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Tesis:

New methods and protection systems for AC and DC power networks

  • Autor: GRANIZO ARRABÉ, Ricardo

  • Título: New methods and protection systems for AC and DC power networks

  • Fecha: 2018

  • Materia: Sin materia definida

  • Escuela: E.T.S. DE INGENIEROS INDUSTRIALES

  • Departamentos: AUTOMATICA, INGENIERIA ELECTRICA Y ELECTRONICA E INFORMATICA INDUSTRIAL

  • Acceso electrónico: http://oa.upm.es/52936/

  • Director/a 1º: PLATERO GAONA, Carlos Antonio

  • Resumen: This thesis focuses on the study and development of new protection methods and systems for power cables used in both AC and DC grids, from generation to distribution or transmission power networks. In AC networks there are many protection functions that allow clearing ground faults from the measurements of currents and voltages obtained from the measurement devices as current transformers, voltage transformers or both. In some applications at transition stations from overhead lines to underground lines, the up and running protection relays can detect the ground fault and develop a correct tripping of the circuit breakers associated to such overhead or underground line. Those protection relays don´t know actually if the ground fault has happened at the overhead side or at the underground side. This circumstance is extremely important in order to allow having auto-reclosing attempts or not. If the order to switch on again the circuit breakers and restore power supply is developed under active ground fault conditions, the consequences can be dramatic if the permanent ground fault is at the underground line side. If the isolation and dielectric characteristics of the underground cables were already relatively damaged, a direct attempt to switch on the circuit breakers will cause a permanent damage in such cables forever and they will have to be replaced by new ones with the consequent lose of power supply for long time. This work proposes new protection methods that identifies if the ground fault has happened at the overhead line side or at the underground cable side. It can release the autoreclosing attempts to minimize the power oscillations of the grid as much as possible when the ground fault has happened at the overhead line side, or block them when the ground fault happened at the underground cable side. So far, no protection relays manufacturers have developed any relay with these characteristics, just the commonly named as SOTF (Switch On To Fault) which is described in the state of the art. During the development of this thesis, some patents which are property of the Universidad Politécnica de Madrid have been obtained for both AC applications. One of them (P201431962: Method to Determine a Ground Fault Emplacement in Circuits with Transitions Overhead Line to Underground Cable Line) has been recently licensed (dated on 23-January-2017) to DSF Tecnologías para Motores y sus Aplicaciones, S.L.with contract nº NL170020064, as they are the Spanish Distributor of the German company Woodward-Seg which designs, develops and manufactures protection relays for low, medium and high AC voltage networks for the last forty years, and showed high interest in including the concepts of this patent in a new protection relay. One of the challenges of this work was to develop new cable protection methods that could be implemented in protection relays in the easiest possible way and, at the same time, with the lowest possible investments. All new methods request the application of current and voltage transformers or sensors actually in use worldwide. In DC grids there are not so many protection functions in operation because high voltage DC networks are not used widely. Nevertheless, power transmission lines also have protection systems to clear ground faults. Normally, DC power links are one-to-one and the main protections are included in the AC/DC converter at the DC side. Multiterminal grids are not installed in a great deal so no big experience is known when facing up the challenge of erecting these kinds of new DC grids. Thinking in new multiterminal grids in DC, more protections functions must be developed in order to grant full selectivity in such grids. New protection concepts for DC cable power networks have been evaluated, simulated and tested with good results for ground faults at the positive or negative cables. In the same way that happened with the research for AC networks, during the development of this thesis, other patents which are property of the Universidad Politécnica de Madrid were obtained for DC power cable protection. The most important task of this work was to develop new protection methods that could be implemented in protection relays in the easiest possible way and, at the same time, with the lowest possible investments. All new methods request the application of standard current sensors actually in use worldwide. This document starts with the description of the actual protection functions used for detecting and clearing ground faults at AC and DC power networks. It follows a complete description of the new detecting methods that includes their operating principles. The simulation tool used for signal analysis of currents and voltages is also described including simulation and laboratory test results. At the end of this document, the main conclusions and improvements in cable ground fault detection for AC and DC networks developed in this work are written. Future research lines are also described and included.