Research On Distortion Detection And Tracking Control Strategy For Unified Power Quality Conditioner

With the progress of the times, power plays a more and more important role in social production and people’s daily life. With the growing use of diversified load, power quality problems caused by nonlinear load are increasing. At the same time, as the emerging of electronic equipment, people’s requirement of power quality is more and more strict so high quality of power has become the demand of the times. Thus, Unified Power Quality Conditioner (UPQC), which is able to handle a variety of power quality problems at the same time, has got more and more people’s attention and research.After a large number of relevant data and reference has been read and learned, many aspects about power quality have been introduced in detail. Firstly, this paper introduced the causes, types and harm of power quality problems and characteristics of power electronic devices, which can improve power quality. Then, we studied working mechanism of each module of UPQC and analyzed the power flow within the system when common voltage changes happened. Afterward, based on the switching function, we established the mathematical model for UPQC series voltage compensation module, the parallel side current compensation module and dc energy storage module. The mathematical model was used as mathematical basis for later detection algorithm and control strategy. Finally, we studied the detection method and the control strategy, which are the important parts and the focus of the research.The distortion detection of UPQC includes detection of power grid voltage distortion and current distortion. In the case of grid voltage distortion, the detection algorithm of p-q and ip-iq based on instantaneous reactive power theory can not detect the current distortion accurately. To ensure the UPQC voltage compensation and current compensation in sync, UPQC voltage and current detection must be conducted under the unified benchmark at the same time. In view of this, the detection algorithm based on synchronous rotating coordinate transformation and improved p-q detection algorithm, which can both detect voltage distortion and current distortion accurately, were emphaticly analysed.The control strategy of UPQC includes voltage control strategy and current control strategy. Firstly, hysteresis control and voltage space vector control were analysed in this paper. Due to unstable switching frequency and interphase interference in three-phase three-wire system when three-phase voltage and current were controlled respectively, this paper proposed two control strategies, which respectively applie to voltage compensation and current compensation. Voltage compensation module offsets voltage by controlling voltage. This is a linear constraint relation. In each switching cycle, two non-zero voltage vectors, which UPQC series inverter output, were chosen to compose compensation voltage vector accurately by controlling the effective time of the two non-zero voltage vectors. Current compensation module offsets current by controlling the speed of the current variation. This is a nonlinear constraint relation. In each switching cycle, an optimal voltage vector, which UPQC series inverter output, was chosen. At the same time, In order to improve the work efficiency, the difference between the compensation current and instruction current was filtered through the fixed frequency hysteresis. When the difference is greater than ring width, current compensation begins to work. Otherwise, the optimal voltage vector was the same to the last switching cycle. Through the PSIM9.0 simulation, proved that this kind of control strategy for UPQC is feasible and effective.Finally, this paper preliminarily designed the basic hardware circuit of the UPQC experiment platform. The design idea of the main program and the overall structure were analyzed. Relevant program flow chart were proposed.