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Natural esters are nowadays attracting a lot of utilities which are thinking of installing (or have already installed) the first large power transformers with positive expectations in terms of thermal and long term performance. Instrument transformers manufactures are anyway still resilient to experiment with such fluids, because of the lower advantage-disadvantage ratio related to their application (higher costs not immediately translated into outstanding performance). In a previous investigation, we discovered an interesting so far unknown failure mechanism, which seemed not to be related to manufacturing errors, but to a possible charge trapping related phenomenon. Pulsed Electro-Acoustic measurements have been carried out on simple impregnated sheets samples to compare the different behavior between mineral oil and natural esters. Preliminary obtained results highlight a very low voltage injection thresholds in both cases, and a comparable injected charge, but the depolarization trends seems to be lower in the case of natural impregnated paper even if the investigation is still far to be considered concluded.

Recent papers have shown controversial aspects regarding nanofluids conduction properties. Therefore, in this work, we have analyzed these apparent incongruencies proposing a simple field model and invoking the possibility charge carriers may be directly transferred from one nanoparticle to the following one when the distances are small enough. The results of the model show that the postulated conduction mechanisms may explain the measured conductivities.

A previous paper proved that conductive nanoparticles-based nanofluids have a better performance than the host mineral oil, but the obtained results still left doubts about their origin and if they are really statistically significant. In this work, we analyzed the impulse breakdown strength of different concentration nanofluids, and the effect moisture has on sinusoidal AC breakdown tests. The results highlight a concentration dependent behavior of the breakdown measurements, and a stronger resistance of nanofluids against moisture contamination.

The research on nanofluids has shown important improvements of the electrical performance, but so far the attention has only been focused on their use in transformers. The target of this work is to understand if nanofluids can be applied as insulators for power electronics modules, where the major stresses are caused by square waves or PWM voltages. Breakdown events in non-uniform field conditions and different voltage configurations have been deeply investigated revealing the effects of space charge mechanisms. While for DC fields the nanofluids contribution is positive, a completely different conclusion can be evinced from sinusoidal and square waves, where the situation even worsens at higher frequencies. Schlieren images of breakdown events seem to confirm the postulated hypotheses.

This paper provides some insights on aging due to PD activity occurring in insulation cavities. In particular, it is observed that the variation of PD amplitude may drastically decrease or increase during aging, thus challenging the interpretation of PD magnitude and repetition rate as diagnostic quantities. Explanations are proposed, based on a series of chemical-physical measurements, as Raman spectroscopy and SEM observations. At the beginning, aging leads to the formation of crystals on the cavity walls, which decrease PD activity, while for longer aging times also pitting and electrical treeing is observed. This would lead to speculate about the diagnostic validity of spot off-line measurements.

A recent investigation on the DC stability of nanofluids has revealed some interesting phenomena regarding the polarization current; some unexpected slow peaks appeared after the voltage reached a precise value. In this work we have deepened and analyzed the possible mechanisms which could cause such experimental results. While in low concentration nanofluids a charge trapping mechanism seems to be the prevailing one, at higher concentration, the reduced nanoparticles distances and the lower energy barrier between two consecutive nanoparticles is able to activate the tunneling charge transfer among them, thus increasing their apparent mobility.

Traditionally mineral oil is employed in power transformer as a cooling medium and as an electrical insulator. Recently, massive research work has been carried out to improve the thermal and dielectric characteristics of mineral oil; nanofluids, i.e. nanoparticle-based liquid insulation, seem a feasible alternative. However, concerns have been raised about the dielectric withstand performance of nanofluids after ageing. In this work, an investigation has been carried out to analyze the thermal ageing performance of nanofluids based on silicone dioxide, tin oxide and ferrofluid along with mineral oil. Based on previous results, the selected nanofluids concentration is 0.2 g/l in all tests, i.e., the concentration that allowed to achieve the best dielectric withstand characteristics. To characterize the dielectric withstand properties of the different fluids, we measured the Breakdown Voltage (BDV) and Partial Discharge Inception Voltage (PDIV) of test samples for AC, DC positive and negative voltage. The results illustrate that, under AC voltages, aged nano fluids shows better performance than the base oil (Shell Diala D). In particular, the nanofluid based on silicone dioxide (SO) shows the best performance. The results reported here seems to confirm the applicability of nanofluids in power transformers.

We have compared the DC conduction of a nanofluid based on mineral oil and magnetite nanoparticles with that of the base mineral oil using a point/plane electrode configuration. The results show that the nanofluid has higher conduction currents and that transition from ohmic to tunneling-controlled conduction occurs at higher fields. This latter evidence might explain why nanofluids improve partial discharge inception voltage under AC conditions. After some tests we observed that the nanofluid is not stable under DC: the nanoparticles are separated from the base fluid by coulombic forces and dielectrophoresis. We have compared the DC conduction of a nanofluid based on mineral oil and magnetite nanoparticles with that of the base mineral oil using a point/plane electrode configuration. The results show that the nanofluid has higher conduction currents and that transition from ohmic to tunneling-controlled conduction occurs at higher fields. This latter evidence might explain why nanofluids improve partial discharge inception voltage under AC conditions. After some tests we observed that the nanofluid is not stable under DC: the nanoparticles are separated from the base fluid by coulombic forces and dielectrophoresis.

In this work, we compare the dielectric withstand performance of nanofluids prepared using Shell Diala D as a base fluid, and magnetite, graphene oxide and silicone dioxide as nanoadditives. The ac withstand capability of the nanofluids was investigated as well as partial discharge inception voltage, partial discharge and repetition rate under ac, dc positive and dc negative voltages. The results indicate that, for all nanofluids, nanoparticle concentrations around 0.2 g/l enhance dielectric withstand properties under quasi uniform fields. Under divergent fields, partial discharge characteristics are improved under ac conditions. Under dc conditions silica nanofluid performs better than mineral oil, but the other two nanofluids do not perform well. In this work, we compare the dielectric withstand performance of nanofluids prepared using Shell Diala D as a base fluid, and magnetite, graphene oxide and silicone dioxide as nanoadditives. The ac withstand capability of the nanofluids was investigated as well as partial discharge inception voltage, partial discharge and repetition rate under ac, dc positive and dc negative voltages. The results indicate that, for all nanofluids, nanoparticle concentrations around 0.2 g/l enhance dielectric withstand properties under quasi uniform fields. Under divergent fields, partial discharge characteristics are improved under ac conditions. Under dc conditions silica nanofluid performs better than mineral oil, but the other two nanofluids do not perform well.

In previous works, we have shown that partial discharge activity in a cavity can deplete oxygen, thus leading to partial discharge suppression. In order to suppress as much as possible partial discharge activity, it is thus essential to improve the oxygen impermeability. In this study, sandwich-like samples of cross-linked polyethylene (XLPE) have been coated with organic-inorganic hybrid coatings containing graphene oxide (GO) in order to hinder the oxygen permeability. The samples containing a cavity having fixed geometry were subjected to electrical ageing above partial discharge inception voltage, while partial discharges were monitored. The electrical aging test was carried out in two cycles. Between cycles, the samples were extracted from the test vessel (full of silicone oil, to prevent surface discharges) and exposed to air, to investigate whether the coating could effectively prevent oxygen ingress in the cavity. Next, aging tests were carried out till breakdown. The effect of GO is thus evaluated indirectly through PD activity behavior, and by statistics of the breakdown times. Experimental results show the effectiveness of the coating and highlight the dependence of partial discharge degradation rates on coating composition.