Power Reversal Algorithm for Resonant Direct Current Transformers for DC Networks

Abstract

The bidirectional resonant DC transformer has all the desired features to drive the development of the DC power distribution networks of the future. This power-electronics converter provides galvanic isolation between DC buses, has desired open loop properties, and follows the system's power flow naturally, requiring only a power reversal algorithm to allow a bidirectional operation and smooth transitions. This paper proposes a solution to tackle the power reversal of the DCT with a dynamic algorithm, considering the system behaviour and the power converter characteristics. The proposed algorithm executes the power reversal in few switching periods and removes the DCT losses during no-load condition. Then, using low voltage prototype of DC transformer, experimental results along the paper demonstrate how this power reversal algorithm is applied under various operating conditions.INDEX TERMS LLC converter, DC transformer, DCT, resonant converter, power reversal, power transition.

Dynamics analysis for DC transformers integration in hybrid AC‐DC power distribution networks

“…In order to operate, several key features are added to the controller scheme in Figure 3. A power reversal method (PRM) determines the active stage to have bidirectional power trans- fer [20,21] by comparing the DC bus voltages. An idle mode (IdM) detection method is deployed to limit the losses when no load is connected and an overcurrent limiter (OL) [22] can limit the power when high powers are being processed.…”

“…The model responses are based on the transfer functions G dvc (s) and G Vdc,P (s) whose expressions can be found in (21) and H(s) is computed via (16) using the expressions of R ′−1 DCT and C ′ provided in (22).…”

On Features of Direct Current Transformers

“…During this transient (soft-start initialization period), the DCT is strongly non-linear and partly discontinuous behavior until the modulation indices return to 0.5. For that reason, one important feature of the soft start strategy for DCTs is the ability to change the soft-start time duration accordingly to the power demand [1]. This initialization period can be set by monitoring power or voltage, matching the system's dynamics, and improving the transient dynamics.…”

“…In literature, there are several ways to identify the power direction. Some use the voltage gain and voltage measurements [17], [15], or resonant currents [18], or dc currents [1]. Any of these could be used to identify the power direction and properly issue the power reversal command for the DCT.…”

“…Essentially, the PRMs use current and voltage measurements to determine which power stage should be switching in order to guide the current, and in case of power reversal, the PRM block detects the change and set a command to alternate the active power stage. This feature of DCT is extensively investigated in [1].…”