Stabilizing Controller Design for a DC Power Distribution System Using A Passivity-Based Stability Criterion

In modern times there has been an increased penetration of power electronic converters into Power Distribution Systems. In particular, there has been a strong interest in DC Power Distribution Systems as opposed to conventional AC Power distribution systems. These DC Power Distribution Systems are enabled by power electronics converters. The strong interest is motivated by improvements in power electronic converter technology, like advances in power semiconductor devices, magnetics, control, and converter topologies which have made possible to build high-performance converters at low cost. In many systems, such as cars, ships and airplanes, there has also been a trend towards the replacement of a number of older mechanical and hydraulic systems with electrical power-electronic-based systems, since these systems provide a number of advantages such as increased system flexibility, reliability, long life expectancy and decreased weight, size, and cost. Together with these advantages, DC Power Distribution Systems offer system-level challenges related to system stability issues and design of individual converter controllers to guarantee proper operation of the interconnected system. System-level stability issues may arise due to interactions among feedback-controlled power converters, which are part of such a large interconnected system. These feedback-controlled power converters exhibit negative incremental input impedance within their control bandwidth. As a result, a power converter that was satisfactorily performing when tested as a standalone unit may experience degradation in performance when connected as part of a system. While the analysis and design of a single power converter and its controls is well understood, in a DC Power Distribution System the situation is different. Analyzing and designing a complex multi-converter system in such a way as to guarantee both system stability and performance is a complex problem that was not fully solved in the past. Difficulties stem from a lack of adequate analysis and design tools, limited understanding of the problem, difficulties in applying the existing stability criteria, and the need for stabilizing converter controllers. To tackle all these difficulties, the present work proposes two tools to address system level stability issues in DC Power Distribution Systems: the Passivity-Based Stability Criterion (PBSC) and the Positive Feed-Forward (PFF) control. The PBSC is proposed as a tool for stability analysis in a DC Power Distribution System. The criterion is based on imposing passivity of the overall DC bus impedance. If passivity of the bus impedance is ensured, stability is guaranteed as well. The PBSC, which imposes conditions on the overall bus impedance, offers several advantages with respect to existing stability criteria, such as the Middlebrook criterion and its extensions, which are based on the minor loop gain concept, i.e. an impedance ratio at a given interface: reduction of artificial design conservativeness, insensitivity to component grouping, applicability to multi-converter systems and to systems in which the power flow direction changes, for example as a result of system reconfiguration. Moreover, the criterion is very designed-oriented because it can be used in conjunction with the second tool proposed in this dissertation, the PFF control, for the design of stabilizing virtual damping networks. The PFF controller design formulation guarantees both stability and performance (a challenge not fully solved in the past, as previously stated). By designing the stabilizing virtual impedance so that the bus impedance passivity condition is met, the approach results in greatly improved stability and damping of transients on the DC bus voltage. Simulation validation is performed using a switching-level-model of the DC power distribution system. Experimental validation is carried out on a DC power distribution system built in the laboratory

Online wideband identification of three-phase AC power grid impedances using an existing grid-tied power electronic inverter

In smart grid applications, online wideband monitoring of AC power grid impedances is a key enabler of a set of capabilities, such as active filter tuning, adaptive control of inverters, and monitoring of local grid status, from which stability margins can be calculated. To evaluate the effectiveness of online wideband monitoring of AC power grid impedances in smart grid applications, this paper presents the implementation, in an embedded controller, of a recently proposed online Wideband System Identification (WSI) technique, validated via Hardware In the Loop (HIL) real-time simulation. The identification technique exploits an existing grid-tied inverter for the estimation of wide bandwidth AC grid impedances, on top of the original power conversion function. This is accomplished by super-imposing a small-signal Pseudo Random Binary Sequence (PRBS), a digital approximation of white noise which is wide bandwidth in nature, on the inverter switching commands so that all frequencies of interest can be excited at once. Then, after postprocessing, the wideband AC grid impedance can be extracted using appropriate cross correlation techniques. The present work focuses on the identification of balanced three-phase AC power grid impedances in the three-phase reference frame

Siting Study Framework and Survey Methodology for Marine and Hydrokinetic Energy Project in Offshore Southeast Florida

Dehlsen Associates, LLC was awarded a grant by the United States Department of Energy (DOE) Golden Field Office for a project titled 'Siting Study Framework and Survey Methodology for Marine and Hydrokinetic Energy Project in Offshore Southeast Florida,' corresponding to DOE Grant Award Number DE-EE0002655 resulting from DOE funding Opportunity Announcement Number DE-FOA-0000069 for Topic Area 2, and it is referred to herein as 'the project.' The purpose of the project was to enhance the certainty of the survey requirements and regulatory review processes for the purpose of reducing the time, efforts, and costs associated with initial siting efforts of marine and hydrokinetic energy conversion facilities that may be proposed in the Atlantic Ocean offshore Southeast Florida. To secure early input from agencies, protocols were developed for collecting baseline geophysical information and benthic habitat data that can be used by project developers and regulators to make decisions early in the process of determining project location (i.e., the siting process) that avoid or minimize adverse impacts to sensitive marine benthic habitat. It is presumed that such an approach will help facilitate the licensing process for hydrokinetic and other ocean renewable energy projects within the study area and will assist in clarifying the baseline environmental data requirements described in the U.S. Department of the Interior Bureau of Ocean Energy Management, Regulation and Enforcement (formerly Minerals Management Service) final regulations on offshore renewable energy (30 Code of Federal Regulations 285, published April 29, 2009). Because projects generally seek to avoid or minimize impacts to sensitive marine habitats, it was not the intent of this project to investigate areas that did not appear suitable for the siting of ocean renewable energy projects. Rather, a two-tiered approach was designed with the first step consisting of gaining overall insight about seabed conditions offshore southeastern Florida by conducting a geophysical survey of pre-selected areas with subsequent post-processing and expert data interpretation by geophysicists and experienced marine biologists knowledgeable about the general project area. The second step sought to validate the benthic habitat types interpreted from the geophysical data by conducting benthic video and photographic field surveys of selected habitat types. The goal of this step was to determine the degree of correlation between the habitat types interpreted from the geophysical data and what actually exists on the seafloor based on the benthic video survey logs. This step included spot-checking selected habitat types rather than comprehensive evaluation of the entire area covered by the geophysical survey. It is important to note that non-invasive survey methods were used as part of this study and no devices of any kind were either temporarily or permanently attached to the seabed as part of the work conducted under this project