Design Considerations for a High Power Medium Frequency Transformer for a DC-DC Converter Stage of a Solid State Transformer

摘要

In recent years, the solid state transformer concept has challenged the conventional low frequency transformer. The conventional transformer cannot store energy and its output is easily distorted as a result of perturbations at its input. In same manner, disturbances from the output unit such as harmonics along with reactive power, as well as load transients are reflected back to the input of the conventional transformer. The size of the low frequency transformer is significantly larger. The Solid state transformer challenges the traditional low frequency transformer in that it eradicates the aforementioned drawbacks and provides multifunctional features.;In this thesis a reliable model to design and optimize a high power medium frequency transformer for a dc-dc converter that forms part of a solid state transformer is researched and established. The aim is to use this model to investigate how high can be the operating frequency for a medium frequency transformer to achieve maximum efficiency and minimum volume. The dc-dc converter consists of a transformer that provides isolation between a medium-voltage circuit and a low-voltage circuit in a distribution system, and power semiconductor devices. Transformer operation at medium frequency reduces size and volume due to the inverse relationship of transformer area product and frequency. However, at medium frequency, the transformer is less efficient as a result of increased losses due to skin and proximity effects and the temperature rise constraint. Unlike low power magnetic cores where there are standard sizes and dimensions, high power magnetic cores for medium frequency maybe designed depending on demand or in certain cases, using limited dimensional references. Thus, an optimised transformer design for high power medium frequency relies on how its dimensions are defined. The characteristics expected of a core material for high power medium frequency are that it should have a high saturation flux density; low core loss and the material should continuously operate at high temperatures. The findings revealed that the frequency can be as high as 10 kHz to achieve maximum efficiency and minimum volume. An optimum design depends upon the flux density, the winding current density, the numbers of primary turns, the operating frequency and the power level of the transformer. There is no point operating above 20 kHz as there is very little reduction in volume and the winding loss results to increased temperature and reduces the efficiency of the transformer.