For a marine 690 Vac bidirectional converter based on a three-level flying capacitor, in order to make it start stably and transition to its normal working state, the flying capacitor must be pre-charged. This paper proposes a pre-charging strategy based on single-tube control which is suitable for three-level flying capacitors.

The on-off of the three-level outer tube is controlled on the basis of the flying capacitor, and efficient and fast pre-charging control is performed.

When this strategy is applied to a 690 Vac bidirectional converter, the pre-charging of the flying capacitor is completed within 1 second in Buck mode and less than 5 seconds in Boost mode. After pre-charging, the voltage of the flying capacitor remains at the value of half of the voltage of the high voltage side, and the voltage of the input side capacitor remains at the value of the input side voltage.

After experimental verification, the proposed pre-charging strategy based on controlling the outer tube of the three-level flying capacitor can be applied to a 690 Vac bidirectional converter device without the need for additional pre-charging power devices, which not only simplifies the topology but also ensures the reliable and fast pre-charging of flying capacitors.