Negative-Sequence Current Capability in Low-Capacitance Cascaded H-Bridge Static Compensators With Optimal Third- Harmonic Circulating Current Injection

• Published in: IEEE transactions on power electronics 2023-04, Vol.38 (4), p.4544-4558
• Main Authors: Ramos, Ezequiel Rodriguez, Leyva, Ramon, Farivar, Glen G., Townsend, Christopher D., Pou, Josep
• Format: Article
• Language: English
• Subjects: Automatic voltage control; Capacitance; Capacitance bridges; Capacitors; Cascaded H-bridge (CHB); circulating current injection; compensation region of negative-sequence currents; Compensators; Current injection; Electric potential; Harmonic analysis; Linear programming; multilevel converter; optimization methods; Oscillators; STATCOM; static compensator (STATCOM); Steady-state; thin dc capacitor; Unbalance; Voltage
• ISSN: 0885-8993; 1941-0107
• DOI: 10.1109/TPEL.2022.3233033

The cascaded H-bridge (CHB) low-capacitance static compensator (LC-StatCom) has a limited negative-sequence current injection capability compared to a conventional CHB StatCom, due to the comparatively larger oscillations on the capacitor voltages. Placing limits on the ability to provide negative-sequence current to the grid for a given capacitor size is required to prevent overmodulation. This article shows that these limits can be maximized by injecting an optimal third-harmonic circulating current. The effects of injecting the third-harmonic current on the capacitor voltages, both in balanced and unbalanced grid voltage conditions, are analyzed. Specifically, the procedure allows the knowledge of the potential capability of the LC-StatCom when injecting negative-sequence currents. The procedure is based on a linear programming approach. Although the procedure is applicable to any CHB StatCom, regardless of the submodule capacitor size, it is particularly important in LC-StatComs. To corroborate the proposed analysis, simulation and experimental results under balanced and unbalanced grid voltage conditions are presented.