Molecular Dynamics Simulation and Conductivity Mechanism in Fast Ionic Crystals Based on Hollandite NaxCrxTi8-xO16

Molecular Dynamics Simulation and Conductivity Mechanism in Fast Ionic Crystals Based on Hollandite NaxCrxTi8-xO1

A New Numerical Approach to the Calibration and Interpretation of PEA Measurements

In this paper, a new approach for interpreting raw PEA signals is proposed. The theory behind acoustic wave propagation in ideal (no attenuation and no dispersion) materials and in real materials (with attenuation and dispersion) initially free of space charge is described. A simulation model has been developed for the following: (1) the acoustic signal formation in the PEA apparatus, the transmission of the acoustic waves, their attenuation/dispersion and detection; (2) the instrumental effects of having a capacitive piezoelectric sensor driving a 50 Ohm input impedance amplifier. The various layers of a PEA system were considered in the model and the effect of each layer on the acoustic wave propagation is analyzed. Since the model allowed raw PEA data to be simulated, it can be used to identify potential sources of error in interpreting real PEA measurement data, such as acoustic mismatch between PEA layers, electrode material effects, pulse voltage, etc. The results showed good agreement between the simulated and experimentally obtained data in the case of space charge free samples

Space charge behaviour in epoxy laminates under high constant electric field

The development of space charge in insulating materials is one of the main causes of their electrical ageing. The pulsed electro-acoustic method is often used to determine space charge distribution, but the signal analysis in the case of laminate structures is much more complex to analyse. In this paper the authors describe and use a simulated signal in order to study laminates made of epoxy resin and fibre mat. The relatively large conductivity of the fibres compared with that of the resin seems to produce a rapid charge dissociation and recombination in the fibres. Under voltage the presence of fibres close to an electrode seems to promote charge injection

Measuring a possible HVDC insulation killer: Fast charge pulses

The increase of rated voltages for modern HVDC polymeric cables up to about 400 kV, thanks to high performance insulating materials and higher design fields, may raise long-term reliability concerns due to space charge accumulation. In particular, heterocharge build up can enhance locally the electric field at the electrode/insulation interface. Use of Voltage Source Converters (VSC), which do not permit polarity inversions, allows a limited amount of injected charge to be tolerated, but only if it leads to homocharge accumulation. However, if injected charge migrates towards the opposite electrode and extraction is delayed, heterocharge can build up, thus affecting eventually electrical insulation. This paper shows that at electric field values of 20-40 kV/mm, i.e. in the range of cable design fields, small charge packets, injected from the electrodes and crossing the insulation very fast as coherent charge pulses, can lead to significant heterocharge accumulation in the presence of a partially-blocking electrode. Even in the absence of a noticeable heterocharge accumulation, the electric field perturbation caused by each charge pulse can provide a non-negligible life reduction of cable insulation. Therefore, the development of proper techniques able to detect and analyze these fast pulses is of utmost importance not only for scientific reasons, but also for design and reliability issues

Measuring a possible HVDC insulation killer: Fast charge pulses

The increase of rated voltages for modern HVDC polymeric cables up to about 400 kV, thanks to high performance insulating materials and higher design fields, may raise long-term reliability concerns due to space charge accumulation. In particular, heterocharge build up can enhance locally the electric field at the electrode/insulation interface. Use of Voltage Source Converters (VSC), which do not permit polarity inversions, allows a limited amount of injected charge to be tolerated, but only if it leads to homocharge accumulation. However, if injected charge migrates towards the opposite electrode and extraction is delayed, heterocharge can build up, thus affecting eventually electrical insulation. This paper shows that at electric field values of 20-40 kV/mm, i.e. in the range of cable design fields, small charge packets, injected from the electrodes and crossing the insulation very fast as coherent charge pulses, can lead to significant heterocharge accumulation in the presence of a partially-blocking electrode. Even in the absence of a noticeable heterocharge accumulation, the electric field perturbation caused by each charge pulse can provide a non-negligible life reduction of cable insulation. Therefore, the development of proper techniques able to detect and analyze these fast pulses is of utmost importance not only for scientific reasons, but also for design and reliability issues

The effect of dc poling duration on space charge relaxation in virgin XLPE cable peelings

International audienceThe effect of DC poling time upon the time-dependent decay of space charge in insulation peelings of Cross-linked Polyethylene (XLPE) cable that had not previously experienced either electrical or thermal stressing is investigated. Two DC poling durations were used, two hours and twenty six hours at an electric field of 50kV/mm and at ambient temperature. Space charge was measured in the two samples investigated both during space charge accumulation and throughout its subsequent decay. The results show that the length of DC poling plays an important role in the subsequent decay. Despite the fact that both samples have had the same amount of space charge by the end of both short and long poling durations the time dependence of the space charge decay is different. Most of the charge stored in the sample that had experienced the short time poling decays rapidly after voltage removal. On the other hand the charge that is stored in the sample with the long DC poling duration decays slowly and its decay occurs in two stages. The data, which is analysed by means of the detrapping theory of space charge decay, implies that the charge stored in the material has occupied energy states with different trap depth ranges. The two poling durations lead to different relative amounts of charge in each of the two trap depth ranges. Possible reasons for this are discussed

Dielectric Response of Nano Aluminium Tri-hydrate Filled Silicone Rubber

The dielectric response of a nano-ATH (Aluminium Tri-hydrate) filled silicone rubber is investigated over the frequency range 10−4 to 103 Hz and compared with that of the micro-ATH composite. An equivalent circuit has been used to decompose the dielectric response into: a loss peak process, a quasi-dc (Q-DC) process, a dc-conductance and constant high frequency capacitance. It was found that the Q-DC and loss peak were a result of the ATH filler with the loss peak more clearly resolved in the nano-ATH filled material than in the micro-ATH material where it is weaker and partly obscured by the Q-DC process. In contrast to the micro-ATH material the characteristic frequencies of all the dielectric responses in the nano-ATH filled samples have the same activation energy. The physical mechanisms of the Q-DC process and loss peak are discussed in terms of long range transport between nano-ATH clusters and the dipolar behaviour of isolated clusters respectively