Helena Martín

Hierarchical control of power plants with microgrid operation

Microgrids are key elements to integrate renewable and distributed energy resources as well as distributed energy storage systems. In the last years, efforts toward the standardization of these microgrids have been made. In this sense, this paper presents the hierarchical control which consist of three levels: i) the primary control is based on the droop method, including an output impedance virtual loop; ii) the secondary control allows to restore the deviations produced by the primary control; and iii) the tertiary control manage the power flow between the microgrid and the external electrical distribution system. Results from a hierarchical-controlled microgrid are provided to show the feasibility of the proposed approach.

Variable Structure Control in Natural Frame for Three-Phase Grid-Connected Inverters With LCL Filter

This paper presents a variable structure control in natural frame for a three-phase gird-connected voltage-source inverter with LCL filter. The proposed control method is based on modifying the converter model in natural reference frame, preserving the low-frequency state-space variables dynamics. Using this model in a Kalman filter (KF), the system state-space variables are estimated allowing us to design three independent current sliding-mode controllers. The main closed-loop features of the proposed method are: first, robustness against grid inductance variations because the proposed model is independent of the grid inductance; second, the power losses are reduced since physical damping resistors are avoided; third, the control bandwidth can be increased due to the combination of a variable hysteresis comparator with the KF; and fourth, the grid-side current is directly controlled providing high robustness against harmonics in the grid. To complete the control scheme, a theoretical stability analysis is developed. Finally, selected experimental results validate the proposed control strategy and permit illustrating all its appealing features.

Study of Reduced-order Models of Squirrel-cage Induction Motors

Abstract This article examines several reduced-order models of squirrel-cage induction motors. The full-order model and its reduced-order models are used to calculate motor speed behavior during symmetrical voltage sags. The stability predicted by several reduced-order models of induction motors during symmetrical voltage sags with different depths and durations is also explored. Moreover, simulations of 36 motors of different power ratings are performed, and results are compared with simulations made with the classical fifth-order single-cage model and its corresponding reduced-order models. The main conclusion is that for speed stability evaluation purposes, even the reduced first-order double-cage model gives better results than the fifth-order single-cage model, which neglects the deep-bar effect.

A Family of Gradient Descent Grid Frequency Estimators for the SOGI Filter

This paper applies the adaptive gradient descent method to the second-order generalized integrator (SOGI) filter in order to find an online estimation algorithm for the grid frequency, which leads to the proposal of three possible estimators. One of them is identical to the frequency-locked loop algorithm reported in the literature, which proves that it should be understood as a gradient descent estimation algorithm and not as a “frequency locked loop.” The proposed gradient descent estimators are simple and suitable to be implemented into a digital processor with small computational burden. However, due to the SOGI characteristics, the estimators show to be especially sensitive to subharmonic and dc-offset voltage distortion. These problems are removed adopting a cascaded double SOGI approach, which strongly increases the rejection capability to harmonics and enhances the response to voltage sags. Simulation and experimental results are provided to validate the proposed contribution.