| In electric power delivery systems, engineers need to properly design the protecting devices based on knowledge of the fault current. For an accurate calculation of the fault current, it is necessary to know the zero-sequence impedance of the underground pipe-type cables. In this dissertation, we present an improved method for computing the zero-sequence impedance. A set of integro-differential equations is formulated, and a finite element solution technique is developed to solve the problem. Making use of a properly set reference of the magnetic vector potential, the homogeneous Dirichlet boundary condition is implemented in the finite element solution procedure. In the development of the numerical technique, special attention is paid to the nonlinear magnetic characteristic of the steel pipe. Both Fourier series fitting and cubic spline interpolation are considered to represent the B-H curve of the steel pipe. In this work, the Fourier series fitting technique is chosen and incorporated with an iterative solution procedure, which is developed together with the finite element solution technique to determine the permeability of the pipe. To validate the numerical technique developed, the numerical results are compared with measured data. In addition, numerical tests are performed to show the stability of the finite element solution technique.; As a byproduct of this work, the magnetic field in the region exterior to the pipe-type cable is investigated. An equivalent model valid in the region exterior to the pipe-type cable is constructed. The computational results of the magnetic field are compared with the existing measurement data to validate this method. |
