Sistema de emulação de rede com conversor multinível em cascata e controle preditivo por modelo para teste de recursos energéticos distribuídos

Abstract

This Doctoral Thesis proposes a grid emulation system for Distributed Energy Resources (DER) testing using a cascaded multilevel converter, with the aim of mitigating problems related to two-level converters that exist in most grid emulators, such as low efficiency, greater presence of harmonics and bulky output filter. For this purpose, a method called Geometrical Modulated Model Predictive Control (GM2PC) is used for three-phase inverters using LC filters, which was developed in this work to solve problems found in the conventional MPC technique, such as variable switching frequency, high computational burden and spread harmonic spectrum. The aim of the method is to ensure the desired voltage synthesis by limiting the current through the filter inductor. In addition, the method presents low computational burden, fixed switching frequency, deals with the overmodulation issue, does not present weighting factor or cascaded control structure, and also has the ability to distribute the PWM signals among the modules of cascaded multilevel converters in a fast and efficient manner. To achieve this, the discrete-time modeling of the used configuration will be presented, followed by a convex optimization problem formulation with two quadratic inequality constraints to minimize the output tracking error. To solve the problem, each constraint is treated as a circular area on the space vector diagram, one for voltage synthesis and the other for current limiting. The Karush-Kuhn-Tucker conditions are used to obtain an unconstrained solution which identifies the mode in which the method should operate. The GM2PC method will be used in this work with a 5-level cascaded multilevel converter to demonstrate the operation of the grid emulation system prototype through commonly used voltage perturbation in DER testing. The results demonstrate that the method is able to work when using linear load, non-linear load absorbing or supplying power, unbalanced load, under short-circuit condition, besides being able to emulate grid impedance. The harmonic contents and execution times of the proposed method were also analyzed in comparison with other methods and for different converters. Finally, a theoretical proposal of a real grid emulation system with high power for DER testing is presented.