The Duality Droop Control for Grid-Tied Cascaded Microinverter

Abstract

Cascaded Connected Microinverter (CCM) system takes the advantage of adapting low voltage stress submodules to build the high voltage output, which makes it easier and safer to achieve for many applications. The distribution of active and reactive power in the CCM system has always been interdependent, resulting in additional communication components in different submodules. Such communication components within different submodules can be avoided by droop control. Droop control is widely adopted in parallel inverter system, and it originates from the synchronous generator that active power is controlled by adjusting synchronous generator's frequency and reactive power is controlled by adjusting its output voltage. However, the traditional droop control is not suitable for the cascaded microinverter inverter system. Therefore, it's necessary to modify the droop control to make it suitable for Cascaded Microinverter system, and therefore a control method called inverse droop control is adopted for cascaded inverter system under island mode. However, it requires a large feeder inductor when it's grid connected since every submodule works as voltage source inverter. In this paper, a duality control method that feedbacks each submodule's active power and reactive power to adjust its inductor current amplitude and frequency respectively is proposed. Compared with traditional cascaded inverter system that's controlled by inverse droop control method, the big line frequency feeder inductor is saved.