Contents Introduction
Due to the progress of energy transition, the structure, functionality and operation of the electrical power grid is changing [1]. The replacement of conventional power plants by decentralized energy generation systems using renewable energy sources is significantly changing the requirements for distribution and transmission grids. In addition, new dynamic loads, which are predominantly integrated into subordinate grids via converters, such as charging stations for electro mobility, can cause fluctuations in the grid voltage. This results in increasing harmonic and flicker effects and thus an additional impact on the grid voltage. There are several approaches to minimize these effects in future grids. A scenario discussed in research and among grid operators is a large-scale expansion of the grid. Further possible solutions to handle such effects as well as a general overload of individual grid sections include a significant expansion of the grid [2], [3] as well as increasing controllability. The most common way to adjust the grid voltage is to use tap changers. For variable adjustment of the transmission ratio of a conventional transformer, a mechanical tap changer is used to change between different taps of the variable winding. Due to this mechanical solution of voltage adjustment, only discrete voltage levels are possible and the control dynamic is noticeably limited. Additional possibilities to compensate and improve the voltage quality in transmission and distribution grids are various flexible ac transmission systems (FACTS) [4], [5], solid-state [6], [7] and hybrid transformers [8]. Furthermore, the requirement for additional system services like the provision of synthetic inertia, primary control power, synthetic excitation time constant and reactive power is increasing due to the further reduction of conventional power plants. Because transformers are the interface between different grids and voltage levels and are distributed over the entire electric power system, they offer the ideal position to implement extended controllability for the power grid. Solid-state transformers offer a high level of controllability, but with the disadvantage that the semiconductors have to be designed for the maximum transmission power. Furthermore, due to the small overload capacity of semiconductor modules, this topology leads to a significant reduction of the short-circuit power in the subordinated grid and therefore requires adjustments to the installed protection systems. In addition, the availability of such a transformer is critical, as the entire system fails if a single component - e.g. controller or semiconductor - breaks down. Higher operational reliability can be achieved by using hybrid transformers. If the converter system fails, a conventional transformer remains in place, so only the controllability is lost but not the power supply of the subordinate system. However, the hybrid transformer presented in [8] offers reduced scalability, because the series converter must be designed for the full rated current whereas the control voltage is small. This paper presents a new topology of a hybrid transformer, a three-phase power electronically controlled transformer, resulting by the integration of a back-to-back converter system in the magnetic circuit of a conventional three-winding transformer.
