Mr Google Fgt
Earth Faults in Extensive Cable Networks
Electric Distribution Systems
Anna Guldbrand
Licentiate Thesis Department of Measurement Technology and Industrial Electrical Engineering
2009
Department of Measurement Technology and Industrial Electrical Engineering Faculty of Engineering Lund University Box 118 221 00 LUND SWEDEN http://www.iea.lth.se
ISBN:978-91-88934-49-9
CODEN: LUTEDX/(TEIE-1057)/1-129/(2009) © Anna Guldbrand, 2009 Printed in Sweden by Media-Tryck, Lund University Lund 2009
Abstract
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
Electric Distribution Systems
Anna Guldbrand
Licentiate Thesis Department of Measurement Technology and Industrial Electrical Engineering
2009
Department of Measurement Technology and Industrial Electrical Engineering Faculty of Engineering Lund University Box 118 221 00 LUND SWEDEN http://www.iea.lth.se
ISBN:978-91-88934-49-9
CODEN: LUTEDX/(TEIE-1057)/1-129/(2009) © Anna Guldbrand, 2009 Printed in Sweden by Media-Tryck, Lund University Lund 2009
Abstract
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
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