I. Ghinello

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

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.

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

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.

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

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.

An investigation of the endurance of capacitors supplied by nonsinusoidal voltage

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.

High-field conductivity measurements and search for threshold in PET films

In the paper results of measurements of DC charging current performed at different values of electrical field and temperature on PET films are presented. The aim is to investigate the threshold of space charges trapping, detected on the basis of the appearance of a maximum in the charging current vs. time characteristics. The values of threshold (function of temperature) thus derived are compared with the results of accelerated life tests performed, under DC voltage and temperature, on the same films until the occurrence of electrical breakdown. It is shown that a relationship exists between the threshold of charge trapping, detected by charging current measurements, and that of electrical breakdown, deriving from multi-stress life tests.

Accelerated capacitor degradation due to nonsinusoidal voltage supply and reliability of insulation systems

Reliability of capacitors for power factor compensation can be considerably affected by the quality of supply voltage. The presence of harmonic voltages and currents, in fact, can accelerate capacitor degradation to a significant extent, particularly when harmonic magnitudes are amplified by resonance conditions. The purpose of this paper is to infer self-healing capacitor reliability through life testing, modeling and failure rate evaluation. Statistical processing of life-test data shows that magnification of voltage peak due to harmonics is the main factor for ageing acceleration. A probabilistic life model is derived on the basis of this observation. The model enables failure rate, thus reliability, to be estimated for power capacitors. A working example shows how quantities useful for reliability inference in electrical plants, as MTTF, can be derived from the proposed model.

An investigation of conduction mechanisms at high-field in PET

The high-field electrical conduction, up to 60 kV/mm, of PET films is investigated in this paper. The behavior of time-to-peak, observed in transient charging current, vs electrical field fits well with the SCLC model, showing a power relationship. An exponential law is obtained between time-to-peak and temperature. Steady state regime provides charging-current density vs. electrical field plots which show clear transition from an ohmic behavior, at low field, to the SCLC mechanism. The threshold for this transition is a function of temperature.

An investigation on performance of XLPE compounds for high-voltage DC application

This paper presents the results of test procedures aiming at electrical characterization of polymeric materials for DC-insulation application. Electrical life tests at room temperature and space-charge observations were performed on specimens of two materials, i.e. a reference XLPE and a XLPE-based compound, in order to compare the behavior under both constant and periodically-inverted DC voltage. The compound differs from the base material due to the introduction of additives intended to modify conductivity, and thus space-charge trapping dynamics. The effect of additives is inferred coherently by both endurance estimation and space charge measurements, thus confirming that an increase of charge mobility can lead to an improvement of insulation performance in the presence of voltage inversion. The test procedures here discussed allow characterization to be achieved, and, thus, a feedback to be provided to insulating-material manufacturers, in reasonable times.

Accelerated degradation of capacitor PP films under voltage distortion

The effect of voltage distortion, that is, of nonsinusoidal supply voltage, on aging acceleration of PP films for ac capacitors is here investigated. The voltage endurance of PP films subjected to life tests under various combinations of sinusoidal and distorted voltage is derived as function of characteristic parameters of the supply-voltage waveform, i.e. peak and rms voltage, as well as waveshape slope. Life tests are performed in oil, in the absence of significant partial discharges, in order to focus the investigation on intrinsic aging. Comparisons are made with the results obtained for low-voltage capacitors. It is shown that for both capacitors and PP films, the prevailing degradation accelerating factor is voltage-peak amplification.

Electrical properties for diagnosis of sulfide-tree growth in cables under dc stress

Electrical properties, that is, AC electric strength and DC conductivity, are measured at different aging times in cable models immersed in sea water with hydrogen sulfide and supplied at two voltage levels. Test conditions are studied in order to favour growth of electrochemical, or sulfide, trees. Cable models have copper conductor and polyethylene insulation. It is shown that both electric strength and conductivity significantly vary with tree-growth time, providing tools for cable degradation diagnosis.