This paper presents a series converter in an application with a Custom Power Active Transformer (CPAT) which is a power electronics integrated transformer providing services to the grid and load. The active transformer structure adds shunt and series windings into three single-phase transformers. This structure improves a substation with the ability of dynamically regulating voltage and current through isolated power electronics converters. Additionally, the active transformer includes a Battery Energy Storage System (BESS) in order to perform voltage control with the series converter and an islanding mode or even a black start with the shunt converter. Throughout the paper, the stability analyses of the control algorithm regarding the series converter is presented and the proposed topology is validated through hardware-in-the-loop simulations as well as in a 200kVA prototype.