Technical Challenges of Smart- and Microgrids

Transcription

1 1 Renewable Efficient Energy II Conference Vaasa, Finland March 2012 Technical Challenges of Smart- and Microgrids Sampo Voima and Kimmo Kauhaniemi University of Vaasa, Finland Abstract Today s passive distribution networks are in the beginning of the transition towards future Smart Grids. A Smart Grid is a network that can employ a wide range of different active resources that include distributed generation (DG) and energy storages connected together with telecommunication. One aspect of Smart Grids is Microgrid which can be explained as a network capable of controlled island operation. This paper gathers and addresses some of the technical challenges faced when transitioning towards Smart Grid. Some of the technical challenges faced are protection, integration of energy resources, load control, stability to name a few. Ultimately Smart Grids enable full integration of energy resources and efficiently delivers sustainable, economic and secure electricity. Introduction Distribution networks have remained unchanged for decades. The infrastructure of distribution networks has been passive and followed the same concepts. Today, however, the long lasted concepts are changing. The passive distribution networks of today are in the beginning of transition towards future Smart Grids. The distribution network is transforming into an instrumented, interconnected and intelligent energy distribution system [1]. Drivers for change are on one hand external, cleaner and more sustainable way of producing electricity which will enable greenhouse gas reduction and on the other hand internal with aging infrastructure [1], [2]. The change in distribution network partly comes from the increasing number of distributed energy resources (DER) and active resource connected to the network. Inevitably, the distribution network is getting increasingly complex and this generates many interesting research challenges. Smart grid does not have a single clear definition and there is a great deal of variation what is considered when speaking of smart grid. One of the many definitions for Smart Grid is given by The European Technology Platform SmartGrids that states [3]: "A SmartGrid is an electricity network that can intelligently integrate the actions of all users connected to it - generators, consumers and those that do both in order to efficiently deliver sustainable, economic and secure electricity supplies". However, smart grid is not only the concept of developing smart meters or home automation, rather there is much more to consider [2]. In [1] it is expected that smart grid should be characterized by self healing actions and improved power quality. Furthermore, smart grid means a way of operating the network using communications and active resources etc. Nevertheless, one of the defining characteristics of the smart grid is that it enables active participation by consumers [1]. The concept of smart grid seems very attractive, but there are still many challenges before fully integrated smart grids can be enabled. Future smart grid challenges will include integration of DER, protection, demand response, peak load shaving etc. Each of the previous categories is facing a series of challenges which will have to be solved before full realization of smart grids is enabled. Also collaboration among utilities, governments, industries, and academia will be essential [1]. To achieve this numerous technological innovations will be required. These will come from multiple fields

2 2 and disciplines as are the challenges including real world aware systems of systems, modeling, analytics, optimization, security, silicon technology, and physical science [1]. Getting to fully utilized smart grid is a step by step procedure with different challenges in different development stages. In the following chapters some of the most important technical challenges are discussed. Active Resources Active resources (distributed generation, load, storage, and electric vehicle) are new and interesting addition to the grid. Active resources change the way how networks are considered; the traditional networks change from passive to active. The active resources considered here include distributed energy resources (DER), energy storage (ES), and demand response (DR). To accommodate all the different resources that can be added to the network requires installing new infrastructure to support them. Although, in [4] it is stated that smart grid is capable of meeting growing demand without need to add new infrastructure. This may very well be the case for primary systems but indeed more supporting infrastructure is needed to enable the full integration of smart grids and enable it to reach its full potential. Operation of electric system requires a perfect balance between electricity generation and load at all times. Active resources can be used to help to balance if the balance between generation and load is not perfect. Both generation and load levels can change rapidly and unexpectedly due to many reasons [5]. Energy storages are needed to balance imbalances in the network with power generation and load. Another reason considering energy storage is that they could be used for peak power reduction by supplying power to the network and loading the storages again when consumption is at lower level. Moreover, in microgrids their role becomes more important as they are often used with microgrids to maintain the voltage and frequency in the microgrid when the generation cannot follow the load variations. Energy storages can also provide ride-through capabilities when there are dynamic variations of primary energy [6]. Energy storages can also be used to balancing out the power output of distributed generation (DG) units. Technology wise a number of different types of energy storages are available right now such as batteries, super capacitors and flywheels [6]. Electric vehicles (EV) are becoming more widespread and their number is increasing. A significant question to be answered is how smart grids should be designed to support a large number of EVs plugged into the electric grid. The question becomes harder with fluctuating production of some DER units such as wind and solar energy [1]. Energy storage adds a new dimension to generation planning since it provides a recourse to buffer the intermittency of other sources. Large scale integration of EVs would lead to a new large load on the grid. Charging strategies and the charging infrastructure for EVs becomes important, but integration of EVs brings up also the possibility to use EVs as energy storage devices [1], [7]. A concept where EVs are used as energy storages is called vehicle-to-grid (V2G). V2G concept opens up possibilities to use EVs for power balancing purposes when the battery is discharged and used as energy storage. In [5] is it said that demand response (DR) can be defined as the changes in electric usage by end-use customers from their normal consumption patterns. Change in consumption patterns can be reduction or shift of demand as a response to incentives as well as energy supply problems or outages [1]. DR includes all intentional changes to consumption patterns that are intended to alter the timing, level of instantaneous demand, or the total electricity consumption [5]. DR makes it possible to manage energy usage down to individual networked appliance. Furthermore, [1] regards V2G as a particular case of DR. DER integration DER integration will be crucial in forming smart grids. DER includes both DG and energy storage. Also controllable load, virtual generation and demand response can be included in the definition of DER. DER units are dispersed geographically in nature and some forms of production, mainly energy from the wind and sun are especially sensitive to weather conditions and therefore they are intermittent in nature [1]. Adding DG units to the network brings up challenges with protection which will be looked at in the following chapter, in addition at least the following challenges are created: [8]

3 3 changes in grid topology steady state voltage rise voltage fluctuations voltage dips. When additional sources of current are added to the network the network topology can change for example when normally radial network starts to remind closed ring or meshed topology networks because of the added source for current. In a study done in [8] it was concluded that both induction and synchronous generators in steady state raise the voltage of the feeder they are connected to. The study further concludes that the impact of induction generators is less than that of synchronous ones and that synchronous generators may have to operate under-excitated to absorb reactive power and thus keeping the voltage rise to minimum or even lowers it. Furthermore, [8] state that voltage can fluctuate for example due to sudden and big fluctuating power output of renewable generation. The voltage fluctuating problem may become severe if the generation is large compared to the capacity of the distribution system and loads. Sudden voltage dips may occur when starting up a DG [8]. According to study done in [8] induction generator can cause voltage dips up to 40 % for several seconds and that a soft-start circuit may be needed to reduce the start up current and therefore the voltage dip of induction generator. In passive distribution networks the policy of installing DER has focused on connection rather than integration and the DER has been installed with fit-and-forget mentality [9]. With this policy they have no active management and therefore lack functionality for system support. In [9] it is suggested that DER (including generation and load) could be aggregated into controllable virtual power plants (VPP). Operating DER as a VPP could also be considered as a special case of demand response. VPP could include a system of large number of small scale DERs into one controllable larger generating unit. The VPP can be compared to a transmission connected generating unit as shown in Figure 1 where a system of small scale DER is shown as one bigger controllable resource [9]. When large number of small scale production is represented with a single VPP it gains characteristics that a transmission network connected plant has, for example schedule of generation, generation limits, operating cost characteristics, contribute to system management and so on. Without integrating small scale units into VPP the DER units do not have sufficient capacity, flexibility or controllability to gain these attributes [9]. Figure 1. Characteristics of DER as a VPP [9]. Challenges for creating VPPs come from market viability and integration as well as from technical solutions. As the VPP is composed of a number of DER with various technologies, types, sizes and geographical locations getting all these together as VPP will be a great challenge. A great deal of technical data has to be taken into account when operating a VPP. Communication requirements and infrastructure comes as an added challenge of VPPs. Protection The transition towards smart grids brings up new requirements and challenges for protection that needs to be adaptive for changes in the network topology and configuration along with connecting of active resources. The protection challenges relating to smart grids come from adding active resources, especially DG, to the grid. This further causes the network topology to change. The effects of adding active resources to networks are numerous including changing power flow and increased or decreased short circuit levels among other effects. The effects of DG on the distribution network protection also depend on the type of DG unit used. DG units can be divided into three categories: synchronous

4 4 generator, asynchronous generator and converter interfaced. The first two are rotating machines with different fault current producing characteristics. Synchronous generator can sustain high fault current for longer periods of time while the fault current produced by an asynchronous generator decays very rapidly. Converter interfaced DG units have their short-circuit current producing capabilities limited to approximately 2-3 times their rated current [10], [11]. More and more of the DG units in the future are expected to be connected via converter interfaces. The low short-circuit current producing capability creates challenges for protection as the current may be so low that it will not be detected with traditional overcurrent protection. The effects of DG on the distribution network protection have been extensively introduced for example in [12] which lists some of the most interesting challenges caused by DG: selectivity problems sensitivity problems unintentional islanding autoreclosing problems. Selectivity problems come from falsely tripping of DG or the feeder. Selectivity problems may occur when the DG unit is feeding fault parallel with feeding substation and contributing to the fault current and cause either the DG or the feeder overcurrent relay to trip. Sensitivity problems mean that fault is not detected or it is tripped slower than what were the initial protection settings. In all situations unintentional islanding should be prevented and removed as fast as possible. In suitable circumstances, however, it is possible that DG remains feeding a part of the network without a connection to the main grid. Unintentional islanding is not allowed for example due to safety reasons thus a specific protection method is required to prevent this. The main aim of autoreclosing is to remove faults by applying as short supply interruption as possible for the fault arc to distinguish. DG causes challenges for protection and can cause autoreclosing sequence to fail by maintaining voltage and therefore the fault arc at the fault location. Autoreclosing sets fast requirements for disconnecting of DG units during the dead time of an autoreclosing sequence. With telecommunication based intertripping scheme this could be possible by relying on intertripping sequence initiated from the start for example of a feeder. Self-healing is a feature strongly related to protection and automation. Self-healing means, for example, that it is possible to continue to supply part of the network after contingency by taking backup supply into use or by operating part of the network as microgrid. The protection and automation system should be able to take action in order to continue to deliver power to areas not affected by contingency after it has occurred. Communication This chapter identifies some challenges relating to communications technology area with the smart grid. The existing grid lacks communication capabilities required from smart grid applications [13]. The smart grid applications require interoperability between different components of the power system which can be provided by linking them together with communication and information technology [13]. This is supported in [13] and [14] which both identify reliable and real-time two-way communication as key factors for the smart grid infrastructure. In [14] it is stated that telecommunication infrastructure is needed because many smart grid applications discuss optimizing local generation and controllable loads to improve efficiency and allot for greater use of sustainable generation. These applications rely on accurate knowledge of real-time network conditions [14]. It is said in [15] that the communications network will facilitate advanced control and monitoring and also support extension of participation of generation, transmission, marketing and service provision to new interested parties. With the integration of advanced technologies and applications a huge amount of data from different applications will be generated and transferred across the communications network and it will be a great challenge to adequately dealing with the volume of data generated [13], [14]. Therefore, [13] stresses the point that it is very critical for electric utilities to define the communications requirements and find the best communication infrastructure to handle the output data and deliver a reliable, secure and cost-effective service throughout the total system.

5 5 However, [15] lists the key challenges for communications is to provide robust, secure and interoperable networks. While in [13] the key challenge is that the overall smart grid system is lacking widely accepted standards and this situation prevents the integration of advanced applications, smart meters, smart devices, and renewable energy sources and limits the interoperability between them. Furthermore, according to [13] standards that allow interoperability for the overall system is critical prerequisite for making the smart grid system a reality. Today the IEC standard is looking like it will be a complete telecommunication solution for the future, still a possibility remains that new applications will require specific protocols [14]. The requirements for smart grid communications can be difficult to define as the network is constantly evolving and the final form of the network is not yet known and it still can change over time. The constantly evolving communications network sets challenges for developing communications solutions as the communication architecture is evolving alongside. One way of dealing with this problem would be to analyze various architectural options for future energy grid and to identify communications requirements between key building blocks such as generation, transmission, distribution, and consumers (including residential, commercial and industrial) [15]. The two most fundamental requirements that need to be considered according to [14] are the throughput of the channel (often called the speed, or bandwidth) and the latency of the channel (delay). The speed of telecommunication has big effect on the protection and to the functions that can be achieved with telecommunication based schemes. Telecommunication signal transfer delays have been studied in [16], where it was concluded that device-to-device data transfer delays with GOOSE messages including the actual data transfer as well as the data processing are 30 ms or less. Also important considerations are reliability and security [14]. Nevertheless, a solution that suits all cases can be impossible to find as every smart grid application will have different expectations from the communications networks, in terms of reliability, Quality of Service (QoS) and capacity [15]. As any communications network the communications in smart grid is also vulnerable to security risks. The risks include manipulating of services to homes and business for instance by gaining the control of smart meters and disrupting load balancing by sudden increase or decrease of power demand [15]. The communications network therefore requires substantial precautions for its security [15]. The cyber security threats and possible solutions are further discussed for example in [14]. Microgrid The increased number of DG units installed to distribution networks opens up new possibilities to use part of the network as microgrids; a controlled island capable of autonomous island operation. Microgrids can be describes as part of a network containing DER including DG and ES which are capable of producing the energy needed by the loads in the microgrid. Normally microgrids are operated parallel with the utility grid. However, for example during outages in the utility grid they are capable of operating in an island mode. This characteristic allows increased reliability, security of supply and furthermore contributes directly to one of the features of smart grids, the self-healing capability. Selfhealing in distribution networks means that the network is able to reduce or even prevent power outages. For microgrids self-healing capability realizes with outages in the utility grid while the microgrid is continuing its operation in island mode. The microgrid is able to restore/continue supple for the customers inside the microgrid. Microgrid can be small household grid or larger for example medium voltage (MV) feeder with island operation capability as depicted in figure 2 where also several other possible microgrid configurations are demonstrated. Microgrids are one key potential concept to realize active distribution networks of the future [17].

6 6 Figure 2. Different possible microgrid configurations [17]. Key technical challenges for microgrid defined in [17] are: successful transition to island operation power quality management during normal and island operation microgrid protection during normal and island operation. Transition to island may take place either intentionally or unintentionally [17]. Successful transition into island operation requires power balance inside the island after transition. The transition can be more easily secured by keeping the power balance inside the island during normal operation. In any case successful islanding will require fast and robust response from control system [17]. ES technologies can be used in the microgrid to ease the difference in power balance or in cases where the generation and loads of the microgrid cannot be exactly matched. ES provides faster control than for example microturbines and fuel cells which have low response to control signals. Furthermore, ES stabilizes and permits DG units to run at a constant and stable output, despite load fluctuations. In addition, ES provides the ride-through capability when there are dynamic variations of primary energy [6]. The main control functions of a DER unit are voltage and frequency control and/or active/reactive power control [18]. In grid-connected mode the supporting network can be considered as an electric slack bus that will supply/absorb any power discrepancy in the microgrid. During island operation this kind of control is not possible and in practice the control functions of an islanded microgrid can be achieved by using some non-critical loads (such as space heating) as controllable load. Further, in an autonomous mode of operation load/generation shedding is often required to maintain the power balance, voltage and frequency within allowed limits and thus make stable operation possible. The overall protection scheme must cover both the grid connected and islanded modes of operation. For grid connected mode the traditional overcurrent based protection may be sufficient. However, the main challenge with microgrid protection, especially during island operation, is the lack of high fault current. Low fault currents are due to low shortcircuit current producing capability of converters as explained earlier. This may prevent or slow the operation of traditional overcurrent based protection. With microgrid protection some issues are needed to be taken into account as

7 7 explained in [19]: a) the detection of utility grid faults and transition to island operation has to be done so fast in every possible fault situation, that the stability of the microgrid can be maintained after islanding, b) the voltage and frequency control principles in microgrid as well as management strategies of energy storages, c) control principles of the converters in island operation, and d) earthing arrangements must be such that safety is secured in every situation. Conclusions Transition of today s passive distribution networks to smart grids retains many exciting challenges. This paper addressed some of the key technical challenges that are needed to overcome to achieve the smart grids of the future. Seamless integration of active resources and DER will be challenging task not only for integration of all the different resources but also for communication. The communication in smart grids needs to be robust and reliable while keeping up with changing communication infrastructure and increased volumes of data. Protection of smart grids needs to be adaptive to changes in network topology and to connecting and disconnecting of distributed generation. Microgrids are an interesting alternative way of using parts of distribution networks as controlled islands which can play an important role in the future smart grid. There are still many technical challenges remaining to be solved before smart grids can be fully realized. References [1] M.G. Rosenfield, The smart grid and key research technical challenges, 2010 Symposium on VLSI Technology (VLSIT), June [2] M. Hashmi, S. Hänninen & K. Mäki, Survey of smart grid concepts, architectures, and technological demonstrations worldwide, 2011 IEEE PES Conference on Innovative Smart Grid Technologies (IGST Latin America), October [3] European Technology Platform SmartGrids, Strategic Deployment Document for Europe s Electricity Networks of the Future, Final Report, 20 April Available at: [4] A. Zahedi, Smart Grid Opportunities & Challenges for Power Industry to Manage the Grid more efficiently, 2011 Asia-Pacific Power and Energy Engineering Conference (APPEEC), March [5] M.H. Albadi, E.F. El-Saadany, Demand Response in Electricity Markets: An Overview, IEEE Power Engineering Society General Meeting 2007, June [6] B. Kroposki, R. Lasseter, T. Ise, S. Morozumi, S. Papathanassiou, N. Hatziargyriou, Making Microgrids Work, IEEE Power and energy Magazine, vol. 6 n. 3, May-June 2008, pp [7] B.F. Beidou, W.G. Morsi, C.P. Diduch, L. Chang, Smart Grid: Challenges, Research Directions and Possible Solutions, nd IEEE International Symposium on Power Electronics for Distributed Generation Systems, June [8] J. Driesen, R. Belmans, Distributed Generation: Challenges and Possible Solutions, IEEE Power Engineering Society General Meeting 2006, June [9] D. Pudjianto, C. Ramsay, G. Strbac, Virtual Power Plant and System Integration of Distributed Energy Resources, IET Renewable Power Generation, vol. 1 n. 1, March 2007, pp [10] S. Chowdury, S.P. Chowdury, P. Crossley, Microgrids and Active Distribution Networks, The Institution of Engineering and Technology, 2004, London, United Kingdom, 4-5, [11] T. Loix, T. Wijnhoven, G. Deconinck, Protection of Microgrids with a High Penetration of Inverter-Coupled Energy Sources, CIGRE/IEEE PES Joint Symposium on Integration of Wide-Scale Renewable Resources Into the Power Delivery System, July [12] K. Kauhaniemi, L. Kumpulainen, Impact of Distributed Generation on the Protection of Distribution Networks, Developments in Power System Protection, Amsterdam 5-8 April, [13] V.C. Güngör, D. Sahin, T. Kocak, S. Ergüt, C. Buccella, C. Cecati, G.P. Hancke, Smart Grid Technologies: Communication Technologies and Standards, IEEE Transactions on Industrial Informatics, vol. 7 n. 4, November 2011, pp [14] D.M Laverty, D.J. Morrow, R. Best, P.A. Crossley, Telecommunications for Smart Grid: Backhaul solutions for the Distribution Network, IEEE Power and Energy Society General Meeting 2010, July [15] M. Sooriyabandara, J. Ekanayake, Smart Grid Technologies for its realization, IEEE International Conference on Sustainable Energy Technologies (ICSET) 2010, Kandy, Sri Lanka, 6-9 December 2010.

8 [16] O. Rintamäki, K. Kauhaniemi, Applying Modern Communication Technology to loss-of-mains Protection, 20 th International Conference on Electricity Distribution, Prague 8-11 June, [17] H. Laaksonen, Technical Solutions for Low-Voltage Microgrid Concept, Ph.D. dissertation, University of Vaasa, [18] F. Katiraei, R. Iravani, N. Hatziargyriou, A. Dimeas, Microgrids Management, IEEE Power and energy Magazine, vol. 6 n. 3, May-June 2008, pp [19] H. Laaksonen & K. Kauhaniemi, Microgrid Network Concept of the Future, Proc. of Renewable Efficient Energy, Nordic Conference on Production and Use of Renewable Energy, Vaasa, Finland, 9-11 July