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Abstract-In this paper, a method for the parallel operation of inverters in an ac-distributed system is proposed. This paper explores the control of active and reactive power flow through the analysis of the output impedance of the inverters and its impact on the power sharing. As a result, adaptive virtual output impedance is proposed in order to achieve a proper reactive power sharing, regardless of the line-impedance unbalances. A soft-start operation is also included, avoiding the initial current peak, which results in a seamless hot-swap operation. Active power sharing is achieved by adjusting the frequency in load transient situations only, owing to which the proposed method obtains a constant steady-state frequency and amplitude. As opposed to the conventional droop method, the transient response can be modified by acting on the main control parameters. Linear and nonlinear loads can be properly shared due to the addition of a current harmonic loop in the control strategy. Experimental results are presented from a two-6-kVA parallel-connected inverter system, showing the feasibility of the proposed approach.

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In this paper, a novel wireless load sharing controller for parallel connected online UPS inverters is proposed. As opposed to the conventional droop method, the proposed method achieves stable steady-state frequency and phase and an good dynamic response is obtained. A virtual output impedance is proposed in order to reduce its line impedance impact and to properly share nonlinear loads. Experimental results are presented from two 6-kVA UPS inverters controlled by DSP boards, showing the feasibility of the proposed approach.