Method for controlling a parallel line-interactive uninterruptible power supply system

A method for controlling a parallel line-interactive uninterruptible power supply (UPS) system having multiple line-interactive UPSs parallelly connected, wherein each line-interactive UPS has a switching element, a power output terminal connected to a load through the switching element, and a controller controlling the switching element to switch on or off and acquiring a switching status from the switching element, wherein the method is executed by the controller and comprises steps of:

• switching the switching element;
• acquiring the switching status from the switching element and recording a switching time of the switching element associated with the switching status;
• determining if the switching element is switched again;
• if the switching element is switched again, adjusting the switching time of the switching element associated with the switching status to be located within a preset range of an input voltage cycle;
• and
• communicating with the controllers of other line-interactive UPSs to synchronously operate all the switching elements.