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Abstract
Transformers are critical components in power systems and their failure can cause long interruption of power supply. The condition of a transformer can be monitored by performing thermal analysis. The use of non-linear devices, such as rectifiers and converters, draws harmonic currents that increase losses in transformers, thereby increasing their operating temperature. In this article, a new numerical approach is presented for determining the rise in hot spot temperature in a 5-kVA, 400/400-V dry-type three-phase transformer laboratory prototype. The key novelty is that the additional winding eddy current loss due to non-linear loads is considered in the numerical modeling. The winding eddy current loss corresponding to harmonic distortion is estimated by conducting experiments and calculations. Numerical simulations are carried out for a wide range of non-linear loads using a commercial computational fluid dynamics package, FLUENT 6.3. The proposed numerical methodology is validated by performing experiments on the transformer for possible non-linear loads and comparing the measured hot spot temperature with the simulated values. Correlation equations for rise in hot spot temperature as a function of total harmonic distortion are presented, which can be used for estimating the life of transformers when connected to different types of loads.
NOMENCLATURE
| C | = | specific heat (kJ/kgK) |
| g | = | acceleration due to gravity (m/s2) |
| I | = | current (A) |
| Ih | = | current of harmonic order h (A) |
| IR | = | fundamental component of current (A) |
| Kag | = | aging factor |
| PEC | = | winding eddy current loss (W) |
| PEC − R | = | winding eddy current loss at fundamental frequency (W) |
| PSC | = | input power during the short-circuit test (W) |
| PT | = | total loss (W) |
| p | = | pressure (Pa) |
| Q | = | heat equivalent of core loss/copper loss (W) |
| = | volumetric heat generation (W/m3) | |
| R | = | resistance (Ohm) |
| T | = | temperature (K) |
| V | = | volume of the individual windings/core |
| n | = | normal |
| u | = | velocity vector (m/s) |
| u, v, w | = | velocity components, x-, y-, z-directions (m/s) |
| x, y, z | = | Cartesian co-ordinates (m) |
| θ | = | dimensionless temperature rise (T(HST − h) − T∞)/(T(HST − R) − T∞) |
Greek Symbols
| ρ | = | density of fluid (kg/m3) |
| α | = | thermal diffusivity (m2/s) |
| ν | = | kinematic viscosity (m2/s) |
| β | = | isobaric cubic expansivity of fluid (1/K) |
| φ | = | phase |
Subscripts
| HST − h | = | hot spot temperature corresponding to harmonic distortion |
| o | = | stagnation |
| HST − R | = | hot spot temperature corresponding to fundamental frequency |
| s | = | solid |
| ∞ | = | ambient condition |
| st | = | static |
| atm | = | atmosphere |
| 1, 2 | = | primary and secondary |
Abbreviations
| HST | = | hot spot temperature |
| THD | = | total harmonic distortion |
| Num | = | numerical |
| Corr | = | correlation |
Additional information
Notes on contributors
Mabel Ebenezer
Mabel Ebenezer received her bachelors and masters degrees in electrical engineering from College of Engineering, Kerala, Trivandrum, India, in 1993 and 1995, respectively. Currently she is an associate professor of Department of Electrical and Electronics Engineering, College of Engineering, Trivandrum, Kerala, India. Her areas of interest are energy management, power system dynamics, and condition monitoring of machines.
Rajkumar Mattacaud Ramachandralal
Rajkumar Mattacaud Ramachandralal received his Bachelor degree in mechanical engineering from College of Engineering, Trivandrum, Kerala, India, in 1994 and his Master degree in 1996 from College of Engineering, Trivandrum, Kerala, India. He obtained his Ph.D. from College of Engineering, Trivandrum, Kerala, India, in 2012. His areas of interest are heat transfer, fluid mechanics, and CFD.
Chandramohanan Nair Padmanabha Pillai Sarasamma
Chandramohanan Nair Padmanabha Pillai Sarasamma received his Bachelors and Masters degrees in electrical engineering from College of Engineering, Trivandrum, Kerala, India in 1975 and 1978, respectively. He obtained his Ph.D. from Indian Institute of Technology (IIT) Delhi in 1992. Currently he is a professor and head of the Department of Electrical Engineering, Amrita School of Engineering, Kollam, Kerala, India. Earlier he worked with Government Engineering Colleges in Kerala as faculty member and as a principal. He also worked as head of the R&D wing of Energy Management Centre, Government of Kerala and as a director, Agency for Non-Conventional Energy and Rural Technology (ANERT), Government of Kerala. His fields of interest are power system dynamics, energy management, and energy conservation.