A Multilevel DC to Three-Phase AC Architecture for Photovoltaic Power Plants

This paper presents a photovoltaic (PV) inverter architecture composed of stackable dc to three-phase ac converter blocks. Several such blocks, each containing a converter power stage and controls, are connected in series on their ac sides to obtain transformerless medium-voltage ac interfaces for PV power plants. The series-connected structure is made possible by a quadruple active bridge dc-dc converter that provides isolation between the PV input and each of the three ac-side phases within each block. Furthermore, since incoming PV power is transferred as constant balanced three-phase ac power, instantaneous input-output power balance bypasses the need for bulk energy storage. To streamline implementation and maximize system scalability and resilience, decentralized block-level controllers accomplish dc-link voltage regulation, maximum power point tracking, and ac-side power sharing without centralized means. The proposed architecture is validated by simulations of a PV string to medium-voltage ac system consisting of six blocks and on a proof-of-concept hardware prototype that consists of three cascaded converter blocks.</p

Grid integration of offshore wind power : standards, control, power quality and transmission

Offshore wind is expected to be a major player in the global efforts toward decarbonization, leading to exceptional changes in modern power systems. Understanding the impacts and capabilities of the relatively new and uniquely positioned assets in grids with high integration levels of inverter-based resources, however, is lacking, raising concerns about grid reliability, stability, power quality, and resilience, with the absence of updated grid codes to guide the massive deployment of offshore wind. To help fill the gap, this paper presents an overview of the state-of-the-art technologies of offshore wind power grid integration. First, the paper investigates the most current grid requirements for wind power plant integration, based on a harmonized European Network of Transmission System Operators (ENTSO-E) framework and notable international standards, and it illuminates future directions. The paper discusses the wind turbine and wind power plant control strategies, and new control approaches, such as grid-forming control, are presented in detail. The paper reviews recent research on the ancillary services that offshore wind power plants can potentially provide, which, when harmonized, will not only comply with regulations but also improve the value of the asset. The paper explores topics of wind power plant harmonics, reviewing the latest standards in detail and outlining mitigation methods. The paper also presents stability analysis methods for wind power plants, with discussions centered on validity and computational efficiency. Finally, the paper discusses wind power plant transmission solutions, with a focus on high-voltage direct-current topologies and controls

Levelized Cost of Energy-Oriented Modular String Inverter Design Optimization for PV Generation System Using Geometric Programming

Levelized cost of energy (LCOE) is a commonly used metric to assess the cost-to-benefit ratio over the lifetime of an energy resource, such as photovoltaics (PV); however, power electronics engineers tend to rely on metrics such as efficiency and power density, which do not guarantee lifetime cost optimality. Recent work has shown that an LCOE-focused optimization approach can yield improved system designs, leading to improved lifetime performance with balanced lifetime cost and energy generation. This paper outlines an LCOE optimization framework for PV power electronics that uses geometric programming. The large number of circuit parameters and nonlinear nature of the system equations pose significant barriers. Our approach allows for decoupling the design variables, which, in turn, enables superior computational efficiency and a near-optimal solution. By incorporating the power electronics design process and magnetic loss mechanism into the convex design framework, the optimization engine yields practically implementable parameters for a PV converter that minimizes LCOE. An optimization example for a cascaded modular PV inverter architecture is presented that suggests 3.35% LCOE improvement can be achieved by the new power electronics and the advanced optimization. The proposed optimization framework can be applied to other power generation systems to evaluate the effect of the power electronics design on system lifetime costs and efficiency. &nbsp;</p

Representative levels of blood lead, mercury, and urinary cadmium in youth: Korean Environmental Health Survey in Children and Adolescents (KorEHS-C), 2012–2014

AbstractBackgroundThis study examined levels of blood lead and mercury, and urinary cadmium, and associated sociodemographic factors in 3–18 year-old Korean children and adolescents.Materials and methodsWe used the nationally representative Korean Environmental Health Survey in Children and Adolescents data for 2012–2014 and identified 2388 children and adolescents aged 3–18 years. The median and 95th percentile exposure biomarker levels with 95% confidence intervals (CIs) were calculated. Multivariate regression analyses were performed on log transformed exposure biomarker levels adjusted for age, sex, area, household income, and father’s education level. The median exposure biomarker levels were compared with data from Germany, the US, and Canada, as well as the levels of Korean children measured at different times.ResultsThe median levels of blood lead and mercury, as well as urinary cadmium were 1.23μg/dL, 1.80μg/L, and 0.40μg/L (95% CIs, 1.21–1.25, 1.77–1.83, and 0.39–0.41, respectively). The blood lead levels were significantly higher in boys and younger children (p<0.0001) and children with less educated fathers (p=0.004) after adjusting for covariates. Urinary cadmium level increased with age (p<0.0001). The median levels of blood mercury and urinary cadmium were much higher in Korean children and adolescents than those in their peers in Germany, the US, and Canada. Blood lead levels tended to decrease with increasing age and divergence between the sexes, particularly in the early teen years. Median levels of blood lead and urinary cadmium decreased since 2010.ConclusionSociodemographic factors, including age, sex, and father’s education level were associated with environmental exposure to heavy metals in Korean children and adolescents. These biomonitoring data are valuable for ongoing surveillance of environmental exposure in this vulnerable population

Photovoltaic ac parameter characterization for dynamic partial shading and hot spot detection

This study investigates ac parameter characterization to detect partial shading within a series string of photovoltaic (PV) cells. Partial shading detection is needed to prevent hot spotting, a condition that reduces panel power performance and accelerates cell degradation. A PV cell is comprised of series and parallel resistances and parallel capacitance, which are affected by voltage bias, illumination, and temperature. When the string is under a maximum power point tracker or constant voltage controller, partial shading results in a clear increase in capacitance that is detectable by measuring the ac impedance magnitude in the 10-70 kHz frequency range. When the shaded cells become moderately negatively biased, there is a clear increase in parallel resistance. These trends in the PV cell physical characteristics can be used to develop partial shading detection methods that utilize the switching ripple of the power converter that controls the PV string