1,496 research outputs found
A Mathematica Package for Computing N=2 Superfield Operator Product Expansions
We describe a general purpose Mathematica package for computing Superfield
Operator Product Expansions in meromorphic superconformal field theory.
Given the SOPEs for a set of ``basic" superfields, SOPEs of arbitrarily
complicated composites can be computed automatically. Normal ordered products
are always reduced to a standard form. It is possible to check the Jacobi
identities, and to compute Poisson brackets (``classical SOPEs''). We present
two explicit examples: a construction of the ``small'' superconformal
algebra in terms of superfields, and a realisation of the
superconformal algebra in terms of chiral and antichiral fermionic superfields.Comment: 15 pages, LaTeX. Minor corrections, particularly to Mathematica
output Out[6],Out[9] in section 4. Available through anonymous ftp from
ftp://euclid.tp.ph.ic.ac.uk/papers/ or on WWW at
http://euclid.tp.ph.ic.ac.uk/Papers
Gauging Conformal Algebras with Relations between the Generators
We investigate the gauging of conformal algebras with relations between the
generators. We treat the --algebra as a specific example. We show that
the gauge-algebra is in general reducible with an infinite number of stages. We
show how to construct the BV-extended action, and hence the classical BRST
charge. An important conclusion is that this can always be done in terms of the
generators of the --algebra only, that is, independent of the realisation.
The present treatment is still purely classical, but already enables us to
learn more about reducible gauge algebras and the BV-formalism.Comment: 10 pages, LaTex, This paper is based on proceedings for the Sixth
Seminar on Quantum Gravity, Moscow, 12-17 June '95 and the Conference on
Gauge theories, Applied Supersymmetry and Quantum Gravity, Leuven, 10-14 July
'9
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The catalytic oxidation of biomass to new materials focusing on starch, cellulose and lignin
Biomass is a renewable class of materials of growing interest amongst researchers aiming to achieve global sustainability. This review focuses on the homogeneous catalysis of the oxidation of biomass, in particular starch, cellulose and lignin. Often such catalytic reactions lead to depolymerisation of the material as happens in Nature with for example brown rot fungi. This depolymerisation can be desirable or not, and control in industrial applications is thus important to obtain the desired outcome. The two main oxidants in use are O2 and H2O2 and their use is described as appropriate. Industrial oxidation catalysis is highly significant in the bleaching of cellulose-containing materials due to its high volume application in the paper, pulp and laundry industries. Here, the presence of a ligand on the oxidising metal ion has a significant effect on the catalyst selectivity and stability. In addition to the bleaching of cellulose-containing materials, the oxidation of cellulose, starch, lignin and lignin model compounds are discussed with a focus on generating even more hydrophilic materials which have important applications or materials which may be further modified. Finally developing applications of biomass are described such as new support materials for catalysts, as supports for sensors and nanomaterials for microbial culture
An area-time efficient FPGA-implementation of online finite-set model based predictive controllers for flying capacitor inverters
Recently there has been an increase in the use of model-based predictive control (MBPC) for power-electronic converters. Especially for flying-capacitor multilevel converters (FCC) this offers an interesting possibility to simultaneously control output current and the capacitor voltages. The computational burden however is very high and often restrictive for a good implementation. In this paper a time and resource efficient design methodology is presented for the FPGA implementation of FCC MBPC. The control is fully implemented in programmable digital logic. Due to a parallel processing for the three converter phases and a fully pipelined calculation of the prediction stage an area-time efficient implementation is realized. Furthermore, this is achieved by using a high-level design tool. The implementation aspects for 3, 4 and 5-level FC inverters are discussed, with a focus on the 4-level case
Flying-capacitor multilevel converter voltage balance dynamics for pure resistive load
Multilevel converters need voltage balancing to be able to generate an output voltage with high quality. Flying capacitor converter topology has a natural voltage balancing property. Voltage balance dynamics analytical research methods reported to date are essentially based on a frequency domain analysis using double fourier transform. These complicated methods are not truly analytical, which makes an understanding of parameter influence on time constants difficult. In this paper, a straightforward time domain approach based on stitching of switch intervals piece-wise analytical solutions to a DC modulated H-bridge flying capacitor converter is discussed. This method allows to obtain time-averaged discrete and continuous voltage balance dynamics models. Using small-parameter approximation for pure resistive loads, simple and accurate expressions for voltage balance time constants are deduced, revealing their dependence on load parameters, carrier frequency and duty ratio
Self-precharge in single-leg flying capacitor converters
Flying Capacitor (FC) multilevel pulse width modulated (PWM) converters are an attractive choice due to the natural voltage balance property. During start-up of the converter, care has to be taken that the power switches are not exposed to voltage overstress due to uncharged capacitors. A flying capacitor self-precharge technique is proposed which, by making use of natural balancing and a DC-bus rate control, makes the capacitors balance with a zero average load current. The DC-bus rate control depends on the capacitor voltage balance dynamics. The regular PWM natural balancing technique gives good results for even-level single-leg converter self-precharge, for odd-level converters a special switching pattern is necessary
Improved natural balancing with modified phase shifted PWM for single-leg five-level flying-capacitor converters
Flying capacitor converters (FCCs), as most multilevel converter topologies, require a balancing mechanism of the capacitor voltages. FCCs have the valuable property of natural voltage balancing when a special modulation technique is used. The classic methods, like Phase-Shifted Pulse Width Modulation (PS-PWM), result in very slow balancing for some duty ratio ranges. Previous work showed that for a single-leg five-level FCC one time constant is infinite for a zero desired output voltage. In this paper, a modified PS-PWM scheme for a single-leg fivelevel FCC is presented which results in faster balancing over the total duty ratio range. The modified PS-PWM scheme is studied, resulting in an averaged voltage balancing model. This model is verified using simulations and experiments. The modified PS-PWM scheme solves the slow balancing problems of the normal PS-PWM method for odd-level FCCs, while maintaining the passive control property, and it provides a self-precharge capability
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