A novel control strategy combined by both of energy optimal and time optimal control for voltage sag compensator

Voltage sags are an important power quality problem. Enhancing compensating capability is the key technique in voltage sag compensators. In this paper, a transformer-less series voltage sag topology without energy storage capacitors is proposed first. This topology is cost-effective by eliminating the large injection transformer and energy storage capacitors that are used in conventional series injection devices. This topology can both be used in three-phase three-line and three-phase four-line systems, and can compensate symmetrical three-phase voltage sags down to 37%, or one or two phase sags down to zero if others at least one phase is rated. Secondly, this paper proposes and illustrates a new compensation control strategy which combines energy optimal control with time optimal control to enlarge the compensating time and draw a minimum amount of energy from the compensator during sags. And then, an asymptotically angle rotation method is proposed to avoid sudden phase jump of the load-side voltage. Simulation results demonstrate the good features.