An uninterruptable power supply system for providing power to a load coupled to a utility power source , said uninterruptable power supply system comprising: a doubly-fed induction generator comprising a stator coupled to the load , and a rotor , said stator and said rotor magnetically coupled together, wherein said doubly-fed induction generator is configured to generate an auxiliary power output; a rechargeable energy storage system ; a first inverter coupled between said rotor and said rechargeable energy storage system ; a prime mover coupled to said rotor ; and a controller communicatively coupled to said doubly-fed induction generator , said prime mover , and said first inverter , said controller configured to: cause said prime mover to be turned off when the utility power source is functioning properly; detect a power disturbance associated with the utility power source ; in response to the detected power disturbance, in a first operational mode while said prime mover is still turned off, control said first inverter to provide an excitation input to said rotor , wherein said doubly-fed induction generator is further configured to operate as a transformer to generate, from a rotating magnetic field induced into said stator by the excitation input from said first inverter , the auxiliary power output to be provided to the load ; and while in the first operational mode, activate said prime mover to operate the uninterruptable power supply system in a second operational mode. a doubly-fed induction generator comprising a stator coupled to the load , and a rotor , said stator and said rotor magnetically coupled together, wherein said doubly-fed induction generator is configured to generate an auxiliary power output; a rechargeable energy storage system ; a first inverter coupled between said rotor and said rechargeable energy storage system ; a prime mover coupled to said rotor ; and a controller communicatively coupled to said doubly-fed induction generator , said prime mover , and said first inverter , said controller configured to: cause said prime mover to be turned off when the utility power source is functioning properly; detect a power disturbance associated with the utility power source ; in response to the detected power disturbance, in a first operational mode while said prime mover is still turned off, control said first inverter to provide an excitation input to said rotor , wherein said doubly-fed induction generator is further configured to operate as a transformer to generate, from a rotating magnetic field induced into said stator by the excitation input from said first inverter , the auxiliary power output to be provided to the load ; and while in the first operational mode, activate said prime mover to operate the uninterruptable power supply system in a second operational mode. cause said prime mover to be turned off when the utility power source is functioning properly; detect a power disturbance associated with the utility power source ; in response to the detected power disturbance, in a first operational mode while said prime mover is still turned off, control said first inverter to provide an excitation input to said rotor , wherein said doubly-fed induction generator is further configured to operate as a transformer to generate, from a rotating magnetic field induced into said stator by the excitation input from said first inverter , the auxiliary power output to be provided to the load ; and while in the first operational mode, activate said prime mover to operate the uninterruptable power supply system in a second operational mode. <EOS>