Distributed Control And Realization Of Cascade STATCOM System

Cascade static synchronous compensator( STATCOM) has been widely applied in medium/high voltages high-power application to regulate reactive power of grid, due to its simple structure, rapid dynamic response and low output harmonics, etc. However, the number of cascade modules will increase with the increase of the voltage class and facility capacity. In traditional centralized control system(CCS), the links between the central controller and each of the cascade modules are point-to-point connected, and the central controller takes care of all operations, thus resulting in low reliability, poor redundancy and flexibility, etc. Compared with CCS, distributed control mode is better suited in this cascade system for its theory of multilayer control. Based on the cascade STATCOM system, a distributed control scheme is designed and deeply researched in this thesis to improve modularization and extension of the cascade system.The thesis is mainly aimed to realize the distributed control of cascade STATCOM system. First of all, the thesis introduces the topology with star configuration, working principles and basic control strategies of the cascade system, including the decoupled control method of active and reactive power and the DC link voltage balancing control methods for all of the cascade modules. The topology and control methods are simulated and verified through Matlab/Simulink tools theoretically. Secondly, considering the special requirements i.e. real-time and reliability of data transmission, strict synchronization among modules and fast fault response in distributed-controlled electronic facilities, a communication subsystem is designed, including the ring structure based hybrid communication topology, frame structure with escape character inserted and the modified UART communication protocol, and then applied in this distributed-controlled cascade system. Further description of detailed different communication process based on the type of signal is provided. Then distributed controllers with two layer architecture are designed according to distributed control principle, as well as the embedded software framework. The distributed controllers are connected only through the optical interfaces, which improves the isolation of signal and reduce interference with each other. Finally, according to the actual requirements of the project, a cascade STATCOM experimental platform of 100V/100 kVar is established. The corresponding communication experimental results, with communication rate set to 5Mbps, verify the realtime and reliability of data transmission, and all of the cascade modules can be operated in strictly synchronous mode. The fault response time is just 8.65 μs, which satisfies the requirements of electronic devices. Moreover, further experimental results of grid-connected operation and reactive power regulation with DC link voltage balancing control confirmed the feasibility and effectiveness of distributed control scheme experimentally.