Abstract
In this article, a novel approach to determine the model of an independently controlled neutral leg is proposed. Based on this model, a cascaded control method is developed to provide a steady neutral point connection for a three-phase four-wire inverter and to balance the two dc capacitors voltages of a split dc-bus. Validation of the model is realised by simulations and the control has been tested by experiments. Results have shown good performance even with a high level of neutral current, which demonstrate the ability of the system to work within an unbalanced load or grid.
Highlights
T HE spreading of renewables and distributed generation in last decades has led to increase the use of grid-connected inverters
AC source voltage and frequency may differ from the grid values, and they can vary according to the working point
In order to connect AC sources to the grid, two stages are generally used: a first inverter is used as a rectifier and the second inverter connects the so-obtained DC bus to the AC grid
Summary
T HE spreading of renewables and distributed generation in last decades has led to increase the use of grid-connected inverters. If the neutral current is high and, especially, if it has a DC component, a fourth leg is needed to manage the neutral connection (conventional neutral leg topology) [15,16,17,18] In this configuration, the neutral leg cannot be independently controlled, and the overall system control strategy becomes rather complicated [19]. A novel approach to determine the ICNL analytical model, which considers capacitor parasitic resistances and the influence on the DC-link terminals of the neutral current, is carried out. Once the transfer function between the neutral leg current and the fourth-leg duty cycle is determined and validated, a cascaded control of the capacitor’s voltage unbalance is designed and tuned.
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