This paper investigates the progressive loss of winding clamping pressure resulted from thermal aging in power transformers. Firstly, out-of-plane stress-strain characteristics of aging pressboards are investigated experimentally and relationship between mechanical properties of pressboards and aging hours is established accordingly. Then, a thermal-mechanical model is built to correlate the aging time with temperature based on the first-order reaction kinetics. Based on the above investigations, the correlation between temperature and mechanical properties of pressboards is obtained and the progressive loss of clamping pressure is predicted. The results show that with the progressive softening of pressboards from aging, the clamping pressure drops linearly under 60 degrees C and nonlinearly over 60 degrees C. The remaining clamping pressure of a transformer winding with 30 years' service history is only 80.12% and 51.59% of its initial pressure under 50 degrees C and 60 degrees C, respectively.
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