Abstract

Small-signal stability of ac power systems is able to be assessed by measuring their terminal impedance, and a transformerless series injector is very attractive for wideband measurement of load impedances. A typical implementation of the transformerless series injector is paralleled H-bridge (PHB) converters, where dc voltage imbalancewill occurwhen system current magnitude is larger than a critical point, and thus the operation region of the series injector is seriously restricted. To address this issue, this paper presents a deep analysis on the mechanism behind dc voltage imbalance for PHB-based series injector, and it is revealed that a positive feedback exists in the dc voltage balancing loop under high system current magnitude. Then an enhanced control scheme is proposed to balance dc voltages in full system current range, where a reactive component is injected in the output voltage while reactive circuiting current is introduced among H-bridge converters for redistributing active power. Furthermore, the system behavior with the proposed scheme is comprehensively analyzed and the reference design for the injected reactive component magnitude in the output voltage is offered. Finally, the imbalance mechanism and the proposed scheme are validated by simulation and experimental results.