The progress in VSCs and the present development of ZSCs have promoted in the last decade an important improvement in the electrical installations of renewable energy, distributed generation and microgrids, and the use of the high voltage technology in direct current (HVDC) for the electric power transmission produced in the offshore wind plants. However, the presence of these converters may cause instabilities in the electrical installations at frequencies below 2 kHz due to the converter control interaction with poorly damped resonances. The present development of VSC and ZSC MVDC grids imposes the analysis of this stability issue. In these circumstances, the specific goal of the project is the study of the stability in ZSC MVDC systems with renewable energy technologies and energy storage systems due to the interaction between the system resonances and the converters. The aim of the project will be the analysis of the possible instabilities and the proposal of solutions to the stability problem. For this, the work takes into account the following tasks. The MVDC grid characterization in the s-domain and the analysis of the MVDC grid frequency response and resonances. It consists in the MVDC grid modelling, the determination of the MVDC grid equivalent impedance from the converter buses, the characterization of the resonance frequencies below 2 kHz and the sensitivity analysis of resonances due to variations of the grid parameter values. The characterization of ZSCs and their control. It consists in the ZSC modelling with Thevenin or Norton circuits, the characterization of the equivalent impedance of these circuits, the determination of the frequency range for which the ZSCs are not passive and the sensitivity analysis of the converter passivity behaviour due to the variations of the ZSC parameter values. The study of the instability phenomenon in the ZSC MVDC systems from the MVDC model and the Thevenin or Norton circuits of the ZSCs. A stability analysis of the ZSC MVDC grids from the impedance-based representation of the system will be carried and a numerical study of the system stability will also be performed considering variations in the parameter values of the grid and converter in order to predict the possible system instabilities. The proposal of solutions which avoid or reduce the instabilities in ZSC MVDC systems. Modifications of the converter electrical and control parameters for reducing the grid instability will be studied and proposed. The experimental validation of the ZSC Thevenin/Norton models and the proposed procedures and solutions to the instability issue. VSC and ZSC MVDC experimental grids will be made in the laboratory for validating the Thevenin and Norton models of ZSCz, studying system stability conditions, applying the proposed solutions into the developed prototype and checking their effectiveness to ensure grid stability.
Plan Estatal de Investigación Científica y Técnica y de Innovación 2017-2020
Programa Estatal de I+D+i Orientada a los Retos de la Sociedad