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The Concept of Microgrid

By the improvement of technology, electricity components which are related with generating, distributing, reliability and resilience issues require improvements as well. Transmission of electricity is one of the most significant elements of those requirements. The generated electricity is connected to homes, industries, farms and all consumers via transmission and distribution networks which are called as grid. In point of renewable energy adaptation and reliability of power supply one of the most promising technical approaches is the concept of Microgrid.

A microgrid is a small scale of electricity supply network which consists of low voltage (LV) distribution systems that are designed for supplying electrical and heat loads with distributed energy resources, storage devices and flexible loads to the small communities like suburban localities, industrial sites, municipal regions, etc. Microgrids can both operate as interconnected to the main grid in non-autonomous way or disconnected from the main grid in autonomous way. The concept of microgrid is emerged because of several reasons. First of all, gradual development of distributed generation (DG) systems such as micro-turbines, photovoltaics, wind turbines and fuel cells bring about lots of advantages for meeting growing customer needs and they are also providing different economic, environmental and technical benefits. Distributed energy resources have generally small capacities and due to its lower energy density, they can be directly connected to LV networks, and they needed to be located close to the loads, so they are generally located at users’ sites and in terms of distribution system planning and operations, the integration of DG brings about some challenges, the vital one is about configuration of power lines and protective relaying in most of the existing grids, which is operated and designed on uni-directional power flow. The penetration of power flow generated by DGs used to be small enough to be regarded as simply reduction in load, but this will not stay same in upcoming times due to huge grow in DG penetrations. Actually, physical wires and transformers can carry power flow in both directions, however DG might negatively affect systems’ reliability, power quality and safety.[1] Hence, in order to facilitate full integration of distributed generation into the system, there is a need for a different type of network architecture which is able to control and manage the generation and associated loads as a subsystem .[2] Secondly, distributed generation, storage and uninterruptible power supplies can be vital for some kind of consumers like hospitals, military bases, data centers, college campuses, etc. Reliability level of conventional grids are inadequate for such consumers, in different parts of the world, big electricity shortages occurred which stem from conventional grid faults, and those shortages caused huge losses in those countries.[3] So, a new type of network architecture needs to have an islanded operation mode which can be operated as disconnected to main grid and increase the reliability of power supply.[4] Also, carrying power to suburbs and outer lands generally become problematic. Due to long way of transmission, there is considerable amount of loss in the transferred power. This loss causes both negative economic impacts and decrease reliability of power. Therefore, islanded network can be used in sites which are far away from power plants. Distributed generations in those places would be much reliable and less costly, so they can reduce the loss and increase the service quality.[5] Although these are only a few reasons, they show how important is the emergence of new kind of network architecture.

There are few constituent parts of microgrids. First one is distributed generation units including microturbines, photovoltaics, wind turbines, fuel cells, etc. Distributed generation units have low power generation potential in comparison with conventional power plants and their generation is not feasible to transport far away, but they are suitable for microgrids. Second one is energy storage systems which are highly important for the future of the concept. The main problem of renewable energy powered microgrids is maintaining the power supply and load mismatches. Energy storage systems can play crucial role for this problem, for instance when the weather is less cloudy than forecasted, PV panels might generate more electricity than actual demand, so surplus electrical energy can be stored and might be dispatched properly later in the event of electricity shortage, so sustainability in electricity supply can be achieved by this way. Third part is electrical loads which are prioritized depending on its urgency, so critical loads that are related with essential processes are more particularly met than non-critical loads. Since the capacities of distributed energy resources are limited, microgrid arranges its units to meet the highest critical load’s demand, after that it tries to meet the demands of less critical loads. If any shortage in supply occurs, then microgrid takes power from the main grid. [6] Last part is the controller part of the microgrids, this part behaves like the brain of the microgrids. Controller part matches the load with generation in both connected and disconnected modes. It optimizes integration, ensures that distributed generation improves cost efficiency, maintains reliability and manages frequency and voltage. It controls real-time responses and provides fault protection, and if any problem occurs, it connects or disconnects microgrid to the main grid. That can also make predictions and forecasting analysis.[7]

In order to understand how would a microgrid operate, a typical microgrid configuration is provided in Figure1.1[8] below,

A typical Microgrid configuration

As it can be seen from the graph provided, electricity and heat loads are supplied by three radial feeders. (A,B and C) Microsources and storage devices are connected to feeders A and C through microsource controllers. Feeder B is connected to non-priority electrical loads, while other feeders are connected to loads which require uninterrupted power supply that are supposed to be priority loads as mentioned above. The grid-connected microgrid is coupled with medium voltage main grid through the point of common coupling (PCC) circuit breaker (CB4) as per standard interface regulations. CB4 in the graph is the key point of the microgrid, so depends on selected operation mode or in case of an emergency, CB4 connect or disconnect the whole microgrid from main grid. CB1, CB2 and CB3 are operating breakers which connect or disconnect feeders A, B, C respectively. To ensure reduction in line losses and to get good voltage profile, microsources on feeders A and C are placed far from the Microgrid bus. When microgrid is disconnected from the main grid, it operates in stand-alone mode. In this case microgrid operates as an autonomous system and all microsources start feeding all the loads in feeders A, B and C or A and C start supplying power for only the priority loads and feeder B remains to ride through disturbance. [8] This is how a microgrid operates in typical cases.

In a nut shell, how microgrids are differ from the power systems we see today is as follows[9], First of all, microgrids can operate in both grid connected and grid disconnected modes. Secondly, DG sources have much smaller capacity in comparison with conventional power sources, they generate in low voltage and power can be directly fed to the utility distribution network. Thirdly, DG sources are generally based on renewable energy sources which are located close to the loads that bring about negligible line losses. Also, active control and two-way power flow in distribution are possible and distribution can be regarded as a transmission resource. Finally, in microgrids there is high dependency on standards and they have ordinate processes.

The State of Art in Microgrid Research

The concept of microgrid started with first DC microgrids that were built in 1886, and thereafter evolution of electricity market incentives for microgrids removed, however today the world needs microgrids than ever before.[10] A remarkable amount of researchers work on this concept to make it feasible for using. The state of art in microgrid researches can be examined by different parts of the concept. From the point of distributed generation, renewable DGs are one of the extensively studied topics in the literature. As it is mentioned in previous parts, the adaptation of renewable energy into microgrids has some challenges. Latest developments about wind power in microgrids are as follows, researches show that in order to decrease the effects of the high variability in the power output of the wind turbines, aggregate wind generation or much greater amounts of storage or both are needed.[11] Also, for wind power plants, pitch angle controller and a rotor speed controller which are used in active power adjustment to help with frequency regulations are introduced, for stabilizing the microgrid voltage during short circuit faults, STATCOM (Static Synchronous compensator) is discussed further.[12] In terms of solar energy, researchers generally study on maximum power generation from the PV in microgrids.[13] One of the main objectives is enhancing solar energy generation by power point tracking, which can be described as a unique point that generates the maximum power on the I-V characteristic of the solar cells, and researchers developed a fuzzy-logic controller for maximum power point tracking of PV systems and also suitable network designs for PV adaptation are examined. Another important part of researches on microgrids consist of energy storage systems. (ESS) As it is mentioned in previous parts, importance of ESS is increasing proportional with the penetration of renewable energy generation in microgrids. Recent studies on ESS aim to increase efficiency of batteries and also some control mechanisms are developed for power balancing and voltage regulations among ESS and loads when the microgrid disconnects from main grid.[14] The following batteries are the major types of energy storage applied in microgrids. Lead-Acid Batteries are the most widely used type of rechargeable batteries. They are great choices for uninterruptible power supply with their low cost, high efficiency and good surge capability, also they are easy to recycle. Lithium-Ion Batteries are the best commercialized batteries in terms of power density. The voltage of a lithium ion battery cell is twice that of a lead-acid battery, however high cost of the lithium prevent widely usage of this battery type in power systems. Redox-Flow Batteries have long lifetime and exchangeable electrolytes make it easy to charge electric cars. However, energy density of Redox-Flow batteries is very low which makes it hard to apply for grid storage. Sodium-Sulfur Battery has already been operated in some countries. The installed capacity of the battery is about 250MW and it has advantages of high power density, long lifetime and high efficiency. But, when the battery is operating, high temperature, around 350 ⁰C, is required to liquefy the sodium. This property makes it difficult to apply such kind of batteries in grid storage systems, because of its high operational cost. Flywheel energy storage system (FESS) is one of the most promising storage types for the microgrids which stores energy in kinetic form. It can supply immediate power support for a renewable energy based microgrids, when there is excessive electricity flywheels can be charged by electric motors, also it can act as generator when discharging. It has advantages of high power density, high conversion efficiency, long lifetime, low maintenance cost, short response time and it is environmental friendly. Their drawbacks are the small capacity and high power loss due to friction. The Supercapacitor consists of numerous two-layer capacitors arranged in parallel or in series. They are formed when the electrode is in touch with the electrolyte. For energy storage in microgrid requires a stack of many single cells connected in series. Supercapacitors have high power density and high energy conversion efficiency with the storage capacity of 20-1000 times higher than a common capacitor.[15] Also, promising studies on high energy density storage batteries and high power density storage ultra-capacitors is reflecting another side of ESS.[16] Table-1 and Table-2 show the comparison of various ESS technologies.[15]

Table-1

Table-1 Comparison of Characteristics for Different Types of ESS

Table-2

Table-2 The Advantages and Disadvantages of Different Types of ESS

Potential Benefits and Drawbacks

The benefits of Microgrids should be examined from different perspectives. From the end user point of view Microgrids supply both thermal and electricity needs, also enhance local reliability, reduce in gaseous and particulate emissions due to widely usage of renewable energy resources and close control of the combustion process for thermal power plants. Close relation between end user and micro-sources might increase the environmental and energy consumption awareness of the end users and might have a contribution to decrease carbon food-step of individuals. Microgrids improve reactive support of the whole system and develop power quality by decentralisation of supply, they have better match of supply demand, enhancing the voltage profile, reducing the large transmission and generation losses. Also, they have potential to be lower costs of energy supply and ensure energy supply for critical loads utilizing on site generation. Economically, widespread use of detachable microsources may reduce the energy prices in the power market

From the utility point of view, thanks to microgrids, 20% of the world population which can not access to electricity might reach it[17] Microgrids reduce the fuel consumption, hence they are much better for environment which is also important for end users. Also, it decreases the demand for transmission and distribution (T&D) facilities, also T&D losses decrease considerably. Capability of grid independence lets power to be transmitted to far places and islands, also it provides secure power supply. During maintenance and repair services, there will not be electricity shortage for whole network which is also great benefit from end user’s point of view. Microgrids can be used as supportive service to other networks during congestion or any faults occurred on other networks, it is also beneficial for grid operators. They also can be regarded as controlled entity which can be operated as cluster of load or generator within the power system, so it is more secure and reliable.

As it can be understood from above, the concept of microgrid has so many benefits, however there are some drawbacks and tough challenges needed to be overcome. First of all high cost of distributed energy sources and installation of them can be regarded as the biggest barrier against the microgrids and also technical challenges, especially ability to transition from grid-connected to grid-disconnected mode has so many technical difficulties that are needed to be solved. Perfect distribution of electricity requires so many perfect control systems which technically looks hard to achieve. There are also financial challenges. Business cases for microgrids are needed to be developed to secure funding for microgrids. Without creating satisfactory business models, financing of microgrid will stay as a problem.[18] After disconnection of microgrid from main grid, who will control the energy prices? The current electricity market will lose its power, so there will be consequences of this. Since microgrids are comparatively a new area, in most countries there are lack of standard legislation and regulations. Hence, administrative and legal barriers are other challenges that should be taken into account.

Can We Do It?

As it can be understood obviously from benefits of the concept of microgrids, they will make the world a better place to live, however the question is whether microgrids are likely to feature prominently in the power systems of the future? Can we overcome the challenges?

In terms of high cost of distributed energy sources, it should be done with the subsidies and encourages from government bodies and efforts of NGOs for promoting microgrids to individuals. These supports should continue until environmental and carbon capture goals are reached by transition to microgrids. Technical challenges should be solved by working in collaboration, hence global forums and summits have been done for a few years [19], the communication among researchers, investors and governmental bodies should increase in such kind of events. In those events, standards and protocols for integration of microsources and their participation in conventional grids and deregulated power markets should be argued, latest technical improvements and challenges should be shared in scientific level, also safety and protection guidelines should be laid down. By working in collaboration, it is not hard to believe that transition to micrgrids will be done.

Although it is not easy to achieve these goals and cope with challenges, I believe that the concept of microgrids will be our future power systems. If the world wants to achieve the carbon emissions targets[20] which were defined in Kyoto, the world mostly needs to transform in renewable power generation which requires microgrids. In another aspect, the markets in emerging countries[21] are developing gradually, and it is expected that it will keep growing in next decades, however as it was mentioned in previous parts, some parts of those countries have no access to the electricity which can be regarded as the biggest barrier against development. For sure, nothing can prevent this emergence and electricity will reach the furthest parts of those countries. So, as it is discussed in previous parts microgrids will play a vital role in this emergence. I expect that, microgrids will cover most part of the world until 2020, which is the target year of Kyoto Protocol. Furthermore transition from microgrids will be towards smartgrids, which is currently studied. Smart grids can be regarded as another concept which intelligently integrate the actions of power system. They will have better control systems and feedback algorithms, so it will bring about better grid management. We can explain this as smarter microgrids, I claim this by the help of promising research and projects which are mentioned in previous parts. Therefore, microgrids and similar concepts like smartgrids can be regarded as
the future of the power systems.

References

[1] KASSAKIAN, J.G. et al. (2011) “The Impact of Distributed Generation and Electric Vehicles” The Future of the Electric Grid. An Interdisciplinary MIT Study. Available from: https://mitei.mit.edu/system/files/Electric_Grid_Full_Report.pdf [Accessed: 21st Oct 2015]

[2] SCHWAEGERL, C., TAO, L. (2014) “The Microgrids Concept”. Page 2. In HATZIARGYRIOU, N. (ed) Microgrids Architectures and Control. Publisher: John Wiley and Sons Ltd. United Kingdom

[3] News related with the electricity shortages

http://www.nydailynews.com/news/national/northeast-blackout-2003-ten-year-anniversary-article-1.1426561

http://www.huffingtonpost.com/2009/11/10/brazil-blackout-largest-c_n_353217.html

http://edition.cnn.com/2015/03/31/middleeast/turkey-power-outage/

[4] SCHWAEGERL, C., TAO, L. (2014) “The Microgrids Concept” Page 4. In HATZIARGYRIOU, N. (ed) Microgrids Architectures and Control. Publisher: John Wiley and Sons Ltd. United Kingdom

[5] SCHWAEGERL, C., TAO, L. (2014) “The Microgrids Concept” Page 3. In HATZIARGYRIOU, N. (ed) Microgrids Architectures and Control. Publisher: John Wiley and Sons Ltd. United Kingdom

[6] B.Zhao, Y. Shi, X.Dong, W. Luan and J.Bornemann, “ Short-Term Operation Scheduling in Renewable-Powered Microgrids: A Duality-Based Approach” IEEE Transaction On Sustainable Energy, Vol 5, No 1, Jan 2014

[7] SANCHEZ, I. “Microgrid Technology: Enabling Energy Reliability and Security – Opportunities in Campus, Commercial & Industrial Communities” MAYA Smart Energy Consulting Presentation. [Online] Available from: http://www.districtenergy.org/assets/pdfs/03AnnualConference/Monday-A/A5.2SANCHEZIvette-Sanchez-IDEA.pdf [Accessed: 24th Oct 2015]

[8] CHOWDHURY, S.,CHOWDHURY, S.P. and CROSSLEY, P. (2009) “A Typical Microgrid Configuration” In  Microgrids and Active Distribution Networks. Publisher: The Institution of Engineering and Technology. London, United Kingdom.       

[9] MOHN, T. “Microgrid Research Opportunities A look at the state of the art and what’s needed” General Microgrids Balance Energy. Presentation in Microgrid Rodeo Summit. [Online] Available from: http://www.utexas.edu/research/cem/RODEO%20Pres/MOHN%20Presentation.pdf [Accessed: 25th Oct 2015]

[10] ASMUS,P., CORNELIUS,A. and WHEELOCK,C.(2009) “Islanded Power Grids and Distributed Generation for Community, Commercial, and Institutional Applications” PikeResearch Cleantech Market Intelligence, Research Report. [Online] Available from: http://www.missioncriticalmagazine.com/ext/resources/MC/Home/Files/PDFs/WP-MICROPike_Research-ExecutiveSummary.pdf  [Accessed: 25th Oct 2015]

[11] A. M. Giacomoni, S. Y. Goldsmith, S. M. Amin, and B. F. Wollenberg,“Analysis, modeling, and simulation of autonomous microgrids with a high penetration of renewables.” in Proc. IEEE Power Energy Soc. General Meeting, Jul. 2012, pp. 1_6.

[12]  B. H. Chowdhury, H. T. Ma, and N. Ardeshna, “The challenge of operating wind power plants within a microgrid framework,” in Proc. Power Energy Conf. Illinois (PECI), Feb. 2010, pp. 93_98.

[13] PARHIZI,S., LOTFI,H., KHODAEI,A. and BAHRAMIRAD,S.(2015) “State of the Art in Research on Microgrids: A Review”

[14] R. Pawelek, I. Wasiak, P. Gburczyk, and R. Mienski, “Study on operation of energy storage in electrical power microgrid_Modeling and simulation” In Proc. 14th Int. Conf. Harmon. Quality Power (ICHQP), Sep. 2010, pp. 1_5.

[15] GAO, D.W., “Basic Concepts and Control Architecture”(2015) In Energy  Storage for Sustainable Microgrid. Publisher: Elseveir Ltd., Oxford, United Kingdom

[16] H. Zhou, T. Bhattacharya, D. Tran, T. S. T. Siew, and A. M. Khambadkone, “Composite energy storage system involving battery and ultracapacitor with dynamic energy management in microgrid applications” IEEE Trans. Power Electron., vol. 26, no. 3, pp. 923_930, Mar. 2011.

[17] Percentage of the world which has no access to electricity is available from: http://data.worldbank.org/indicator/EG.ELC.ACCS.ZS

[18] Stefferud, K. “Microgrid Benefits and Challenges”(2013) [Online] Available from: http://www.enernex.com/blog/post-7-microgrid-benefits-and-challenges/ [Accessed: 26th Oct 2015]

[19] Summit and Forums about Microgrids http://www.microgridglobalsummit.org/2015/html/program.htm  http://www.microgridinnovation.com/

[20] The carbon emission targets of the countries  http://www.c2es.org/international/history-international-negotiations/2020-targets

[21] The list of emerging countries http://www.iawp.org/joiniawp/countrylist.htm

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