Design of Modular Multilevel Converter for Enhancement of Electrical Energy Efficiency in Smart Grid System

Abstract

Power distribution networks frequently experience power quality issues due to transient voltage sag originating from the upstream grid. Voltage sag is one of the major concerns of modern industry, as it can interrupt sensitive electrical loads and in the worst case cause production problems. Various technologies are used to mitigate power quality issues due to transient voltage sag such as DVR, SVC, and UQPC but they increased the complexity and cost of the system. MMC is a state-of-the-art power electronics-based technology with outstanding features of power quality and low cost. The aim of this thesis is to explore the feasibility of the power quality conditioning system based on a back-to-back Modular Multilevel Converter (MMC) to overcome voltage sag and ensure satisfactory power delivery to the distribution network. In this thesis, an MMC is designed to mitigate voltage sag due to symmetrical and asymmetrical faults in the upstream AC grid using the integrated energy of its sub-modules. In addition, the designed MMC is highly scalable and reliable, having low harmonic distortion and reactive power support. The significant outcomes of the proposed MMC-based voltage sag mitigation are cross-referenced with the other methods adapted for voltage sag mitigation in the literature. The designed converter is also one of the best options for future DC grids due to its many advantages. The designed converter is fault-tolerant and meets the challenges of Fault Ride-through (FRT) capability to avoid short-term outages caused by faults in AC or DC networks. In this research, a Fault Ride-through (FRT) strategy is proposed by converting 10% redundant submodules to full-bridge submodules during a fault scenario. The proposed strategy is framed in conjunction with the DC circuit breaker to accomplish economically viable operations and respond quickly to the system during the pole-to-pole and pole-to-ground DC faults. It is concluded that the proposed FRT strategy is economically viable. Moreover, the power loss of semiconductor devices within the submodules of the designed MMC, losses of the IGBT module, and the free-wheeling diode are analyzed when the switching frequency, power factor (p.f) and modulation index of the system are changed. The power losses of MMC have been examined for the four-quadrant operations i.e. inverter (inductive), rectifier (inductive), rectifier (capacitive), and inverter (capacitive). The evaluation of the power losses has been carried out employing PLECS to examine the losses of the designed MMC. In addition, the power loss of the designed MMC is compared with the power loss of the other conventional MMCs. It is concluded that the power losses of the designed MMC are less than those of the other conventional MMCs. Finally, it is concluded that the contributions of this research can be used in many practical and industrial applications. In terms of industrial specifications, this research meets the power quality requirements of industrial load for its efficient and economical operation meeting power quality standards. It provides a FRT strategy that economically overcomes the short-term outages caused by faults in AC and DC distribution networks. It can also be used for HVDC Back-to-back systems operating in islanded mode.