O. L. Hestad

Field dependence on the molecular ionization potential and excitation energies compared to conductivity models for insulation materials at high electric fields

The aim is to improve the understanding of high-field phenomena (such as preinception currents/conduction, streamer initiation and propagation) in insulating materials in terms of the molecular properties of the substances involved. In high electric fields, ionization is a likely process, and in all such processes, the ionization potential is an important parameter. A fundamental question is how these processes depend on the electric field, and therefore, based on the interaction between a negative point charge and a molecular cation as modeled by density functional theory, a field-dependent model for the ionization potential is developed. In addition, the first excitation energies as a function of the electric field are calculated using time-dependent density functional theory. It is demonstrated that empirical high-field conduction models for cyclohexane and n-tridecane can be explained in terms of the difference between the ionization potential and the first excitation energy. It is also suggested that...

Effects of N,N-dimethylaniline and trichloroethene on prebreakdown phenomena in liquid and solid N-tridecane

Prebreakdown phenomena in 0.1 M N,N-dimethylaniline (DMA) and 0.1 M trichloroethene (TCE) in n-tridecane have been investigated experimentally in liquid and solid phases in a needle-plane geometry with impulse voltage. Light emission and charge injection from the high-field electrode were measured. Results have been compared with results obtained in neat n-tridecane and in cyclohexane with the same additives. Adding TCE to n-tridecane increases the p re-inception current and decreases the inception voltage for negative and positive polarity. The increase in pre-inception current is probably caused by trap assisted conduction, and may be responsible for the decrease in inception voltage. TCE had no effect on propagation of streamers in n-tridecane. The main effect of DMA in n-tridecane is to enhance the propagation of streamers and electrical trees with positive polarity. This is in line with what has been reported for cyclohexane, and is explained by the low ionization potential of DMA. Charge injection for negative streamers is reduced by the addition of DMA in n-tridecane, while the emitted light is increased. This indicates that some of the energy otherwise used for propagation is emitted as light. A low electronic excitation energy for DMA at 3.8 eV supports this hypothesis. The additives have the same effect on prebreakdown phenomena in solid and liquid phases with positive polarity, and thus the same mechanism is suggested to be responsible for electrical treeing and streamer inception and propagation. The scatter is generally found to increase when going from liquid to solid phase, which is explained by the inhomogeneity in the solid phase. The transition to solid phase with negative polarity typically results in an even larger scatter than for positive polarity. The effect of additives seems to be secondary to the morphology at the point electrode with negative polarity.

N-tridecane as a model system for polyethylene: comparison of pre-breakdown phenomena in liquid and solid phase stressed by a fast transient

Pre-breakdown phenomena in pure n-tridecane in liquid and solid state have been investigated experimentally in a needle-plane geometry with impulse voltage. Light emission and charge injection from the high-field electrode were measured. Pre-inception currents in liquid and solid state were recorded and compared to finite element calculations to find plausible high-field conduction models for n-tridecane and cyclohexane. n-tridecane was chosen as a model system for polyethylene based on the known similarities in energy bands for polyethylene and alkanes. The results obtained in pure n-tridecane were compared to similar experiments on cyclohexane and polymer systems. High-field conduction in liquid n-tridecane was found to have a similar field dependence as previously reported for cross linked polyethylene (XLPE). Conduction currents and inception probabilities were found to be almost independent of polarity in frozen and liquid n-tridecane, which is also in line with results obtained for XLPE. Conduction currents were observed at lower voltages in n-tridecane than in cyclohexane, reflecting the lower space charge limited field (SCLF) in n-tridecane. Inception of streamers in liquid cyclohexane occurs at lower voltages than in liquid n-tridecane, and a stronger polarity dependence for all phenomena was observed in cyclohexane compared with n-tridecane. A possible explanation for the differences between n-tridecane and cyclohexane is given based on the field needed for electron avalanches and the SCLF in the two liquids. Inception voltages, light emission and charge injection were found to be similar in liquid and solid n-tridecane. This indicates that the same processes are responsible for inception and propagation of electrical trees in solids and streamers in liquids when the material is stressed by a fast transient. The similarity between the measured properties in solid and liquid phases leads to the conclusion that electrical treeing mainly takes place in the amorphous regions of the solid phase.

New experimental system for the study of the effect of temperature and liquid to solid transition on streamers in dielectric liquids: application to cyclohexane

An experimental setup for dielectric testing of insulating liquids in the temperature range from -60°C to +250°C has been built. The system uses point-plane geometry, a sensitive differential charge measurement technique, a computer controlled temperature regulator, a photomultiplier and a pulse generator with a fast rise time. This paper presents a detailed overview of the functionality of the system and results obtained in cyclohexane under positive and negative polarity below and above freezing point. Pre-inception current, inception probabilities, streamer charge and emitted light from streamers are reported. The results found in cyclohexane at room temperature compares well to results obtained in other similar setups. A temperature dependence on the injected streamer charge for liquid cyclohexane under positive polarity was observed. No polarity differences were observed for any of the measured quantities in frozen cyclohexane.

High frequency dielectric response of paper/oil insulation

Dielectric response of oil impregnated kraft paper has been measured in the frequency range 8 Hz to 2 MHz in a custom designed test cell. Temperature was varied between -17 and 60degC, allowing master curves for 20degC condition to be established for the frequency range 10 mHz to 10 MHz. Paper types and moisture was varied. The dielectric response showed losses in the low frequency region dominated by conduction and in the high frequency region, by a loss peak located around 10 MHz. The activation energy for the two relaxation mechanisms was found to be different: about 0. 9 eV for the conduction influenced part and between 0.4 and 0.7 for the part influenced by the high frequency peak. The low frequency part of the dielectric response is very dependent on the water content, while the high frequency part seems quite unaffected by water.

Photoionization model for the transition to fast mode streamers in dielectric liquids

The transition to fast mode streamers is explained through a model that includes a photoionization mechanism. The model assumes that photons are created in the head of the streamer, and that the photoionization cross section is dependent on the electric field through field dependent ionization potential. The transition in speed occurs when radiation from the most active electronically excited state can cause ionization.

Thermal decomposition of cyclohexane by Kinetic Monte Carlo simulations and its relevance to streamer formation

Kinetic Monte Carlo was used to simulate the decomposition of cyclohexane at temperatures up to 10 000 K. Rate constants needed for the simulations were obtained from literature and combined with differences in Gibbs free energy between the reactants and products. The degree of decomposition and an approximate heat capacity for cyclohexane were combined with finite element analysis to find the initial radius of a newly formed streamer channel. The initial channel was found to be of the order of 0.05-0.1 μm, expanding to a few μm after relaxation.

Simulation model for the propagation of second mode streamers in dielectric liquids using the Townsend-Meek criterion

A simulation model for second mode positive streamers in dielectric liquids is presented. Initiation and propagation is modeled by an electron-avalanche mechanism and the Townsend--Meek criterion. The electric breakdown is simulated in a point-plane gap, using cyclohexane as a model liquid. Electrons move in a Laplacian electric field arising from the electrodes and streamer structure, and turn into electron avalanches in high-field regions. The Townsend--Meek criterion determines when an avalanche is regarded as a part of the streamer structure. The results show that an avalanche-driven breakdown is possible, however, the inception voltage is relatively high. Parameter variations are included to investigate how the parameter values affect the model.

Streamer inception in cyclohexane above and below freezing point

Streamer inception as a function of temperature was studied in pure cyclohexane in needle-plane geometry under impulse voltage with a positive needle electrode. The temperature of the sample volume was regulated to 0, 20, 40 and 60degC, where 0degC is below the freezing point of cyclohexane (6degC). A sensitive differential charge measurement, with sensitivity down to 0.1 pC, and a photomultiplier was used to study the discharges in the liquid and solid phase. Streamer inception probabilities, total injected charge and photomultiplier readings were compared at the four different temperatures. Only small differences were observed as a result of increasing the temperature from 20degC to 60degC. This consisted in a small decrease in injected charge and a slightly lower pre-inception current when the temperature was increased. When going from liquid to frozen cyclohexane a marked difference was seen for all properties measured.