Fabrizio Negri

The effect of magnetite, graphene oxide and silicone oxide nanoparticles on dielectric withstand characteristics of mineral oil

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.

Behavior of nanofluids under DC divergent fields

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.

Analysis of conduction currents in nanofluids

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.

Partial Discharge and life behavior of rotating machine wire insulation under PWM waveforms: The influence of inverter characteristics

Partial Discharge (PD) topic in adjustable speed drives has received growing attention in the last decades because of the higher stresses introduced by Pulse Width Modulation (PWM) voltages compared to sinusoidal power supply.

Influence of ageing on dielectric characteristics of silicone dioxide, tin oxide and ferro nanofluids based mineral oil

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.

Effect of graphene oxide-based nano-coatings on electrical ageing

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.

Partial discharge and aging phenomena in insulation systems of rotating machines fed by power electronics

This paper deals with partial discharge measurements carried out on specimens of Type I and Type II insulation systems for rotating machines. Partial discharge (PD) patterns under sinusoidal and repetitive impulse voltage, considering two level and five level inverters, are presented showing that phenomenology is deeply different, but that increasing the number of inverter steps, PD patterns closer to those obtained under sinusoidal supply voltages can be obtained. The different characteristics, magnitude and repetition rate of PD varying the voltage supply waveshape have significant impact on life and insulation system reliability, for both Type I and Type II insulation.

Analysis of the charge-nanoparticle interaction in nanofluids

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.

Noise rejection and partial discharge identification in HVDC insulation systems

This paper shows techniques for singling out noise in partial discharge (PD) measurements starting from the well-known time-frequency (T-F) map extracted from impulse shape and statistical processing of features extracted from PD-pulse distribution features. A case study shows that the noise rejection procedure which is proposed, very effective in AC, is also promising in DC, and that some kind of automatic identification of noise and PD is feasible resorting to statistical processing of recorded pulses.

An investigation on partial discharge time-behavior in epoxy resin cavities

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.