Signal Transmission on Power Lines.: Analysis of Power Line Circuits

In the analysis of power-line networks over a large frequency span, the time-domain method used in programs like EMTP (Electromagnetic Time domain Program) can not be used. A more rigorous analysis method must be employed. The correct analysis method (assuming TEM-mode propagation) for multiple coupled transmission line circuits like power line circuits, is based on finding the eigenvalues and eigenvectors for the matrix ZY, where Z and Y are the impedance- and admittance matrices for the coupled lines in W/m and S/m.

In this paper a flow-graph representation of the relations beween voltage- and current-vectors is presented for a general coupled line circuit. This flow-graph can be used to find any transfer function on a multiple coupled lossy transmission line circuit. In some cases, however, it is difficult to find the eigenvalues correctly.

A correct analysis can thus not be made. An approximate analysis method must be usedIn this paper it is shown that an eigenvectormatrix, that is the propagation modes, can be chosen almost arbitrarily if the frequency interval of interest lies below the frequency, where the line length is about 1/3 times the wavelength for the propagation mode with the lowest phase velocity.

The propagation constants corresponding to the chosen eigenvector matrix (the quasi modes) are found iteratively, which is a much simpler procedure than finding the eigenvalues.