Construction times of new power plants are subject to uncertainty in deregulated electricity markets and they have an important impact on the planning of the expansion of the transmission network. In this paper, we develop a model of transmission expansion planning that is based on Robust optimization and considers the uncertainty in the instant when new power plants are connected into the electricity grid. Using this transmission expansion planning model, we assess the impact of postponing the connection time of some new power plants over the system cost and the optimal network expansion plan. By obtaining the optimal solutions of the proposed model for different levels of robustness, we are able to rank the new power plants with respect to how much the delay of each project negatively affects the system cost. We illustrate our methodology and the performance of two solution approaches proposed to solve our model by using a 34-node stylized version of the main Chilean grid. In the case of the main Chilean grid, the results obtained indicate that some of the projects whose delays negatively affect the most to the system cost are the renewable power plants projected for the next ten years. This is important from the regulator viewpoint because it allows the assessment of renewable energy policies that accelerate the installation of renewable new power plants also in terms of their impact over the transmission expansion planning.