Research On Key Techniques Of Cascaded H-Bridge Multilevel STATCOM

Keeping system voltage level stable is a basic requirement for electricity generation,transmission and distribution systems, and this can bring the users stable, reliable and cleanelectric energy. Rapid and dynamic reactive power compensation is very important. Havingmany merits, the Static Synchronous Compensator (STATCOM) represents thedevelopment tendency of reactive power compensation devices, and it has been paid moreand more attention. Cascaded H-Bridge STATCOM is the best choice of high voltage largecapacity STATCOM application at present, and it has a broad development prospect andspecial research value. The key technologies of cascaded H-Bridge STATCOM involvesynchronization algorithm, modulation strategy, dynamic reactive power andgrid-connected current control, control and protection under unbalanced conditions and dcvoltage balance control, etc. To solve the problems existed in the research field ofsynchronization algorithm, control under unbalanced conditions and dc voltage balancecontrol, this paper works in the following three areas.First, the frequency-biased characteristic of abc/dq transformation is discussed. Theimproved dq_MAF_open synchronization algorithm based on predictive filter has two maindefects: the frequency adaptive range is too narrow, and the amplitude attenuation onuneven harmonic spot is not enough. To make up these two defects, a novel wide frequencyadaptive range open-loop synchronization algorithm based on FIR filter is proposed. Theoperation principles and design methods of all parts of this algorithm are analyzed in detail,and specific designing steps are given. The frequency adaptability of the predictive filterimproved dq_MAF_open synchronization algorithm and the proposed algorithm arecompared by simulation results, which imply the superiority of proposed method. Second, the mathematical model of star cascaded STATCOM is built. The specialactive power biased phenomenon of cascaded STATCOM under two typical unbalancedconditions are analyzed. The principle that the conventional active current regulationmethod cannot correct the biased active power is proved by mathematic method. And themain principles and limitations of existing two active power correction methods areanalyzed. A zero sequence voltage solution method is proposed. By quantitatively analyzingthe valid compensation region of zero-sequence voltage injecton method, the relationshipbetween STATCOM negative sequence current compensation ability and k and gridconnected inductor L is obtained. The validity of theory analysis is proved by thesimulation results of a10kV/±50MVar star cascaded STATCOM.Third, this paper presents a novel dc capacitor voltage distributed control system (DCS)for cascaded H-Bridge multilevel STATCOM. The DCS can simplify the communicationsystem design, without injuring the voltage balancing performance. The system total powerbalance relation and each link power change before and after introducing vbare analyzed.Considering the effect of main link DC voltage close-loop controller, the quantitativerelation between vband link loss diversity is built. The correctness of this quantitativerelation is proved by simulation. A hardware and software solution is presented. Thevalidity of DCS is verified by the experimental results of a single phase three-moduleseven-level cascaded STATCOM prototype.