Control Method Of Cascaded Statcom Based On Unified Control Of Compensation And Balancing

ABSTACT:Recent years have witnessed huge reactive power polution in power sys-tems, which is increasing endangers power system devices. Thus the local reactive pow-er compensation has become an urgent task in power systems. To meet the challenge, cascaded multi-level STATCOM has seen wide applications in engineering practices due to its advantages such as highly modularization, good output voltage waveforms and easy-to-control. But as its cascaded structure has multiple independent DC capac-itors, voltage imbalance among capacitors in operation emerges, which impairs effec-tiveness of local reactive power compensation of STATCOM devices and may even lead to unstable operation. Therefore, the control technique to balance DC capacitor volt-ages becomes one of the keys to the study on cascaded STATCOM. The goal of this thesis is to study a cascaded multi-level STATCOM control technique addressing DC capacitors’voltage imbalance.Traditional control techniques combine the effort to balance capacitor voltages with the power adjustment module. Being an exception, this thesis controls reactive pow-er compensation and capacitor voltage balancing unifiedly, combining with the pulse width modulation(PWM) technique. The thesis proposes a novel cascaded multi-level STATCOM control technique based on a uniform algorithm. With this technique, indi-vidual tuning of each H bridge module’s modulation factor is no longer needed as just three PWM controller exist in the whole device.The history of Multi-level converter topology applied to STATCOM is first intro-duced. Principles of STATCOM is then explained. And then by doing a comparative analysis, the cascaded multi-level converter is introduced, summarizing advantages of the cascaded STATCOM. A thorough analysis on possible reasons of the imbalance of DC capacitor voltages has been done, which are later verified by simulations. With those reasons for imbalance in mind, a study on several common DC capacitor voltage control method is carried out. Their advantages and disadvantages have been analyzed.In the next part of the thesis, the mathematical model of the cascaded STATCOM is constructed. The main circuit parameters and the control technique of a lOkV,2MVar cascaded25-level STATCOM are designed. The upper level of the control technique deployes power decoupling, and the lower level deployes a cascaded PWM control technique which is based on a unified algorithm. Compared with traditional control techniques, the control technique reduces the load on the main controller and thus re-duces the overall load of the system, because its voltage balancing technique can be implemented in FPGA. It also simplifies the design process of the main controller’s control parameters, and improves the stability of the system.In the final part, steady state characteristics and dynamic characteristics of the aformentioned cascaded25-level STATCOM are simulated in MATLAB. The simula-tion results indicate the superior performance of both the reactive power compensation functionality and the DC capacitor voltage balancing effects of the cascaded STAT-COM.