Control Scheme for a Cascaded Multilevel Converter Used in Low-Voltage-Ride-Through Tests of Grid-Connected Wind Turbines

This paper describes the implementation of a control scheme for a symmetrical cascaded multilevel converter used as a fault generator for low-voltage-ride-through (LVRT) tests of grid-connected wind energy conversion systems (WECSs). The converter is able to emulate the main faults that can happen in real power systems, with no significant impact on the grid during the LVRT tests. The proposed control scheme is divided in two levels: a central unit and local controllers in each converter power cell. The cells are composed of a back-to-back single-phase full-bridge converter and a local controller, a Digital Signal Processor (DSP), that controls the DC bus voltage through the grid-side converter and performs the modulation of the output-side converter. The central controller is also a DSP, that generates the voltage references and the synchronization signals for the power cells. Experimental results of LVRT tests are shown to demonstrate the good performance of the converter and the proposed control scheme.