Publication Detail

Investigating the influence of zero sequence impedance in ship ungrounded electric networks

J. M. Prousalidis
2010
14 pp.

In shipboard installations ungrounded earthing is selected because it provides continuous power supply, even in the case of single-phase earth faults, which cannot be accomplished by low resistance earthing in the neutral point of wye-connected generator or transformer windings. However, the electric network is actually earthed via the high impedance capacitances of all the equipment installed onboard. Therefore, as already discussed in literature, the 'ungrounded' earthing type of shipboard installations has to be treated as 'grounded' via capacitive elements. This paper aims to show, for the first time, an analytical description of statements given in previous publications, ie, that in all analyses performed, it is the entire zero sequence (homopolar) impedance that has to be taken into account. This homopolar impedance includes, among others, leakage capacitances of the entire electrical installation. Therefore, an investigation of the influence of the zero sequence impedance is performed in representative case studies, all referring to unbalanced operating conditions. Within this frame, analytical expressions of voltages and currents with respect to zero sequence impedance are yielded. Moreover, the significant adverse effects of leakage capacitances on the occasion of an actual ship earthing network, are highlighted.

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