The aim of the project is to develop and verify a new state-of-the-art high power density energy storage system with supercapacitors and bi-directional DC/DC converter. The system is intended to be used as a power cache in power supply systems with bi-directional power flow (traction supply systems, high-power multidrive systems, active filters, etc.). The main parts of this system are the supercapacitor stack and interface converter with dedicated control, protection and supervision systems. The converter transfers power from high-voltage source to supercapacitors operating at a lower voltage level. In case of interruptions of the main supply the power is transferred from the energy storage to the load. The main challenges here are the improved efficiency and increased power density (power per unit volume) of the bi-directional DC/DC converter. The circuit schematic proposed for the bi-directional DC/DC converter is the Dual Active Bridge (DAB) with three-phase auxiliary AC link. The proposed topology utilizes two identical three-phase inverters (in input and output stages of the converter), each operating in a six-step mode with controlled phase shift. Use of two active three-phase inverters allows both a bi-directional power flow and control at a fixed frequency. A substantial and very broad application fields is transportation, where the proposed DC/DC converter could store the regenerative braking energy and utilize it at high power demands (acceleration of trams), thus optimizing the efficiency of the traction supply system. The hardware novelties to be studied and tested during project work are the high-frequency isolation transformers with planar structure and low leakage inductance, last generation power MOSFETs and low loss IGBTs, new cooling methods, and isolation materials. One of the main challenges of research is the development of design methodology and control algorithms for the DAB converter to improve the efficiency and reliability of the system. Application of such an energy storage system requires serious tests on different operating conditions, such as high power peaks, power fall and oscillations. This is essential in the selection of appropriate elements for the DC/DC converter and other schemes (filtering, protection etc.) that are related to safe and efficient operation of energy storage. Since digital control of bi-directional power switches is used, the effectiveness, reliability and safety mostly depend on the control system. That is the reason why the redundant protection and diagnostic algorithms must be considered for the converter during charging and discharging of supercapacitors.