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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%.

The age of the majority of power transformers installed in the western electricity network reaches 30 to 60 years and replacement on short term seems eminent. A technically sound policy concerning the replacement of these assets requires a model that estimates the life expectancies of individual components and from that calculates parameters related to the behavior of a population of assets as a whole. A probabilistic approach is adopted and is applied to thermal degradation of the transformer paper insulation. In this paper, we will focus on the determination of the population reliability from individual reliabilities. These individual reliabilities are based on Arrhenius modeling of paper insulation degradation, including the inherent uncertainty in the parameters involved. A statistical failure model is used to obtain the population reliability figures. The modeling method is demonstrated on two populations of power transformers in The Netherlands to evaluate the different replacement alternatives. Using the model, strategies can be defined to maximize transformer utilization and postpone replacement. The downside is the need to replace the complete fleet in a relatively short time afterwards.

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.

This paper presents an approach for obtaining science-based answers to practical vacuum device problems. The question "How to translate scientific knowledge to design and operation practice" is rephrased to the more productive question "What research activities are required to provide science-based answers to practical problems!" Such activities involve the study of operational mechanisms in realistic, geometries with the help of appropriate diagnostics. Within this framework, recent research developments on vacuum insulator flashover are reviewed, with emphasis on the implications for device performance and design. Secondly, an overview is given of recent research activities performed at the Eindhoven University of Technology. By studying the relationship between flashover mechanisms and insulator performance, and the relationship between insulator performance and design, we aim at improved device performance, the formulation of guidelines, and the development of device diagnostics, for dc and ac vacuum devices.

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. >

The age of the majority of power transformers installed in the western electricity network reaches 30 to 60 years and replacement on short term seems eminent. A technically sound policy concerning the replacement of these assets requires a model that estimates the life expectancies of individual components and from that calculates parameters related to the behavior of a population of assets as a whole. We will illustrate the approach by applying it to a well-known degradation process: thermal degradation of the transformer paper insulation. In this paper we focus on the determination of the population reliability from individual reliabilities. These individual reliabilities are based on Arrhenius modelling of paper insulation degradation including its inherent uncertainties. A statistical failure model is used to obtain the population reliability figures. We demonstrate the modelling method by applying it to an existing population of power transformers in the Netherlands, to evaluate the expected replacement wave. Further, the model is applied to analyse alternative replacement scenarios.

The age of the majority of power transformers applied in the western electricity network varies between 25 and 50 years. Depending on the load history and time of operation, replacement on short term is imminent. A technically sound policy concerning the replacement of these assets must be based on knowledge of (i) the life expectancy or reliability of individual components, (ii) how these failure probabilities cumulate to a replacement wave, and (iii) how to manage an expected replacement wave. The population reliability is obtained from individual transformer reliabilities using Arrhenius based modelling of paper insulation degradation. This modelling technique includes measures to cope with inherent uncertainties in available data. Population reliability figures are obtained using an adapted k-out-of-N failure model. The modelling method is applied to existing populations of power transformers in The Netherlands, to evaluate their expected replacement wave.

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. >

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.

In this paper we present the diagnostic approach and the techniques developed by KEMA for condition based maintenance of transformers, switchgear, generators and cables. The diagnostics include UHF and VHF PD-detection techniques, dynamic measurements on moving parts including the electric drives used (switchgear, tap changers) and loss measurements. The condition-assessment techniques involve pattern recognition, fingerprint techniques and the application of performance indicators and knowledge rules obtained from field experience. As an example our approach will be illustrated with two techniques employed to assess the dielectric condition and the remaining life of transformers.

Power equipment such as turbines, generators, transformers, switchgear and cables represent a major capital investment in power stations and grids. With a high duty-cycle, or repeated short duration overloading, aging of such components may be appreciable. In practice, however, many components perform well even after their depreciation term has expired. As a result, extension of service time is a serious option for reduction of investment costs. On the other hand, life extension may increase the risk of failure, depending on the operation and maintenance practice. Adequate condition assessment (CA) enables to determine effective strategies for operation, replacement, thereby economical catastrophes. Before elaborating on CA-techniques, the authors specify three different types of condition relevant to power transmission and distribution equipment: technical, strategic and economical condition.

In this paper, we will present the results of a numerical study of the avalanche to streamer transition in an overvolted uniform field gap in atmospheric N2 and dry air. The model used is fully two-dimensional in the solution of the representative set of continuity equations, as well as in the determination of the space-charge field, gas-phase photoionization and cathode photoelectron emission. The spatio-temporal development of the electron density as well as the temporal development of the cathode and anode directed streamer velocities are presented and will be discussed.

As part of a study on high-voltage design concepts used in microwave tube technology, a number of different insulator designs were studied. This work demonstrated the presence of surface charge and its effect on the voltage holdoff performance and the conditioning process. Insulators may be characterized by the breakdown voltage, the conditioning speed, and the dependence on surface charge. Insulators with anode field enhancements gave the best performance. Field enhancements at the cathode side were less harmful if stepped insulator shapes were chosen. Effective conditioning required at least a limited number of breakdowns. With sufficient conditioning breakdowns all insulator geometries tested reached an averaged breakdown field exceeding 12 kV/mm.<>

Voltage transients generated in GIS by the operation of circuit breakers or disconnect switches induce high voltages across interruptions of the enclosure. In this paper we discuss such interruptions and their necessity. We present fast measurements of voltage and current at the GIS/cable-interface of a 150 kV substation, where the enclosure is interrupted to permit current measurement. Circuit breaker operation causes a single voltage transient across the interruption with an amplitude up to 40 kV, a duration of 200–300 ns, and a risetime of about 5 ns. Disconnect switch operation causes multiple transients, each having a smaller amplitude and a longer risetime. The voltage across enclosure interruptions is adequately suppressed, and breakdown is completely eliminated, when the interruption is bridged with resistors. The resistors do not affect the current measurement.

A report is presented on theoretical and experimental methods suitable for evaluating the high-voltage (HV) performance of spacecraft traveling-wave-tube (TWT) components. The field calculations, pressure calculations, and experimental methods used are presented, discussed, and illustrated, and some general conclusions and recommendations are formulated regarding the design, operation, and testing of spacecraft HV TWT components. >

A model, describing the time-dependent behaviour of a noble gas MHD generator plasma, has been set up. With this model it is possible to calculate the relaxation for ionization or recombination as a response to a stepwise temperature development, once the initial and final conditions are given. In the model radiative transitions and a deviation from Maxwellian electron distribution are included. Radiation causes an enhancement of both the ionization relaxation time and the recombination relaxation time. A non-Maxwellian electron distribution results in an increase of the relaxation time for an ionizing plasma because of a depletion of the high energy electrons. A decrease of the relaxation time for a recombining plasma is caused by an excess of high energy electrons. The relaxation time is strongly dependent on the seed ratio and the temperature step.