The world we live in today is being troubled by the concerns on global warming, pollution and CO2 emissions. RE systems offer means of generating cleaner and sustainable energy. However, there are lots of challenges that must be tackled so that RE resources could be utilized to their full potential. RE resources are mostly dispersed and different generation approaches should be used to harvest the maximum potential out of those sources. This is contradictory to the traditional concept of central generation and distribution over large distances. For this reason, existing grids are not entirely compatible for excessive integration of DG units. On top of that, micro-scale implementation of known generation plants such as micro hydroelectric power plants, diesel generators and etc, have similar aspects since they are also distributed and their generation capacities are much smaller than their traditional giant counterparts.
In order to achieve a cleaner, reliable, and secure power generation, transmission and distribution system, the various challenges brought about by this new grid structure and
management system shall be tackled with similar research projects. The outputs of studies on microgrids will aid in the development of secure, reliable, and stable real-life small-scale networks with greater penetration of RE sources. This will aid in achieving a more reliable, secure and cleaner energy without compromising from environment protection and similar concepts.
This chapter summarized the current status of the research on microgrids, their management and operation. As it is stated above, microgrid protection has received relatively less attention as compared to other research fields. There is a knowledge gap regarding;
i) monitoring the structural changes occurring in microgrids due to connection changes or new deployments,
ii) adjusting parameters of protective devices according to these changes to ensure proper and reliable operation,
iii) standardizing the much-needed communication between grid components in microgrids, and
iv) interaction between different protection schemes applied on microgrids.
Identifying this research opportunity, author has focused his research on these topics. Consequently, the research work presented in the coming chapters of this thesis is an effort to address these knowledge gaps pertaining to microgrid protection systems. In a nutshell throughout this research work, a versatile microgrid protection system with communication support [55] has been developed. The operating parameters are calculated in on-the-fly and no prior knowledge of the microgrid structure is required [145]. This system can be extended to accommodate various types of components such as fault current limiters [146] or EVs [147], accept new deployments and extract the new microgrid structure with adaptive computing [7] and can be implemented with international communication standards[148]
such as IEC 61850 [8] and IEC 61850-7-420 [12]. In order to accommodate all types of grid components, necessary extensions have been developed for this standard, e.g. for FCLs [149] and EVs [147].
Chapter 3
Conceptual Design of Centralized Microgrid Protection System
Publications pertaining to this chapter:
1) Taha Selim Ustun, Cagil Ozansoy, Aladin Zayegh, "A microgrid protection system with central protection unit and extensive communication," in Proceedings of 10th International Conference on Environment and Electrical Engineering (EEEIC), 2011, vol., no., pp.1-4, 8-11 May 2011, Rome, Italy, ISBN 978-1-4244-8781-3.
2) Taha Selim Ustun, Cagil Ozansoy, Aladin Zayegh, "A central microgrid protection system for networks
with fault current limiters," in Proceedings of 10th International Conference Environment and Electrical Engineering (EEEIC), 2011, vol., no., pp.1-4, 8-11 May 2011, Rome, Italy, ISBN 978-1-4244- 8781-3.
3.1. Introduction
The first step of the methodology assumed in this research work is to devise a conceptual design for the developed microgrid protection system. This process is the foundation stone of the overall research work on which the other methodology steps shall build. Therefore, during this period, different possibilities in microgrid operation, equipment used in power grids and different operation cases have been investigated. This has been done with the aim of thoroughly understanding the operational situations that could occur in microgrids and designing the protection system in a comprehensive manner. Unlike most of the studies
reported in the literature, instead of focusing on a particular microgrid topology or a particular case thereof [73, 90, 150-153], this research strives to achieve a flexible protection system. It is an aim of this research that the designed protection system can be used in microgrids with different topologies and set of components. It is also desired that the varying operating conditions, connection/disconnection of components and new deployments can be handled with this new microgrid protection system.
In order to serve this purpose, different DG types have been investigated. It is a known fact that rotating-machine type DGs such as wind turbines or diesel generators have different fault current contributions as compared to IIDGs such as PVs or Fuel Cells [14, 15, 153, 154]. Same type of DGs have different fault current contributions which depend on their operating capacity [155] and might be manipulated with auxiliary elements such as Fault Current Limiters (FCLs) [156]. There are devices which may provide power as well as demand it, such as EVs or storage devices [14]. All this information has been taken into account during the conceptual design process.
The overall conceptual design has been developed in a fashion to respond to the demands of new generation microgrids with their various operating conditions and dynamic set of grid components. Furthermore, this design has been tailor-made to enable the implementation of the underlying communication exchange using the IEC 61850 standard. This is essential for a protection system which is intended to be used on different topologies where same devices might have different manufacturers and models. This design is suitable for automated grid structure detection whereby the changes occurring in the microgrid topology can be monitored automatically and necessary actions could be taken. These above-mentioned two factors have been paid special attention since they are the building blocks of the plug-and-
play concept in power networks. When these two features are used, this conceptual design can implement plug-and-play for new grid components connected to the microgrid.
The next section of this chapter explains the fault current challenges in microgrids. Different protection challenges have been explained along with their representations on microgrid topologies. Then, Section 3.3 reveals the details of the conceptual design of the developed microgrid protection system. Section 3.4 sheds light on a very important device, i.e. FCL, which is bound to be used frequently in microgrids, and its integration to the conceptual design. EVs and storage devices have the ability to operate as a load or a generator and they have been implicitly included in the conceptual design in terms of individual generators and loads. EVs and their implementation shall be discussed in detail in Chapter 6.