Effect of Cellulose Contamination on the Breakdown Voltage and Thermal Generated in PFAE under Lightning Impulse with DC Voltage Superimposed

This research investigates the effects of cellulose particle contamination on the breakdown voltage (BDV) and thermal profiles of palm fatty acid ester (PFAE) under lightning impulse (LD with DC voltage superimposed. Commercial cellulose microcrystalline powder was dispersed into the transformer oil as a contaminated by-product to replicate a bridge with 0.004 wt% particle concentration. A+10kV DC power supply is placed between the electrodes to artificially create and retain the cellulose bridge skeleton, with the occasional LI voltage waveforms ofl.2/50 $\mu$ s shot in accordance with the IEC 60230 test setup. The lightning impulse breakdown voltage (LIBDV) was determined using the IEC 60897 rising-voltage method. The LIBDV results were analysed and compared with clean oil (CO) using Weibull cumulative breakdown probability. The electrical field distribution was determined using finite element analysis (FEA), and the thermal profile was obtained via numerical calculation. The breakdown voltage decreased by 3% when the cellulosic particles were contaminated with PFAE. The cellulose particles become more prominent during bridge formation with a further reduction of 16%. The FEA results show that the electrical strength of the bridge contaminated process sample (B_CPS) has exceeded the critical field of CO, causing the breakdown events in the PFAE. The lightning impulse thermal generated (L
ITG
), which was influenced by the electrical field strength, has contributed to the heat releases in the liquid. In conclusion, the presence of cellulose particles and their bridge structures has a detrimental effect on liquid strength, reducing the LIBDV by up to 19% and releasing the L
ITG
by 53% of the PFAE. The findings can be used as a guideline for diagnosis, condition monitoring and assessment of insulation integrity to help extend the life span of transformers.