Induced voltages in the sheaths of crossbonded a.c. cables

When groups of single-conductor concentric cables are used in power systems, it is necessary both to bond the sheaths together at the various joint shells and to earth them at intervals to ensure that there are no large potentials present on them. This arrangement leads to large circulating sheath currents and losses. Various arrangements have been devised to eliminate the sheath losses, the one which is probably most in favour in Britain being the Kirke-Searing scheme. In this scheme, the cable conductors are transposed at regular intervals along a run, and the sheaths are bonded and transposed at the same intervals. While the arrangement eliminates the circulating currents and losses under balanced conditions, it does not prevent relatively large potentials from being present at intermediate points, and it causes appreciable voltages to be present across the insulators inserted in the sheaths at the joints. It is of interest to know the magnitudes of the sheath voltages which are likely to be present in the absence of voltage limiting devices. Some analytical work has been done on this subject for both balanced and unbalanced steady-state conditions, and there are also some experimental results relating to transient conditions. There is, however, little analytical work available on the latter topic. This paper extends the analysis of steady-state conditions by the use of symmetrical-component techniques, and explains the underlying reasons for the high transient sheath voltages which have been detected in the past.