A cascaded converter using hybrid cells and H-bridge structure

This paper introduces a cascaded converter structure using hybrid two-level cells to form a single-phase seven-level inverter. Compared to various conventional and existing multilevel (MLI) topologies, this topology requires a lower number of devices. It uses several DC sources integrated in two-stage cells to provide the required voltage. In this topology, the DC-link condensers are not necessary. The modes of operation and modulation of the structure proposed are described in depth. Finally, in the MATLAB/Simulink platform the new topology is evaluated and the results reported.

Inter-cluster voltage balancing control of a delta connected modular multilevel cascaded converter under unbalanced grid voltage

A new inter-cluster DC capacitor voltage balancing scheme for a delta connected modular multilevel cascaded converter (MMCC)-based static synchronous compensator (STATCOM) is presented. A detailed power flow analysis of applying negative sequence current (NSC) and zero-sequence current (ZSC) injection methods in addressing the issue of inter-cluster DC voltage imbalance under unbalance grid voltage is carried out. A control scheme is proposed which integrates both inter-cluster methods using a quantification factor QF. This is used to achieve the sharing of the inter-cluster active power between the NSC and ZSC injection methods. An accurate method of determining the quantification factor is also presented. The proposed method offers better sub-module DC capacitor voltage balancing and prevents converter overcurrent. The influence of unbalanced grid voltage on the delta connected MMCC-based STATCOM rating using this integrated cluster balancing technique is investigated. The control scheme is verified with a 5 kV 1.2MVA MMCC-STATCOM using 3-level bridge sub-modules, and the results show the advantages of the proposed method over other inter-cluster methods.