Negative-Sequence Current Range in Low-Capacitance Cascaded H-Bridge STATCOMs with Peak Arm Current Limitation

The cascaded H-bridge (CHB) low-capacitance static compensator (LC-STATCOM) with delta configuration has the capability to inject positive- and negative-sequence currents simultaneously. However, to maintain the energy balance among the converter arms, a fundamental-frequency zero-sequence current must flow within the delta ring. This may involve greater peak arm current values, which, in turn, limits the range of positive- and negative-sequence currents that the STATCOM can provide. In the LC-STATCOM, the inherent large twice-fundamental-frequency oscillation on the capacitor voltages also can bound its operating range. This paper analyses the operating range constraints in LC-STATCOMs and analytically derives its operating limits. Analytical derivations show that the allowable steady-state region corresponds to a polyhedron in Cartesian coordinates. The obtained polyhedra, for different desired performances, represent a new tool that shows the operating limits of STATCOMs and can assist in their design. The paper presents simulation results to corroborate that the operational limits correspond to those shown by the derived polyhedron.