In this paper, a current sharing method based on the circular chain control (3C) method is proposed for controlling parallel inverters of unequal ratings in uninterruptible power supply (UPS) applications. Due to its circular structure, 3C is one of the most convenient methods which can be used in UPS as well as microgrid systems. However, the conventional 3C control strategy is only applicable to inverters of equal power ratings. The proposed method not only retains the circular structure of the 3C method, but also provides adaptability for the parallel operation of inverters with different power ratings. Moreover, this method adds hot-swap capability to the parallel inverter. A two-loop control structure is used to control the inverters. For proper current sharing, currents of inverters are conveyed in a circular structure with appropriate gains through control links. Simulation and experimental results for linear and nonlinear loads verify the effectiveness of the proposed strategy.

Distributed power generation systems use power electronics converters as interface for connection to grid. The connection through an LCL filter offers certain advantages, but it brings also the disadvantage of having a resonance frequency. Power electronic converter control is challenging due to filter resonance peak. In this paper a three level ultra-efficient converter called T-Type converter is connected to grid through an LCL filter and its injected current is controlled by PR controller. The resonance peak of the filter is also damped using filter capacitor current feedback. A step by step mathematical design procedure for controller and filter capacitor current feedback coefficient is investigated in details. Simulations are carried out in MATLAB/Simulink environment and results depict suitable performance of the system with designed parameters.