Electromagnetic Field Calculation Of 1.35kV/300kVA Medium Frequency Transformer

Compared to the traditional traction transformer, power electronic traction transformer(PETT) is smaller and lighter, because of the increased operating frequency, which is good for energy conservation. Medium frequency transformer (MFT) is a key part of PETT. However, leakage inductance and stray capacitance have adverse effect on its performance. And smaller size may lead to insulation damage. Therefore, it is very important to calculate the stray parameters accurately and design compact but reliable insulation structure for PETT.A 1.35kV/300kVA MFT model is used in this paper. The equivalent circuit of MFT is analyzed firstly. Then stray capacitance and leakage inductance are calculated. Compact main insulation structure is designed finally. A general stray capacitance calculation method is deduced based on the principle of electrostatic energy storage. Taking skin and proximity effect into consideration, the magnetic field strength distribution is inferred based on magnetic field energy storage principle. Electrostatic and magnetic Simulation models of are established respectly in the finite element software ANSYS. And distributions of current, voltage and magnetic field intensity are got. Then leakage inductance and stray capacitance are obtained by the electromagnetic macro. Neglecting the influence of stray capacitance, the leakage inductance of different frequency is measured by digital electric bridge in condition of short circuiting the secondary side. Three measuring methods of stray capacitance is compared, and the stray capacitance of the model is measured by resonance method using impedance analyzer. The results got through the 3 methods are matched up generally. To sum up. the value of stray capacitance is related to structural sizes of MFT and voltage distribution. While leakage inductance is not only related to structural parameters, but also decreases with the increase of operating frequency. Lastly, to make the main insulation structure more compact, a simplified electrostatic field simulation model is established in ANSYS. An angel ring is set on the end of high voltage winding. Different influencing factors of Emaxare analyzed through the scanning method and the optimum sizes are found. The distance between the high voltage winding and iron yoke is shorten to 11mm and the distance between two windings is shorten to 12mm. The optimized Emax is 7.98kV/mm, less than initial discharge value. This insulation structure is compact and reliable.