We develop new optimization methodology for planning installation of Flexible Alternating Current Transmission System (FACTS) devices of the parallel and shunt types into large power transmission systems, which allows to delay or avoid installations of generally much more expensive power lines. Our methodology takes as an input projected economic development, expressed through a paced growth of the system loads, as well as uncertainties, expressed through multiple scenarios of the growth. We price new devices according to their capacities. Installation cost contributes to the optimization objective in combination with the cost of operations integrated over time and averaged over the scenarios. The multi-stage (-time-frame) optimization aims to achieve a gradual distribution of new resources in space and time. Constraints on the investment budget, or equivalently constraint on building capacity, is introduced at each time frame. Our approach adjusts operationally not only newly installed FACTS devices but also other already existing flexible degrees of freedom. This complex optimization problem is stated using the most general AC Power Flows. Non-linear, non-convex, multiple-scenario and multi-time-frame optimization is resolved via efficient heuristics, consisting of a sequence of alternating Linear Programmings or Quadratic Programmings (depending on the operational cost dependence on the power injected by the generators) and AC-PF solution steps designed to maintain operational feasibility for all scenarios. Computational scalability and other benefits of the newly developed approach are illustrated on the example of the 2736-nodes large Polish system. One most important advantage of the framework is that the optimal capacity of FACTS is build up gradually at each time frame in a limited number of locations, thus allowing to prepare the system better for possible congestion due to future economic and other uncertainties. NOMENCLATURE Parameters: N l Number of power lines in operation N b Number of buses in the system M Number of loading scenarios representing given time frame N Number of scenarios representing planning horizon T Number of time frames representing horizon t = 1..T Index of a decision point a = 1..M Index of a scenario at time frame t P r t,aOccurrence probability of a scenario a at time frame t x 0 ∈ R N l Vector of initial line inductancesVector of line apparent power limitsVector of maximum (minimum) allowed voltages C SC ∈ R Cost per Ohm of a series FACTS device C SV C ∈ R Cost per MVAr of a shunt FACTS device N years ∈ R Planning horizon Optimization variables (operational, scenario dependent): V ∈ R N b Vector of bus voltage magnitudes θ ∈ R N b Vector of bus voltage angles P G ∈ R N b Vector of generator active power injections Q G ∈ R N b Vector of generator reactive power injections x ∈ R N l Vector of line inductances modified by SC devices ∆x ∈ R N l Vector of series FACTS settings ∆Q ∈ R N b Vector of shunt FACTS settings Optimization variables (investment, scenario independent): ∆x t...