Random vibration testing of a quasi-zero-stiffness isolator for distribution transformers and the effects of composite damping parameters

To address low-frequency vibration and structure-borne sound transmission in distribution transformers, this study investigates a prototype quasi-zero-stiffness (QZS) low-frequency isolator with a composite particle damping unit. Broadband vertical random-vibration tests over 2-2000 Hz were conducted to evaluate the effects of particle medium and filling ratio, while a simplified nonlinear dynamic model was used to qualitatively assess the influences of structural stiffness, equivalent damping, initial oblique-spring angle, and excitation amplitude. Rubber particles, quartz sand, and hybrid particles were tested at 30 % and 60 % filling ratios using acceleration PSD ratio and isolation efficiency as the main indices. The transmission values reported in the tables are acceleration PSD ratios rather than amplitude transmissibility. Both particle medium and filling ratio significantly affect the frequency-domain response. Across the six conditions, the isolation efficiency ranges from 10.07 to 31.21 dB at 100 Hz and from 16.34 to 40.73 dB at 200 Hz. Quartz sand at 60 % filling performs best, reaching 31.21 dB and 40.73 dB, respectively. Quartz sand and hybrid particles generally show better performance at 60 % filling, whereas rubber particles show reduced performance. These results provide a reference for composite damping design and particle-medium selection in QZS isolators for distribution transformers.