Nonlinear Decentralized Feedback Linearizing Controller Design for Islanded DC Microgrids

This paper aims to design decentralized controllers for different components in islanded DC microgrids. The major components in the DC microgrid as considered in this paper include a fuel cell, solar photovoltaic (PV) unit, and battery energy storage system (BESS) along with critical and non-critical loads. The main control objective is to maintain the power balance within the DC microgrid through the regulation of the common DC-bus voltage. The controllers are designed based on the dynamical models of the fuel cell, solar PV unit, and BESS. The feedback linearization technique is employed to obtain the control laws, which simplifies the original dynamical models and decouples different components in the form of several subsystems. In this way, the feedback linearization technique allows different components in DC microgrids to achieve the desired control objectives by using only the local information (i.e., in a decentralized manner). The performance of the proposed decentralized controllers for different components is evaluated on a test DC microgrid under different operating conditions. Simulation results demonstrate that the proposed control scheme performs in a much better way as compared to an existing proportional integral controller.     

Properties of Entire Functions Over Polynomial Rings via Gröbner Bases

 In this paper it is shown that the extension ideals of polynomial prime and primary ideals in the corresponding ring of entire functions remain prime or primary, respectively. Moreover, we will prove that a primary decomposition of a polynomial ideal can be extended componentwise to a primary decomposition of the extended ideal. In order to show this we first prove the flatness of the ring of entire functions over the corresponding polynomial ring by use of Gr?bner basis techniques. As an application we give an elementary proof of a generalization of Hilbert's Nullstellensatz for entire functions (cf. [10]). Received: August 8, 2002; revised version: January 29, 2003 Keywords: Flat module, Gr?bner basis, Entire function, Hilbert's Nullstellensatz.     

Investigation of nanopore evolution in ion track-etched polycarbonate membranes

Single heavy ion tracks in polycarbonate foils were chemically etched in an electrolytical cell under various conditions (different temperatures, etchant concentrations, and applied potentials), and the pore evolution was monitored by measuring the current through the membrane. Different zones of the latent tracks could be identified via changes in the radial etching rate with time. Further it was found that the shape of the radial etching rate versus time curves depends on temperature, etchant concentration, and applied voltage. The functionalities are attributed to etching products (double-charged diphenylol-propane anions), which are adsorbed on the pore walls and, thus, affect the further etching process.     

Influence of dike breaches on flood frequency estimation

Many river floodplains and their assets are protected by dikes. In case of extreme flood events, dikes may breach and floodwater may spill over into the dike hinterland. Depending on the specific situation, e.g. time and location of breach, and the capacity of the hinterland to contain the floodwater, dike breaches may lead to significant reductions of flood peaks downstream of breach locations. However, the influence of dike breaches on flood frequency distributions along rivers has not been systematically analysed. In order to quantify this influence, a dynamic–probabilistic model is developed. This model combines simplified flood process modules in a Monte Carlo framework. The simplifications allow for the simulation of a large number of different scenarios, taking into account the main physical processes. By using a Monte Carlo approach, frequency distributions can be derived from the simulations. In this way, process understanding and the characteristics of the river–dike–floodplain system are included in the derivation of flood frequency statements. The dynamic–probabilistic model is applied to the Lower Rhine in Germany and compared to the usually used flood frequency analysis. For extreme floods, the model simulates significant retention effects due to dike breaches, which lead to significant modifications of the flood frequency curve downstream of breach locations. The resulting probabilistic statements are much more realistic than those of the flood frequency approach, since the dynamic–probabilistic model incorporates an important flood process, i.e. dike breaching, that only occurs when a certain threshold is reached. Beyond this point, the behaviour of the flood frequency curve is dominated by this process.