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Contactless capacitive sensors are applied to monitor(over)voltages near the overhead line terminations of asubstation or at cable to line transitions. The sensor response isthe signal time derivative when loaded with a resistiveimpedance and the waveform is restored by integration. As partof this differentiating/integrating (D/I) measuring concept, theuse of open-air sensors results in excellent EMC characteristicsbut the inherent cross-coupling to other phases has to be dealtwith. Three applications are presented: 1) For a 150 kV cabletermination the partial discharge activity needs to be related tomomentary phase voltages; 2) Measured slow front overvoltageat a 380 kV cable termination from line energizationare compared with predictions from numerical simulation; 3)The perspectives of employing the D/I method for (very) fastfront overvoltages near a 380 kV circuit breaker are examined.

Condition based assessment of OLTCs has become more popular in recent years, and the importance of reliable and accurate diagnostic techniques is being realized. The work described in this paper aims to improve existing diagnostic techniques, develop new ones and combine some techniques (hybrid) in order to realize a highly reliable set of diagnostics for tap changers. Topics addressed in this paper include motor power measurement, dynamic resistance measurement and vibro-acoustic measurement, the latter designed and tested as part of this project. Preliminary results show that in particular a combination of aforementioned techniques may lead to improved condition assessment and may lead towards automated and online monitoring of tap changer operation.

Design of cables for operation in a low pressure environment is challenging, when applications also demand lightweight and flexibility, and material choice is restricted. The operating pressures in combination with relevant distances, can be situated near gas breakdown and cause partial discharges, making long-term damage conceivable. An experimental setup is developed, based on partial discharge detection in a vacuum system, to characterize the partial discharge inception voltage of three coaxial cable designs in an argon and nitrogen environment at controllable pressure. In order to cope with outgassing behavior, a lumped element model is presented to simulate the internal pressure distribution along the cable as a function of settling time. Microscopic cross sections of the cables are analysed with electrostatic voltage simulations. Using scaled Paschen curves, expected partial discharge inception is determined and compared with measurements. Quantifying cable performance, in terms of minimum PD inception voltage and pressure, in relation to its design is feasible, but also deviations occur which are discussed.

Traditionally, extra high voltage (EHV) transmission systems consist exclusively of overhead lines (OHL) but in the recent years the utilization of high voltage AC (HVAC) power cables is increasing due to environmental, political and operational aspects. To study the impact of HVAC power cables on temporary overvoltages and slow front transients, a simulation model of the large-scale network is required. Guidelines concerning the modeling detail for the surrounding network need to be established to ensure sufficient accurate results. Existing guidelines for purely OHL transmission networks may not be adequate since the integration of HVAC power cables shift the harmonic impedance of the network to lower frequencies and that can affect the extent of the needed simulation model. Much effort can be saved from obtaining detailed data for an extensive but proper model, but a less detailed model might not describe the network behavior with sufficient accuracy. In this study, simplification options are considered, for which the time to construct and analyze a simulation model can be reduced. The simplification options are compared with the reference model (full detail), and judged based on three indicators for the level of accuracy: frequency and amplitude of the main resonance, and maximum over-voltage. As a result, guidelines on the extent of the simulated network and in which detail the model has to be built are proposed. The inverse approach, i.e. increasing level of detail until simulation results hardly change leads to similar results, but requires equivalent short-circuit networks for the rest of the grid. The method is tested for the Dutch 380 kV grid where an underground cable connection of 10.8 km length is introduced in 2013 and a second connection is planned for 2017.

Summary High frequency signal components from partial discharge in an underground power cable propagate according to available modes provided by the cable structure. Which mode or combination of modes is detected depends on type and installation of sensors. Without the ground return impedance, the earth screen voltage is assumed to be equal to the ground potential and all current will flow through phase conductor(s) and earth screen. However the soil may contribute to the transmission line propagation modes and affect the detected partial discharge signals. This paper investigates the effect of the ground return path on partial discharge detection at cable ends. Existing theory and modeling (modal analysis, ground impedance) are customized for the analysis. Transmission line parameters measurements approve the modelings. Time domain signal propagation simulation shows that the ground mode has a minor contribution to the signal propagation. The implication for partial discharge detection is discussed. Copyright © 2016 John Wiley & Sons, Ltd.

Transients from lightning strikes can enter underground cables at overhead line to power cable transitions. Possible overvoltages on these surges at cross-bonding connections of the cable screens are of major concern. A model is developed for modelling overvoltages from transient signal propagation through a combined cross-bonding cable and box. This model is applied to the first Dutch 400 kV cable connection. Such model incorporates model parameters whose values depend on design details of the cross-bonding box. The values for these model parameters are extracted from the measured transmission and reflection signal on steep pulses injected into the actual cross-bonding box configuration. The model combines transmission line description for the cross-bonding cables with mainly inductive behaviour of the cross-bonding box. The obtained results are verified by measurements. The model is applied to investigate overvoltages induced at the cross-bonding cable and box on 1.2/50 µs impulse voltage injection representing a lightning impulse voltage applied to the core conductor of the cross-bonding cable. Furthermore, the effectiveness of surge arresters to reduce overvoltages at the cross-bonding cable screen is demonstrated by simulations with this model in PSCAD.

Underground low voltage (LV) power cables have a long life span, and so far minimum effort was put in methods for condition assessment. However, a slow but steady increase in the amount of faults is being observed and future de-central power generation may further put pressure on the reliability of the LV grid. Therefore, condition monitoring for LV cable systems is under investigation. With the occurrence of intermittent faults, fast transients from discharges may be observed before an outage occurs. To estimate if such signals are measurable, a model of the cable valid up to 50 MHz was derived. Numerical simulations have been used to obtain the parameters of the multi-conductor transmission line model, with special consideration of the copper screen configuration. Measurements on a 60 m cable segment have been performed to verify the model, showing good agreement. This model will be used to investigate signal propagation through a LV grid with focus on the feasibility to base LV condition monitoring on signals which may originate from upcoming cable faults.

Higher actuator voltages are required for future generation lithography machines to increase productivity. The operating pressures in combination with relevant insulation distances, can be situated near gas breakdown and cause partial discharge activity, making long-term damage conceivable. High voltage cone-shape connectors with pliable insulation can be applied for the low and medium vacuum range. However, materials and space are restricted making a cone shaped connector not ideal. In this paper, a flat connector concept is presented with a pliable insulation material allowed to be used for the aforementioned application. The permittivity of this material is measured and simulation results of the connector reveal critical design points. A prototype connector is made and its partial discharge inception voltage is characterized for the low and medium vacuum range. The results are discussed and compared with simulations.

Aluminium is widely used as conductor material for power cables, but corrosion taking place under specific circumstances can have impact on its reliability. Aluminium corrosion under the influence of an alternating current was studied experimentally. Submerged cable segments with inflicted damage exposing the conductors resulted in a continuous corrosion of the conductor material. Effects of temperature and applied voltage were studied by observing corrosion of an aluminium wire. These parameters strongly affect the corrosion rate. Comparison with copper showed that aluminium is far more susceptible to corrosion. The impact of corrosion on the reliability of the cables is discussed.

For the study of switching transients in Extra High Voltage (EHV) networks it is common practice to model a relatively small part of the EHV network. The downstream network is either disregarded or modeled as a parallel RC or RL circuit, based on the powerflow, directly at the secondary side of the transmission transformer. In this paper an investigation for the extent and model type of the downstream network (below 380 kV) is performed. A detailed model for an example downstream network is developed. Different simplifications and equivalent models are compared in frequency domain and in time domain, during cable energization. The impact of the lower voltage network on both the maximum overvoltage and the harmonic content during energization is assessed. Moreover, components that are of major importance are identified and proper equivalent models for the downstream network are proposed. Simulation results show that in particular the type of the 150 kV network (underground cable or overhead line) affects greatly both the maximum overvoltage and the harmonic content during energization. On the other hand, the parameters of end customer loads have only minor effect.

Traditionally, Extra High Voltage (EHV) transmission systems consist exclusively of Overhead lines (OHL) but in the recent years the utilization of High Voltage AC (HVAC) underground power cables (UGC) is increasing due to environmental, political and operational aspects. The application of EHV cables will influence the operation of the transmission system in both steady and transient state. Therefore, formulation of accurate cable simulation models is of great importance in order to understand the new phenomena introduced. This paper presents a comparison between two cable models, a detailed and a more simplified one, as well as a sensitivity analysis on several cable parameters. In this way, guidelines can be extracted concerning the parameters that have larger impact on the cable response and must be modeled in detail and the ones where simplifications can be made for achieving better computational speed. The simulations are performed using software package PSCAD/EMTDC.

Models in high frequency range for underground power cable connections are essential for the interpretation of partial discharge (PD) signals arising e.g. diagnostic techniques. This paper focuses on modeling of power cable joints. A lumped parameter odel and a cascaded transmission line model are proposed based on scattering parameters (S -parameters) measurement on a 10 kV oil-filled PILC-PILC straight cable joint in the frequency range of 300 kHz-800 MHz. It is shown that the lumped model is suitable for up to 10 MHz while the transmission line model can cover the whole frequency range. The cascaded transmission line model is applied to simulate the reflection on a 150 kV single core XLPE straight joint. Comparison between measurement and simulation indicates that the model parameters (characteristic impedance and propagation coefficient) can be matched to predict the joint’s propagation characteristics.

In countries with a high ambient temperature and strong solar irradiation, transformer winding hot-spot temperature may increase over its maximum permissible limit. This can considerably reduce the insulation life of the transformer by enhanced degradation of the paper insulation. According to current loading guides, for each 6 K increase in working temperature, the ageing rate increases with approximately a factor two. Therefore, it is important to take into consideration the impact of the sun on the power transformer thermal behavior. In this paper, a modified hot-spot temperature model is presented to account for the effect of transformer winding temperature rise by solar irradiation. The effects of solar irradiation on transformer winding paper insulation are shown by comparing the degree of polymerisation (DP), the fault probability and the remaining life. Here, the fault probability is defined as the probability that the estimated DP-value at a certain moment in time is below a certain end-of-life criterion (threshold value). An additional winding hot-spot temperature rise of 9 K during the summer and a temperature rise of 6 K during the winter may occur in countries with strong solar irradiation. This may result in a reduction of the remaining lifetime by up to 40%.

Matched filtering can be applied to recorded signals for maximum detection sensitivity during online partial discharge (PD) measurements in medium-voltage cable circuits. PD waveform predictions are combined with a noise spectrum estimation to construct matched filters. The PD waveform is specific for each cable circuit under test and the predictions are based on online system identification measurements and a standard cable model. For some cable circuits, for instance with a ring-main-unit along the cable, the predicted waveforms only approximate the real waveforms due to signal distortion not accounted for in the model. This results in suboptimal detection sensitivity. This paper describes an automated procedure for updating the matched filters based on detected PD signals. The algorithm analyzes detected pulses to judge whether the current set of matched filters perform optimally. If not, the current set of matched filters is updated, increasing the PD detection sensitivity and potentially also the location accuracy. At the same time, recognition of recurring disturbances is improved and better rejection criteria can be defined. The method is tested on signals measured on live cable connections.

Uncertainty analysis is an essential part of models that estimate remaining lifetimes of power transformers. Uncertainty can be included in modeling the insulation aging process. If the uncertainty becomes too large, the insulation condition can, in principle, be determined by measuring the DP value, and more accurate projections can then be made. A drawback of the model presented here is the required input data, i.e., average values of dynamic load and ambient temperature, and maximum values, because the degradation rate is not a linear function of temperature. This information is not available for most transformers. However, even if the loading data are available only over times when the load approaches the rated value, a reliable estimate of the paper degradation can still be made. The observation that tap-changer failure is still the major transformer failure mode supports the simulated results presented above, which show that a transformer failure peak due to paper insulation failure is expected only after several decades of service. Although prediction of transformer performance may be considered speculative because of uncertainties in the model parameters and incomplete service data, it can serve as a tool to compare different maintenance and replacement strategies. Thus annual replacement strategies appear superior to equal loading because the latter provides no guidance on the order of replacement. It has also been shown that transformers should be replaced before the beginning of the failure wave and that variation in annual load growth between transformers leads to spreading of the replacement wave. A disadvantage of a distributed transformer loading is the reduction of the average remaining lifetime due to relatively fast degradation of the more heavily loaded transformers. Comparison of these strategies requires a technical reliability model, which is the foundation of risk-based asset management.

Accurate location of the origins of partial discharges in power cable systems, based on arrival times, is imperative for the identification and assessment of defects. This paper evaluates different time-of-arrival algorithms in order to determine which method yields most accurate location under different circumstances. These methods are based on trigger level, Akaike Information Criterion, energy criterion, Gabor's signal centroid and phase in frequency domain. Several criteria are defined by which the algorithms are evaluated. These criteria include the sensitivity to noise, pulse shape and effect of load impedance. The sensitivity of the methods upon varying these quantities is evaluated analytically and by means of simulations. Further, the methods are tested on a medium-voltage cable system by injecting PD pulses in a cable with one joint. Each algorithm is applied to the measured pulses and the resulting location is compared with the known joint location. From the results the energy criterion method and the phase method show the best performance.

In medium- and high-voltage cables with a helical structure of the earth screen current pulses from partial discharges travelling along the cable are observed to follow the spiralling structure. The resulting tangential current produces an axial magnetic field which can be detected by an inductive sensor without having to make any change to the cable. A specially developed inductive sensor placed around the outer sheath of an intact cable is made selectively sensitive to the high-frequency signals (± 100 MHz) arising from the partial discharges. The discharge pulses are recorded by a fast digitizer (bandwidth of 600 MHz). In laboratory experiments partial discharges with a minimum magnitude of about 5 pCfrom electrical treeing in short, medium-voltage cables could be detected. Location by means of transit time measurements was performed with an accuracy of about 5 cm. In one case, the polarity of the discharge signal was successfully used to locate the discharge to within 1 cm.

Transients upon faults can be analyzed by the time domain approach adopted e.g. in available EMTP based software. When the connection becomes complex and the effect of circuit parameters must be systematically evaluated, the efficiency drops and simplifications are required to limit computation time. For a new connection (Randstad380) the complexity is related to the embedded mutually coupling cables (2 circuits, 2 cables per phase) and the use of cable joints over varying distances. This paper applies an alternative approach that analyzes transients upon faults in frequency domain. The modeling procedure is presented in detail and it is applied to the Randstad380 south-ring connection. Comparison is made with PSCAD/EMTDC modeling based on a simplified transmission line configuration, which is suitable for time-domain simulation. The results show that the presented method is able to effectively and accurately analyze fault transients.

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. >

A power cable model for partial discharge (PD)pulse propagation requires knowledge of all factors influencingthe transmission line parameters of the cable (characteristicimpedance, attenuation coefficient and propagation velocity). These frequency dependent quantities are related, amongst others, to the dielectric properties of the semiconducting layers. This paper discusses methods to estimate the characteristicimpedance, attenuation coefficient and propagation velocity using the cable geometryand the material properties of conductor, conductor shield, XLPE and semi-conducting layers. Measurementson cable samples show that the cable characteristics can be approximated withsufficient accuracy for PD signal propagation modeling, e.g. estimating the PD magnitude at their origins.

This paper presents the remaining lifetime calculation of power transformers paper insulation and consequently of power transformers. The calculations are performed based on two models, which are related to the thermal degradation of the cellulose winding paper insulation: the common IEC loading guide and a paper degradation model. The paper insulation model's prediction can be improved by involving data from furfural analysis. The remaining lifetime is extracted from the fault probability (reliability) of the paper insulation. The two models are brought together, to aid the asset manager in the decision making process. A probabilistic approach is used, which can be coupled to analysis in terms of risks, benefits, costs, and availability by the asset manager.

A series of experiments on the capacitive current interruption using an air-break disconnector (DS) with an Auxiliary Interrupter (AI) is presented. The AI contacts open with much higher speed (average llm/s) than the main contacts of the DS (0.2-0.6m/s). This paper mainly focuses on optical observation of the arc recorded by a high-speed camera. The result shows: (i) The arc brightness is synchronous with the arc current; (ii) The arc brightness increases with the higher current level to be interrupted I d ; (iii) With higher I d , the afterglow of the arc remnants decay slower.

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. >

For specific circuits the accuracy of online partial discharge (PD) detection and location (PD-OL, [1]) systems is impaired by complications in the cable circuit configuration, such as ring-main-units (RMUs) along the cable under test. For maximum sensitivity matched filtering is applied for extracting PD pulses from the recorded signal. The matched filters are based on a series of prior online system identification measurements and a standard cable model. Especially, with an RMU along the cable, the matched filters only approximate the real waveforms due to signal distortion. This results in sub-optimal detection sensitivity. This paper describes automated procedure for updating the matched filter bank based on measured (large) PD signals. The algorithm analyzes detected pulses to judge whether the current templates perform optimally. If not, a new template is created increasing the PD detection sensitivity and the location accuracy, and also improving the recognition of recurring disturbances. The method is tested on signals measured on a live cable connection.

Online partial discharge (PD) detection and location systems for medium-voltage (MV) cables have been developed over the past years and are currently introduced by utilities on an increasingly large scale. This paper addresses a few of the new challenges related to the integration of these systems in various cable networks. The effect of ring-main-units (RMUs) and substations on the propagation of PD signals is studied to determine feasible options for PD monitoring on two consecutive cables connected through an RMU or a substation. Different algorithms for PD pulse arrival time estimation are analyzed for robustness and PD origin location accuracy. An algorithm based on the signal energy proved to be most suitable for complex circuits where the PD shape is not only distorted by the propagation through the cable, but also by complex impedances in the RMUs along the cable under test and each cable end. Matched filters for PD detection can be improved using measured PD signals. This results in improved detection sensitivity and location accuracy.

Online partial discharge (PD) monitoring systems are traditionally installed at a single mediumvoltage (MV) cable connection between two ring-mainunits (RMUs). It is more efficient to monitor two or more consecutive cables using a single monitoring system. Moreover, practical experience with the PD-OL system [1], shows that for substations, with many parallel MV cables, and RMUs installing the inductive sensor may be hampered or even impossible. in this paper the influence of RMUs and substations on the propagation of PDs is studied. An RMU or substation can be modeled as a combination of complex impedances representing switchgear, transformer and MV cables. A PD pulse from a cable encounters a load impedance that does not match the cable's characteristic impedance, resulting in partial reflection and partial transmission transmission to other cables. Models for RMUs and substations are proposed and verified by measurements. Feasible options for online PD monitoring through RMUs or substations are determined. .

Capacitive current interruption with air-break disconnectors in a high-voltage network is an interactive event between circuit and arc with a variety of interruptions and re- ignitions. In order to investigate this transient phenomenon, a series of interruption tests was performed at KEMA High Power Laboratory. In this paper, a brief analysis of the interruption process is presented and is compared with experimental data from the test. Typical wave shapes of voltages across the capacitances, disconnector and current through the disconnector are given. Re-ignition voltage and energy input to the arc on re- ignition are also investigated. Comparison shows that the test data are in good agreement with simulation. It is concluded that besides higher interruption current and higher power supply level, a lower ratio between source side and load side capacitance leads to more severe interruption and longer arc duration. In the end, the actual status of IEC recommendations on testing, that has taken into account this arc-circuit interaction, will be discussed.

Accurate location of partial discharges in power cable systems, based on arrival times, is critical for the identification and assessment of defects. This paper evaluates different time-of-arrival algorithms in order to determine which method yields most accurate location under different circumstances. These methods are based on threshold, Akaikepsilas information criterion, energy criterion, Gaborpsilas signal epoch and phase in frequency domain. Several criteria are defined by which the algorithms are evaluated. These criteria include the sensitivity to noise, pulse shape and effect of load impedance. The sensitivity of the methods upon varying these quantities is evaluated analytically and by means of simulations. From the results the energy criterion method and the phase method show the best performance.

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.

In this paper the concept of an integral transformer lifetime model is presented. The model provides the best possible prediction of future behaviour given the data available. It treats remaining life in terms of future failure probability, thereby giving better support to the decision taking process than a mere remaining life estimate. The core of the model is a generic description of ageing processes, coupled to a probabilistic approach. The approach presented utilizes various techniques to reduce the uncertainty that is inherent to modelling processes with incomplete knowledge of the operational history. One technique couples the process model to externally measurable quantities; another technique involves a sensitivity analysis, which shows what additional input data gives the most efficient way to improve accuracy. We will illustrate the approach by applying it to a well-known degradation process: thermal degradation of the transformer winding insulation.

The area of pulsed-power technology covers a broad range of voltages and pulse durations. State of the art systems have voltages up to a few hundred kilovolts at pulse lengths down to the nanosecond range. Special applications require extreme field strengths in the range of 1 GV/m. The design of a compact source meeting the above requirement aims for 1 MV, 1 ns pulses. In this paper, we propose a concept for a novel compact transmission line transformer. To verify theoretical analyses and understanding, a model of the transmission line transformer has been simulated by time domain electromagnetic field solver. An optimized transmission line transformer has been built and first measurements have been performed. The equivalent simulation model agrees with the experimental results. The effect of secondary mode suppression by means of ferrite has been studied. Both simulations and experiments showed that ferromagnetic cores around each individual transmissions line in our design can be omitted

A new and unique measuring system is presented for the online acquisition and analysis of partial discharges (PD-s) in medium-voltage power cables. The system is able to locate the origin(s) of PD-s by using two inductive sensors, each at one cable end. The measuring system is called PDOL, which stands for PD detection On-line with Localization. A patented solution is used for the time synchronization of the data intake at both cable ends and for the on-line calibration. One computer at a remote location collects data from various PDOL measuring systems for final presentation and interpretation. This paper discusses the basics of PDOL and some first measurement results.

In this work coupling of communication frequencies within the CENELEC A-band to medium voltage cables is studied. Inductive and capacitive signal coupling is considered not only on basis of technical performance, but also on basis of practical aspects as installation. The effectiveness of coupling schemes depends on the impedances of installed equipment at the cable terminals. The frequency response of a 10 kV, 400 kVA cast resin distribution transformer is investigated. In the frequency range of interest, the behaviour is best described as a capacitance of typically 1 nF. The signal transfer over a 4 km paper cable terminated by various load impedances to mimic real equipment is studied. From the results it is concluded that for inductive coupling performance can be low within the CENELEC A-band, but can become competitive if frequencies up to 1 MHz are allowed. Further, the location of signal coupling has a strong effect on the noise spectrum.

The development of an on-line PD detection system has its main challenges in the area of sensors and signal processing techniques. If localization is included by means of simultaneous signal detection at both cable ends, the problem arises of synchronizing the measuring systems at both sites accurately. One method, already successfully applied in off-line PD measurements, is the use of GPS. Besides its high costs, this system has one main disadvantage: it requires an external antenna in the line of sight of several satellites. This requirement may be a problem for on-line measurements because of the long measuring time. Another option to perform the synchronization is the injection of high-frequency pulses into one cable end and detecting them at the other end. Taking the propagation time along the cable length into account, this results in accurate synchronization of the measuring sides, without being dependent on any other system. This paper discusses several aspects of the pulse injection technique. The obtained resolution realized during field experiments with the method of pulse injection is within the resolution obtainable by means of GPS (100 ns).

For railways systems the overhead contact wire is normally electrically insulated from the portals or poles by means of insulators. The size of these insulators adds to the required distance between the contact wire and the grounded portals. In tunnels and under viaducts this distance is kept to a minimum for economical reasons. A possible option to decrease the required space is to abandon the concept of insulators, but to use electrically insulating beams as catenary supports. A major point of concern in the application of fibreglass-reinforced synthetics is the electrical behaviour with respect to aging. In this paper DC conductivity measurements are presented on a synthetic fibre glass-reinforced beam, which is in service for about twenty years. The voltage was varied up to 25 kV, and the current could be measured with sensitivity better than 1 nA during the field measurements, corresponding to over 10/sup 13/ /spl Omega/. Reference measurement on a small sample of the same material showed a conductivity of five orders of magnitude lower. The surface conductivity can be attributed to deposited material, probably carbon and metal from the catenary, in conjunction with moisture.

Partial discharge activity is measured at AC voltages close to breakdown of a 5 mm thick spacer. Patterns as a function of the phase angle of the applied voltage are observed which can be ascribed to charging processes on the insulator. Besides a stable regime of discharges, sudden changes in behaviour may also occur. These patterns often emerge as bursts (few seconds or minutes) after which they either quench or lead to a full breakdown.

In this contribution we discuss a number of practical implications from recent studies on high- voltage design concepts used in microwave tube technology. We show how the insights gained can be used in the design of, for example, insulators and cables, and how conditioning procedures and operating conditions (operating pressure, insulator charging) should be reflected in the design.

The experimental setup used to measure electron spectra at well defined detection angles for grazing incidence doubly charged ion-surface collisions at keV-energies is described. Electron spectra are reported for the rare gas ions colliding with a Cu(110)-surface. The spectra are analyzed in terms of various spontaneous ionization processes using a newly developed model. It is found that double capture followed by atomic auto-ionization on the incoming trajectory and Auger-capture processes in which the first and the second hole in the doubly charged projectiles are successively filled are the main processes contributing to the electron spectra. From a comparison of model calculations with measured spectra it is concluded that the metal electrons cannot adapt adiabatically to the sudden changes of the charge state of the projectile in front of the surface. A parameter characterizing the partly diabatic behavior is determined. The variation of spectra upon adsorption of a monolayer of oxygen on th...

Transmission line transformers (TLTs) are usually used for impedance match. In this paper, we present a novel circuit topology based on multiple switches and TLTs for both the impedance match and synchronization of multiple switches. In contrast to the Marx pulse generator, our proposed pulsed power systems produce high levels of pulsed power not only at higher voltages but also at higher currents.