Generation and Transmission Expansion Planning (GTEP) to achieve decarbonisation targets in the power sector requires installing relevant shares of production from Renewable Energy Sources (RES). GTEP optimization models determine expansion plans over a long-term horizon so as to minimize the total investment and operation cost. In the presence of high shares of non-dispatchable RES power plants, operation costs must be evaluated taking into account the different operating conditions due to the intermittency of RES power generations. In this work we consider a GTEP model for a power system based on thermal, solar and wind power generation and we introduce a novel approach to select representative days (RDs), discretized in hours, in order to obtain accurate estimates of the operational costs. The RDs are not connected, in order to get a decomposable problem. A procedure is introduced to assign the ON/OFF status of every thermal unit at the beginning of each RD. Numerical results on a test case of suitable dimension show that the proposed method provides expansion plans very close to those obtained by the complete hourly model, while dramatically reducing computational costs.