Analysis and design of a three-phase PLL structure for utility connected systems under distorted utility conditions

This paper presents a new phase detecting method by using a digital three-phase PLL (phase locked loop) system for utility- connected devices under distorted utility conditions. The PLL systems have been used in several equipment applications such as, UPS systems, active filters, PWM rectifiers, etc, in which phase angle Information of utility voltage is necessary. The PLL system proposed in this work is based on instantaneous power theory. The analysis is performed considering a three-phase stationary reference frame model, which uses the instantaneous active power as the PLL reference. To detect the positive sequence component of the voltage in an unbalance there-phase system, the method of symmetrical coordinates is used. To validate the theoretical analysis a model of the PLL system is presented and its dynamic response, obtained from digital simulation, is discussed considering distorted voltage conditions. Mathematical analysis of the PLL system is made and a design procedure to determine the controller gains is presented.