Contents I. Introduction
In today's power grids, many transient power quality problems appear. Traditional regulating devices do not have very good effects on this. A power quality regulating device that can continuously regulate and respond quickly is required. The unified power quality regulator is an ideal device. In 1996, Japanese scholar H.AKAGI first proposed the concept of unified power quality conditioner (UPQC) [1]. UPQC is composed of series and parallel parts and DC side, which can simultaneously control voltage drops and harmonics. The research on UPQC mainly focused on the structure, control method. Literature [2] proposed a new type of UPQC topology using series double converters and three single-phase coupled transformers. The three transformers are not directly connected on the changing side. The UPQC of this structure can compensate for the positive, negative, and zero sequence at the same time. The voltage harmonic components can improve the safety and reliability of the system. The UPQC introduced in literature [3] adopts a modular multi-level converter structure, which can improve the voltage temporary suppression capability. The UPQC topology proposed in literature [4] is based on photovoltaic integration and adopts a notch filter phase-locked loop control algorithm to correctly detect and eliminate voltage and current disturbances. When the grid voltage distortion is severe, it can avoid Zero crossings. The UPQC structure proposed in literature [5] reduces the switch topology. This structure only uses 10 switches, and at the same time minimizes its utilization rate without increasing the switch VA rating. The UPQC of literature [6] is a four-linear structure with the bridge arm as the neutral point. Literature [7] is a research on traditional PI control. Literature [8] introduced a power angle control method based on a synchronous reference frame, which uses the load current after decoupling to compensate the load voltage and reduce the amount of extra calculations.
