The article presents the results of the transients investigation of DC-DC converter as a part of energy storage system in the frequency-controlled electric drive, which works in hard start-up and braking modes with a large number of cycles per hour. The ultracapacitor is used as an energy storage element, which is connected to DC link of frequency converter through the bidirectional DC-DC converter. The block diagram of alternating current electric drive is described. The circuit diagram and the equivalent circuit of the energy storage system, and also the simulation results of processes in the system «energy storage device - frequency converter DC link» in order to identify the specific properties of the system as a control object are shown. In the article, a detailed analysis of the system responses to the control and disturbing actions under different operating conditions of the electric drive are considered. As a control action, a control voltage signal of the DC-DC converter is used, the disturbance for this system is the current from the voltage source inverter DC link of the frequency converter. Processes in the system are considered in three modes of operation, which simulate the starting, braking and sequential operation of these modes in a frequency-controlled electric drive. In conclusion, the specific properties of DC-DC converter are formulated. The presented investigation results show the operation modes features of DC-DC converter as part of an energy storage system. This information can be used for choosing the structure and the method of controllers synthesis for the automatic control system of bidirectional DC-DC converter.