Global Loss Evaluation Methods for Nonsinusoidally Fed Medium-Frequency Power Transformers

Medium-voltage conversion systems, such as traction and grid-connected converters, are continuously evolving toward higher power densities. Consequently, volume, weight, and material reductions are becoming major design issues, which lead the research focus toward high-/medium-frequency isolated power conversion systems. An optimized design of these conversion systems requires a detailed transformer-loss evaluation considering both copper and core losses. This paper presents a simple and flexible methodology to analyze global medium-frequency transformer losses in an isolated dc–dc converter fed by general nonsinusoidal waveforms. Various loss evaluation approaches are considered, pointing out their validity and limitations by means of finiteelement simulations and experimental tests

Transient Thermal Model of a Medium Frequency Power Transformer

Nowadays medium voltage conversion systems, like traction and energy distribution systems, aim to increase power density. Volume, weight and material reduction are gaining the market today. Due to the development of new high power semi-conductor devices a new working frequency range is conceived, where magnetic component reduction is feasible. However, along with the reduction of magnetic component size, the cooling surface is reduced, increasing the equilibrium temperature of the magnetic components. The core element of these new conversion systems is the medium frequency power transformer. In this paper a dynamic thermal model is introduced, with equivalent non-linear temperature dependent thermal resistances. Besides, heat sources are determined taking into account non-sinusoidal excitations, and frequency dependent effects. For our case study, the model shows natural heat dissipation limits, pointing out the cooling requirements throughout the design process

Analysis of Empirical Core Loss Evaluation Methods For Non-Sinusoidally Fed Medium Frequency Power Transformers

Nowadays medium voltage conversion systems, like traction and energy distribution systems, aim to increase power density. Volume, weight and material reduction are gaining the market today. Due to the development of new high power semiconductor devices a new working frequency range is conceived, where magnetic component reduction is feasible. The core element of these new conversion systems is the medium frequency transformer. For an optimized design of the conversion system, the correct determination of transformer core losses is essential. Sinusoidal approaches, accurate enough for line frequency transformers, do not cope with the non-sinusoidal waveforms of future medium voltage conversion systems. In the past few years several empirical methods derived from the original Steinmetz equation have been introduced to determine magnetic core losses for non-sinusoidal waveforms. This paper proposes extended expressions for core loss determination in case of rectangular three level voltage waveforms, typical of high power converters. These expressions are compared in simulation and with experimental results obtained in the same conditions using a reduced scale dc-ac conversion system prototype that includes a 61.6 kVA/2 kHz transformer. It is shown that some of these techniques allow a considerable loss estimation accuracy (error below 12% for any ratio)

Photovoltaics in microgrids. An overview of grid integration and energy management aspects

The microgrid vision contains several aspects, and a commonly admitted one is a portion of grid with its own means of production and energy flow controls. Photovoltaic (PV) generation is geographically the most distributed means of electricity production. In this sense, the integration of PVs in microgrids seems natural. The intermittency of PV generation can be compensated not only by using energy storage technologies but also by demand-side management and exchanges with other power networks: the main grid and surrounding microgrids. Many aspects still have to be investigated in the fields of power electronics, information communications technologies (ICTs), protections, and power quality (PQ) issues, to make this association a reality