Impact of Reactive Power Fluctuation on the Performance and Insulation Loss-of-Life Rate of Industrial Power Transformers: A Case Study of the PT Semen Tonasa Electrical System

Power transformer performance is strongly influenced by reactive power (VAR), yet the quantitative link between VAR fluctuation and insulation loss-of-life in heavy industrial systems remains under-documented. This study analyzes that link for six 70 kV power transformers (three at the Tonasa IV and three at the Tonasa V substations, 31.5–40 MVA) in the electrical system of PT Semen Tonasa, combining SCADA operational data, an ETAP load-flow simulation, and an analytical ageing-rate calculation based on IEC 60076-7. Varying reactive power from 1 to 10 MVAR at a fixed active power of 10 MW increased the loading level from 31.41% to 44.19% of the 80% operating limit, consistent with S = √(P²+Q²). SCADA records showed current rising and primary-side voltage falling as VAR increased, with all voltages (66.68–68.95 kV) remaining within ±10% of nominal. For the three Tonasa V units, hotspot temperatures of 65.42–75.23°C and loss-of-life rates of 0.022–0.069 pu/day were obtained, well below the 98°C/V = 1 design reference; Transformer #1 aged roughly 3.1 times faster than Transformer #2, reflecting its higher average loading (63.4% vs. 36.5% of the operating limit). The extrapolated remaining-life figures (79.7–250 years) are indicative rather than literal, given the short (3–5-day) SCADA window; the loss-of-life rate is therefore treated as the primary, more defensible indicator. Reactive-power compensation via capacitor banks or static var compensators is recommended to limit current, hotspot temperature, and insulation ageing.