This paper proposes a grid impedance monitoring system for distributed power generation electronic interfaces. The system estimates the grid equivalent impedance and voltage source from the voltage measurements performed at the point of common coupling. The estimation algorithm is based on a recursive least-squares algorithm implemented in the complex field. Simultaneously, the system evaluates the quality of the estimation, minimizing its influence on the grid and detecting islanding situations. The proposed system performance has been evaluated under experimental testing.

This paper presents a current control scheme for a voltage-source converter connected to the grid through an LCL filter. The proposed current control is based on the linear quadratic (LQ) Gaussian (LQG) servo controller technique, which combines the LQ regulator and the Kalman filter (KF). The use of the KF instead of a Luenberger estimator improves the quality of the estimation of state variables in noisy environments, increasing the noise immunity of the control loop. This paper focuses on giving some criteria and practical hints to choose both the KF and LQ design parameters to achieve null steady-state tracking error, channel decoupling, and noise immunity. Furthermore, this paper analyzes the steady-state KF approximation that allows a significant decrement in the computational complexity of the algorithm at nearly no cost in accuracy. Simulation and experimental results prove the correct operation of the proposed LQG current control and the possible advantages of using KF to estimate the state vector.

This paper presents the real-time (RT) power-hardware-in-the-loop (PHIL) emulation of the two most popular variable-speed wind turbines (WT): the doubly-fed induction generator WT with partial-scale power converter and the permanent-magnet synchronous machine WT with full-scale power converter. RT-PHIL emulation offers a realistic, controlled, and easily reconfigurable test environment for analyzing grid integration problems, in which classic tools frequently show a lack of capacity. The proposed discrete-time models and implementation methodology result in creation of platform-independent models suitable for application on any RT platform. A voltage-source converter is used as PHIL power interface between the RT simulation and a microgrid testbed. This test environment is of interest for verification of high-level control strategies for wind farms and for evaluation of the response of hardware systems, such as active filters or static synchronous compensators (STATCOMs) connected to a grid (or microgrid) in presence of wind power generation. The RT discrete models of both WTs are demonstrated in a grid-connection experiment.

Recently, the topic of grid integration of distributed energy resources (DERs) has been closely linked with developments in power electronics research. The experimental evaluation of control strategies for power interfaces usually requires a significant allocation of both time and resources, and this is why the power hardware-in-the-loop (PHIL) techniques have been introduced to help accelerate the design and evaluation process. This article presents a PHIL-based microgrid test bed, Smart Energy Integration Lab (SEIL), that has been specifically designed for evaluation studies on the grid integration of DERs, with an emphasis on the system dynamic performance and power dispatch scenarios. The SEIL is equipped with a remotely controlled network configuration, power electronics converters, passive load banks, a battery storage system, and real-time (RT) control and acquisition systems that allow the emulation of several microgrids and connection of several DERs at the same time. To demonstrate the functionality of the SEIL, experimental results for two typical scenarios will be presented: 1) a power dispatch scenario in a microgrid and 2) a wind turbine (WT) operation in grid connection.

This work presents a comparison of three different modulation techniques applied to modular multilevel converters (MMCs). The three modulation strategies studied in this paper are the phase-shifted sinusoidal pulse width modulation (PS-SPWM), the space-vector modulation (SVM) and the nearest level modulation (NLM). This paper focuses on analysing the particularities and implementation of each modulation technique. The modulation technique largely defines the generated harmonic content, making this is a key point that must be studied in depth. The paper briefly describes the three modulation techniques and analyses the harmonics generated by each one of the methods. In addition, the paper presents and compares the digital implementation of the three modulation methods in a Field Programmable Gate Array (FPGA). The proposed approaches are validated using a real processing platform and experimentally evaluated in a real high-power six-level MMC.