Comparative steady-state performance of crossbonded cable systems

Abstract

The paper gives a universal system of steady-state analysis for any crossbonded and/or transposed cable system with unequal lays. It first introduces a nomenclature which enables any conductor or sheath to be identified in geometric and/or longitudinal position in any lay and/or section both for the general case and for four specific cases of crossbonding. It is then shown that the effect of unequal lays on sheath voltages, currents and losses may always be analysed, provided that there is rigorous adherence to the system of nomenclature. The paper is restricted to cases of unbalanced conductor currents in the steady state and does not deal with the additional variables and complexities when the various crossbonded systems have multiple-earthed points and are under external or internal unbalanced fault conditions. A large selection of graphical results is presented for sheath currents, losses and longitudinal sheath voltages of the three main cases of crossbonding. In the particular case of trefoil spacing, taken as an example, the influence of variation of lay length between 0.6 and 1.4 p.u. on the sheath currents and longitudinal induced voltages is shown. The results are discussed in detail since comparative performances for these various systems have not previously been presented. The conclusions extend also the analytical methods, it being shown that all crossbonded systems of whatever complexity can be analysed by the use of rotation matrices and the notation for nonuniform lay length; it follows en passant that normal symmetrical-component methods do not yield a solution, owing to the crossbonded systems having multiple unbalances and six degrees of freedom.