Analysis and control of a transformerless series injector based on paralleled H-bridge converters for measuring impedance of three-phase AC power systems

摘要

Small-signal stability of AC power systems is able to be predicted by measuring their terminal impedances, and the transformerless series injector is attractive for measuring load subsystem impedance in wide frequency range. The series injector paralleling H-bridge converters is studied in this paper, where the DC voltage unbalance mechanism is analyzed by modeling the active power flow of the converters, and it is revealed that a positive feedback exists under the situation of high system current. Furthermore, an enhanced control scheme is proposed to make the series injector operate in full system current range. Finally, the theoretical analysis is verified by hardware test results.