Research On Statcom Compensation Under Unbalanced Load Of Three-Phase Four Wire System Based On DSP

With the continuous development of science and technology, electric energy has become one of the essential energy in people’s production and life. Three-phase four wire system is mainly used in middle and low voltage distribution system power supply in our country. There are a lot of nonlinear load and the power electronic devices in system which brought many electric energy quality problems such as harmonics and unbalanced for grid systems. These problems threat to safe, stable and reliable operation of the power grid. In this paper, the elimination of zero sequence current, negative sequence current and harmonics in three-phase four wire system with unbalanced load conditions is studied. A three-phase four wire STATCOM device is designed to improve the power quality.In this paper, firstly the research background of the three-phase four wire system STATCOM is introduced and the current research status of the subject is described. The work and compensation principle of three-phase four wire system STATCOM is introduced in this paper, three topologies of three-phase four wire STATCOM main circuit are compared and analyzed, the main circuit of the three-phase four leg is selected in the design of the compensation device. The instantaneous reactive power theory and the p-q and ip-iq compensation current detection algorithm which based on the theory are analyzed in detail. According to the characteristics of three-phase four wire system, the method of zero sequence separation is used to detect the compensation current. PI regulation method is used to maintain voltage stability of DC side capacitor. In order to eliminate the system delay in the process of compensation current detection, an improved method of heterodyne value prediction is proposed, which can eliminate the system delay effectively and improve the compensation accuracy. Hysteresis control method and triangular wave comparison control method are analyzed and compared, combined with the advantages of these two methods, variable ring wide hysteresis loop control method is used to track the compensation current command signal and generate PWM control pulse. The simulation model of three-phase four wire system STATCOM is built on the MATLAB\Simulink simulation platform. Through the simulation experiment, the change of the system before and after compensation is compared, and the correctness of the design is verified.Finally, the hardware system and software system of the three-phase four wire STATCOM system are designed, the DSP chip TMS320F28335 which is produced by TI company is the control core, and the experiment platform is built. Through the analysis of experimental results, the three-phase four wire system STATCOM which is designed in this paper can eliminate the harmonic, zero sequence and negative sequence current of the system effectively be fully verified, which has a very positive effect on improving the power quality of the system.