For extra high voltage transmission lines, corona considerations result in the use of expanded ACSR, hollow copper conductors or bundle conductors. For one conductor per phase it may be desirable to design each phase separately particularly for the commonly used flat horizontal configuration, as the surface voltage gradient is higher for the centre conductor and lower for the outer conductors.
The expression for the surface voltage gradient is simple for the centre conductor, but not so for the outers. An approximate expression has been given which gives fairly accurate results. Keeping in view that many factors influencing corona are known approximately only, the approximation is good enough for practical purposes.
The unequal surface voltage gradient may mean using a smaller conductor on the outer phases than on the centre phase. In India, at its present state of development, it may be worthwhile to try out new ideas and designs which can result in economy, of course, without sacrificing the security or quality of service.
