Jm Jos Wetzer

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. >

HV design of vacuum components

We discuss a number of practical implications from recent studies on HVDC design concepts for vacuum components. These studies dealt with microwave tube technology. The conclusions, however, are valid for a wide range of components. The goal of this work is to provide a scientific basis for the design of HVDC vacuum components. From a study of breakdown and emission mechanisms, and from the measured insulating performance of many different geometries, we have derived guidelines for the design of, for example, insulators and cables. It is further shown how conditioning procedures and operating conditions (operating pressure, insulator charging) should be reflected in the design. We discuss a number of practical implications regarding insulator design, conditioning, vacuum vs. air operation, HV cables in vacuum and potting. >

The effect of insulator charging on breakdown and conditioning

As part of a study on HV design concepts for microwave tubes, a number of different insulator designs have been studied. Analysis of the measured DC current, partial discharge activity and breakdown voltage shows that surface charging of insulators is a key mechanism in the breakdown process and in the conditioning process. Insulator parameters are not only the breakdown voltage, but also the conditioning speed and the sensitivity to gas exposure or charge leakage. In all these respects insulators with a field enhancement at the anode are superior. Field enhancements at the cathode are less harmful if stepped insulator shapes are chosen. Effective conditioning requires at least a limited number of breakdowns. With sufficient conditioning breakdowns, all insulator geometries tested reached a breakdown field exceeding 12 kV/mm. >

Electron avalanches influenced by detachment and conversion processes

The authors analyze the avalanche growth in electronegative gases which exhibit detachment and conversion processes in addition to ionization and attachment processes. The study is based on an evaluation of the model of H.F.A. Verhaart and van der Laan (1982). Humid air is taken as an example to demonstrate, with this model, the effects of detachments and conversion processes on the avalanche electron distribution across the gap and on the external current waveform. The model is also used to determine swarm parameters from fast swarm experiments. The results show that detachment and conversion affect the electron distribution in the avalanche and can thereby influence the streamer breakdown threshold. The neglect of these process in the analysis of swarm experiments results in measured values for the effective ionization coefficient that can may substantially differ from the true values. >

Time-resolved avalanche current waveforms in octafluorocyclobutane

Avalanche currents have been measured by means of a time-resolved swarm method in c-C/sub 4/F/sub 8/ at pressures between 0.7 and 27 kPa. The recorded avalanche current waveforms are, in comparison with those of most insulating gases, unusual. Whereas, in most gases, avalanche current waveforms are characterized by a clear distinction between the electron contribution (high amplitude, short duration) and the ion contribution (low amplitude, long duration), the waveform in c-C/sub 4/F/sub 8/ is significantly different. During the first electron transit time an electron component can only be observed at low pressure (below 1.3 kPa) and even then is strongly intermixed with the ion contribution. Above approximately 1.3 kPa the waveform seems completely ion-dominated. >

Different avalanche types in electronegative gases

Based on the observation of time-resolved avalanche current waveforms in electronegative gases, three different types of avalanches can be distinguished. The first distinction is that between electron avalanches and ion-dominated avalanches. Electron avalanches are further subdivided into electron avalanches with and without delayed electrons. Delayed electrons are the result of consecutive attachment and detachment processes. Experimental identification of the different avalanche types requires an experimental set-up with a time-resolution in the order of 1 ns. The conventional avalanche model, which involves effective ionization and drift, gives an adequate description only for electron avalanches without delayed electrons. An extended model, which also incorporates electron detachment and ion conversion processes, in principle describes all three kinds of avalanches. When the extended model is applied to the evaluation of measured waveforms, the often-quoted abnormal pressure dependencies in the electron drift velocity and the effective ionization coefficient disappear for the gases studied in this paper.

Bandwidth limitations of gap current measurements

The bandwidth limitations which affect gap current measurements, and which are important for pulsed swarm experiments, are analyzed. Such limitations are caused by the electrode geometry, geometry imperfections and the components and equipment used for signal transport and storage. Recommendations are given to optimize the time resolution. The analysis is applied to a pulsed swarm setup for which a time resolution of 1.2 to 1.4 ns has been achieved.<<ETX>>

Analysis and improvement of HV-components for spacecraft traveling-wave tubes

A report is presented on the analysis and improvement of HV traveling-wave tube (TWT) components for use in spacecraft. The investigation includes a theoretical study, involving electric field and pressure calculations, and an experimental one, involving partial-discharge measurements as a supplement to the standard voltage withstand tests. Improvement of the designs under study has been achieved mainly by the reduction of the cathode electric fields, thus reducing primary emission. A problem inherent in most space tubes is the fact that they are subjected to preflight tests in air, although designed for use in the satellite vacuum. This concerns primarily the outside connections and the HV cable. Because vacuum and air operation impose different and sometimes conflicting design requirements, these preflight tests are not representative of the HV performance in orbit and may even cause insulation degradation as a result of partial discharges. The methods used are presented, discussed, and illustrated, and, based on the results, some general conclusions and recommendations are formulated regarding the design, operation, and testing of HV-TWT components. >

Reliability estimation of paper insulated components

In view of the coming major asset replacement wave of power grid components, new lifetime prediction tools are developed to assist asset managers in making technically and economically sound decisions. This paper discusses the modeling technique based on the concept of quality parameters applied to paper insulated systems. The concept of quality parameters was recently introduced for power transformers to obtain a measurable quantity which relates the actual degradation process to an observable quantity. The present paper discusses a general approach for modeling other power components along the same lines. In case of a power transformer, oil samples can be taken and analyzed in order to estimate the DP (degree of polymerization) value, which is linked to the mechanical strength of the insulation material. This quantity then serves as quality parameter. Using a probabilistic approach based on the physical degradation mechanism a remaining lifetime model was developed. This resulted in a scheme, which is flexible in the sense that any new available data can be included to enhance the prediction reliability. The question addressed here is whether a similar approach can be applied to other power components as well. In particular we have investigated the possible use of quality parameters which are not directly correlated to one degradation mechanism. We will define different kinds of quality parameters and propose a component reliability model that incorporates all of these.

Vacuum insulator flashover: mechanisms, diagnostics and design implications

This paper presents an approach for obtaining science-based answers to practical vacuum device problems. First, developments in research on vacuum insulator flashover are reviewed, with emphasis on the implications for device performance and design. In particular flashover mechanisms, diagnostics and geometry studies are discussed. Secondly, an overview is given of research activities, performed at the Eindhoven University of Technology, which aim at improved device performance, the formulation of guidelines, and the development of device diagnostics, for DC and AC vacuum devices.