Research On Hybrid Cascaded Multilevel Converter Based STATCOM

Static synchronous compensator (STATCOM) has been paid great attention for its good compensation performance. With the expansion of the transmission capacity and load demand of national power grid, STATCOM develops towards direction of high-voltage and high-power. As expansion and redundancy is easy due to its modular and multilevel structure, cascade STATCOM has been widely studied and applied. In recent years, the cascade topology has a development, and the hybrid cascaded multilevel converter emerges and brings a new way to the research of high-power STATCOM. This paper proposes a new type of STATCOM based on hybrid cascaded multilevel converter (HCMC), that is, HCMC-STATCOM. The control strategy, modulation stratedy and DC capacitor parameter design of the HCMC-STATCOM are studied in this paper.The proposed HCMC-STATCOM has two key parts, i.e., the conventional three-phase two-level converter using series-connected IGBTs and the wave-shaping circuit composed of cascaded H-bridge sub-modules. Based on the configuration of HCMC-STATCOM, its basic working principle is briefly analyzed. And the transient mathematical models of the proposed HCMC-STATCOM are established in abc and dq coordinates respectively.This paper introduces the power feedforward decoupling control strategy, and designs the controllers to achieve the coordination and synchronization of the two-level converter and the wave-shaping circuit. Two modulation strategies applied to the wave-shaping circuit are elaborated in detail, that is, the carrier phase-shifting SPWM (CPS-SPWM) and the nearest level modulation (NLM). The start control stratedy of the HCMC-STATCOM is also analyzed in detail. The effectiveness of the control strategy, modulation strategy and start control stratedy are verified by PSCAD/EMTDC simulation.Balanced H-bridge DC capacitor voltage of the wave-shaping circuit is a precondition for normal operation of the HCMC-STATCOM. The DC capacitor voltage balancing control strategies are studied under two different modulation strategies CPS-SPWM and NLM. The effectiveness and respective characteristics of the proposed balancing control strategies are verified by PSCAD/EMTDC simulation.DC capacitor parameters directly affect the overall performance of the HCMC-STATCOM. This paper establishes the DC capacitor ripple voltage equations and the criterion for sizing the DC capacitors. Comparisons of the HCMC-STATCOM and the cascaded H-bridge STATCOM are presented in terms of sub-modules and IGBTs count, capacitor number and size, stored energy of capacitors. According to the above analysis, the advantages and disadvantages of HCMC-STATCOM compared to the cascade STATCOM and other STATCOMs are summarized. The effectiveness of the DC capacitor parameter design and comparisons are verified by simulations in PSCAD/EMTDC.