Research On Technology Of Distribution Static Synchronous Compensator

D-STATCOM has been paid more and more attention since it can stabilize the voltage of the common coupling point and compensate the reactive power. But there are still many problems to be resolved. Therefore, a deep research has been done for the voltage control technology and the current control technology of D-STATCOM, the positive sequence voltage and reactive current detection method, as well as the structure design of output filter and its parameter optimization. At the same time, the possible solutions are presented in this dissertation.In order to stabilize the common coupling point voltage, basis of the traditional double loop control method (voltage outer loop and current inner loop), the algebraic relations between the VSI(voltage source inverter) output voltage and the VSI output current in dq coordinate are deduced by using the energy conservation principle, and replace the current inner loop of the double loop control method, and so reduce the PI adjusters. When the load is unbalanced, it will cause voltage imbalance on the common coupling point. In order to solve this problem, the negative sequence feed forward control module is imported, and the new algebraic relations under negative sequence voltage conditions are deduced too. At the same time, to improve the systems's robustness and dynamic characteristics, the fuzzy-neural-network algorithm is adopted to adjust the PI parameters on line which divides the PI parameters into two parts, theωi(k) and the Ki (k). When the tracking error is great, the fuzzy algorithm is chose to adjust Ki (k) quickly, and when the tracking error is small, the neutral-network is chose to adjustωi(k).To compensate reactive power effectively, the direct current control method based on variable structure space vector method is adopted in this dissertation. The sliding mode control rule which can maintain the system stability is deduced when choose I Sq=0 to construct switching surface. However, the sliding mode control rule has control quantity only when the tracking error is not in zero. So, When the power system is in transient, the sliding mode control rule is chose directly to make transient component decrease quickly, and when the power system is in stability, the direct current control method based on voltage space vector method is imported to make the error current reduce to zero by inches and avoid surge. Furthermore, the control method can reduce the switch frequency, decrease the switch loss, and stabilize the direct voltage. The rapid and accurate detection of positive sequence voltage and reactive current is crucial for improving the performance of D-STATCOM. The traditional synchronization coordinate transformation method is analyzed in detail in this dissertation. And it is pointed out that if use this method in unbalanced system, it will bring great examination error. The reason is that it can't obtain synchronous rotate angle under unbalanced system. To solve this problem, a new way has been proposed, in which a single phase voltage's differential is obtained at first, and then the synchronous rotation angle is calculated by the orthogonal relations of the single phase voltage and its differential directly. Because the examination precision is decided by the differentiator performance, the differentiator's actual differential effect is analyzed emphatically in this dissertation, and the results indicate that the digital differentiator designed by difference equation has fine performance. On the basis of discussion above, a new Ip-Iq transformation method by using positive sequence voltage component instead of common coupling point voltage is proposed to ensure that the reactive current can be detected accurately under unbalanced voltage system.In order to filter out the harmonics voltage output by inverter, the performance of existing filters are analyzed. Then the new LCL type filter which connected a small resistance to the capacity is pointed out can suppress the resonance effectively. Furthermore, the output filter's influence to the D-STATCOM is discussed from the ability of reactive power compensation and transient current tracking. According to this, the optimization method of parameters for output filter is proposed by using the advanced genetic algorithm that considering the influence degree of each optimized goal to the D-STATCOM into the fitness function. The simulation results show that the optimization method can meet the requirement of the reactive power compensation better than the ones designed just by experience.Based on theoretical study, The research plan of 50kvar D-STATCOM experimental prototype is proposed at the end of this dissertation. The design technology and experimental results is presented in detail. The experimental results show that the D-STATCOM can attain a satisfactory performance by adopting the proposed techniques.