Application of the direct Lyapunov method for optimum control of a UPFC

This paper presents a new control strategy for a UPFC to improve the transient stability of a power system. It is based on a Lyapunov energy function and uses global parameters that give a global optimum. Because present energy functions do not involve a UPFC´s actions, a new term of a structure-preserving energy function that represents a UPFC´s energy function was constructed. The maximum transient-stability improvement is achieved by setting the angle of the series-injected voltage to a value that maximizes the increase of the potential part of the energy function. The control strategy was tested in both longitudinal and IEEE nine-bus machine test systems. The results show that using the proposed control strategy a maximum transient stability can be achieved and that the angle of the series-injected voltage can change its value just a few times during the period of the first-swing angles´ propagation.