PROJECT TITLE :

Variable Slope Trapezoidal Reference Signal based Control for a dc Fault Tolerant Hybrid Modular Multilevel Converter with Cascaded Full Bridges - 2017

ABSTRACT:

The hybrid modular multilevel converter with cascaded full bridges (HMMC-CFB) may be a recently proposed dc fault tolerant converter topology for top-power applications. It has a director part (DP) consisting of a series of connected 0.5-bridge submodules, and a wave-shaping half (WSP) composed of a series of connected full-bridge submodules (FBSMs). The submodules in DP avoid high voltage stresses and improve synchronization between DP and WSP by slowing down the amendment rate of DP output voltage. This slowdown is achieved by employing a trapezoidal reference signal for DP. The WSP attenuates the voltage harmonics produced by DP. However, the energy exchanged by WSP changes with modulation index, which could cause improper voltage regulation of the submodule capacitors. This paper proposes a new control technique to regulate the energy exchanged by WSP, which uses a variable slope trapezoidal reference signal of DP. To obtain this slope, the feedforward and feedback controllers are used. The feedforward controller gives the price of slope depending on the modulation index (mv) to keep the energy exchanged by WSP nearly equal to zero, whereas the losses and nonideality of the converter are taken care of by the feedback controller. This technique guarantees correct voltage regulation of the submodule capacitors for all modulation indices. The efficacy of the proposed technique is confirmed using simulations and experimentations. This paper conjointly presents a circulating current suppressing controller for HMMC-CFB. The proposed control schemes and dc fault tolerant capability of HMMC-CFB are tested on a grid-connected model engineered in PSCAD.