THE DESIGN AND ANALYSIS OF EXPERIMENTS. By Oscar Kempthorne. New York, John Wiley and Sons, Inc., 1952. 631 pp. $8.50. This book by a teacher of statistics (as well as a consultant for \"experimenters\") is a comprehensive study of the philosophical background for the statistical design of experiment. It is necessary to have some facility with algebraic notation and manipulation to be able to use the volume intelligently. The problems are presented from the theoretical point of view, without such practical examples as would be helpful for those not acquainted with mathematics. The mathematical justification for the techniques is given. As a somewhat advanced treatment of the design and analysis of experiments, this volume will be interesting and helpful for many who approach statistics theoretically as well as practically. With emphasis on the \"why,\" and with description given broadly, the author relates the subject matter to the general theory of statistics and to the general problem of experimental inference. MARGARET J. ROBERTSON

The Design and Analysis of Experiments

An investigation of the effect of trapped space charges on the aging of polymeric insulating materials subjected to thermo-electrical stress is reported in this paper. Possible scenarios of degradation mechanisms, thermally activated, but accelerated by the presence of space charges, are examined. The model which derives from this approach has some interesting features: it is characterized by electrical and thermal thresholds, its parameters have a physical background, it can be cast into a probabilistic framework. Acceleration of aging due to space charges is attributed to a reduction of the free-energy barrier to degradation, seen as a local partially-reversible reaction, which is caused by energy stored in space-charge centers. The validity of the model is limited to dc voltage, and to the time of formation of microcavity-crazes, rather than to breakdown times, since other mechanisms will occur under electrical field once large enough cavities are formed in the insulation. The model is applied to the results of thermo-electrical life tests performed on PET, showing very good fitting, as well as interesting relationships between parameter estimates and insulation morphology. It is shown that the model can also fit well to ac life data, where it takes on a phenomenological meaning.

The role of trapped space charges in the electrical aging of insulating materials

Considering the title of this paper, one might think that everything has been said, shown, proven and discussed already on ageing and failure of electrical insulating materials, based on at least half century of research work and field feedbacks. However, perhaps there is still room to collect ideas and speculate on apparently proven or brand new concepts, such as the interrelation between ageing mechanisms and physically measurable quantities such as space charge, partial discharges and conduction current. The latter is, indeed, the scope of this paper, which is taken from the Whitehead Memorial Lecture given at the 2010 IEEE Conference on Electrical Insulation and Dielectric Phenomena (CEIDP). On one hand we will revisit the concepts of space charge accumulation in insulating (polymeric) materials, considering also the latest achievements which deal with ultra-fast charge pulses (solitons), on the other hand we will discuss how space charge can contribute to ageing rate under dc, ac or impulsive voltage, directly through electric field alteration, or indirectly modifying a fundamental ageing factor such as partial discharges.The goal is, at the very end, to show how much such basic investigations can have a fundamental impact on insulation design and electrical apparatus long-term performance/ reliability.

Bringing an insulation to failure: the role of space charge

Measurement of dc charging current and space-charge observations, performed on specimens made by low-density polyethylene (PE) and crosslinked PE, are considered for dc electrical threshold investigation. The threshold is inferred from charging-current measurements through steady-state voltage-current characteristics, as well as from the total trapped charge, obtained by space-charge measurements carried out at 0 V and under short-circuit after poling for a pre-selected time. It is shown that both techniques provide similar information regarding the electrical threshold values, which vary for the two tested materials. Being the threshold associated with mechanisms of charge storage, its evaluation can help in insulation design and material characterization when electric field, especially dc, is a significant stress in service. Other significant information for design purposes comes from the rate of charge accumulation as function of the applied field, which is estimated through the charge-field characteristic derived from space-charge measurements.

The electrical degradation threshold of polyethylene investigated by space charge and conduction current measurements

We present a conduction model aimed at describing bipolar transport and space charge phenomena in low density polyethylene under dc stress. In the first part we recall the basic requirements for the description of charge transport and charge storage in disordered media with emphasis on the case of polyethylene. A quick review of available conduction models is presented and our approach is compared with these models. Then, the bases of the model are described and related assumptions are discussed. Finally, results on external current, trapped and free space charge distributions, field distribution and recombination rate are presented and discussed, considering a constant dc voltage, a step-increase of the voltage, and a polarization–depolarization protocol for the applied voltage. It is shown that the model is able to describe the general features reported for external current, electroluminescence and charge distribution in polyethylene.

https://iopscience.iop.org/article/10.1088/0022-3727/37/2/020/pdf

Description of bipolar charge transport in polyethylene using a fluid model with a constant mobility: model prediction

This paper presents the results of life tests performed on models of turn insulating materials of AC rotating machines, having the purpose to infer the aging acceleration effects due to distorted voltage waveforms. PWM speed drives, indeed, have been shown to affect considerably the reliability of AC motors, due to the unusual voltage waveforms supplied to them. These waveforms are significantly different, regarding harmonic content, with respect to those generally considered as the reference for insulation design. The results discussed in this paper show that the main factor affecting insulation aging acceleration is the increase of peak supply voltage, due to wave reflection caused by impedance mismatch of drive and motor, which is applied to turn insulation. Of course, commutation frequency plays also a fundamental role, while less evident is the contribution of rise and fall rate of supply-voltage pulses. A life model is proposed, based on a statistical analysis of life data, which allows evaluation of the degradation acceleration factor.

Influence of PWM-type voltage waveforms on reliability of machine insulation system

A possible way for the detection of electrical threshold in dielectric polymers is illustrated in this paper. On the basis of the consideration that the lowest threshold level for electrical aging is related to build up of steady space charge, high-field current measurements performed at different voltage and temperature values, seeking space charge limited current conditions, can provide estimations of electric field at threshold. This approach is applied to PET films under dc voltage. The values of electrical threshold thus obtained are compared with those derived from conventional accelerated life testing, showing that there is good correspondence of values at the lowest temperature of the test range, while deviation between estimates increases with temperature.

Searching for short-term techniques for the inference of electrical threshold of PET. DC conductivity measurements

Additives have been introduced in the conventional high-voltage grade XLPE, with the purpose to modify the electrical threshold level The electrical threshold is estimated through the steady-state current-voltage characteristic obtained from dc charging current measurements performed at different voltage levels Space-charge distribution observations are carried out, by PEA, in order to confirm the threshold values detected by charging-current measurements, having associated the threshold to the limit value above which space charges are steadily trapped in insulation Significant variations of the electrical threshold levels is actually observed in the two tested additivated compounds, with respect to the reference XLPE The conduction mechanism, phenomenologically attributed to space-charge limited current (SCLC), is rediscussed on the basis of space-charge distribution observations which show deviations from the SCLC theory

Behavior of voltage-current characteristics and threshold indications for XLPE-based materials

This paper presents the results of tests performed on small-size capacitors with the aim of investigating the effect of nonsinusoidal voltage on life performance. Harmonic components, as those detectable in ac networks supplying distorting loads, are added to the sinusoidal supply voltage and life tests are carried out under such waveforms. The effects of voltage and current distortion on accelerated electrical aging are modeled and discussed on the basis of statistical techniques, focusing mainly on the features which affect life performance of capacitors. This approach leads to a relationship between expected life and voltage waveform characteristics.

An investigation of the endurance of capacitors supplied by nonsinusoidal voltage

This paper investigates the influence of harmonic distortion on the life reduction of power capacitors when connected in parallel to power converters operating with randomly-varying loads. A probabilistic life model is employed in order to evaluate the effectiveness of standard design procedures for capacitors operating in distorted regimes. This approach is applied to a working example based on measurement data obtained from a busbar feeding a ski-lift resort. Either a capacitor bank or a system consisting of a shunt filter plus a capacitor bank is considered for reactive power compensation. It is shown that capacitor design made on the basis of voltage derating of 20% may be not sufficient to ensure satisfactory life performance.

I. Ghinello

Papers

Influence of PWM-type voltage waveforms on reliability of machine insulation system

Searching for short-term techniques for the inference of electrical threshold of PET. DC conductivity measurements

Behavior of voltage-current characteristics and threshold indications for XLPE-based materials

An investigation of the endurance of capacitors supplied by nonsinusoidal voltage

Considerations on the life performance and installation practice of shunt capacitors in the presence of harmonics generated by AC/DC converters