The demands on electric power distribution grids have changed substantially compared to the time when the present systems were put into practice. The unremitting growth of electric energy consumption as well as the upcoming large-integration of Distributed En-ergy Resources (DER), based on Renewable EnEn-ergy Resources (RES), e.g. photovoltaic systems, wind generators, biomass, result in a gradually more complex electric network.
The traditional electrical power and energy system consists of bulk generation, a high voltage transmission grid, a medium and low voltage distribution system as well as the costumer. Such large and complex, non-linear system are prone to cascading failures due to single fault in transmission and/or distribution lines. The functionality and readiness of the power and energy system is a pre-requisite for the social and economic welfare of today´s society [1]. For those reasons, future distribution grids dictate new requirements on fault tolerance and service availability: in case of partial system failure, the system should be able to achieve its appointed objectives to the greatest possible extent without human guidance. For example, in the process of isolation of a fault, keeping the non-effective area under power is crucial. The restoration problem is usually a combinatorial problem owning to many combinations of switching operations that scale exponentially with system size. Several methods as centralized techniques, e.g. mathematical program-ming, complex and non-linear optimization, genetic algorithms, particle filtering, heuris-tics, knowledge based systems, etc., decentralized or Multi-Agent Systems (MAS) tech-nology have been proposed in the literature to solve this type of problem. These central-ized approaches mostly incorporate a centralcentral-ized architecture and therefore depend on a powerful central computing facility to handle huge amounts of data resulting in a potential single-point-of-failure [2]. The vision of a grid capable of dynamic optimization of grid performance, rapid response to disturbances and minimization of their impacts as well as fast recovery into a stable operation point with little or no human intervention, is shared by many working groups such as the IntelliGrid Initiative [3] and the European Smart Grids Technology Platform [4]. Consequently, distribution companies are required to in-vest in sophisticated network monitoring and control systems as well as to enhance pro-cesses used at present. One of the main goals of distribution companies is to reduce outage costs. This improves profitability and provides a better quality of electricity distribution.
A decline in outage costs involves different fields, out of which automatic fault manage-ment systems covers one part. This ´smart` concept is replicated throughout the electrical network under the notion of ´Smart Grid` (SG).
1.1 Research and motivation
With the increased deployment of DER into the grid, it becomes complex to manage the network operations. Nonetheless, SG applications help to improve the capability of elec-tricity producers and consumers. This radically changes the way the system and the net-work switching is controlled and implemented in case of faults. It demands the usage of remote measuring, communication and control systems for the switching equipment (lays, breakers and others) in the distribution network. This control has to detect and re-solve the faults in the least possible time in the distribution system; similar to how is realized at the transmission level. Thus, improving the reliability of power systems is an essential goal. This goal can be accomplished by realizing on the most important features of the smart grids, which is its self-healing ability. The centralized operation of Fault location, Isolation and Service Restoration (FLISR) function which is performed manu-ally by human operators will be converted to automated FLISR or self-healing function.
As a result, a system subjected to a fault will be able to automatically and intelligently perform corrective actions to restore itself to the best possible state in order to perform the basic functions without violating any constrains.
This automation is necessary in the network and has become the motivation of the thesis and thus the matter to research about. Given a power distribution network in a faulty state, the self-healing problem consists in finding the sequence of switching operation to reach the optimal operation state. In the case of smart grids, the complexity of smart grids as well as the complexity of power restoration increases because search space in presence of distributed generation, energy storage and mobile loads (electric vehicle) varies at each outage. However, observability of the smart grid network increases with the deployment of smart meters, Intelligent Electronic Devices (IEDs) in primary and secondary substa-tions and remote operable devices.
The thesis mainly consists on a literature study about the smart grid in terms of the self-healing functionality of the smart grids. In chapter two the central idea of self-self-healing utilized in smart grids is introduced. This was complemented with chapter three, where a detailed discussion about the electricity distribution system automatic fault management was conducted in order to create a framework around which the aim of the research is based. In the fourth chapter, the major principles and all the relevant areas concerning fault location, isolation and system restoration as a framework of the self-healing grid is introduced. An extensive review of the existing self-healing architectures and its applica-bility via FLISR algorithms is developed in this chapter. In chapter five, numerous case studies were referred among which a total of five were briefed to understand the context and the environment in which FLISR systems function as well as to bring together any lessons learnt from recent projects and the effect they had.
1.2 Research methodology and materials
The study, due to its novel character, has made use of the qualitative approach and ex-plored the literature published up to date with the analysis of its practical application aspects in present distribution network. The focus of the research was desk-based study in which a broad literature review was conducted by examining and using a number of secondary sources in the form of various information sources containing data that have already been collected and compiled. This included books and latest articles from sources including but not limited to IEEE and others.
The proposed qualitative data collection in form of the present thesis followed a continu-ous process. After a gathering of sufficient amount of information in form of numercontinu-ous books, articles and journal on the various issues on automation for the distribution net-work, a deep analysis of such was accomplished. In some cases analysis of the reports and publication for a better understanding was required. Once the ideas were defined and clear, a categorization into segments of all the information was carried out. This resulted in a preliminary framework with findings for the study, which made it easier to start the execution.