The Influences of Cellulose Bridging on the Electrical Field Strength and Thermal Profile of PFAE under Lightning Impulse Stress with DC Superimposed

This paper investigates the influences of cellulose bridge formation on the Lightning Impulse Breakdown Voltage (LIBDV), electrical filed strength and thermal generated of Palm Fatty Acid Ester (PFAE). A standard lightning impulse voltage (SLIV) waveform of 1.2/50 µs with DC superimposed was set up in accordance to IEC 60230. The commercial cellulose powder were dispersed into PFAE as contaminated by-product to replicate a bridge skeleton with concentrations of 0.004%, 0.008% and 0.012% by weight. The rising-voltage method was used to measure the LIBDV in accordance with IEC 60897 test method. Weibull cumulative breakdown probability to present breakdown results statistically and the results was compared with clean oil (CO). Electrical field strength was simulated using Finite Element Analysis (FEA) and thermal profile was obtained via numerical calculation. The breakdown results showed that the cellulosic particles at 0.004 wt% reduced the LIBDV by 16%. The influence of cellulose concentration on LIBDV become more prominent during bridge formation, with a further reduction for 29% and 31% at 0.008 wt% and 0.012 wt% respectively. The electrical field strength along the bridge lines is dominant factor for the breakdown occurrence, subsequently the cellulose contamination; contributing up to 32% to the instantaneous breakdown of the PFAE. Moreover, the $L_{ITG}$ indicated that, the prominent effect of thermal evacuation process influence by concentration level, where the cellulose accumulation acts as a heat scavenger in electrically stressed by inhibited the thermal to conduct easily by 21%, thus reducing the thermal performance of the oil. Therefore, the finding potentially contributes to the formulation of guidelines for condition assessment and contamination monitoring to minimize common issues in power transformer failures that are attributable to the insulation system, and consequently, achieve optimum transformer insulation integrity by extending HVDC converter transformer life span.