Untersuchungen zum frequenzabhängigen Übertragungsverhalten von Energiekabeln
The dissertation describes the effects of operating frequency reduction on the transmission behaviour of power cables. Strongly reduced current losses, lowered induced voltages in the sheaths and lower charging currents offer new solutions for the transmission of energy by cables. The following facts can be obtained by reduction of the operating frequency: an increase of the current carrying capacity, an enlargement of the transmission distance, lower thermal stress of the cable and better efficiency of the power transmission. The above mentioned advantages are typical for direct current transmission systems, but can now also be achieved for usual alternating current cables by reduction of the operating frequency. The positive effects of frequency reduction are represented for different types of power cables. The second part of the thesis describes the transmission behaviour for high frequencies. Travelling electromagnetic waves in power cables are strongly influenced by the frequency- dependent attenuation. For many applications in overvoltage protection the knowledge of the attenuation is very important. Especially in long cable systems the attenuation can be so high that lightning or switching voltages will be reduced to an innocuous level. Another area is the Power Line Communication (PLC) which uses the energy power grid for signal transmission in a frequency range up to 30 MHz. Here an in-depth analysis of the attenuation of the transmission line is very important. Particularly the admittance of the cable, composed of insulating and semiconducting materials, is very important for the attenuation of high frequencies. Simulation results agree well with measurements at a 20-kV-XLPE cable, if the semiconducting layers between conductor and screen are thoroughly modeled. For the calculations, the material parameters of the commonly used carbon-polyethylene-compound and conductive paper layers were measured. Measurements at a test setup show the attenuation for several plies of conducting paper layers on the inner conductor of a coaxial pipe system. The effect of different numbers of plies on the impulse rise time were investigated.