Thesis Albert PEREIRA
“Design methodology of a medium frequency transformer for high voltage and high power DC-DC converters”
The transmission and distribution of electric power is normally made by ac networks (50 Hz or 60 Hz), where one of the key elements of this infrastructure is the power transformer; used for more than a century, its design is very well understood, with a level of operating efficiency normally greater than 99%.
In recent years, the share of renewable energy has been increasing. Often times the energy generated from renewable sources is produced far from consumption centers, and so transportation in the form of high voltage direct current (HVDC) over long distances is more profitable, due to the lower losses seen than with HVAC after a certain length of transmission line. In this case, we need power converters operating with Medium Frequency Transformers (MFT) from 1 kHz to tens of kilohertz. For these applications, the research of their maximum efficiency in operation is paramount. Increasing the transformer operating frequency has the beneficial effect of reducing its size.
However, a number of problems will appear with this frequency increase, such as: the increase in the losses in the conductors and the magnetic circuit that are related to the frequency; the less well understood winding type (Litz wire and foil) and magnetic materials (ferrites and nanocrystalline) in the MF that are different from those used at 50 Hz; the cooling is more complex because the power density is higher, etc.
In this thesis, a design methodology was developed in order to optimize the design of MFTs with respect to the compromise between accuracy and the length of calculations. In addition, analytical and numerical models were identified that can be used to accurately estimate the performance of an MFT. Furthermore, two MFTs (apparent power: 180 kVA and 1 kVA, respectively) were sized, manufactured and tested in order to demonstrate the domain of validity of the models, and also for optimization of the different models.
This work has enabled the development of a design methodology using the converter specifications and build a simulation with complete model of the transformer, which can then be used to validate an MFT design. We have highlighted: the influence of the technological parameters on the rise of resistance in the foil coils and the influence of the technological parameters on the magnetic properties of nanocrystalline materials.
This work was performed with the group “Materials for Electrical Engineering” Ampère laboratory and funded by the Institute for Energy Transition SuperGrid Institute.
