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Abstract-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 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.

In ThIs arTIcle, The sTaTe of The arT In the variable-speed operation of hydropower plants is reviewed, with a focus on pumped-storage hydropower. relevant literature is reviewed to address the benefits of variable-speed operation for both power systems and hydropower facilities. Two main configurations to enable variable-speed operation-the doubly fed induction machine (DfIM) and the converter-fed synchronous machine (cfsM)are discussed and compared. The article addresses the technology and energy policies of the past, present, and future and points out how the motivation, services, value, and technology of variable-speed hydropower plants have been subject to considerable change.

In ThIs arTIcle, The sTaTe of The arT In the variable-speed operation of hydropower plants is reviewed, with a focus on pumped-storage hydropower. relevant literature is reviewed to address the benefits of variable-speed operation for both power systems and hydropower facilities. Two main configurations to enable variable-speed operation-the doubly fed induction machine (DfIM) and the converter-fed synchronous machine (cfsM)are discussed and compared. The article addresses the technology and energy policies of the past, present, and future and points out how the motivation, services, value, and technology of variable-speed hydropower plants have been subject to considerable change.

In this paper, the influence of slot harmonics on magnetic forces and vibration is studied in a 120-slot/116-pole low-speed PM machine at no-load. It is shown how the lowest mode of vibration is produced at no-load due to slotting. Comparing the cases of open slots, semi-closed slots and magnetic wedges, the effect of slot closure on radial forces and torque production capability is discussed. Magnetic flux distribution in the airgap is computed using finite element analysis. Spatial harmonics due to slotting are investigated in different cases. Maxwell's stress tensor is employed to calculate radial and tangential components of the force density in the airgap. Spatial distribution of the total forces on the teeth and also time-dependent force waveform on one tooth are analyzed and discussed for different cases. It is shown how the magnitude of the lowest mode of vibration is reduced in the case of using semi-closed slots and magnetic wedges. Tangential force density distribution and torque production capability are also discussed. Structural analysis is presented to compute the maximum amplitude of the stator deformations due to the radial forces. Experimental results of the prototype generator are presented verifying the existence of the lowest mode of vibration at no-load because of the slot harmonics.

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