Design, Modeling and Performance Analysis of Smart Transformer for Distribution System

Abstract

The changing architecture of electric power grid from centralized to decentralized form by integration of distributed energy resources (DERs) to ensure a reliable, efficient and environmentally friendly power supply to customers is becoming an opportunity and challenge for power companies. The decentralized electric power grid, poses many challenges to a hundreds of years old conventional transformers as it is a unidirectional power system infrastructure which doesn‟t compensate for harmonics and reactive power. Moreover, the conventional transformers in distribution system are bulky in size and volume; use a mechanical tap changer, which toggles between different tap positions many times per day, reducing life time of transformer by wear and tear. It doesn‟t allow integration of micro grids without investing on additional power converter equipment. The aforementioned short comings of conventional transformers are overcome by emerging technology known as a smart transformer. In this dissertation work, design, modeling and analysis of a three stage multiport smart transformer (ST) based on modular multilevel converter (MMC) is presented in Simulink/PLECS simulation platform using different controllers, modulation techniques and converter configurations. In addition, integration of smart transformer (ST) in to the existing feeder model of ArbaMinch distribution system containing 72 distribution transformers has been done and its impact on voltage regulation, loss minimization and power quality (PQ) improvement has been assessed. Observations of simulation result showed that Fuzzy Inference System (FIS) and Adaptive-Neuro-Fuzzy Inference System (ANFIS) controllers yield better output than Proportional-Integral (PI) controller for front stage of ST. Simulation result of Input-Series-Output-Parallel Dual Active Bridge (DAB-ISOP) configuration for DC-DC stage proved voltage sharing at input side and current sharing at output side reducing device voltage and current stress and increasing availability. The MMC topology for back-end converter yields a reduced voltage total harmonic reduction (THD)value (3.8%), higher efficiency (96.7%) and reduced current stress (61%) than its cascaded H bridge (CHB) counterpart having voltage THD of 13.7% and efficiency of 83% for the same sub-module (SM) used. It has been observed from the simulation result that the maximum voltage drop in the feeder model without ST integration is 781V with voltage regulation of 9.9%. However, the maximum voltage drop after integration of an ST model is reduced to 444V with voltage regulation of 5.4% (improvement by 4.5%). The line loss without ST integration is 1096.7kW, which is reduced to 823.2kW when ST is integrated in to the feeder model (power loss reduction by 273.5 kW).