The large scale applications of power electronics converters will bring more and more new features into power systems, e.g. smaller short circuit capacity, lower inertia, faster controllable reactive power etc. Consequently, two important challenges are the reactive power – voltage control and system protection which would be different from the system dominated by conventional synchronous generators. Moreover, most of the Danish power system is expected to change from overhead lines based grid into underground-cable (UC)-based systems, which will also drastically change the reactive power characteristics and grid short circuit power. Clearly, the PE converters replacing synchronous generators in a cable dominated power grid will seriously challenge the performance of the existing power system voltage control and protection practice. This project will address the above mentioned challenges by conducting the reactive power characteristics analysis, optimal reactive compensation planning/operation, investigation of protection performance and development of new protection methods for the Danish power systems to meet the demand of the transition towards the power electronics and underground cable dominated system in order to guarantee the security of Danish power system in the transition period and in the future.