Jordi de la Hoz

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.

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.

Variable Structure Control for Three-Phase LCL-Filtered Inverters Using a Reduced Converter Model

This paper presents a new concept in active damping techniques using a reduced model of an LCL -filtered grid-connected inverter. The presence of the LCL filter complicates the design of the inverter control scheme, particularly when uncertainties in the system parameters, especially in the grid inductance, are considered. The proposed control algorithm is addressed to overcome such difficulties using a reduced model of the inverter in a state observer. In this proposal, two of the three state variables of the system are obviated from the physical inverter model, and only the inverter-side current is considered. Therefore, the inverter-side current can be estimated emulating the case of an inverter with only one inductor, thus eliminating the resonance problem produced by the LCL filter. Besides, in the case of a distorted grid, the method allows us to estimate the voltages at the point of common coupling free of noise and distortion without using any phase-locked loop based synchronization algorithm. This proposal provides the following features to the closed-loop system: first, robust and simple active damping control under system parameters deviation; second, robustness against grid voltage unbalance and distortion; and third, an important reduction in the computational load of the control algorithm, which allows us to increase the switching frequency. To complete the control scheme, a theoretical stability analysis is developed considering the effect of the observer, the system discretization, and the system parameters deviation. Experimental and comparative evaluation results are presented to validate the effectiveness of the proposed control scheme.

A general MILP model for the sizing of islanded/grid-connected microgrids

Recently, energy systems have experienced a change of paradigm, from a large-scale centralized approach to the in-situ exploitation of renewable sources. Special attention has been paid to microgrids, a particular case of distributed generation where consumer nodes include generation and can be either grid-connected or isolated. This work aims to develop a general model to determine the optimal sizing of an energy system under fixed conditions and to analyze the effect of considering different cycle patterns on the solution. The mixed integer linear programming (MILP) formulation proposes allows determining the best combination of available technologies that satisfies the demand of a given set of scenarios at minimum total cost. The model has been implemented using AIMMS and applied to a case study consisting of a five-member Mediterranean house. The results obtained reveal the need to select the most convenient time cycles for defining the scenarios of the sizing model.

An optimization model for the long-term planning of energy distribution networks

Although design is a problem that has been addressed in the literature, maintaining, upgrading and expanding energy distribution networks along the entire life-cycle is a topic that has received scarce attention. The problem includes considering the long-term dependence of the efficiency of the investments along their life span. This work presents a novel model for the optimization of energy distribution networks considering the decaying performance caused by equipment aging. Increasing maintenance costs have been included to model the decaying performance, thus giving rise to an MINLP formulation. A simplified case study based on a real electricity distribution network has been used as a test bed for the proposed approach. Results show that unrealistic sizing and planning solutions are obtained when the decaying performance is not considered and demonstrate that the proposed MINLP overcomes such limitations.