Research On The Control And Parallel Operation Technology Of Medium Voltage Cascade STATCOM

At present there have been more and more problems with the quality of electric energy due to the wide application of non-linear, impact and unbalanced load. Under medium and high voltage circumstances, STATCOM stands for the latest development trend of reactive power and harmonic suppression and becomes the newest device for quality management of electric energy. As a result, researches on related technology are endowed with great significance. This thesis studied technologies related to single medium-voltage cascading STATCOM, as well as the parallel running of multi-STATCOM under medium voltage. These researches produce a new type of parallel running control technique suited to different working conditions.In the beginning, this thesis analyses problems of reactive power and harmonics in the grid and clarifies the necessity to manage the quality of electric power. Then, based on the on-site running, the author introduces the basic principles of STATCOM and designs the main circuit structure and parameters of STATCOM, including the H-circuit module, the filter module, and the BUS capacitance on direct current side. A mathematical model of STATCOM is built based on a certain structure and the circulating current in parallel running analyzed.Further studies are conducted on single STATCOM based on the mathematical model. Detailed researches are made on the controlling units of single medium-voltage cascading STATCOM, including directive current detection technique, current tracking technique and capacitance & voltage balancing technique on direct current side. Improved dq detection technique is applied in directive current detection. In the ordinary PLL based join a fundamental positive sequence voltage phase lock link, when the distortion of grid voltage, three-phase unbalance to make the detection more accurate. In current tracking, direct current controlling technique suitable for high-capacity cascading STATCOM under medium voltage is applied. Also, the repeating PI double-loop controller is used to achieve the floating tracking of directive current. PI control is applied in BUS capacitance on direct current side to ensure that the fluctuation of voltage and capacitance is within the acceptable range. Finally, a simulation model built through MATLAB proves the superiority of the design.Regarding the problems with parallel running of multi-STATCOM, further study is carried out on the control method of parallel running 6KV STATCOM. A new control method suitable for different working conditions is proposed. For this method, the number of working STATCOM can be set independently and current balance coefficient updated automatically. When there is an error in the system, the current limit control ensures that STATCOM manages the reactive power and harmonics, so that the system can function stably. Finally, simulation analysis is conducted through the simulation model built on MATLAB, which proves the stability and feasibility of this control method.At last, the 6KV cascade STATCOM is simulated and applied in the field. The effectiveness and practicability of the design are verified by the experimental data and the field application effect.