Research On Control Strategy Of DSTATCOM Based On Modular Multilevel Converter

Voltage fluctuation and unbalanced three phase are those power quality problems may cause a huge impact on the safety and stable operation of distribution system. Because of the advantages of smaller size and quicker dynamic response, distribution system static synchronous compensators(DSTATCOM) have been widely used in power quality comprehensive treatment. In recent years, with the increasing power level of the distribution system, the compensation capacity and voltage level requirements of DSTATCOM devices have become more sophisticated. The advantages of highly modular structure and easy to extend make the research of using modular multilevel converter(MMC) as a DSTATCOM more and more significant.This paper analyzes the basic operation principle of MMC at first, and then gets the dq synchronous rotating coordinate system mathematical model and current inner loop control system based on a three-phase two-level VSC modeling methods. Taking into account of large circulating current may increase the current stress on the power switch, causing the capacitor voltage fluctuations and other issues, the expression of secondary harmonic was obtained quantitatively, different kinds of circulating suppressing methods were also analyzed and compared.Based on carrier phase shifted sinusoidal pulse width modulation(CPS-SPWM), the effect of modular numbers, phase different of carriers and operation conditions on output voltage level and the output harmonic content of MMC-DSTATCOM were analyzed in detail. To achieve a smooth start of the device, this paper uses non-controlled rectifier and high frequency rectification to achieve dc capacitor charging and keeps capacitors voltage balance through hierarchical control strategy.Hardware and software were analyzed in detail for the design of MMC-DSTATCOM experimental system, including a plurality of digital signal processor(DSP) of the hierarchical structure, the selection of arm inductor and dc capacitor, the communication between upper and lower controller, and the synchronization of carrier phase and the driving pulse. This paper also verifies of the correctness of CPS-SPWM modulation strategy, dc capacitor voltage balancing control and reactive power generation.Finally, the analysis and experimental verification of parallel compensation performance of MMC-DSTATCOM was done. Taking into account of the complexity of positive and negative sequence double-loop control system, this paper use the method of combining proportional integral(PI) control and repetitive control to achieve the tracking of periodic compensation signal with no steady state error, experiment result shows MMC-DSTATCOM can regulates PCC voltage and suppresses of unbalance of three phase effectively.