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Based on quasi-steady-state model, the coordinated voltage control problem was represented by an optimization objective subjected to differential-algebraic equations with continuous-discrete time variables. A direct dynamic optimization approach from modern control theories was introduced to solve the dynamic optimization problems. The dynamic optimization problem can be described as nonlinear programming by approximating state variable, algebraic variable and control variable profiles by a family of polynomials on finite time intervals. Considering the discrete control characteristics of the ratios of transformers with on-load tap changers, shunt capacitors and load shedding, a quadratic penalty function was employed to handle corresponding discrete variables. The primal-dual interior point (IP) strategy was applied to solve this nonlinear programming model. The simulation results on New England 10-machine 39-bus system show that the proposed approach can determine effective controls to largely enhance long-term voltage stability of power systems.