P.C.T. van der Laan

A fast pulsed power source applied to treatment of conducting liquids and air

Two pilot pulsed power sources were developed for fundamental investigations and industrial demonstrations of treatment of conducting liquids. The developed heavy-duty power sources have an output voltage of 100 kV (rise time 10 ns, pulse duration 150 ns, pulse repetition rate maximum 1000 pps). A pulse energy of 0.5-3 J/pulse and an average pulse power of 1.5 kW have been achieved with an efficiency of about 80%. In addition, adequate electromagnetic compatibility is achieved between the high-voltage pulse sources and the surrounding equipment. Various applications, such as the use of pulsed electric fields (PEFs) or pulsed corona discharges for inactivation of microorganisms in liquids or air, have been tested in the laboratory. For PEF treatment, homogeneous electric fields in the liquid of up to 70 kV/cm at a pulse repetition rate of 10-400 pps could be achieved. The inactivation is found to be 85 kJ/L per log reduction for Pseudomonas fluorescens and 500 kJ/L per log reduction for spores of Bacillus cereus. Corona directly applied to the liquid is found to be more efficient than PEF. With direct corona we achieve 25 kJ/L per log reduction for both Gram positive and Gram negative bacteria. For air disinfection using our corona pulse source, the measured efficiencies are excellent: 2 J/L per log reduction.

Fast current measurements for avalanche studies

The electrical method is often used to study avalanches in gases. In this paper a substantial improvement of the time resolution of the measuring system is reported. To release the primary electrons within a very short time a transversely excited atmospheric pressure N2 laser with a pulse width of 0.6 ns is used. Secondly the high‐frequency response of the entire circuit is analyzed. As one of the results of this analysis a subdivided cathode is used consisting of a central measuring electrode surrounded by a grounded electrode. The diameter of the circular measuring electrode determines a maximum gap distance for reliable measurements, which can be calculated from a theory of Shockley and Ramo. As an example of the high‐frequency response of our experimental setup avalanches in nitrogen are shown and analyzed. Avalanche measurements in air clearly show a dependence on the humidity, presumably caused by rapid detachment and conversion processes.

Prebreakdown currents: Basic interpretation and time-resolved measurements

The relation between the motion of charges in an electrode gap and the current induced in the external circuit is described on the basis of energy considerations and on the basis of the Ramo-Shockley theory. It is shown that both approaches are applicable, and mutually consistent, not only for gas-filled gaps but, if applied properly, also when the gap is partially filled with a solid dielectric. Energy considerations are especially useful for two-electrode gaps. If more than two electrodes are involved, the Ramo-Shockley theory has to be invoked. Based on the theoretical description, fast measuring systems with a subdivided measuring electrode can be constructed (typically 400 MHz bandwidth, 1 ns risetime). Fundamental and electrotechnical bandwidth limitations are discussed. The derived techniques and insights are applied to perform and interpret time-resolved measurements of currents induced by avalanches in homogeneous fields corona discharges in inhomogeneous fields and partial discharges in voids. >

The influence of water vapor on avalanches in air

In the early stages of breakdown of a gas individual avalanches of charged particles are formed. Measurements on avalanche currents with the electrical method can yield new information on discharge processes if the measuring system has a high time resolution (1.3 ns in our case) and if the avalanche is initiated at the cathode by means of a very short (0.6 ns) light pulse. Avalanches in atmospheric air show an aftercurrent caused by delayed electrons which result from detachment. Measurements at various humidities, an extensive analysis and comparison with computer calculations give, for E/p=35 V/cm Torr, the following result. The negative oxygen ions formed lose their electrons gradually with a timeconstant of 83 ns. Collisions however, cause conversion to a nondetaching type of ion. This conversion process has a characteristic time of 32 ns in dry air and is more rapid when water molecules are present, down to 3 ns at a water vapor pressure of 11.25 Torr. The occurrence of these processes, where water m...

A novel circuit topology for pulsed power generation

This paper presents novel circuit topologies based on multiple switches and transmission line transformers (TLTs). The proposed pulsed power circuit topologies produce high levels of pulsed power either at higher voltages or at higher currents. The TLTs are used for impedance matching and synchronization of multiple switches. This paper also presents our first experimental results to verify the circuit topologies.

Transfer impedance of nonmagnetic conduits of various shapes

Metallic conduits provide mechanical support for cables, and may also protect the cables against interference caused by currents induced by external electromagnetic fields. The conduit transfer impedance Z/sub t/ then is a good measure for the provided protection. We studied Z/sub t/ by means of conformal transformations and of the method of moments in a quasistatic, two-dimensional (2-D) approach. Practical U-, H-, L-, and T-shaped conduits are discussed. Conformal transformations provide simple engineering expressions for the mutual inductance part of Z/sub t/ for cable positions inside the conduit. The method of moments formulation results in useful approximations for the crossover frequencies in Z/sub t/ between DC and high frequency. Measurements on U-shaped conduits confirm the calculations.

Symbolic Computation and Exact Distributions of Nonparametric Test Statistics

We show how to use computer algebra for computing exact distributions of nonparametric test statistics. We give several examples of nonparametric statistics with explicit probability-generating functions that can be handled this way. In particular, we give a new table of critical values of the Jonckheere–Terpstra test that extends previous tables.

Reliable protection of electronics against lightning: some practical applications

The classical lightning conductor, which must prevent fire, has to have a sufficiently small resistance. An analogous condition can be formulated for the new challenge: the protection of sensitive electronics against lightning. In this case, the so-called transfer impedance, which gives the interference voltages across a sensitive input per ampere lightning current, must be made small. The arguments for this approach are described. A theoretical description is available and practical experience has been built up over the years, also in high-voltage (HV) research and in power engineering measurements in the field. This approach was used to greatly improve the lightning protection of several installations: a marine radio station, the peripheral equipment of a nuclear power plant, and an electronic siren for a nationwide public warning system. Detailed investigations were requested by our contract partners, together with suggestions for economically acceptable improvements that could be carried out in reasonable time. Later, the correctness of the renovations had to be demonstrated. In the validation measurements, we injected currents into the installation or into relevant parts thereof. The current waveform was chosen fast enough so that inductive effects determined the current distribution. At present, lightning position and tracking system (LPATS) data on lightning strikes near the marine radio station and the nuclear power plant are available to verify the effectiveness of the protection. After our improvements no more damage was reported.

Development of a magneto‐optic current sensor for high, pulsed currents

A description is given of a current‐measurement system based on the Faraday rotation in a one‐turn fiber coil. It is shown that an ultralow birefringence ‘‘spun’’ single‐mode fiber is extremely suitable in a magneto‐optic current sensor. The properties of this fiber were investigated. Measurements of high, pulsed currents (Imax∼1 MA; f=25 kHz), with good accuracy (1%), and in agreement with a conventional method, are presented. Because of the low birefringence, this good accuracy was also reached in the low‐current region (ϑ≤π/4 rad). In order to be able to compute the value of large currents unambiguously from the Faraday rotation, a setup with a beam‐splitter prism and a second detector was constructed. Some measurements were carried out with this setup.

Common-mode currents generated by circuits on a PCB-measurements and transmission-line calculations

Common-mode (CM) currents on cables attached to printed circuit boards of various complexity have been measured. The boards have a continuous ground plane, with tracks on both sides. The experimental results are compared to calculations with a transmission-line (TL) model. Expressions for the coupling parameters in the TL model have been derived with conformal transformations and the method of moments. Measurements up to 1 GHz agree with the TL model, showing that this model can be used as a tool for an early EMC analysis during the lay-out and routing of a board.