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Unformatted text preview: Chapter 2 Microgrid Concept and Applications PART A OF CHAPTER 2: THE NEED FOR DISTRIBUTED CONTROL AND MICROGRID A.1 MIGRATION TOWARD DISTRIBUTED CONTROL Electric power system is the cornerstone of every industrialized nation in the world. As the utility grid ages and the demand for electricity grows, the impact of major interruptions of the electricity infrastructure will be more intense. Costly power outages throughout the world caused by natural disasters such as floods and hurricanes have highlighted the importance of reinforcing the electricity infrastructure. Power outages caused by hurricanes Sandy and Katrina in the united states threw into notice the crucial role of smart grid technology and the need for further investments in more comprehensive data communication and distribution management systems, distributed energy resources, energy storage facilities, additional automation, and further migration toward decentralized operations for the largely centralized power grid. A recent study conducted for the U.S. Department of Energy indicated that sustained power interruptions (those lasting more than 5 min) in the United States incur costs of more than $26 billion annually. Hurricane Sandy left approximately 7.5 million customers without power across 15 states and Washington, D.C., after it hit the eastern shore of the United States. Authorities announced that restoring the power to the New York City transit system was the biggest challenge to that city. A few days after Sandy, there were still nearly 3 million customers in New Jersey and New York without power and 700,000 more customers whose daily lives were distracted by power outages in 11 other states, from Massachusetts to Virginia and as far west as Michigan. As the result of electric power shortages in Manhattan, people walked to coffee shops that still had power to charge their cell phones. The electric supply deficiency in New Jersey disabled gas stations, as the fuel pumps were out of power. Even when fuel was available, many residents lined up for hours to get fuel for home generators. In a similar situation, Connecticut residents in 2011 suffered intense power outages from the effects of tropical storm Irene followed by an October snowstorm that downed thousands of trees, left several towns without power for almost two weeks, and cost towns millions of dollars. The widespread power outages in the wake of hurricane Sandy cast light on the weakness of the conventional power system that has served for over a centurythe centralized electric power system (Figure 1) and highlighted the benefits of migration toward distributed generation systems (Figure 2). Figure 1: Centralized electric power system Figure 2: Distributed Generation Systems The electrical grid is tending to be more distributed, intelligent, and flexible. The use of distributed generation (DG) of energy systems makes no sense without using distributed storage systems to cope with the energy balances. The smart grid concept has been developed to cope with the penetration of distributed generation (DG) which can be realistic if the final user is able to generate, storage, control, and manage part of the energy that will consume. This change of paradigm, allows the final user be not only a consumer but also a part of the grid. The smart grid, will deliver electricity from suppliers to consumers using digital technology to control appliances at consumer's home to save energy, reduce cost and increase reliability and transparency. In this sense, the expected whole energy system will be more interactive, intelligent, and distributed. The use of smart grid for microgrid development as the paradigm for the massive integration of distributed generation will allow technical problems to be solved in a decentralized fashion, reducing the need for an extremely ramified and complex central coordination and facilitating the realization of smart grid. Microgrids have existed for many decades in remote communities where the interconnection with the main power grid is not feasible due to technical and/or economical reasons. Due to their scalability, competitive investment costs and flexible operation, fossil‐fuel generation technologies have been the most common choice for supply of electricity in these remote grids. However, with the demonstrated technical and economical feasibility of greener generation technologies based on wind, solar, hydrogen and hydro power, integrating DG has become a priority in microgrids. Furthermore, policies have been developed globally, including feed‐in tariffs, renewable portfolio standards, tradable green certificates, investment tax credits and capital subsidies, among others, to promote the use of renewable energy technologies. In Europe, the UK is aiming for 15% of its electricity to be generated from renewable energy sources by 2015/16, which represents an increase of around 10% compared to the existing share; Germany, with a more aggressive policy, targets a 25‐30% share by 2020, and 50% by 2030. According to the trends observed in 2009 for the EU, nearly 55% of the new installed capacity based on renewable sources corresponds to wind and solar‐Photovoltaic (PV) intermittent generation (39% and 16%, respectively). In the US, the state of California has set a target of 33% for the retail load to be served from renewable sources by 2020. In Canada, the province of Ontario has an aggressive policy for the promotion of energy conservation and investments in renewable energy sources, as part of an overall climate change action plan; according to the Ontario Green Energy Act (2009), renewable energy sources are granted long‐term contracts with predefined feed‐in tariffs in order to reduce the risk for investors, and progressively phase out the existing coal‐fired generators. A.2 MICROGRID CONCEPT The concept of microgrid was first introduced as a solution for the reliable integration of DERs, including Energy Storage Systems (ESSs) and controllable loads. Such microgrids are perceived by the main grid as a single element responding to appropriate control signals. Microgrids are self‐controlled entities interconnecting on‐site DERs and operating in either grid‐connected or islanded modes. A schematic diagram of a generic multiple‐DER microgrid is shown in Figure 3. A microgrid can be characterized based on the following prominent features: A cluster of loads and DERs; A single self‐controlled entity perceived by the main grid (utility grid); Connected to the main grid at PCC (Point of Common Coupling); Being able to operate in grid‐connected and island (autonomous) models and handling the seamless transition between the two modes; Microgrid is a cluster of loads, DGs, and ESSs operated in coordination to reliably supply electricity, connected to the host power system at the distribution level at a single point of connection, PCC. From a grid perspective, the microgrid concept is attractive because it recognizes the reality that the nation’s power distribution system is extensive, old, and will change only very slowly. The microgrid concept enables high penetration of DER without requiring re‐design or re‐engineering of the distribution system, itself. Figure 3: Schematic diagram of a microgrid with multiple‐DERs The goal is to accelerate the realization of many benefits offered by smaller‐scale DG, such as their ability to supply waste heat at the point of need (avoiding extensive thermal distribution networks) or to provide higher power quality to some but not all loads within a facility. When a strong coupling between the operation of different energy carrier systems (heating, hot water, etc.) exists, microgrids can integrate and operate all these energy carriers in coordination. The smaller size of emerging generation technologies permits generators to be placed optimally in relation to heat loads allowing for use of waste heat. The coordination of multiple DG units throughout a bulk power system is considered as a Virtual Power Plant (VPP) solution. Such applications can more than double the overall efficiencies of the systems. Different initiatives around the world aim to further develop the concept of microgrid through research, development, and demonstration (RD&D). A.3 BENEFITS OF MICROGRID INTEGRATION Each innovation embodied in the microgrid concept (e.g., intelligent and hierarchical control, smart switches for grid disconnect and resynchronization) was created specifically to lower the cost and improve the reliability of smaller‐scale DG systems (i.e., systems with installed capacities in the 10’s and 100’s of kW). A smart microgrid that can operate in both grid‐tied as well as islanded modes typically integrates the following seven components: ✔ It incorporates power plants capable of meeting local demand as well as feeding the unused energy back to the electricity grid. Some microgrids are equipped with thermal power plants capable of recovering the waste heat, which is an inherent by‐product of fossil‐based electricity generation. The combined heat and power (CHP) plants or cogenerators would recycle the waste heat in the form of district cooling or heating in the immediate vicinity of the power plant. ✔ It makes use of local and distributed ESS to smooth out the intermittent performance of renewable energy sources. ✔ It incorporates smart meters and sensors capable of measuring a multitude of consumption parameters (e.g., active power, reactive power, voltage, current, demand, and so on) with acceptable precision and accuracy. Smart meters should be tamper‐resistant and capable of soft connect and disconnect for load and service control. ✔ It incorporates a communication infrastructure that enables system components to exchange information and commands securely and reliably. ✔ It incorporates smart terminations, loads, and appliances capable of communicating their status and accepting commands to adjust and control their performance and service level based on user and/or utility requirements. It also services a variety of loads, including residential, office and industrial loads. ✔ It incorporates an intelligent core, composed of integrated networking, computing, and communication infrastructure elements, that appears to users in the form of energy management applications that allow command and control on all nodes of the network. Microgrids would be capable of identifying all terminations, querying them, exchanging data and commands with them, and managing the collected data for scheduled and/or on‐demand transfer to the higher‐level intelligence residing in the smart grid. Figure 4 depicts the topology of a smart microgrid. Figure 4: Microgrid Components A.4 DIFFERENCES WITH THE MAIN GRID OPERATION The key differences between a microgrid and conventional power utility operations are as follows: (1) Microsources in microgrids are of much smaller capacity with respect to the large generators in conventional power plants. (2) DG power generated at distribution voltage can be directly fed to the utility distribution network. (3) Microsources in microgrids are normally installed close to customer premises so that electrical/heat loads can be efficiently supplied with satisfactory voltage and frequency profile and negligible line losses. The technical features of a microgrid make it suitable for supplying power to remote areas of a country where supply from the national grid system is either difficult to avail due to the topology or frequently disrupted due to severe climatic conditions or man‐made disturbances. From grid point of view, the main advantage of a microgrid is that it is treated as a controllable entity within the power system. Microgrid can be operated as a single aggregated load which ascertains its controllability and compliance with grid rules and regulations without hampering the reliability and security of the power utility. From customers’ point of view, microgrids are beneficial for locally meeting their electrical/heat requirements. They can supply uninterruptible power, improve local reliability, reduce feeder losses and provide local voltage support. From environmental point of view, microgrids reduce environmental pollution and global warming through utilization of low‐carbon technology. However, to achieve a stable and secure operation, a number of technical, regulatory and economic issues have to be resolved before microgrids can become commonplace. Some problem areas that would require due attention are the intermittent and climate‐dependent nature of DER generation, low energy content of fuels and lack of standards and regulations for operating microgrids in synchronism with the power utility. A.5 MANAGEMENT AND OPERATION OF MICROGRIDS Suitable telecommunication infrastructures and communication protocols must be employed for overall energy management, protection and control. Management and operational issues of microgrids are listed as follows: (1) Active and reactive power balance must be maintained within the microgrid on a short‐term basis. (2) A microgrid should operate as stand‐alone in regions where utility supply is not available or in grid‐
connected mode within a larger utility distribution network. Microgrid operator should be able to choose the mode of operation within proper regulatory framework. (3) Generation, supply and storage of energy must be suitably planned with respect to load demand on the microgrid and long‐term energy balance. (4) Supervisory control and data acquisition (SCADA) based metering, control and protection functions should be incorporated in the microgrid MCs and LCs. Provisions must be made for system diagnostics through state estimation functions. (5) Economic operation should be ensured through generation scheduling, economic load dispatch and optimal power flow operations. (6) System security must be maintained through contingency analysis and emergency operations (like demand side management, load shedding, islanding or shutdown of any unit). Under contingency conditions, economic rescheduling of generation is done to take care of system loading and load‐
end voltage/frequency. (7) Temporary mismatch between generation and load should be alleviated through proper load forecasting and demand side management. The shifting of loads might help to flatten the demand curve and hence to reduce storage capacity. A.6 TECHNICAL AND ECONOMICAL ADVANTAGES OF MICROGRID INTEGRATION The development of microgrid is very promising for the electric energy industry because of the following advantages: Environmental issues: It is needless to say that microgrids would have much lesser environmental impact than the large conventional thermal power stations. The successful implementation of carbon capture and storage schemes for thermal power plants will drastically reduce the environmental impacts. However, environmental benefits of microgrid in this regard are as follows: (i) Reduction in gaseous and particulate emissions due to close control of the combustion process may ultimately help combat global warming. (ii)
Physical proximity of customers with microsources may help to increase the awareness of customers towards judicious energy usage. Operation and investment issues: Reduction of physical and electrical distance between microsource and loads can contribute to: (i) Improvement of reactive support of the whole system, thus enhancing the voltage (ii)
(iv) profile. Reduction of T&D feeder congestion. Reduction of T&D losses to about 3%. Reduction/postponement of investments in the expansion of transmission and generation systems by proper asset management. Power quality: Improvement in power quality and reliability is achieved due to: (i)
(iv) Decentralization of supply. Better match of supply and demand. Reduction of the impact of large‐scale transmission and generation
outages. Minimization of downtimes and enhancement of the restoration process through black start operations of microsources. Cost saving: The following cost savings are achieved in microgrid: (i) A significant saving comes from utilization of waste heat in CHP mode of operation. Moreover, as the CHP sources are located close to the customer loads, no substantial infrastructure is required for heat transmission. This gives a total energy efficiency of more than 80% as compared to a maximum of 40% for a conventional power system. (ii)
Cost saving is also effected through integration of several microsources. As they are locally placed in plug‐and‐play mode, the T&D costs are drastically reduced or eliminated. When combined into a microgrid, the generated electricity can be shared locally among the customers, which again reduces the need to import/export power to/from the main grid over longer feeders. Market issues: The following advantages are attained in case of market participation: (i) The development of market‐driven operation procedures of the microgrids will lead to a significant reduction of market power exerted by the established generation companies. (ii)
The microgrids may be used to provide ancillary services. (iii)
Widespread application of modular plug‐and‐play microsources may contribute to a reduction in energy price in the power market. (iv)
The appropriate economic balance between network investment and DG utilization is likely to reduce the long‐term electricity customer prices by about 10%. A.7 ISSUES WITH MICROGRIDS INTEGRATION The microgrid concept is a quite appealing alternative for overcoming the challenges of integrating DER units, including renewable energy sources, into power systems. However, in order to allow seamless deployment of microgrids, several issues still remain unsolved. Currently, effort is being put into the design of special control and protection schemes that ensure reliable, secure and economical operation of microgrids in either grid‐connected or stand‐alone mode. In order to successfully integrate DERs, many challenges must yet be overcome to ensure that the present levels of reliability are not significantly affected, and the potential benefits of distributed generation are fully harnessed. In this sense, the main issues include: High installation cost for microgrids is a great disadvantage. This can be reduced by arranging some form of subsidies from government bodies to encourage investments which should be done at least for a transitory period for meeting up environmental and carbon capture goals. There is a global target set to enhance renewable green power generation to 20% by 2020 and to reduce carbon emission by 50% by 2050. Lack of technical experience in controlling a large number of plug‐and‐play microsources. This aspect requires extensive real‐time and off line research on management, protection and control aspects of microgrids and also on the choice, sizing and placement of microsources. Lack of proper communication infra‐structure in rural areas is a potential drawback in the implementation of rural microgrids. Economic implementation of seamless switching between operating modes is a challenge since available solutions for reclosing adaptive protection with synchronism check are quite expensive. Reliable and economic operation of microgrids with high penetration levels of intermittent generation in standalone mode of operation. Development of new voltage and frequency control techniques to account for the increase in power electronics interfaced distributed generation. Reengineering of the protection schemes at the distribution level to account for bidirectional power flows. Schedule and dispatch of utility units, under supply and demand uncertainty, and determination of appropriate levels of reserves. Design of appropriate Demand Side Management (DSM) schemes to allow customers toreact to the grid’s needs. Design of new market models that allow competitive participation of intermittent energy sources, and provide appropriate incentives for investment. A.8 TYPIC...
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