Earth Faults in Extensive Cable Networks
Electric Distribution Systems
Licentiate Thesis Department of Measurement Technology and Industrial Electrical Engineering
Department of Measurement Technology and Industrial Electrical Engineering Faculty of Engineering Lund University Box 118 221 00 LUND SWEDEN http://www.iea.lth.se
CODEN: LUTEDX/(TEIE-1057)/1-129/(2009) © Anna Guldbrand, 2009 Printed in Sweden by Media-Tryck, Lund University Lund 2009
The amount of underground cable in the Swedish rural distribution systems has increased considerably since the 2005 Gudrun hurricane. The resulting new rural networks combine the long line sections of the traditional rural networks that they replace, with the use of cable, common in urban networks. The introduction of long cables is a technology change and as such, it influences the distribution system earth fault behaviour. This work explains why the earth fault behaviour of electric distribution systems with long cables is different from that of conventional systems consisting of short cable feeders. The analysis in this work is carried out by use of circuit theory. The obtained results are compared to and found in accordance with time simulations. In terms of equivalent impedance and earth fault behaviour, the main difference between the new rural cable distribution systems and conventional urban systems, is that the zero sequence series impedance of the rural systems is not necessarily negligible. Since the zero sequence series impedance is partly resistive, the equivalent impedance of the system has a resistive component that cannot be compensated for by use of conventional resonance earthing. The zero sequence resistance damps the resonance of the system and by that influences the earth fault behaviour. The damping might result in large low-impedance fault currents and difficulties to detect high-impedance faults. The influence of the zero sequence series impedance on the equivalent impedance and the earth fault behaviour depends on the fault location. The zero sequence series parameters of cables are much different from those of overhead lines. Consequently, fundamental frequency and harmonic resonance is reached for considerable shorter cables than overhead lines. In addition, feeders that combine overhead lines and underground cables might give rise to series resonance. The zero sequence parameters of underground cables, and by that the earth fault behaviour, depend on the cables properties as well as the cable installation. One important finding of this work is that the zero sequence impedance is not necessarily proportional to the cable length. iii
Distributed compensation increases the shunt impedance of underground cables so that the influence of the series impedance decreases. If the local compensation coils are accurately dimensioned, the series impedance is negligible and does not contribute to the equivalent system resistance and the resonance damping. There are resistive losses in the local Petersen coil, which to some extent damp the resonance. The equivalent zero sequence resistance is however considerable smaller than that of systems with central compensation. Distributed compensation can thus be considered as an efficient way to make the earth fault behaviour of systems with long cables similar to that of traditional systems with short cables.
I am sincerely grateful for all the help and encouragement I have received while working on my research project. First and foremost, I would like to express my gratitude towards my supervisor Dr Olof Samuelsson for his help throughout this work. I am particularly grateful for (and impressed by) his ability to always give guidance and support based on my interests and needs. During this work, I have also had the privilege to receive guidance from Professor Sture Lindahl. I appreciate that Professor Lindahl has shared some of...
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