The Study of Remaining Operational Life of Power Transformers

Abstract

Power transformers are critical and expensive components of electrical power networks. Their paper-oil insulation system degrades over time due to heat, moisture, and oxygen. Once the insulation fails, the transformer cannot be repaired on site, making accurate remaining life estimation essential for utility engineers deciding when to schedule maintenance or replacement. This bachelor thesis studies and applies the main engineering methods used to estimate the remaining useful life (RUL) of oil-immersed power transformers. Four calculation methods are studied and applied:

(1) the IEEE C57.91 and IEC 60076-7 thermal models, which calculate the hotspot temperature and the rate at which insulation life is consumed under a given load [7,8,16];

(2) the Arrhenius-based degree of polymerisation (DP) kinetic model, which predicts how quickly the cellulose paper insulation loses mechanical strength [17,18];

(3) chemical diagnostic methods including dissolved gas analysis (DGA) [6,12,23,24,25], furan compound analysis [30], and oil quality testing [9,32], which provide indirect evidence of insulation condition without opening the transformer;

(4) frequency domain spectroscopy (FDS) [11,15], which estimates the moisture content of the paper insulation.

All four methods are combined in a weighted health index (HI) formula that gives a single score for the overall transformer condition [3,10,19,20,21]. Full worked numerical examples are presented for each calculation step, and results are compared against published data from real 500 kV and 750 kV transformers [1]. The study confirms that neglecting moisture content leads to overestimates of remaining life by 20-40 % [3,4,22], highlighting the importance of including moisture measurement in any complete transformer condition assessment [3,4,11].