Electrical ageing of composites: an industrial ... - Schneider Electric

Schneider Electric 2007 - Conferences publicationsand 40µm, even though we had convinced ourselves asto their absence. In other words considering fig1. we arealready at time t 3 whilst we thought we were at t 0 .Shortly after manufacturing, the gas pressure in a voidmight be less than atmospheric and might not be air.This situation would be even worse, as the mean freepath would be greater. Bigger undetectable voids couldthus exist. They would however gradually fill up toatmospheric pressure via air diffusion, and thereafterdischarge violently. This might take days or evenmonths. This point is worth pondering… Increasingsensitivity and/or applied field, improves things, butremember that for an avalanche to grow normally, theelectrons must be able to freely follow the direction ofthe electric field lines. If this is not the case, such as insinuous/tortuous channels, even longer “elongatedvoids” might not be detectable.One is justified in wondering whether the precedingdiscussion has any real practical importance. Considertherefore, the following case of a 6µm length void filledwith atmospheric air. This corresponds to a maximumof 3 mean free paths. In the pure gas we would thus notexpect to produce more than 8 electrons by collisionalionisation, no matter how high the applied field.However each of these electrons will strike thedownstream surface with an energy corresponding tothe voltage drop across the final accelerating step. For asmall gas void in epoxy with an applied electric fieldequal to 2kv/mm, the “average” impact energy will beabout 20eV. This might be enough to do damage to thepolymer chain structure. Under 50HZ ac conditions, thisbombardment will continue as a repetition of “bursts”throughout the life of the insulation. Assuming anaverage of only 2 discharges per cycle, leads to about3×10 10 impacts per year or 10 12 over the 30 year lifespan. Hence an almost “negligible” and virtuallyundetectable discharge might have a strong cumulativeeffect in practical applications. In conclusion, we cangenerally not be sure that our samples do not containvoids in the 0 – 30µm range.Electrical ageing driven by this might be occurringwithout us ever suspecting. However, the basic theorysays that ageing is via extension of the initial microvoid,so at some instant in time the PD level should riseabove the noise level of the experimental set up, andcontinue to rise. This experiment is so simple to do thatone can wonder that it has not been done many times,because without this result the “standard scenario” cannot be validated. To try to obtain this validation, wehave had an accelerated ageing experiment running nowfor about 18 months at 6 times the normal applied field,with samples at temperatures of 40°C, 80°C and 120°C.So far we have observed no evolution of the PD level.ELECTROLUMINESCENCEWe also decided to study the electric field induced lightemission of our filled samples, to see how this couldhelp us in our quest. We rapidly discovered that photonemission exists and rises rapidly with electric field,Fig.7. We also discovered a strong thermally stimulatedluminescence for higher temperatures, which was linkedto oxidation of the hardener, (removed in the curvesshown), [4]. Although plotting on a linear scale seemedto indicate the presence of a threshold, this becomesquestionable when plotting with Log-log coordinates asshown. For ambient temperature the signal falls belowthe noise level of the detector for about 2kV/mm.In reality the overall photon detection “efficiency” is notsimple to determine. As photons are emitted in alldirection we can only hope to capture a smallpercentage depending on the design of the opticalarrangement. Furthermore the efficiency of the detectorsthemselves is less then 20%, so the sample could beproducing more than ten times as many photons as thosewe actually “count”. Also we are intrinsically assumingthat photon absorption by the polymer itself isnegligible for our 1mm thick samples. All the same let’smake the following rough estimation. 1 photon detectedper second corresponds to about 10 9 photons per year.Fig.7: Electric field induced light emission for a filled sampleThe question we now have to ask ourselves once moreis, “has this light emission got anything to do withelectrical ageing”? For instance, a light emitting diode,(LED), produces extremely high numbers of photonspermanently, but we do not normally consider that theproduction mechanism involved actually “ages” thecomponent. Knowing the emission wavelength andusing Planck’s constant, we can estimate the energyassociated with a single photon. For a wavelength of500nm, this turns out to be about 2.5eV. To decide ifthis has anything to do with electrical ageing we need toknow what mechanism is responsible for liberating thisenergy as light, and how it was gained from the electricfield. Taking an average hopping length of 5nm, we findthat for our nominal conditions, the voltage differenceper hop, is about 10mV. The average carrier impactenergy is thus about 0.01eV which seems unlikely toproduces the observed effect.So where does this, “2.5eV” light come from ? Carrierrecombination ?, Deep trap filling ? One option is that itis linked to the presence of micron size “voids”. In thiscase some carriers would be able to make “jumps” ofseveral microns, rather than a few nanometers and404