Protection challenges for the overhead / underground / undersea double circuit 400 kV EHV GIS Aliveri - System transmission line

This paper presents the protection and exploitation related issues arisen at the cases of use of "mixed transmission circuits" in the power transmission systems. Under this term transmission corridors / circuits are meant, where overhead lines of different types, cables; underground or undersea type etc in series are erected. In this context nowadays and mainly because of environmental, ecological and financial resaons transmission circuits are not homogenous. For example, in order to avoid the annoyance and hence the reaction of the communities, transmission lines are constructed in a mixed manner. This has as a result various phenomena to appear and the task of their operation and protection for those circuits to be more complicated. The scope of this presentation is the recently installed 400 kV mixed circuit that connects the new Aliveri generating plant (GIS type substation) with the rest of transmission system. All relevant issues like type of protection, protection functions applied, fast clearing of cable faults, safe autoreclosing after automatic open of the circuit, fault positioning are analyzed and presented.