Microgrids and Active Distribution Networks
2: University of Manchester, Manchester, UK
Microgrids and Active Distribution Networks offer a potential solution for sustainable, energy-efficient power supply to cater for increasing load growth, supplying power to remote areas, generation of clean power and reduction in emission of greenhouse gases & particulates as per Kyoto protocol.
Inspec keywords: electricity supply industry deregulation; power system economics; power grids; power system management; smart power grids; distribution networks; renewable energy sources; distributed power generation; power system control; power markets
Other keywords: microgrids; market participation; generation technology; power system control; power industry deregulation; smartgrids; grid integration; ancillary services; power system operation; electricity delivery network; renewable energy; economic viability; active distribution networks; power system management
Subjects: Distributed power generation; Distribution networks; Power systems
- Book DOI: 10.1049/PBRN006E
- Chapter DOI: 10.1049/PBRN006E
- ISBN: 9781849190145
- e-ISBN: 9781849191029
- Page count: 320
- Format: PDF
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Front Matter
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1 Distributed generation and Microgrid concept
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Around the world, conventional power system is facing the problems of gradual depletion of fossil fuel resources, poor energy efficiency and environmental pollution. These problems have led to a new trend of generating power locally at distribution voltage level by using non-conventional/renewable energy sources like natural gas, biogas, wind power, solar photovoltaic cells, fuel cells, combined heat and power (CHP) systems, microturbines, and Stirling engines and their integration into the utility distribution network. This type of power generation is termed as distributed generation (DG) and the energy sources are termed as distributed energy resources (DERs). The term 'Distributed Generation' has been devised to distin guish this concept of generation from centralised conventional generation. The distribution network becomes active with the integration of DG and hence is termed as active distribution network.
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2 Distributed energy resources
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Renewable or non-conventional electricity generators employed in DG systems or Microgrids are known as distributed energy resources (DERs) or microsources. One major aim of Microgrids is to combine all benefits of non-conventional/ renewable low-carbon generation technologies and high-efficiency combined heat and power (CHP) systems. In this regard, the CHP-based DERs facilitate energy efficient power generation by capturing waste heat while low-carbon DERs help to reduce environmental pollution by generating clean power. Prospective DERs range from micro-CHP systems based on Stirling engines, fuel cells and micro turbines to renewables like solar photovoltaic (PV) systems, wind energy conver sion systems (WECS) and small-scale hydroelectric generation. Choice of a DER very much depends on the climate and topology of the region and fuel availability. Possibilities of using biofuels and application of various storage technologies like flywheel batteries and ultracapacitors are also being investigated across the globe in the field of Microgrid research. Most of the countries are coming up with schemes to support the exploitation of the renewable/non-conventional energy resources for meeting up global carbon commitment.
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3 Impacts of Microgrid
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This chapter discusses Microgrid. Microgrids appear to the main grid as aggregated units of loads and microsources. Microgrids have remarkable impact on existing electricity and gas markets. To harness their benefits fully, their market participation must be encouraged. Suitable market reforms must be made to allow such participation, and financial incentives should be provided for owners to invest in Microgrids. Major changes in conventional electricity market have already been initiated in some countries.
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4 Microgrid and active distribution network management system
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There are some key issues that require extensive research to improvise the design of a Microgrid management system and to make it intelligent in the true sense of the word. Major issues like market reforms, impacts on distribution system, emission reduction, communication infrastructure needs, ancillary services and protection co-ordination have been discussed in detail in Chapter 3. This chapter details how and to what extent these may be taken care of by the Microgrid controllers. Chapter 5 discusses in detail the protection systems in Microgrid.
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5 Protection issues for Microgrids
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A Microgrid is an aggregation of electrical/heat loads and small capacity on-site microsources operating as a single-controllable unit at the distribution voltage level. Conceptually, Microgrids should not be thought of as conventional distribution networks with additional local generation. In a Microgrid the microsources have sufficient capacity to supply all the local loads. Microgrids can operate both in synchronism with the utility (grid-connected mode) and in autonomous power islands (stand-alone mode). The operating philosophy is that under normal condi tion the Microgrid would operate in the grid-connected mode but in case of any disturbance in the utility, it would seamlessly disconnect from the utility at the point of common coupling (PCC) and continue to operate as an island.
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6 Power electronic interfaces
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The microsources are quite capable of contributing significantly to the generation augmentation. Power electronic interfaces are used for preferred microsources, viz. micro-CHPs, wind turbines, PV-arrays and fuel cells. It not only generates utility grade AC power, but also facilitates their overall integration in Microgrids. How ever, these interfaces are quite costly for their complicated technology and system packaging. Power converter designs are normally customised for achieving economic performance. The applicability of electric power converters can greatly be enhanced by proper design to make them rugged, cheap, reliable and interchangeable. Recent trends in the design of power electronic converters include the integration of several components similar to computer architectures and digital electronics. In order to increase the applicability of power electronic converters in distributed power generation and Microgrids with economic viability, research is being focused on the development of modular architecture. It leads to systematic power electronic solutions by the use of pre-engineered components through mass production. This modular approach has been applied to design Bricks-Buses-Software (BBS) framework as proposed by Power System Engineering Research Center-Wisconsin (PSERC) and Wisconsin Power Electronics Research Center (WisPERC). As evident from the nomenclature, this framework consists of three components, viz. (i) modular converter components known as bricks, (ii) connecting elements known as buses and (iii) interfacing elements known as software. The technical and implementation issues of this framework are discussed in the following sections with its advantages and limitations.
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7 SCADA and active distribution networks
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The overall control and management of a Microgrid will have to be implemented through an intricate network of state-of-the-art IED devices interlinked through SCADA and high-speed communication channels. The central con troller for Microgrids will perform functions like energy management, management of ancillary services, metering and protection co-ordination, grounding co-ordination, inter-tripping and fast tripping of circuit breakers through its Energy Management Module (EMM) and Protection Co-ordination Module (PCM). The entire operation of these modules will depend on high speed communication and interoperability between the devices. Therefore, the development of communication standards for microsource sub-station automation with SCADA will have immense significance in designing, developing and implementing EMM and PCM for a Microgrid and an active distribution network. Standardised communication protocols will also help in the integration of several Microgrids to form a quality power park.
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8 Impact of DG integration on power quality and reliability
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CHP applications in DG integration may be very useful to address power quality problems of less sensitive loads because of customer's reluctance to invest in UPS. Critically power quality sensitive customers are not interested to get this benefit of DG integration because of their existing dedicated UPS systems. In order to make DG integration a potentially widespread solution for power quality and reliability problems, the following issues need to be addressed is discuss in this chapter.
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9 Microgrid economics
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Microgrid's market acceptability and its viability are significantly related with several economic issues. The current policy on standby charges, net metering, Microgrid's public utility status as well as the regulatory issues on the capability of small generators to serve neighbouring customers are more akin to distributed energy resources (DERs) but not sufficient to make Microgrid a commonplace. Therefore, Microgrid-related economic issues need to be assessed and addressed in their paradigm to get Microgrid the status of a viable public utility. Regulatory issues in relation to economic issues need to be devised carefully to establish efficient participation of Microgrids in the open market of electricity as well as several ancillary services. Many regulatory barriers need to be revised to establish the viability of Microgrids.
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10 Market participation of Microgrids
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This chapter discusses the market participation of Microgrids. The electricity market reforms have brought about major changes in the market monopoly of the vertically integrated power systems. Three major components of vertically integrated monopoly are generation, transmission and distribution. Although in the restructured environment, the main functions of these three components remain the same as before, new types of unbundling and co-ordination are gradually being established to ensure competition and non-discriminatory open access to all the participants, including sellers and consumers.
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Appendix A: Modelling and performance analysis of microturbine in stand-alone and grid-connected modes
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This section discusses the modelling and simulation of a microturbine generator (MTG) system consisting of a microturbine (MT) coupled with a synchronous generator. The model is then used to perform the load-following analysis of the MTG system in both stand-alone and grid-connected modes. Simulation is done in MATLAB Simulink platform.
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Appendix B: Dynamic modelling and performance analysis of a DFIG wind energy conversion system
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This section describes an integrated dynamic model of a 750 W variable speed doubly fed induction generator (DFIG)-based wind energy conversion system (WECS). Separate mathematical models are developed for wind flow, rotor, gear and the DFIG using MATLAB Simulink. The WECS model is also validated using realistic data. The model is helpful in choosing an appropriate WECS for any given wind regime.
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Appendix C: Software simulation of PEM fuel cell system for dynamic performance analysis
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This section describes two dynamic models of a proton exchange membrane fuel cell (PEMFC) for dynamic performance analysis. The first model uses a proportional-integral (PI) controller to control the fuel flow to the reformer of the fuel cell system and the second model uses a fuzzy logic controller (FLC). The dynamic behaviour of the fuel cell system with a step load change is studied and the outputs are compared for both models. The PEMFC power generation system has three main parts, the fuel processor, power section, power-conditioning unit. The fuel processor converts the fuel into hydrogen and by-product gases. The power section generates electricity using a number of fuel cells. The power conditioning unit (PCU) converts the generated DC power into output AC power with current, voltage and frequency control to meet the demand as per requirement.
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Appendix D: Application of solid-oxide fuel cell in distributed power generation
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This section describes a dynamic model of a 100 kW solid-oxide fuel cell (SOFC) power generation system and its control. This is suitable for studying its performance in distributed generation (DG) systems. The SOFC system is chosen as a distributed energy resource (DER) because of its ability to tolerate relatively impure fuels. It can also be operated at a higher operating temperature. This dynamic model can be used to simulate and analyse the performance of such a system in both stand-alone and integrated modes with other DERs to predict its dynamic behaviour and load following characteristics. With a strategy developed to control the active power and inverter output AC voltage, it is highly efficient and capable of providing good dynamic behaviour and load-following characteristics while maintaining the load parameters. The proposed model is used to simulate a step change in power demand from the inverter side controller to the SOFC-Inverter system. It uses two proportional-integral (PI) controllers separately with the SOFC system to control the fuel flow in accordance with the power demand and to maintain the bus voltage constant at the set point value.
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Appendix E: Modelling and performance evaluation of a stand-alone photovoltaic (PV) plant with maximum power point tracking
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Photovoltaic (PV) plants consist of inverter-interfaced PV arrays. The inverter keeps the AC output voltage at the specified level irrespective of solar irradiance E (W/m2) and ambient temperature T (K). The inverters are provided with maximum power point tracking (MPPT) feature that sets the operating point voltage of the array such that maximum power can be extracted from the array. This section discusses the development of a robust and very simple mathematical model of a polycrystalline PV array in MATLAB Simulink that (i) imposes low computational burden on the system, (ii) has low data storage requirement and (iii) can be represented by standard block set of MATLAB Simulink. At the same time, the model can take into account the variation of the PV array output with solar irradiance and ambient temperature by incorporating the MPPT feature. This feature shifts the operating voltage set point of the model to its maximum power voltage so that maximum power can be extracted from it. The model is used to (i) study in detail the performance of the PV array with varying weather and loading conditions, (ii) simulate a stand alone PV plant for studying the effect on load variation on the AC side bus and (iii) develop a simple load-shedding scheme for the PV plant.
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Appendix F: Setting of market clearing price (MCP) in Microgrid power scenario
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This section proposes and analyses a pricing mechanism for Microgrid energy in the competitive electricity market where the Microgrid CC is made to participate in the bidding process to settle the market cleaning price (MCP). Two important market settlement techniques, day-ahead and real-time, are considered with the marketing strategies for renewable distributed generator (DGs), viz. PV and wind generation. The main idea is to determine the MCP for the dispatch by an aggregate of different types of DGs to an aggregate of different types of consumers. These consumers are categorised as sheddable loads and uninterruptible loads.
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Appendix G: Islanding operation of distributed generators in active distribution networks - simulation studies
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Due to growing power demand and increasing concern about the environmental pollution caused by fossil fuelled-power plants, the distributed generation concept is gaining greater commercial and technical importance all over the world. Distributed generation encompasses the interconnection of small-scale, on-site DGs with the main utility grid at distribution voltage level. DGs constitute non-conventional and renewable energy sources like solar PV, wind turbines, fuel cells, mini/micro hydro, tidal and wave generators and microturbines. These technologies are being preferred for their high energy efficiency (microturbine or fuel cell-based CHP systems), low environmental impact (PV, wind, hydro, etc.) and their applicability as uninterruptible power supplies to PQ (power quality) sensitive loads. Electricity market reforms and advancements in electronics/communication technology are currently enabling the improved control of geographically distributed DGs through advanced SCADA (supervisory control and data acquisition). Research has been carried out on how interconnected DGs can be operated as Microgrids in both grid connected and stand-alone modes.
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Back Matter
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