Abstract

Recently, multiple static synchronous compensator (STATCOM) units have been adopted in power transmission systems in order to obtain a better voltage regulation and to share loads. However, they could possibly interact in a negative way instead of helping each other, due to the improper design of the STATCOM controllers. To analyze this problem, a d-q frame impedance-based stability analysis was used to explore the instability with the presence of STATCOMs, where previous stability-related findings are not applicable directly because of some unique features of STATCOMs. This paper identified the frequency range of interactions in a viewpoint of d-q frame impedances and pinpointed that the ac voltage regulation was the main reason of instability, masking the effects of phase-locked loop on power transmission systems. In addition, due to the high impedance of STATCOMs around the frequency range of interactions, the number of connected STATCOMs was the main contributor to stability instead of the topology of power systems or the locations of STATCOMs. A scaled-down 2-STATCOM power grid was built to verify the conclusions experimentally. This paper is accompanied by a video showing instability between STATCOMs in the experiment.