Evaluation of voltage imbalance on low‐voltage distribution networks considering delta‐connected distribution transformers with a symmetrical NGS

Abstract

The main purpose of this study is to evaluate the effect of distribution transformers with a delta-connected neutral-grounded structure (NGS) on the voltage imbalance of low-voltage distribution networks. The proposed NGS for delta-connected distribution transformers has a symmetrical structure. That is, the stability and reliability of the operating characteristic of the proposed delta-connected NGS are better than other grounding methods such as corner-grounding, mid-tapped grounding, a zig–zag grounding transformer, and an asymmetrical NGS. In this study, the coupling-free equivalent circuit mathematical models for various methods of grounding a delta-connected structure are first developed. Furthermore, these coupling-free equivalent circuits can be simply implemented in the MATLAB/Simulink platform. Next, the IEEE test feeder system is used as a benchmark for verification. To determine the feasibility of the proposed NGS and evaluate its operational performance, a Monte Carlo simulation-based three-phase power-flow method was proposed to compare the operational characteristics of various grounding structures (i.e. system imbalance resulting from asymmetric load conditions or asymmetric transformer connections) such as phase-to-ground voltages, line currents, power consumption, and voltage unbalance factor. The results provide a valuable reference for distribution engineers and building technologies seeking a suitable method of grounding delta-connected power service systems.