This graduate thesis was made for the purpose of acquainting the reader with the problem of protection of high-voltage lines, that is, lines at the voltage level of 400 kV. The work consist of six basic units which are analyzed in following chapters: „400 kV network concept in Croatia“, „Principles of protection of 400 kV overhead lines“, „Distance protection of high-voltage overhead lines“, „APU- Automatic reclosing“, „Estimation of transmission line condition using synchronized Phasor measurement technology – WAM technology“, „WAMPAC system“. The „400 kV network concept in Croatia“ whole describes the condition and concept of high-voltage transmission lines in Croatia. „ Principes of protection of 400 kV overhead lines“ is representing systems protection of high-voltage transmission lines in Croatia. The „Distance protection of high-voltage overhead lines“ chapter is describing the technology of distance protection and all the reasons for it's application on transmission line protection. „APU- Automatic reclosing“ whole is explaining the „APU“ technology and the reasons for it's application in the power system are being highlighted. Next comes the „ Estimation of transmission line condition using synchronized Phasor measurement-WAM technology“ in which the technology of synchronized Phasor measurement is presented. The final „WAMPAC system“ is telling the reader above the possibilities of upgrading the WAM system with protraction and management functions. Future direction of development of the system mentioned above is also represented.