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• Integrated optimization of multi-stage expansion planning and grid operation. • Selection of expansion candidates via linear programming relaxations. • Optimizing the grid operation reduces the need for network expansion. • Need for new lines is substituted by the placement of phase shifting transformers. • Minimization of expansion costs by dynamic multi-stage expansion planning. The ongoing transformation of the power system characterized by an increased share of renewable energy sources requires additional transmission capacities. Nevertheless, the need for new transmission capacities can be reduced by optimizing grid operation. This contribution presents a novel approach to the co-optimization of multi-stage transmission expansion planning and grid operation within a mixed-integer linear programming formulation. The expansion via AC systems and phase shifting transformers (PSTs) is combined with the control of PSTs and the determination of congestion management interventions such as redispatch of power plants or curtailment of renewable energies. The number of potential expansion candidates is reduced by linear programming relaxations. Exemplary results based on a synthetic test system containing 120 buses evidence the impact of the proposed approach on overall system costs. The proposed co-optimization reduces the total costs by 16.5 % when applied instead of the sole expansion via AC technology. [ABSTRACT FROM AUTHOR]