Robert G. Olsen

Inactivating microorganisms using a pulsed electric field continuous treatment system

High-voltage pulsed electric fields (PEFs) can be used to inactivate microorganisms in liquids. Applying PEF technology to food pasteurization is a promising nonthermal method, which may radically change food preservation processes and provide consumers with microbiologically safe, minimally processed, fresh-like products. A continuous-flow system in a laboratory-size prototype was constructed for the nonthermal pasteurization of liquid foods with PEF technology. Major components in the prototype include a high-voltage repetitive pulse generator, a coaxial liquid food treatment chamber, a fiber-optic temperature sensing instrument and a data acquisition system. Microbial inactivation tests were conducted in the continuous PEF treatment system. Repetitive high-voltage pulses with an exponential decaying waveshape were applied to the liquid food which was pumped through the treatment chamber. Test microorganisms selected for inactivation were Escherichia coli, Staphylococcus aureus and Saccharomyces cerevisiae. Over 6-order-of-magnitude reductions in the viability of selected microorganisms were achieved while the food temperature was maintained below 40/spl deg/C.

Characteristics of low frequency electric and magnetic fields in the vicinity of electric power lines

The authors describe the quasi-static and radiation fields of simple three-dimensional sources and proceed to a two-dimensional source. It is shown that the quasi-static approximations result in an error of 0.000002% for fields at 100 m or less from the power line. This is followed by a discussion of how the quasi-static fields are coupled together by Maxwells equations. It is shown that the electric field of a power line can formally be determined from the magnetic fields, but this cannot be done practically. The electric and magnetic fields are assumed to be independent and are related respectively to voltage and current. The conditions under which electric and magnetic fields are related by the free space impedance are discussed. In general, they are not for power line fields. Poyntings theorem is applied to the power line fields to describe the bulk power transferred along the power line as a result of the quasi-static fields and to calculate the insignificant amount of power radiated laterally away from the power line. >

Inaccuracies in numerical calculation of scattering near natural frequencies of penetrable objects

Calculated scattering from a nonconducting cylindrical ring shows great variability with small changes in the material or geometric parameters within certain ranges of these parameters. The observations are explained by a resonance phenomenon in which the operating frequency is found to lie close to a complex natural frequency of the scattering object. This resonance is a real, observable effect predicted by analytical solutions and not the spurious numerical resonances which have been widely discussed and which one wishes to suppress. Attempts have been made to reproduce near-resonance scattering results using the method of moments and finite-difference-time-domain (FDTD) codes. These have failed despite the use of widely accepted discretization densities. Thus, the existence of such resonances requires additional care when interpreting computed results for scattering from similar nonconducting objects have electric and/or magnetic properties. >

A comparison of exact and quasi-static methods for evaluating grounding systems at high frequencies

Recently, it has been suggested that traditional quasi-static methods for evaluating power network grounding systems are not valid at high frequencies. However, the conditions under which exact full wave theory must be used have not been established. In this paper, exact full wave and quasi-static methods are used to evaluate touch and step potentials of a simple ground stake. It is shown that a sufficient condition for quasi-static theory to be valid is if the size of the buried electrode is less than one-tenth of a wavelength in the earth.

On The Exact, Carson and Image Theories for Wires at or Above the Earth's Interface

The relation between the exact theory and Carsons theory for the fields of straight wires above a homogeneous earth is derived. Sufficient conditions for the validity of Carsons equations are given. Cases for which Carsons theory is not valid are discussed. In addition, an image theory useful for magnetic induction problems is given and discussed.

Corona onset as a design optimization criterion for high voltage hardware

Hardware for use on HV systems is designed to be corona free. Recent hardware design activity has been centered around attempts to increase the corona onset voltage by optimizing the electric field distribution on its surface. Corona onset, however, is not only a function of the surface electric field, but also its rate of decay away from the surface, and the temperature and pressure of the gas in which it is immersed. While corona onset is a better criterion, formulas for predicting it have been validated only for simple geometries. The first goal of this work, then, is to validate corona onset conditions for more practical electrode geometries. The second goal is to determine whether the use of corona onset rather than surface electric field as an optimization criterion can result in hardware with a measurably higher corona onset voltage. To test this idea, two electrodes were designed, one using electric field optimization and the second using corona onset optimization. The corona onset voltage of each electrode was then measured and the results compared with predicted values. It was found that the measured results compared favorably with the predicted values and that the use of corona onset optimization can result in a modest but measurable increase in corona onset voltage.

Electromagnetic wave propagation on a thin wire above earth

The electromagnetic properties of a current on a thin horizontal wire above a flat lossy earth is reviewed. Attention is given to the historical development of the topic, starting with the seminal work of Carson. Particularly, the importance of Professor Waits contribution to the initial understanding of the modal and radiative behaviors of the current, as described in his influential paper (see Radio Sci., vol.7, p.675, 1972), is discussed in detail. A description of Waits full wave analysis is provided to show how that analysis justified many of the assumptions embedded in Carsons result and how that same analysis later led to a fuller understanding of the currents spatial spectrum. Although no rigorous proof is known to exist with respect to the completeness of this spectrum, a necessary condition for completeness is offered; the basis of this condition is the measurable input conductance of the wire. The paper concludes with a short discussion on the various extensions of Waits classic work that have been provided over the last two decades.

Application of a corona onset criterion to calculation of corona onset voltage of stranded conductors

The conditions under which corona first occurs for any set of electrodes are an important design consideration since corona can limit the performance of any given configuration of electrical conductors. Here, the 60 Hz ac corona onset voltage for a stranded wire in air is calculated. The criterion involves the characteristics, i.e. composition and pressure, of the gas in which the conductor is immersed and the electric field in the vicinity of the conductor surface. It is found that the calculated corona onset voltage agrees reasonably well with experimental data and that its behavior as a function of overall conductor radius and number of strands is as expected.

A new method for real-time monitoring of high-voltage transmission-line conductor sag

The amount of power flowing through a high-voltage transmission line is proportional to its voltage and the current flowing through it. To increase power flow without modifying the line to support increased voltage, the current must be increased. There is a limit to this, however, since increasing the current causes the conductor temperature to increase and, hence, the conductors to elongate and sag. In this paper, a method to simply and inexpensively measure the amount of conductor sag and, through simple calculation, the average conductor core temperature is described, and the results of a field test are summarized. The method involves attaching two ends of a grounded wire of high electrical resistance to an appropriate location on each of two transmission-line towers and measuring the current induced on the wire by the nearby transmission-line conductors. Information from this measurement is a critical input to any method for dynamically rating transmission lines.

A simple theory for optimizing finite width ELF magnetic field shields for minimum dependence on source orientation

The effectiveness of single layer, finite width, planar extremely low frequency (ELF) magnetic field shields is strongly dependent upon the orientation of the field sources. Since source information is difficult to obtain, the issue of designing shields which are independent of source orientation is important. Here, a simple analytic model for shielding by multiple layer, finite width, planar shields constructed from perfect electric and perfect magnetic material is presented. This is augmented by a study of conditions for which the perfect material approximation is valid. The simple model is used to determine strategies for designing shields which are independent of source orientation. It is found that two layer perfect electric/magnetic shields perform significantly better than single layer shields.