Design and Analysis of Functionally Graded Delta-Type Spacer in a Three-Phase Green Gas-Insulated Busduct Under Various Abnormalities

Gas-insulated busducts (GIBs) require spacer designs that can maintain a controlled electric field distribution under high-voltage operating conditions. Local field enhancement can occur at critical regions of the spacer, particularly in the presence of manufacturing and service-related defects. The shift toward environmentally sustainable power equipment is driving the replacement of sulphur hexafluoride (SF6), whose global warming potential is very high, in gas-insulated busducts (GIBs). Green gas mixtures such as g3 (C4F7N/CO2) offer a low-GWP alternative for such systems. This study presents a numerical investigation of a functionally graded material (FGM) delta-type spacer for a three-phase green gas-insulated busduct under protrusion, void, and delamination conditions. Three FGM grading configurations, namely, low-graded (GL-FGM), high-graded (GH-FGM), and U-shaped graded (GU-FGM), are investigated with three permittivity cases for each configuration. A recessed metal insert (MI) is incorporated near the enclosure end to control the electric field concentration at the critical region. The electric field stress is evaluated using the finite element method for the R, Y, and B conductor phases. In the initial analysis, the MI reduces the electric field stress by approximately 24.65% for the R phase and 39% for the Y and B phases at the investigated voltage levels. Under defect conditions at 765 kV, the MI consistently reduces the electric field stress across the investigated FGM configurations and conductor phases. The results also show that the C-3 grading generally produces lower electric field stress than C-1 and C-2. The numerical findings indicate that combining FGM grading with a recessed MI can reduce local electric field concentration in the proposed three-phase GIB delta-type spacer.