As part of transmission network expansion planning (TNEP), a technical and economical assessment of several planning alternatives must be performed in order to ensure fulfillment of the network security criteria and to estimate the alternatives' expected operating costs. This task requires performing load flow calculations for different operating points (OPs) of the power system. Due to the high computational burden, considering all possible OPs is simply not possible. As a consequence, only a set of representative OPs is usually taken into account. Mostworks in the TNEP focus on issues related to optimization algorithms and modeling, neglecting the selection process of the representative OPs. Furthermore, most works only consider a few OPs, providing little or no insight about the criteria used in the selection process or about the error made when evaluating planning alternatives using a limited number of OPs. In this work, a novel methodology for selecting representative OPs to consider within the TNEP is presented. The proposal pays special attention to critical situations, where the network security may be endangered. Furthermore, the methodology allows quantifying the error made when evaluating network operation using a limited number of representative OPs.