O.L. Hestad

Dielectric response of oil-impregnated cellulose from 0.1 mHz to 3 MHz

Pressboard and Kraft paper with and without moisture, with and without added acids have been tested with frequency domain spectroscopy up to 1 kHz. Kraft paper with three moisture levels has also been tested at various ageing levels. Moisture and low-molecular carboxylic acids cause shifts in the permittivity vs. frequency curves. In addition, in paper but not pressboard, moisture is associated with an extra relaxation in the low-frequency part. High-molecular carboxylic acids have little effect on the permittivity. The same is the case for reduction in chain length of the cellulose polymer. The method should be useful as a non-invasive field test method for destructive agents in transformers, but not for the ageing (chain scission) itself. The measurements up to 3 MHz are so far inconclusive, but have yielded a few results that blend in well with the measurements below 1 kHz.

Electrical conductivity of medium voltage XLPE insulated cables

Improved knowledge on the charge transport phenomena in XLPE is important to control the field distribution in the cable insulation, and thus further improve the reliability of future HVDC cables. Polarization and depolarization currents was measured on two medium voltage cables at 40, 60, and 80°C with an applied average electric field varying from 2.5 to 19 kV/mm. The conductivity of the insulation was calculated based on the measured quasi steady state current. The two cables were identical apart from the XLPE material (95mm2 Cu conductor, 3.4 mm insulation). The field dependence of the conductivity was found to be non-linear for the entire field range. Measured currents were fit to the Poole-Frenkel and hopping conduction. The fit to Poole-Frenkel gave a relative permittivity of 2.9, while the fit to hopping conduction gave an average hopping distance of 19 nm. The best fit to the measurements was obtained with Poole-Frenkel conductivity. The temperature dependence follows an Arrhenius relation with activation energy of approximately 1.4-1.6 eV.

Numerical simulation of positive streamer propagation in cyclohexane

A numerical 3D simulation of positive streamer propagation in a liquid, using cyclohexane as a reference case, in a needle-plane geometry is presented. The physical basis of this model is a version of the Townsend-Meek streamer criteria, known from gas discharges, adapted to a liquid containing an additive. The propagation of the streamer is modelled in the following way: Electrons located in an electric field above a given threshold (liquid dependent) create electron avalanches in the liquid. The electron avalanches leave behind positive ions that will move the high field region further into the liquid due to the field created by the high charge concentration. Subsequent avalanches in the liquid neutralize this region, but create a new highly ionized region, thus the ionization region (streamer head) moves further away from the needle, while still being connected to the needle by a weakly conducting plasma channel. The stochastic nature of streamer inception, propagation and branching is modeled through a random distribution of seed electrons in the liquid. Key results obtainable from the model includes streamer shape/degree of branching, speed (instantaneous and average), and inception delay of positive streamers as a function of applied voltage. The presented model is crude as it does not take into account the energy required for phase transition, and the dynamics of the plasma channel trailing the streamer head. In addition it is based on a rough estimate of the field in the liquid. This paper presents the model in detail.

Streamer initiation in cyclohexane, Midel 7131 and Nytro 10X

Streamer initiation has been studied in cyclohexane, Midel 7131 (ester based dielectric liquid) and Nytro 10X (mineral transformer oil). Experiments were conducted in point-plane geometry under 40 ns/740 /spl mu/s impulse voltage. The tip electrode was electrochemically etched to approximately 2.0 /spl mu/m, the electrode gap was approximately 18 mm. A sensitive differential charge measurement technique was used to measure the injected charge. Shadow graphics was used to acquire images of streamers, and a photomultiplier was utilized to study light emission under positive polarity in Nytro 10X. The experimental setup is capable of measuring injected charges down to 0.1 pC, the shadow graphics has an image resolution of 1 /spl mu/m and the photomultiplier can detect light down to the order of single photons. For all three liquids a small pre-inception current at voltages below streamer inception was found with positive point polarity; no emitted light from this current in Nytro 10X was detected. No preinception current was observed under negative polarity in any of the liquids.

Conductivity and permittivity of midel 7131: effect of temperature, moisture content, hydrostatic pressure and electric field

Knowledge of the conductivity and permittivity of materials used in subsea connectors is needed to predict the electric field distribution in the connectors. The synthetic ester Midel 7131, being one of several dielectric liquids used in subsea connectors, has been chosen as a relevant candidate material. The conductivity and permittivity of Midel was measured by an IRLAB conductivity meter and an IDA insulation diagnostic analyzer. The temperatures were 30, 60 and 90 °C, the hydrostatic pressures were 0.1, 1 and 10 MPa (1, 10 and 100 bar), the water content varied between 20 and 5600 ppmw, and the applied electric fields varied between 2.5 V/mm and 9.4 kV/mm. The results show that the conductivity increases linearly with increasing levels of water content, whereas the temperature dependency follows an Arrhenius relation. The conductivity was found to increase slightly with increased pressure, the differences between samples were higher, rendering this increase insignificant. Measurements of the permittivity showed a linear increase in permittivity with increasing water content. The measured permittivity was found to follow the Clausius-Mossotti relation when varying the temperature and hydrostatic pressure. Based on the dependence of the conductivity and permittivity on water content, it seems that water molecules in Midel have a reduced degree of mutual interaction compared to liquid water.

Field and temperature dependent conductivity of syndiotactic polypropylene effect of temperature and electric field

Syndiotactic polypropylene (s-PP) is an extrudable polymer with properties which should make it highly suitable as high voltage cable insulation, including high AC breakdown strength, flexibility and a high melting point. However, for HVDC applications it is important to improve the knowledge of charge transport phenomena which control the field distribution. This paper presents electrical conductivity measurements performed on Rogowski shaped test objects and model cable. The polarisation and depolarisation currents were measured at the temperatures 60, 80, 100, and 120 °C with an applied electric field varying from 6 to 20 kV/mm. The electrical conductivity was calculated based on the quasi-steady polarization current. The field dependence of the conductivity was found to be non-linear for the entire field range. The temperature dependence for Rogowski shaped objects with painted silver electrodes followed an Arrhenius law with activation energy of approximately 1.68 eV, which is similar to values reported for XLPE. For model cable with polymeric semi-conductive electrodes the average activation energy was also 1.68 eV. In addition the estimated conductivity was also fitted to the Poole-Frenkel and hopping conduction mechanisms.

Condensation of water vapour in XLPE insulation at different cooling rates and pressures

Temperature cycling and temperature gradients can cause super saturation of water vapor in the insulation. Condensation of water can then occur and high hydrostatic pressures can be established within water droplets embedded in the polymer. Water treeing is closely related to the condensation of water, and the main purpose of this paper has therefore been to examine the effect of different cooling rates and pressures on water condensation in XLPE insulation during temperature cycling. 1 mm thick samples cut from a XLPE cable with a super clean grade XLPE (Borealis LE 4201 S) were subjected to temperature cycles from 90 to 65, or 90 to 20degC at cooling rates ranging from 0.12 to 120degC/min. The total pressure during cycling was atmospheric or 100 bars. The insulation was characterized by water content measurements and microscopy SEM examinations. The solubility of water was about 244 g/m 3 at 90degC independent of the applied pressure. The results show that a relatively small but significant increase of about 65 g/m3 of water were measured for the fastest cooling rates at atmospheric conditions (1 bar) after 50 cycles. No significant change was measured for the slow cooling at the same conditions. An even smaller, but not significant, increase was measured at 100 bars, indicating that the high external pressure to some extent suppresses the formation of micro-voids. It is likely that the relatively low water content after the temperature cycles was due to a very low density of condensation nuclei within the insulation avoiding the formation of micro-voids. This was confirmed by the SEM analysis

Modeling the transition to fast mode streamers in dielectric liquids

A simplified model for photoionization, modeling fast streamer propagation, is combined with an existing model for slow streamers, based on electron avalanches. Transitions from fast mode to slow mode, and from slow mode to fast mode, are investigated.

Field dependent conductivity of n-pentane

A sensitive differential charge measurement technique was used to measure the conduction current from a sharp needle in a needle plane geometry in n-pentane. The current was measured at voltages up to the threshold for streamer inception in n-pentane with a positive needle polarity. The measured current was compared to finite element models to link the recorded charge with conductivity models for the liquid. A good fit was obtained using the classical Poole-Frenkel conductivity model.

The effect of temperature cycling on mechanical and electrical properties of syndiotactic polypropylene

Polypropylenes (PP) are a candidate to replace cross-linked polyethylene (XLPE) in medium voltage cables. This is due to its recyclability and potentially lower energy consumption during production. To prove to be an alternative, the PPs must meet the same requirements as for XLPE. The maximum service temperature for XLPE is 90 °C. The syndiotactic PP (sPP) was chosen for this study. sPP has a significantly higher melting temperature (~130 °C) than for other alternatives, such as LDPE/MDPE, but softening and morphology changes during service could be challenging. Thin sPP discs were therefore subjected to 90 °C and also thermal cycling between 20 and 90 °C with heating and cooling rates of ±0.2 °C /min and ±3.5 °C /min. Pristine sPP has low conductivity, losses and good mechanical properties. The temperature treatment changed the mechanical and electrical properties slightly. The tensile strength was lowered and the melt peak changed, but the total crystallinity did not change (24-26 %). The dielectric losses increased by a factor 2.5 for thermally aged samples. The conductivity was low and almost unchanged compared to reference for samples subjected to temperature cycling. Heat treatment weakens the electrical and mechanical parameters.