Static var compensators (SVCs) are used for voltage and reactive power control in power systems. In this paper, we will consider the structure of SVCs that mainly consists of Y -connection of mechanically-switched capacitor (MSC) and delta connection of thyrister-controlled reactor (TCR). First, the control model of SVC was established in this paper. Then, a novel optimal nonlinear voltage controller for SVCs is proposed. The proposed SVC voltage controller consists of a nonlinear function and a conventional PI controller. The improved simplex method (SPX) is presented to adjust and optimize the parameters of the nonlinear PI controller in real-time, and to make transient state response procedure of SVC optimum. The integration of time multiplied absolute error (ITAE) is adopted as the optimized objective function of SPX. Simulation and engineering application results show that the proposed voltage control method is able to track reference voltage value of SVC immediately. It also demonstrates that the whole SVC control system can synthetically compensate reactive power. In this paper, we will consider the structure of SVCs that mainly consists of Y -connection of mechanically-switched capacitor (MSC) and delta connection of thyrister- The proposed SVC voltage controller consists of a nonlinear function and a conventional PI controller. The improved SVC voltage controller consists of a conventional PI controller. simplex method (SPX) is presented to adjust and optimize the parameters of the nonlinear PI controller in real-time, and to make transient state response procedure of SVC optimum. The integration of time multiplied absolute error (ITAE) is adopted as the optimized objective function of SPX. Simulation and engineering application results show that the proposed voltage control method is able to track reference voltage value of SVC immediately. It also demo nstrates that the whole SVC control system can synthetically compensate reactive power.