Johannes Burkard

Due to the increasing integration of renewable energies 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, hybrid transformers can be applied to ensure a high power quality by controlling voltage, active and reactive power dynamically. For the application in conventional grids, hybrid transformers have to withstand considerable overvoltages and overcurrents during voltage surges or short circuits. Although hybrid transformers recently gained increasing research interest, protection requirements and concepts have not been discussed yet. These aspects are, however, essential to evaluate the practical potential of hybrid transformers. In this paper, protection requirements are derived and a protection concept is developed and verified by simulations and experiments for a 100kVA hybrid distribution transformer. Although the protection concept has a considerable impact on the design and performance of the converter, it increases the total losses and volume of the hybrid transformer only insignificantly. An efficiency of 98.64 % is achievable for a hybrid transformer including protection, which is only slightly lower than the efficiency of 98.48 % for a comparable conventional transformer. This renders the HT a promising solution for today's and future AC grids.