An Integrated Synchronization and Control Strategy for Parallel-Operated Inverters Based on <i>V</i>–<i>I</i> Droop Characteristics
Wenyuan Cao, Minxiao Han, Xiahui Zhang, Yajuan Guan, Josep M. Guerrero, Juan C. Vásquez
Abstract
Voltage–current ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> – <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> ) droop control is becoming a promising alternative to achieve proper power sharing among the parallel-operated voltage-sourced inverters (VSIs). Compared with the conventional droop control, this method has a simpler structure and a better dynamic performance because it saves the power loop (including the low-pass filters). However, due to the absence of the power loop related to the system frequency, the synchronization of VSIs becomes an intractable issue. To tackle this, a communication-free algorithm to synchronize and control the VSIs is proposed. Initially, the principle of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V–I</i> droop control is presented straightforwardly based on the virtual impedance. Then, a synchronization algorithm is incorporated into the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V–I</i> droop controller for accurate power sharing. This algorithm adaptively adjusts the phase step of each VSI according to the phase differences between VSI and the ac bus. In this way, the phase differences of each VSI to the ac bus eventually converge to an identical value, thus realizing the synchronization of VSIs. A detailed parameter design method is also implemented considering both system stability and power quality. Finally, the proposed strategy is validated experimentally using a three-parallel-VSIs system.