Autonomous Power-Source Regulation in Series-Connected Low-Voltage Microinverters

This paper is focused on a photovoltaic system architecture based on series-connected low-voltage ac (LVac) microinverters. In contrast to standard microinverters, LVac microinverters do not require high step-up conversion stages, which leads to potentials for improved efficiency and reduced cost. This paper proposes a distributed autonomous control strategy for the LVac microinverters, where each module is operated autonomously by a controller that regulates instantaneous output power. Modeling of system dynamics is challenging due to the nonlinear and time-varying nature of the system. Under quasi-static assumptions, a linearized small-signal model is obtained and solved along the operating points traversed during a line cycle. The modeling approach and controller design are verified by simulations and experiments on a scaled-down system consisting of three series-connected LVac microinverters, demonstrating appropriate ac voltage sharing across the modules. In certain cases, including a low compensator gain in the control loop, significant total harmonic distortion (THD) is observed in the ac line current. In order to reduce the distortion, a pre-distortion technique is used in each LVac control loop to compensate the sinusoidal input reference. The experimental results demonstrate that the predistortion technique reduces the THD to 3%-4% across a wide operating range, without the need for additional electro-magnetic interference filter stages.