A controller for controlling a power supply system that includes a plurality of uninterruptible power supplies (UPSs) (102, 104, 106), a ring bus, and at least one load electrically coupled to the plurality of UPSs (102, 104, 106) and the ring bus, said controller comprising: a processor; and a memory device communicatively coupled to said processor, said memory device storing executable instructions and configured to cause said processor to: calculate an output voltage frequency for each UPS of the plurality of UPSs; control operation of each UPS (102, 104, 106) based on the respective calculated output voltage frequencies; characterized in detecting that said at least one load has been disconnected from one UPS of said plurality of UPSs (102, 104, 106); and synchronize, in response to the detection, the output voltage frequency and an output voltage phase angle of said one UPS with a voltage frequency and voltage phase angle of said ring bus; wherein to detect that said at least one load has been disconnected, the controller is configured to detect that said at least one load has been disconnected based on whether an output voltage of said one UPS is negative and whether a DC link voltage of said one UPS is above a predetermined voltage threshold; and wherein to calculate an output voltage frequency for each UPS, said executable instructions are configured to cause said processor to calculate the output voltage frequency for a UPS as the sum of a predetermined frequency and the product of a slope and an output power of the UPS. a processor; and a memory device communicatively coupled to said processor, said memory device storing executable instructions and configured to cause said processor to: calculate an output voltage frequency for each UPS of the plurality of UPSs; control operation of each UPS (102, 104, 106) based on the respective calculated output voltage frequencies; characterized in detecting that said at least one load has been disconnected from one UPS of said plurality of UPSs (102, 104, 106); and synchronize, in response to the detection, the output voltage frequency and an output voltage phase angle of said one UPS with a voltage frequency and voltage phase angle of said ring bus; wherein to detect that said at least one load has been disconnected, the controller is configured to detect that said at least one load has been disconnected based on whether an output voltage of said one UPS is negative and whether a DC link voltage of said one UPS is above a predetermined voltage threshold; and calculate an output voltage frequency for each UPS of the plurality of UPSs; control operation of each UPS (102, 104, 106) based on the respective calculated output voltage frequencies; characterized in detecting that said at least one load has been disconnected from one UPS of said plurality of UPSs (102, 104, 106); and synchronize, in response to the detection, the output voltage frequency and an output voltage phase angle of said one UPS with a voltage frequency and voltage phase angle of said ring bus; wherein to detect that said at least one load has been disconnected, the controller is configured to detect that said at least one load has been disconnected based on whether an output voltage of said one UPS is negative and whether a DC link voltage of said one UPS is above a predetermined voltage threshold; and wherein to calculate an output voltage frequency for each UPS, said executable instructions are configured to cause said processor to calculate the output voltage frequency for a UPS as the sum of a predetermined frequency and the product of a slope and an output power of the UPS.