The Analysis Of Unbalance Compensation Capability Of The Cascaded STATCOM With Delta Configuration And Its Control Strategy

STATCOM(static synchronous compensator, STATCOM) is a parallel compensation device connected to the grid. It can be used for the transmission grid, and also for the distribution network. STATCOM chain structure has been widely applied in the unbalanced load compensation, for its fast dynamic response speed, good harmonic performance, compact structure, significant economic benefits, etc.. Because the triangular shape STATCOM contains zero-sequence current flow path, it is more suitable for the unbalanced load compensation. However, the triangle cascaded STATCOM is a complex FACTS controllers, it has some problem that need to be solve, such as its control, and capacitance-voltage design method 、the main circuit current inner loop decoupling control、reactive power control outer imbalance compensation range、the node voltages control and the balance control.In this paper, making the triangle chain STATCOM compensation for reactive power compensation and negative sequence current as the research object, analyzing the working principle、the mathematical model and the main circuit parameters. Through Coordinate transformation and switching function method, getting its mathematical model in the positive, negative and zero sequence order. On that basis, controlling the STATCOM device based on the feed-forward decoupling method in each sequence coordinate, that the positive and negative sequence components are controlled in synchronous rotating coordinate system, the zero-sequence component in the instantaneous coordinate control. Then, for the triangle chain STATCOM DC capacitor voltage imbalance problem, analyzing the generation mechanism of DC capacitor voltage imbalance,and hierarchical control methods is proposed to suppress the DC side capacitor voltage unbalance. The basic idea is to achieve the overall power balance by positive sequence current, to achieve white power balance by zero-sequence current injection means,and to achieve power balance in phase by changing output voltage amplitude and phase of each H-bridge. Finally, through the analysis of the achieving power balancing mechanism of the triangle STATCOM, deducing the calculation method of the negative sequence current compensation size,when rated reactive current is compensated. In Matlab / simulink, 10 k V/ 10 MVA triangle STATCOM device is built, and the simulation results are giving in this paper.Simulation results show that the proposed current inner loop control strategy,the DC voltage control strategy of triangle chain STATCOM are accurately and effectively under unbalanced condition. The calculation method of negative sequence current compensation range laid the theoretical foundation for the design and application of STATCOM, and the project has some practical significance.