Nonlinear control strategies for cascaded multilevel STATCOMs

Two internal nonlinear control strategies based on the feedback linearization technique for cascaded multilevel static compensators (STATCOMs) are presented. The strategies depend on the control capability of the converter output voltage and are suitable for line frequency-switched converters. The first strategy considers a STATCOM where the voltage is set independently of the dc link voltage. Fast reactive power control within subcycle time response is achieved. The second strategy is constrained to a voltage whose amplitude remains proportional to the dc link voltage. Despite this limitation, the proposed strategy allows full stabilization of the STATCOM dynamics and relatively fast control of the reactive current (within one cycle). This may be adequate for most STATCOM applications. Simulation results, using power system computer-aided design/electromagnetic transient program (PSCAD/EMTP), presented for both strategies confirm the effectiveness of the control schemes to impose linear STATCOM dynamics. Preliminary experimental results from a five-level prototype are presented for a converter using fixed control angles.