This thesis consists of nine chapters in total and it is organized as follows:
Chapter1 gives an overview of the thesis, its aims of the research and its contribution to the knowledge. It also sheds some light on the research techniques and methodologies used in the research. Chapter 2 provides a comprehensive literature review for new concepts in the
power engineering field such as DGs, microgrids, their implementations, interactions with one another and the larger power network. These concepts have been introduced recently and their implementations might differ in different parts of the world. This literature review is intended to provide the required background information, especially from microgrid protection perspective. Although relatively much work is done in other aspects of microgrids such as stability and islanding detection, not much effort has been reserved for fault levels and fault current calculations.
Following this review and the detection of the research field, Chapter 3 depicts the details of a conceptual microgrid protection system which is proposed to fill the knowledge gap outlined in the previous chapter. It reveals the connection of grid components with MCPU over communication lines, the process for collecting grid data and its analysis, the calculation of operating points and updating them in relevant grid components. It is also mentioned in this section, how this conceptual design is kept flexible and how several generators with different grid connection interfaces such as rotating machines, IIDGs or DGs with FCL can all be handled in single design.
The hallmark of microgrids is their dynamic structure which is bound to change very often. Therefore, Chapter 4 explains how the developed protection scheme is designed to automatically monitor changes happening in the microgrid and adapt the operating points accordingly. This is achieved by representing electrical networks as a graph with Object- Oriented (OO) models. Then, the steps of automatic operation through Dijkstra‟s algorithm are examined where, in an unprecedented manner, this shortest-path-finding algorithm is utilized to extract the microgrid structure. This automatic operation is also applicable for new deployments and this serves as a solid step towards plug-and-play purposes.
Chapter 5 analyzes the mathematical approach to fault levels and protective devices coordination calculations. This approach is formed to match with the flexible conceptual design and automated operation explained above. In a unique manner in microgrid research, Chapter 6 gives the full communication framework of the developed microgrid protection system according to IEC 61850 communication standard. This type of standardized communication ensures that any grid component will be compatible with the proposed microgrid protection system for communication purposes, regardless of its manufacturer or model. Also in this chapter, several extensions to this standard have been proposed for some crucial devices which are bound to be indispensible in microgrids in the near future.
Chapter 7 gives the details of implementation of the protection system with several simulation works performed and presents the results. Chapter 8 gives an insight about how the developed protection scheme can be used as a roll-back strategy or an additional support for other protection schemes in case of a failure. This chapter also provides valuable information about the interaction between the developed protection system and others, should they exist in the same microgrid. Finally, Chapter 9 summarizes the whole research work, highlights the contributions made and draws the conclusions.
Chapter 2
Literature Review on Microgrid Concept and Its Implementation
Publications pertaining to this chapter:
1) Taha Selim Ustun, Cagil Ozansoy, Aladin Zayegh, "Recent developments in microgrids and example
cases around the world--A review," Renewable and Sustainable Energy Reviews, Elsevier, vol. 15, pp. 4030-4041, 2011
2.1. Introduction
The prominence of generating electric power in very large steam-powered central power stations seems to have ended. The increased concerns for the environmental impacts of centralized coal-fired generation, most importantly those that relate to high CO2 emissions,
are the main factors driving the transition towards small-scale decentralized generation of power. Decentralized (distributed) generation of electricity most favorably occurs from renewable sources that are located on the distribution system close to the point of consumption. Governments and industries all around the world are increasingly looking for ways to reduce the greenhouse emissions from their operations with a major focus on the use and installation of sustainable distributed energy systems [1].
The need for far more efficient electricity management systems has given rise to the development of innovative technologies and groundbreaking ideas in power generation and
transmission. The trend is to increase the share of Distributed Generator (DG) in the electricity supply. DG may also comprise Renewable Energy (RE) systems such as solar, wind, fuel cells and wave, which are promising cleaner technologies leading to reductions in greenhouse gas emissions and in effect aiding in the remedy of the global warming problem [13]. Consequently, governments and energy regulation authorities worldwide are encouraging more deployments of RE based DGs. However, higher penetration of micro- sources, i.e. small scale PV panels, wind turbines, and diesel generators into the grid changes the traditional “radial” structure of the grid. This revolutionary change in the structure
triggers many problems which were previously unknown to the grid operators and power engineers [2]. There are now various micro-sources at different penetration levels in the grid and this new structure invalidates the traditional power flow control methods. Moreover, DGs also make contributions to the fault currents around the network. Hence, in case of a fault, the transient characteristics of the network become completely different [3]. These are only a few of the issues that have arisen in relation with the revolutionary changes occurring in the grids and the way they are operated.
There are still many technical challenges that must be overcome so that DGs can be cost- effectively, efficiently and reliably integrated into existing electric power systems. Existing distribution systems are not designed for significant penetration of DG. Distribution systems were traditionally designed with the assumption of a passive network. The interconnection of decentralized renewable energy generation systems to such networks inevitably changes the characteristics of the system and presents key technical challenges such as circuit protection coordination, power quality, reliability, and stability issues that must be overcome. Controlling a huge number of geographically dispersed DGs in a large network is a daunting challenge for the safe, reliable, and effective operation of the network.
The search for alternative energy sources and more efficient utilization of the energy as a means of tackling the global warming concerns will require fundamental changes in the Electrical Engineering (EE) field explicitly in relation to the matters associated with the Transmission and Distribution (T&D) of this renewable electricity. Although T&D grids have been around for many decades, DG and RE concepts have recently become irreversibly popular. As a result, many research and development needs have evolved as a necessity to enable the scaling up of the implementation and uptake of renewable energy systems giving them recognition and equal status in energy sector investment processes.
Microgrids, which are small entities in a power system network, are capable of coordinating and managing DGs in a more decentralized way thus reducing the need for the centralized coordination and management of such systems [5, 9]. This is highly recommended in [9], where it is claimed that such a scheme would permit DGs to provide their full benefits. Yet, there are still many research and development needs associated with the microgrids [10].
Microgrid is a very exciting research field in the power engineering field and it has many different aspects which are complete research fields in their own. Furthermore, microgrids resemble an intersection zone of several concepts such as network operations, protection, power electronics, distributed generation, renewable energy etc. [14]. Consequently, it is not a surprise to see more researchers focusing on microgrids and more publications appearing in the literature.
This chapter presents a detailed review of the literature with regard to microgrids by outlining the existing knowledge as well as the problems and challenges being encountered. It also provides an overview of the current research and development work being carried out all over
the world. It gives an insight into some real-life implementations of microgrids. Finally, it focuses on knowledge gaps yet to be addressed and possible future work in this field of Power/Electrical Engineering. Quite naturally, special attention has been paid to microgrid protection, recently proposed schemes and knowledge gaps since the work presented in thesis pertain to the same research field.