Flexible transformers for distribution grid control

Increased load demand and penetration levels of renewable energies are imposing great challenges in maintaining the voltage profile of the distribution feeders at high quality. Various solutions such as volt/VAR control and tap changers are employed in the system to regulate the secondary voltage within a typical band of ± 5%. This paper presents a novel device for distribution grid control. The proposed device, called flexible transformer, consists of a fractionally-rated converter and a slightly modified medium voltage (MV) distribution transformer with additional taps. The converter can be wired to either the primary or the secondary side of the transformer, resulting two different configurations. The proposed device successfully meets the requirements of great performance, long lifetime, high efficiency, and low cost for typical utility assets. This paper presents the concept of the flexible transformer as well as the schematic and operating principle of both configurations. Simulation and experimental results of the proposed device rated at 7.2 kV / 50 kVA is also provided.