This paper investigates the role of fast-acting energy storage systems in transmission expansion planning, by allowing higher transfers through the network during normal operation. This is achieved by considering the ability of energy storage systems to provide real and reactive power reserves after forced single-circuit outages to prevent line overloading and voltage level violations in post-contingency states, and by applying the corrective N - 1 security criterion. A computational tool is presented to solve the multi-year transmission expansion problem with multiple scenarios of availability of renewable energy sources. The model is solved using the FICO Xpress software. The 2022-2037 Chilean transmission expansion plan is used as a case study, given the high need for flexibility to integrate 29.5 GW of new solar and wind generation capacity several hundred kilometers from its load center, with a system peak demand of 16.5 GW. The results obtained show that fast-acting energy storage systems reduce the cost of the investment plan by USD 712 million (-18%) mainly because it requires 5 GWh less of conventional storage capacity (-19%), allowing the system operator to increase the usage of the existing transmission network, and providing the central planner with a deferral option for the construction of new transmission lines.