Abstract-Low-voltage ride-through (LVRT) requirements demand wind-power plants to remain connected to the network in presence of grid-voltage dips. Most dips present positive-, negative-, and zero-sequence components. Hence, regulators based on symmetrical components are well suited to control gridconnected converters. A neutral-point-clamped topology has been considered as an active front end of a distributed power-generation system, following the trend of increasing power and voltage levels in wind-power systems. Three different current controllers based on symmetrical components and linear quadratic regulator have been considered. The performance of each controller is evaluated on LVRT requirement fulfillment, grid-current balancing, maximum grid-current value control, and oscillating power flow. Simulation and experimental results show that all three controllers meet LVRT requirements, although different system performance is found for each control approach. Therefore, controller selection depends on the system constraints and the type of preferred performance features.Index Terms-Distributed power generation, grid interface, multilevel conversion, three-level inverter, wind-power system.

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The increasing installed wind power capacity has caused wind power generation to become a significant percentage of the entire electric power generation. As a consequence, the power system operators have included wind power plants regulation to improve the control of the overall power system, both in steady-state and transient operation. Therefore, wind power systems are required to verify the grid connection requirements stated by the power system operators. In presence of grid voltage dips, the low voltage ride-through requirement compliance produces a mismatch between the generated active power and the active power delivered to the grid. The conventional solution assumes that the active power surplus is dissipated in a dclink resistor. In this work, a control scheme for the back-toback neutral-point-clamped converter is proposed. Under grid voltage dip, the controllers for generator-side and grid-side converters work concurrently to meet the low voltage ridethrough requirement by storing the active power surplus in the turbine-generator mechanical system inertia while keeping constant the dc-link voltage. Simulation and experimental results verify the proposed control scheme.Index Terms-Wind energy conversion, low voltage ridethrough, neutral point clamped converter.

This paper presents a novel approach for the connection of renewable energy sources to the utility grid. Due to the increasing power capability of the available generation systems, a three-level three-phase neutral-point-clamped voltagesource inverter is selected as the heart of the interfacing system. A multivariable control law is used for the regulator because of the intrinsic multivariable structure of the system. A current source (playing the role of a generic renewable energy source) is connected to the grid using a three-level inverter in order to verify the good performance of the proposed approach. Largeand small-signal d-q state-space averaged models of the system are obtained and used to calculate the multivariable controller based on the linear quadratic regulator technique. This controller simultaneously regulates the dc-link voltage (to operate at the maximum power point of the renewable energy source), the mains power factor (the power is delivered to the grid at unity power factor), and the dc-link neutral-point voltage balance. With the model and regulator presented, a specific switching strategy to control the dc-link neutral-point voltage is not required. The proposed controller can be used for any application, since its nature makes possible the control of any system variable. The good performance of the presented interfacing solution in both steady-state and transient operation is verified through simulation and experimentation using a 1-kW neutral-point-clamped voltage-source-inverter prototype, where a PC-embedded digital signal processor board is used for the controller implementation.Index Terms-Distributed power generation, grid interface, multilevel conversion, multivariable control, photovoltaic power system, three-level inverter, wind power system.

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