Converter system nonlinear modelling and control for transmission applications - part II: CSC systems

The high-power self-commutated voltage-source converter (VSC) and current-source converter (CSC) are the key control devices in high-voltage direct current, flexible ac transmission systems, and distribution flexible ac transmission systems. To achieve the expected control objectives, suitable control strategies must be implemented based on the available devices, system models, and control techniques. The self-commutated ac/dc converters control the electrical power by generating controllable ac fundamental and dc average outputs. These controllable outputs are controlled by the conducting state combinations of the converter switching devices, driven by their gate signals. The gate signals are specified by fundamental parameters of frequency, amplitude, and phase angle. The converter system model for describing the relation between the system-state variables and the gate signal parameters is essential for the converter system control strategies. The companion paper (Part I) derives the state variable equations for the transmission systems using voltage-source-type converters. Part II is for the transmission systems using current-source-type converters. The self-commutated converter systems provide control flexibility of active and reactive powers, but their nonlinearity makes their control difficult. The linearized state equations using feedback linearization are presented to enable the controller design by using linear control theory

Iodine-129 in Seawater Offshore Fukushima: distribution, inorganic speciation, sources, and budget.

The Fukushima nuclear accident in March 2011 has released a large amount of radioactive pollutants to the environment. Of the pollutants, iodine-129 is a long-lived radionuclide and will remain in the environment for millions of years. This work first report levels and inorganic speciation of 129I in seawater depth profiles collected offshore Fukushima in June 2011. Significantly elevated 129I concentrations in surface water were observed with the highest 129I/127I atomic ratio of 2.2 &times; 10&minus;9 in the surface seawater 40 km offshore Fukushima. Iodide was found as the dominant species of 129I, while stable 127I was mainly in iodate form, reflecting the fact that the major source of 129I is the direct liquid discharges from the Fukushima NPP. The amount of 129I directly discharged from the Fukushima Dai-ichi nuclear power plant to the sea was estimated to be 2.35 GBq, and about 1.09 GBq of 129I released to the atmosphere from the accident was deposited in the sea offshore Fukushima. A total release of 8.06 GBq (or 1.2 kg) of 129I from the Fukushima accident was estimated. These Fukushimaderived 129I data provide necessary information for the investigation of water circulation and geochemical cycle of iodine in the northwestern Pacific Ocean in the future.</p