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This paper discusses results of an EPRI contract on control design for an ac/dc transmission system to increase power transfers limited by dynamic stability. The control is acbieved by modulating the rectifier dc current or power and the inverter dc voltage. Coordinated active and reactive power modulation is accomplished by Linear Quadratic (LQ) control design. Improved performance is realized by utilizing reactive power coupling between dc and ac networks to modulate dc and ac active power. For the demonstration system, power modulation with coordinated voltage modulation is shown to be about ten times more effective than power modulation alone. Control interactions between electromechanical modes of oscillation are eliminated by Kalman filtering. Filtering permits excellent control performance with a minimum of system measurements. Modern control technology is applied within realistic system constraints, such as measurements.