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In this work, a novel control method for transformerless H-bridge cascaded STATCOM is explained. FLEXIBLE ac transmission systems (FACTS) are being increasingly used in power system to enhance the system utilization, power transfer capacity as well as the power quality of ac system interconnections. As a typical shunt FACTS device, static synchronous compensator (STATCOM) is utilized at the point of common connection (PCC) to absorb or inject the required reactive power, through which the voltage quality of PCC is improved. In recent years, many topologies have been applied to the STATCOM. Among these different types of topology, H-bridge cascaded STATCOM has been widely accepted in high-power applications for the advantages of quick response speed, small volume, high efficiency, minimal interaction with the supply grid and its individual phase control ability.
There are two technical challenges which exist in H-bridge cascaded STATCOM to date. First, the control method for the current loop is an important factor influencing the compensation performance. Second, H-bridge cascaded STATCOM is a complicated system with many H-bridge cells in each phase, so the dc capacitor voltage imbalance issue which caused by different active power losses among the cells, different switching patterns for different cells, parameter variations of active and passive components inside cells will influence the reliability of the system and even lead to the collapse of the system.
This paper presents a transformerless static synchronous compensator (STATCOM) system based on multilevel H-bridge converter with star configuration. This proposed control methods devote themselves not only to the current loop control but also to the dc capacitor voltage control. With regards to the current loop control, a nonlinear controller based on the passivity-based control (PBC) theory is used in this cascaded structure STATCOM for the first time. To enhance robustness and simplify the controller design, a passivity-based controller (PBC) based on error dynamics is proposed for STATCOM. In terms of dc capacitor voltage balancing control, there are three pivotal issues: overall voltage control, clustered balancing control, and individual balancing control. As to the dc capacitor voltage control, overall voltage control is realized by adopting a proportional resonant controller. Clustered balancing control is obtained by using an active disturbances rejection controller. Individual balancing control is achieved by shifting the modulation wave vertically which can be easily implemented in a field-programmable gate array.
Two actual H-bridge cascaded STATCOMs rated at 10 kV 2 MVA are constructed and a series of verification tests are executed. The simulation results prove that H-bridge cascaded STATCOM with the proposed control methods has excellent dynamic performance and strong robustness. The dc capacitor voltage can be maintained at the given value effectively.
Simulation designing tools:
Matlab software, sim power systems, power sources, power electronics switches.
Configuration of the proposed scheme: