Reactive power limits of Cascaded H-Bridge STATCOMs in star and delta configuration under negative-sequence current withstanding

Abstract

Flexible AC Transmission System devices (FATCS) and in particular Static Synchronous Compensators (STATCOM) play a key role in the evolution of the modern power grids to the future smart grids. The STATCOM is used in an increasingly wider variety of scenarios in which the operation under unbalanced conditions stands out. This paper analyzes and compares the reactive power limits of Cascaded H-Bridge (CHB) STATCOMs in star (YCHB) and delta (DCHB) configuration to withstand negative-sequence current. Zero-sequence voltage for the YCHB and zero-sequence current for the DCHB are injected in order to correct the intercluster uneven active power distribution and thus to preserve the dc-link voltage balancing. Both solutions will have an impact on the power rating of the converter. This work clearly quantifies the reactive power limits of each CHB STATCOM configuration depending on the current unbalance at converter terminals, by means of a systematic procedure. Improved explicit expressions of the zero-sequence current in the DCHB are also provided. Experimental results obtained from a real-scale set-up in a MV laboratory validate the theoretical analysis.