No-load energization of very long EHV mixed overhead-cable lines

Recently, in many countries EHV AC cable lines are used as means to overcome the difficulties in construction of new transmission lines. However, the coexistence of overhead and cable lines in transmission systems, in the so called “mixed” lines, raises a number of issues regarding the behaviour of such systems both in transient and steady-state. In particular, with the increment of the operating voltage of transmission systems, the behaviour of such mixed systems during switching operation becomes of special interest. The purpose of this study is to investigate the no-load energization of very long EHV mixed lines in two main configurations, both characterized by an high difference in the lengths of the first and the last stretch. The work addresses this issue from two different aspects: the switching transient and the quasi-steady state condition. The analysis will show the suitability of those configurations which have the majority of cable away from switching end, even if some important observations in transient analysis will be pointed out, especially regarding the “best end” switching issue: in particular, difference in length between the first and the last stretch affects the multiple reflections and the equivalent two-port circuit constants, then significantly influencing the opened end overvoltages.