A stochastic model for contact surfaces at polymer interfaces subjected to an electrical field

Morphology of the contact area between solid insulation materials ultimately determines the short- and long-term electrical properties of the complete insulation system. The main purpose of this paper is to propose a statistical model to examine the real area of contact between solid dielectric surfaces and secondly to verify and correlate the model outputs with experiments. The model computes the real area of contact, number of contact spots and average cavity size at the interface as a function of elasticity, contact force, and surface roughness. Then, using the average cavity size and the Paschen's law, the discharge inception field of the cavity (CDIE) is estimated. AC breakdown strength (BDS) testing of solid-solid interfaces was carried out, where cross-linked polyethylene (XLPE) samples with four different surface roughnesses were tested at various contact pressures. Following the increased contact force, the calculated average cavity size decreased by a factor of 4.08-4.82 from the roughest to the smoothest surface, corresponding to increased CDIEs by a factor of 2.01-2.56. Likewise, the experimentally obtained BDS values augmented by a factor of 1.4-1.7 when the contact pressure was elevated from 0.5 MPa to 1.16 MPa. A linear correlation between the CDIE and BDS was assumed, yielding a correlation coefficient varying within 0.8-1.3. When the 90% confidence intervals were considered, the range reduced to 0.86-1.05. This correlation suggests that interfacial breakdown phenomenon is strongly related to the interfacial cavity discharge. Hence, the proposed model is verified with experiments.A stochastic model for contact surfaces at polymer interfaces subjected to an electrical fieldsubmittedVersio

Full-scale case study of a road crossing thermal bottleneck in buried MV cable installation

Infrastructures such as road crossings, are often limiting the ampacity of a cable link due to poor thermal properties. In order to fully utilize the true loading capacity of the cable, thermal modelling with correct input parameters should be applied. In this work, a full scale artificial road crossing have been constructed and characterized, and temperature measurements have been compared with calculations performed according to IEC 60287 and finite-element analysis (FEA). It was found that calculations according to IEC provide a good fit to measured temperature profiles by adjusting the unknown mean thermal resistivity of the material layers outside the pipe. The thermal properties of road construction materials can be challenging to measure, and in order to provide accurate ampacity calculations further work should be focused on this.publishedVersio

Experiences With cable faults located at metallic screen connections

Overheating of metallic ground screen connections at accessories of single core cables are the root cause of severe insulation failures in the distribution network. The overheating is due to a high transition resistance of the connections combined with high induced currents in the ground screens during operation. For copper screened cables equipped with a metallic laminate, overheating causing failures are also observed randomly along the cables often at locations close to cable straps or cleats. Currently there are no international standards with complete tests to determine the ampacity of ground screen connections for different cable system designs making future installations more reliable. Furthermore, product information from the suppliers indicates that basis for design and test criteria is not uniform, and tests are not performed concerning relevant operation and fault scenarios. A research project is launched in Norway, to test different ground screen connections and then to provide guidelines to ensure proper future installations. A new CIRED working group is proposed to provide recommendations for complete tests of ground screen connections lacking in current international standards and brochures.publishedVersio

Effect of Material Elasticity on the Longitudinal AC Breakdown Strength of Solid-Solid Interfaces

acceptedVersio

Effects of Frequency and Temperature on Partial Discharge Characterization of Stator Windings

acceptedVersio

Condition Assessment of Hydrogenerator Stator Bar Insulation Using Partial Discharge Measurements

This paper presents results from laboratory measurements of partial discharge (PD) activity in 50 cm long samples cut from the mainwall section of old hydrogenerator stator bars. All stator bars were manufactured in 1976 and samples were taken after 35 years in service from both the low and high voltage sections of the generator, as well as non-energized back-up bars. The PD activity, using a phase resolved (PRPDA) measuring system, was investigated at different test voltages up to 9.6 kV (1.5 U 0 ), frequencies and temperatures in the range 20–155 °C and 0.1-50 Hz, respectively. The service-aged and the unaged reference samples showed a clear difference in voltage frequency dependence. It was, however, not possible to distinguish between service-aged bars from high and low electric stress. The observed frequency and temperature dependences are discussed with respect to theoretical assumptions regarding possible void degradation and surface conductivity.Condition Assessment of Hydrogenerator Stator Bar Insulation Using Partial Discharge MeasurementsacceptedVersio

A Deterministic Breakdown Model for Dielectric Interfaces Subjected to Tangential Electric Field

The contact area between solid insulations, namely solid-solid interfaces, affect the dielectric characteristics of an entire insulation system. With the theoretical and experimental studies covered in this paper, we intend to investigate the effects of the elastic modulus, interface contact pressure, and surface smoothness/roughness on the tangential AC breakdown strength (BDS) of contact surfaces at dielectric interfaces that undergo tangential electric stress. Four distinct solid dielectric specimens with various elastic modulus values were employed. The interfaces between the same materials were subjected to AC breakdown and partial discharge (PD) detection tests at different contact pressure values. In addition, the interface surfaces were polished using four different sandpapers of various grits to scrutinize the effect of surface roughness. A deterministic model built around the contact mechanics of solid surfaces was proposed to determine the sizes of the interfacial cavities and to simulate the 3D displacement of the surface protrusions based on the surface roughness, contact pressure, elastic modulus, and hardness of an interface. The estimated sizes of cavities and contact areas were then coupled with a comprehensive breakdown model that addressed cavity discharge and breakdown of contact areas, separately. The results by the model were correlated with the results of the AC breakdown and PD experiments to elucidate not only how cavities were linked at solid-solid interfaces but also the effects of roughness, elasticity, contact pressure, and gas pressure inside cavities.acceptedVersio

A Deterministic Breakdown Model for Dielectric Interfaces Subjected to Tangential Electric Field

The contact area between solid insulations, namely solid-solid interfaces, affect the dielectric characteristics of an entire insulation system. With the theoretical and experimental studies covered in this paper, we intend to investigate the effects of the elastic modulus, interface contact pressure, and surface smoothness/roughness on the tangential AC breakdown strength (BDS) of contact surfaces at dielectric interfaces that undergo tangential electric stress. Four distinct solid dielectric specimens with various elastic modulus values were employed. The interfaces between the same materials were subjected to AC breakdown and partial discharge (PD) detection tests at different contact pressure values. In addition, the interface surfaces were polished using four different sandpapers of various grits to scrutinize the effect of surface roughness. A deterministic model built around the contact mechanics of solid surfaces was proposed to determine the sizes of the interfacial cavities and to simulate the 3D displacement of the surface protrusions based on the surface roughness, contact pressure, elastic modulus, and hardness of an interface. The estimated sizes of cavities and contact areas were then coupled with a comprehensive breakdown model that addressed cavity discharge and breakdown of contact areas, separately. The results by the model were correlated with the results of the AC breakdown and PD experiments to elucidate not only how cavities were linked at solid-solid interfaces but also the effects of roughness, elasticity, contact pressure, and gas pressure inside cavities

Norsk landbruksforskning : ekstra vitamintilskudd til handelsfôrblandinger for høns

Inneholder flere artiklerEffekten av ekstra tilsetning av fettløselige og/ eller vannløselige vitaminer til kommersielle fôrblandiner til høns ble undersøkt i to forsøk. Det ene omfattet bare verpeperioden, mens det andre omfattet både oppals- og verpeperioden. Det ble ikke funnet signifikante utslag i vektøkning, eggproduksjon, fôrutnyttelse, eggkvalitet, rugeresultater eller helsetilstanden. Resultatene tyder derfor ikke på at det er behov for ekstra tilskudd av vitaminer til de kommersielle forblandingene som ble brukt i denne undersøkelsen. Disse blandingene inneholdt imidlertid noe mer vitaminer enn forutsatt

Norsk landbruksforskning : ekstra vitamintilskudd til handelsfôrblandinger for høns

Effekten av ekstra tilsetning av fettløselige og/ eller vannløselige vitaminer til kommersielle fôrblandiner til høns ble undersøkt i to forsøk. Det ene omfattet bare verpeperioden, mens det andre omfattet både oppals- og verpeperioden. Det ble ikke funnet signifikante utslag i vektøkning, eggproduksjon, fôrutnyttelse, eggkvalitet, rugeresultater eller helsetilstanden. Resultatene tyder derfor ikke på at det er behov for ekstra tilskudd av vitaminer til de kommersielle forblandingene som ble brukt i denne undersøkelsen. Disse blandingene inneholdt imidlertid noe mer vitaminer enn forutsatt