Dual converter (circulating-current mode) In a dual converter operated in circulating-current mode, when does the internal circulating current flow?

Introduction / Context:
A dual converter consists of two fully controlled bridges connected in antiparallel on the DC side. In the circulating-current mode, both bridges are kept active so that instantaneous reversal of DC voltage polarity is possible without discontinuity. Understanding when and why circulating current flows is essential for correct reactor sizing and protection.

Given Data / Assumptions:

• Two line-commutated bridges share the same DC terminals.
• Bridges are set to provide slightly different average DC voltages to establish bias.
• A series reactor is present to limit circulating current.

Concept / Approach:
In circulating-current mode, one bridge operates as a rectifier while the other is biased as an inverter. Their instantaneous EMFs are not exactly equal at all times. Any small difference drives current through the interposed reactor, creating a closed loop between the two bridges even if the external load current is momentarily zero.

Step-by-Step Solution:
Identify that both bridges are gated continuously to enable four-quadrant capability.Recognize that the bridges’ instantaneous DC voltages are out of phase, producing a voltage difference.This voltage difference drives a loop current through the reactor and bridges, independent of external load.Therefore, circulating current exists even at zero load current.

Verification / Alternative check:
Waveform sketches show finite reactor current during no-load intervals; turning off one bridge (non-circulating mode) removes this internal current, but sacrifices instantaneous reversal.

Why Other Options Are Wrong:

• “when load current is zero” is incomplete; the key is that it flows even with zero load current.
• “when load current is non-zero” is not the defining condition.
• “only during load transients” ignores steady-state internal loop current.
• “none of the above” is incorrect since the correct condition is listed.

Common Pitfalls:

• Assuming circulating current equals load current; it is an internal loop current limited by the reactor.
• Under-sizing the reactor, leading to excessive losses and device stress.

Final Answer:
even when load current is zero