New control method of a robust NPC converter for renewable energy sources grid connection

This paper presents a new control strategy for the three-level, neutral point clamped (NPC), voltage source converter. The converter can be used in many high-power renewable energy systems such as direct drive wind turbines and hybrid generating units. The developed control algorithm proposes an improved solution to balance DC bus intermediate voltages using space vector modulation (SVM) techniques. It aims to guarantee fault ride-through (FRT) capabilities of renewable energy plants equipped with multilevel converters. Since the main attention of this paper is the grid side converter control, the energy generating system was simplified to an equivalent variable current source. The grid side converter is connected to the grid through an LCL filter. Two controllers were developed to achieve grid currents regulation. The first controller is based on PI-regulation. In the second case, linear quadratic Gaussian (LQG) control is investigated. The control of DC bus voltage is achieved by PI-regulation. Performances of the two controllers and the improved modulation technique are compared and evaluated in terms of accordance with the grid connection requirements (GCR) including, low voltage ride-through capabilities, frequency variation and reactive power control.

► In this study, a new control strategy for the three-level, neutral point clamped (NPC) is proposed.
► In order to achieve grid currents regulation, two controllers were developed and their performances were compared.
► LQG currents controller gives better performances comparing to the PI solution in terms of reference tracking.
► In addition, this paper proposes a simple SVM method that can be used to ensure DC bus voltage control in NPC three-level converters.
► With the proposed SVM technique, the converter can ride-through voltage dips and its performances are in accordance with the GCR.