A finite-element method solution of the zero-sequence impedance of underground pipe-type cable

In order to provide adequate circuit protection for underground pipe-type cable systems from ground fault currents, it is important to be able to determine the zero-sequence impedance of pipe-type cables. For better knowledge of the impedance, a more accurate method needs to be developed. In this paper, we present a numerical method for computing the zero-sequence impedance of a pipe-type cable. This method is developed based on a finite-element analysis. Special attention is paid to the nonlinear B-H characteristic of the steel pipe, and an iterative procedure is employed for determining the permeability varying in the steel pipe. To validate the numerical method presented, measurements are made for the zero-sequence impedance at different current levels. A good agreement is observed between the numerical results and the measurement data.