Solid State Transformer based on Grain-Oriented Electrical Steel Wound Cores

摘要

Considering the progress in semi-conductor technology and magnetic core material performances, an increased attention is paid to the so called Solid-State Transformers (SST). Besides the ability to monitor the direction of the power flow compared to classical 50/60 Hz transformers, these devices, composed of power converters placed each side of a high frequency power transformer, offer many advantages such as small size and low weight. The main focus of this paper is to report about the transformer itself as a key element of the SST. Thin grain oriented electrical steel (GOES) has been chosen for building the core of the transformer. Reasons for this choice are the high saturation polarization capability even at high temperature and its availability on a large scale compared to amorphous and nanocrystalline materials used in current SSTs; thus thin GOES seems to be a good alternative to achieve a technological and economical balance by playing on both frequency and working induction levels to reduce the size of the transformer. The aim of the study is to assess the limits of the GOES use in terms of maximum operating frequency and flux density for power electronic like voltages, offering both high power density and efficiency in reduced volume. The primary and secondary windings of the transformer are respectively connected to standard electronic converters that deliver square shaped voltages. Losses under such regimes far from classical sinus cases have to be assessed. They are the root cause for heating of the device and shall be known before energizing the transformer. Experimental results and finite-element simulation performed on the core structure, show that dynamic core losses are lower for square wave voltages than for sine waves at the same peak flux density and frequency. The maximum winding operating temperature obtained at thermal equilibrium fixes the operating limits of the device. Then to avoid issues due to eventual overheating when energizing the transformer, a thermal model suitable for predicting the temperature at various transformer locations has been developed and used; results are then described.