Due to the increasing integration of distributed generators, the grid underlies conceptual challenges. To ensure an efficient and reliable operation, more controllable assets are expected in the future grid. The transformer as one of the most important components possesses only limited control capabilities today. By combining a conventional transformer with a converter, a hybrid transformer with comprehensive controllability is obtained. In this paper suitable concepts are investigated and an optimum design is determined to compare its potential to conventional and solid state transformers.

Due to the increasing integration of renewable energy sources and power electronic loads into the distribution grid, a deterioration of the grid power quality is expected. Consisting of a low frequency transformer and a fractionally rated power electronic converter, the hybrid transformer can be applied to ensure a high power quality by controlling voltage, current, active and reactive power dynamically. For the application in grids with conventional grid protection infrastructure, hybrid transformers have to withstand considerable overvoltage and -current stresses during voltage surges or grid short circuits. Since the semiconductors are less robust than low frequency transformers with respect to these stresses, the effects of possible fault scenarios and a protection concept are studied in this paper. TABLE I: Specifications of the investigated 400 kVA, 20 kV-400 V HT. Parameter Value S n 400 kVA V MV , V LV 20 kV RMS , 400 V RMS , line to line Controllability ±10% of nominal V (LV side), P and Q S conv , V ser 40 kVA, 23 V Topology 2-level voltage source inverters, back to back DC-link 70 V, 5 mF, actively balanced split DC-link

Studies indicate that modulating enterocyte metabolism might affect whole body glucose homeostasis and the development of diet-induced obesity (DIO). We tested whether enhancing enterocyte fatty acid oxidation (FAO) could protect mice from DIO and impaired glycemic control. To this end, we used mice expressing a mutant form of carnitine palmitoyltransferase-1a (CPT1mt), insensitive to inhibition by malonyl-CoA, in their enterocytes (iCPT1mt) and fed them low-fat control diet (CD) or high-fat diet (HFD) chronically. CPT1mt expression led to an upregulation of FAO in the enterocytes. On CD, iCPT1mt mice had impaired glycemic control and showed concomitant activation of lipogenesis, glycolysis and gluconeogenesis in their enterocytes. On HFD, both iCPT1mt and control mice developed DIO, but iCPT1mt mice showed improved glycemic control and reduced visceral fat mass. Together these data indicate that modulating enterocyte metabolism in iCPT1mt mice affects glycemic control in a body weight-independent, but dietary fat-dependent manner.

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Large inrush currents can occur when power transformers are connected to the grid. In the past, a wide range of countermeasures has been developed whereof the application of power electronic converters recently received more attention. With hybrid transformers combining a conventional transformer with a power electronic converter, the elimination of inrush currents can be realized without considerable additional hardware effort. For such hybrid transformers, a novel mitigation procedure is proposed, which is based on the injection of a synchronous core flux before the transformer is connected to the grid. In addition, dedicated demagnetization strategies applicable for grid connected converters with filter elements are presented. The effectiveness of the proposed procedure is verified by comprehensive simulations and a comparison to a conventional method.