Study of overvoltages in high voltage cables with emphasis on sheath overvoltages

摘要

This thesis is concerned with the calculation of transient sheath overvoltages in power transmission cables. The main part of the work deals with the development of a time domain cable model for transient calculations, but substantial effort is also directed towards the calculation of the electrical parameters needed as input to the cable model. The first part of the thesis presents a time domain travelling wave model for transmission cables and lines. This model takes accurately care of the frequency dependence of the cable/line parameters, including the modal transformation matrix. The model can easily be interfaced to general time domain transient programs. The frequency dependence of the modal transformation matrix is taken care of by carrying out the time domain calculations in the actual phase domain, thereby avoiding the need for a transformation between phase quantities and modal quantities. The second part of the thesis deals with the calculation of frequency domain cable data, which are input to the cable model. Commonly used formulae assume the current distribution over the conductor surfaces to by cylindrically symmetric, and the cable sheaths are treated as tubular conductors. The effect of these assumptions on the resulting sheath overvoltages is investigated in the thesis. Calculated sheath overvoltages are compared to field test results when the frequency domain data are calculated using the above assumptions. The field test involves a system of 3 single core coaxial cables with sheaths constructed as a wire screen with counter helix. The cables are buried underground in a trefoil configuration. Calculations by the Finite Element Method are carried out in order to investigate the effect of the displaced currents (proximity effect) on the calculated sheath overvoltages. The effect of the circular current component in the wire sheaths (solenoid effect) is also studied. 51 refs., 108 figs., 4 tabs.