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Modern grid codes worldwide determine specific requirements for the connection of wind energy conversion systems (WECSs) to the grid. One of the requirements is the ride-through fault capability (RTFC) or low-voltage ridethrough (LVRT), which defines that the plants must not be disconnected from the grid at certain levels of voltage sags and also must contribute to network stabilisation. The LVRT of the doubly-fed induction generator (DFIG) technology is a well-known problem, mainly during unbalanced grid voltage conditions. The present study extends the studies presented in the literature, analysing the DFIG behaviour during asymmetrical voltage sags using a frequency-domain modelling. A modified resonant controller is proposed to improve the system response during voltage sags, and the analysis of the RTFC based on the converter limits is carried out. The proposed strategy is validated with simulation results of a 2 MW-WECS and experimental results of a 25 kW scaled test bench.

This study presents a new methodology for modelling and handling with aggregated harmonic loads and linear loads in the harmonic power flow analysis. This method, so-called variable load method, can be used in balanced and unbalanced systems and for deterministic and stochastic studies. The main types of harmonic loads and their participation coefficients inside the facility are estimated by inspection or based on usual values. In addition, a new type of load model that represents the load variation and minor loads is also defined based on field measurements, in order to build a more adherent harmonic load representation. A stochastic analysis, based on Monte Carlo simulations, is also performed. This sequence of steps makes possible to evaluate if the harmonic distortion has a significant probability of exceeding the limits set by standards because of load variations. This analysis is performed jointly with the bootstrapping method for obtaining the probability density functions of the voltage harmonic distortions. Tests on a real plant show that the proposed approach is able to represent measured data with high precision.

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Resumo-Devido ao alto nível de inserção de usinas eólicas, na rede elétrica, estão sendo requeridos pelos códigos de conexão na rede, que as modernas turbinas eólicas contribuam para a estabilidade da rede elétrica, ajustando-se a freqüência através do controle da potência ativa e a tensão através da regulação da potência reativa. Baseado neste contexto, este trabalho apresenta uma nova estratégia de controle de potência reativa utilizando uma máquina de indução de dupla alimentação, um dos tipos de geradores mais utilizados atualmente em usinas eólicas no mundo. A estrutura e o projeto da estratégia de controle desenvolvida são apresentados, assim como, os resultados experimentais obtidos em uma bancada de testes de 4kW. Palavras-Chave-controle de potência reativa, energia eólica, fontes renováveis, gerador de indução de dupla alimentação.