**ABSTRACT**

Medium frequency transformer (MFT) plays an important role in high power AC-DC transformation in power grid, due to its high power density and high loss density, it brings difficulty to heat dissipation. Thermal reliability is crucial for transformer design, different from the previous work, the thermal characteristics of MFT under different working conditions are concerned and analyzed in this paper. The temperature field model of MFT is established, and the temperature field characteristics of MFT at rated load, no load, overload, fluctuating overload and short circuit conditions are simulated and analyzed, the results show that with the increase of load coefficient, the winding temperature increases obviously, the allowable load coefficient of MFT can be determined by calculating the transformer temperature at different load coefficients, which lays a foundation for thermal reliability and safe operation of MFT.

**KEYWORDS**

PAPER SUBMITTED: 2023-08-07

PAPER REVISED: 2023-09-05

PAPER ACCEPTED: 2023-09-21

PUBLISHED ONLINE: 2023-11-11

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