The main problem in most existing control methods for uninterruptible power supply (UPS) inverters is their excessive complexity. This problem is successfully solved in multi-loop control methods by simple proportional controllers. Nevertheless, using the simple proportional controllers instead of conventional controllers, such as a proportional-resonant (PR) or a proportional-integral (PI) controller, leads to several disadvantages such as the steady state and transient errors. This paper investigates the multi-loop control scheme by proportional controllers using a reference frame transformation for single-phase UPS inverters. The proposed control method uses two nested proportional controllers in the synchronous reference frame. In fact, the steady state error of the UPS inverter in response to the ramp function changes to the steady state error in response to the step function using the reference frame transformation. Excellent tracking performance, no need to any derivative or integrator, fast dynamic response and simplicity of design and implementation are some advantages of the proposed method. The feasibility of the proposed control method is confirmed through simulations in MATLAB/SIMULINK.

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