UNINTERRUPTIBLE POWER SUPPLY, UPS, FOR CONNECTING A MULTIPHASE LOAD TO AN AC SOURCE AND A DC SOURCE

The invention relates to a method for operating an uninterruptible power supply, UPS, (1, N) for connecting a load (2) to an AC source (3) and a DC source (4), the UPS (1, N) comprising a first converter assembly (5) comprising at least two first converters (6) connected on a first side to the AC source (3) and on a second side to a split DC link (7) comprising two halves with a midpoint reference (8), a second converter assembly (9) comprising a second converter (10) connected on a first side to the DC link (7) and on a second side to the load (2), a third converter assembly (11) comprising a third converter (12) connected on a first side to the DC source (4) and on second side to the DC link (7), a first controlled device (15) configured for disconnecting at least two of the first converters (6) from the AC source (3), and a second controlled device (16) configured for connecting at least two of the first converters (6) together, and comprising the step of: Opening the first controlled device (15), and closing the second controlled device (16) such that the first converter assembly (5) transfers energy between the split DC link halves (8) to maintain voltage regulation of the DC link (7) in respect to the midpoint reference (8).