Design of a Protection Concept for a 100-kVA Hybrid Transformer

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 (HTs) can be applied to ensure a high power quality by controlling voltage, active and reactive power dynamically. For the application in conventional grids, HTs have to withstand considerable overvoltages and overcurrents during voltage surges or short circuits. Although HTs recently gained increasing research interest, protection requirements and protection concepts have not been discussed yet. These aspects are, however, essential to evaluate the practical potential of HTs. In this article, protection requirements are derived and a protection concept is developed and verified by simulations and experiments for a 100-kVA 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 HT only insignificantly. An efficiency of 98.48% is achievable for an HT including protection, which is only slightly lower than the efficiency of 98.64% for a comparable conventional transformer. This renders the HT a promising solution for today's and future ac grids.