Erling Ildstad

Understanding water treeing mechanisms in the development of diagnostic test methods

Recently, several diagnostic methods for measurement of permittivity and dielectric loss have been developed for non-destructive testing of water-tree degraded XLPE cables. Critical degradation is found to be associated with a more than proportional increase of the dielectric properties when increasing the applied test voltage above a certain level. In this paper an explanation for this nonlinearity will be presented, using a mechanical approach to the water treeing phenomenon. The water treed insulation is considered to consist of small water filled voids separated by crazing zones. At relatively low water content and low applied test voltage, several of these crazing zones are likely to be closed and insulating. When increasing the test voltage, Maxwell mechanical stresses will cause water to penetrate into the crazing zones, thus making electric contact between the elongated water droplets. Results from finite element method (FEM) calculations of electric fields and losses show that the effect of this will be to enhance the electric field at the tip of these conductive channels and to increase the dielectric losses of the water treed insulation.

Correlation between AC breakdown strength and low frequency dielectric loss of water tree aged XLPE cables

This paper describes experiments performed on laboratory aged 12 kV and service aged 24 kV XLPE cable samples. Results from measurements of residual AC breakdown strength, degree of water treeing and dissipation factors are presented. The dissipation factor at low frequencies was determined by both time and frequency domain measurements. The relationship between these methods are discussed and it is shown that both can be used for assessing the average ageing state of the cable, The results show that water treeing causes reduced residual AC breakdown strength and high and nonlinearly increasing low-frequency dielectric loss.

Application of contact analysis on evaluation of breakdown strength and PD inception field strength of solid-solid interfaces

Interfaces between solid insulating materials are generally weak regions in electrical insulation systems, particularly if the electrical stress is applied in parallel direction. This paper presents a theoretical mechanical model for estimating average size of air-filled voids at the interface as a function of surface roughness and contact pressure. It is argued that the interfacial breakdown strength (BDS) is governed by the discharge inception stress (Ev) of the void with the most likely estimated dimension. The estimated values of the breakdown strength and partial discharge inception electric field strength (PDIE) were compared with the results from measurements using XLPE specimens with interfaces energized in longitudinal direction. The measured ratio of increase of breakdown strength was found to be in good agreement with the estimated breakdown values of the most likely interfaces void. Additionally, it was found out that the air pressure inside voids was not affected by applied contact pressure. The estimated PDIE values was found to be in agreement with the measured values, in the case of rough surfaces, but not for smooth surfaces. The results indicate that the mechanical contact approach using the motif description of surface roughness may improve the understanding of the factors influencing the PDIE of electrically stressed interfaces.

Charging of dielectric barriers in rod-plane gaps

The electric properties of an air gap between a hemisphere-rod covered with 3 mm thick silicone rubber and plane metal electrode have been examined. Observation of discharge activity and measurement of the discharge current have been made under impulse voltage stress. When applying an impulse voltage close to the inception voltage, discharge activity is observed around the hemispheric tip of the rod. Observed light emission seems to be evenly distributed along the hemispheric tip comprising a number of independent discharges similar to discharges in a homogeneous electrode configuration. Applying a conductive layer at the rod tip reduces the discharge activity and measured discharge current considerably. The discharge activity was concentrated close to the tip end of the rod. Relaxation time constant for the deposited charge was measured to be about 4 h. By comparing conductivity of the insulation material, the dominating relaxation mechanism was found to be conduction through the solid insulation.

Electrical treeing caused by rapid DC-voltage grounding of XLPE cable insulation

This report presents results from an experimental investigation using Rogowski shaped test objects with needle implants, either fully insulated or in contact with one of the electrodes. In both cases abrupt grounding caused electric breakdown to be initiated at a voltage much lower than the short term breakdown voltage. Reduced breakdown strength occurred due to increased DC pre-stress level, number of rapid voltage groundings and rate of voltage reduction. The observations are explained by the concept of a field limited space charge injection (FLSC). When slowly increasing the applied voltage a cloud of homo charge is likely to be injected into the high field region around the needle tip resulting in reduced local electric stress preventing electrical tree formation. In case of rapid voltage grounding the injected charge will move in the opposite direction, towards the tip electrode. Due to the high local field enhancement, increased electric conductivity, high dissipated power and electromechanical forces may cause electrical treeing and breakdown

Effect of water content on the conductivity of XLPE insulation

In HVDC cable insulation, the steady state electric field distribution is mainly determined by the following factors: DC conductivity, charge trapping, electron injection and ion formation. Factors which contribute strongly depend upon changes in morphology and chemical features within the insulation. In case of polymer insulated HVDC cables, nonuniform conductivity due to for example temperature gradients in the material will result in space charge formation and can yield an electric field distribution different from what will be expected in case of application of AC voltage. It is expected that the absorption of water will increase the formation of ions and thus increase the conductivity of XLPE insulation. The goal of this paper is to present results from conductivity measurements on XLPE cable insulation with varying water content. Measurements of charging and discharging currents were performed at applied electric stress in the range of 2-20 kV/mm, using Rogowski type test objects equipped with semiconducting polymeric electrodes. The measurements were carried out at temperatures of 40, 60 and 80 °C, at different relative humidities in the range from 10 % to 90 %. The test objects were conditioned in a climate chamber kept at the selected relative humidity and temperature of the subsequent measurement for up to 2 days prior to the conductivity measurements, thus ensuring that equilibrium water content was reached before the measurement started. In order to remove remnant space charge between each test, the test objects were kept grounded for a time 10 times longer than the high voltage charging time. The results show that an increase in water content increases the conductivity of the XLPE insulation. At the highest water content for a given temperature the conductivity was 1.8-3.4 times larger than at the lowest water content. The temperature- and field-dependency of the conductivity did not significantly change with increasing water content. The conductivity was found to increase proportionally to the square root of the water content, indicating dissociation of water as the origin of the increase.

Conduction and partial discharge activity in HVDC cable insulation of lapped polypropylene films

The purpose of the work presented here is to characterise polymeric HVDC cable insulation consisting of lapped thin film polypropylene with respect to DC current conduction and partial discharge activity in air filled butt gaps. The PD activity was found to strongly increase with increasing temperature and applied electric stress. The discharge frequency decreased with time after voltage application and the effect of the void size was found to be of minor importance. These observations agree very well with the proposed theory stating that the discharge frequency will be proportional to the current flowing through the insulation.

The effect of DC superimposed AC voltage on partial discharges in dielectric bounded cavities

Voltage source converters is used in HVDC stations in offshore HVDC transmission systems, between the AC and DC power grid. The AC ripple voltage on the DC side of the HVDC stations can be in the range of 1-10 % of the nominal DC voltage, depending on the size of the filter employed. For offshore HVDC grids, there is a drive to use polymeric insulated cables on the DC side. This work investigates how an AC voltage at power frequency superimposed on DC voltage influence the partial discharge magnitude and repetition rate in artificial cylindrical cavities in polymeric insulation. The AC voltage is kept below the AC partial discharge extinction voltage, and the DC voltage is kept above the DC partial discharge inception voltage. A resistor-capacitor ABC-circuit model is used for prediction of partial discharge magnitude and repetition rate under combined AC and DC voltage. Measurements has been performed on a test object of 3 layers of PET film with 1 mm radius cylindrical cavity in the middle layer. The results indicate that an AC voltage ripple with an amplitude lower than the AC partial discharge extinction voltage will increase the number of large discharges, compared to a DC voltage without ripple, but the repetition rate will be several orders lower than the AC voltage frequency.

Breakdown strength of solid|solid interface

Interfaces between solids are generally considered weak regions in electrical insulation systems. This is particularly so if the electrical stress is applied parallel to the interface. Important parameters, affecting the breakdown strength, are interface pressure, humidity, presence of liquid dielectric and the surface roughness of the solid in contact. The main aim of the work presented here is to examine, theoretically and experimentally, the effect of interfacial pressure and roughness on the tangential breakdown strength. The size and gas pressure of enclosed surface voids were estimated using mechanical contact theory. The dielectric 50 Hz AC tangential strength of XLPE|XLPE interface was investigated for various values of pressure and roughness. The increase in breakdown strength due to increased pressure was largest in case of surfaces with the high degree of roughness. As expected the highest breakdown strength was observed in case of the smoothest surfaces. The estimated results of void size and gas pressure were found to be in good agreement with the experimental observations.

Factors influencing the tangential AC breakdown strength of solid-solid interfaces

The combination of two solid dielectrics (interface) increases the risk of formation of microscopic cavities reducing the breakdown strength (BDS) of the interface considerably, particularly when the electric field has a tangential component. The main purpose of this paper is to investigate the impact of the applied contact pressure and composite elastic modulus on the tangential ac BDS of the solid-solid interfaces experimentally. In the experiments, three different contact pressures were applied using different mechanical loads with two different materials having different elastic moduli, i.e. cross-linked polyethylene (XLPE) and silicon rubber (SiR). Two rectangular prism shaped samples were placed between two vertical Rogowski shaped electrodes either in air or oil. The type of the interface (air/oil) is highlighted duly upon showing the results. Increase in contact pressure caused relatively higher increase in the tangential BDS of dry SiR-SiR (assembled in air) than that of XLPE-XLPE, revealing that elastic modulus facilitated significantly to reduce the mean void size in SiR that in turn improved the tangential BDS. Likewise, the tangential BDS of hybrid interfaces formed by XLPE-SiR specimens increased by 43% compared to that of XLPE-XLPE interface at the same pressure. Additionally, the same set of experiments assembled in oil reveals that the presence of oil enhanced the tangential BDSs around 2-3 times for all three-interface cases. Moreover, with the increase of applied pressure the tangential BDS of air-filled and oil-filled cavities tended to get significantly higher.