Extruded high-voltage direct current cable systems transmit electric power over long distances. Numerical field simulation can provide access to the internal electrothermal behavior of cable joints, which interconnect cable sections. However, coupled nonlinear electrothermal field simulations are still a challenge. In this work, a robust numerical solution approach is implemented and validated. This approach allows for efficient parameter studies of resistively graded high-voltage direct current cable joint designs. It is assessed how the dielectric stress distribution between the conductor connection and the grounded cable sheath is influenced by nonlinear field and temperature dependent electric conductivity of the field grading material. Optimal field grading material parameters, which fulfill the field grading and power loss requirements, are suggested based on the simulation studies.

Ensuring thermal stability is an important aspect of an arrester design process. In this paper, the thermal stability of station class surge arresters is investigated using a coupled electrothermal numerical model. An ungraded 550-kV-station class arrester model is successfully validated against temperature measurements in power grid operation. The arrester cooling rate is introduced to determine the thermal stability threshold. Thermally stable and unstable scenarios are compared. The influence of selected arrester design parameters on the stability threshold is analyzed. It is shown that an improved heat transfer in the arrester air gap increases the thermal stability threshold significantly. Additionally, the nonlinear zinc oxide material characteristics influence the thermal stability threshold strongly. Furthermore, a thermal stability prediction approach based on few, computationally less expensive simulations is proposed. Finally, the results are confirmed for an arrester model that is equipped with a field-grading system. The paper shows that the presented field simulation approach can be reliably used to determine the stability threshold of an arrester, thus, reducing the need for costly laboratory tests.

An electrothermal simulation based procedure for the optimization of the field grading systems of station class surge arresters is presented. The main difficulty consists in the numerical solution of the three-dimensional and transient electric field problem that arises when a field grading system is introduced. Therefore, we first develop an equivalent axisymmetric arrester model that can reproduce the electric field stress in the resistor column with high accuracy. The model employs a special virtual electrode geometry whose shape and position are determined by a multi-parametric optimization procedure. On the basis thereof, a numerical optimization of the three-dimensional grading system for an arbitrarily large arrester can be efficiently performed based on transient, electroquasistatic simulations of its axisymmetric equivalent. This procedure is demonstrated for the standard IEC surge arrester. The detailed electrothermal analysis of this arrester shows that an immense improvement of the field and thermal stress distribution in the resistor column can be obtained.

Many optimization problems in electrical engineering consider a large number of design parameters. A sensitivity analysis identifies the design parameters with the strongest influence on the problem of interest. This paper introduces the adjoint variable method as an efficient approach to study sensitivities of nonlinear electroquasistatic problems in time domain. In contrast to the more common direct sensitivity method, the adjoint variable method has a computational cost nearly independent of the number of parameters. The method is applied to study the sensitivity of the field grading material parameters on the performance of a 320 kV cable joint specimen, which is modeled as a finite element nonlinear transient electroquasistatic problem. Special attention is paid to the treatment of quantities of interest, which are evaluated at specific points in time or space. It is shown that the method is a valuable tool to study this strongly nonlinear and highly transient technical example.

In conventional finite element simulations, foil windings with a thin foil and many turns require many mesh elements. This renders models quickly computationally infeasible. With the use of homogenization approaches, the finite element mesh does not need to resolve the small-scale structure of the foil winding domain. Present homogenization approaches take resistive and inductive effects into account. With an increase of the operation frequency of foil windings, however, capacitive effects between adjacent turns in the foil winding become relevant. This paper presents an extension to the standard foil winding model that covers the capacitive behavior of foil windings.