the battery internal resistance. The results show that the battery internal resistance can affect both frequency and voltage regulation performances, thereby highlighting the coupling between the battery characteristics and control design, and the need for a control design framework that takes this coupling into account.

A mathematical model for incorporating biofeedback into human postural control

BackgroundBiofeedback of body motion can serve as a balance aid and rehabilitation tool. To date, mathematical models considering the integration of biofeedback into postural control have represented this integration as a sensory addition and limited their application to a single degree-of-freedom representation of the body. This study has two objectives: 1) to develop a scalable method for incorporating biofeedback into postural control that is independent of the model’s degrees of freedom, how it handles sensory integration, and the modeling of its postural controller; and 2) to validate this new model using multidirectional perturbation experimental results.MethodsBiofeedback was modeled as an additional torque to the postural controller torque. For validation, this biofeedback modeling approach was applied to a vibrotactile biofeedback device and incorporated into a two-link multibody model with full-state-feedback control that represents the dynamics of bipedal stance. Average response trajectories of body sway and center of pressure (COP) to multidirectional surface perturbations of subjects with vestibular deficits were used for model parameterization and validation in multiple perturbation directions and for multiple display resolutions. The quality of fit was quantified using average error and cross-correlation values.ResultsThe mean of the average errors across all tactor configurations and perturbations was 0.24° for body sway and 0.39 cm for COP. The mean of the cross-correlation value was 0.97 for both body sway and COP.ConclusionsThe biofeedback model developed in this study is capable of capturing experimental response trajectory shapes with low average errors and high cross-correlation values in both the anterior-posterior and medial-lateral directions for all perturbation directions and spatial resolution display configurations considered. The results validate that biofeedback can be modeled as an additional torque to the postural controller without a need for sensory reweighting. This novel approach is scalable and applicable to a wide range of movement conditions within the fields of balance and balance rehabilitation. The model confirms experimental results that increased display resolution does not necessarily lead to reduced body sway. To our knowledge, this is the first theoretical confirmation that a spatial display resolution of 180° can be as effective as a spatial resolution of 22.5°.