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When a classical digital controller is applied to parallel inverters, there is a tradeoff with respect to the stability, dynamic response, and control accuracy. In a practical design, the stability of the system should be considered first. To ensure sufficient margin in stability in a digitally controlled system, the proportional gains in the feedback path are limited. To solve these problems, a linear voltage-control scheme with duty-ratio feedforward is proposed. The control parameters are chosen according to the stable operating condition derived in the z-domain. In this paper, the closed-loop transfer function and output impedance for both the classical controller and the proposed controller are derived theoretically. A comparison reveals the advantages of the proposed control scheme, i.e., a unity closed-loop gain, no phase shift, good current sharing, and low total harmonic distortion of the output voltage. The theoretical results are verified by the experimental setup of a system with two digitally controlled inverters connected in parallel.
Date of Publication: Dec. 2011