Smart Grid and Advance Metering Infrastructure

Transcription

1 Chapter 5 Smart Grid and Advance Metering Infrastructure This chapter presents technical review of smart grid as a case study of application involving remote data access. Smart grid, a major paradigm shift in electrical power system, is introduced and its scopes are identified. Architecture and major functionalities of advance metering infrastructure (AMI) from the point of view of data communication in smart grid is discussed. This is followed by detailed review of DLMS/COSEM standards that are adopted as open protocol standard for communication in AMI. Last section of the chapter presents discussion on communication technologies to be used in AMI. Various challenges to be faced by communication networks in AMI are studied and possible technology options are evaluated. Traditionally an electrical power system consist of remote and centralized large capacity power plant for generating power, long high-voltage transmission lines for transmitting power from generating stations to load centers and distribution networks for supplying power from load centers to consumers. This design of power system, popularly called as grid, is supported with additional infrastructure for monitoring and controlling various parameters of grid with an aim to improve its reliability and efficiency. By time technology in this grid has improved but not significantly. Today when world is moving in a new era of energy consciousness, power system utilities are facing unprecedented challenges with its infrastructure in current form. Growing demands of high-quality power and improved grid reliability, pressing need of alternative source of power generation, stringent regulations, 73

2 environmental concerns and rising customer s expectations have forced utilities to rethink and move towards a better grid structure. Driven by these leading utilities, technology vendors and government organizations worldwide have started their efforts towards improved energy delivery system popularly called as smart grid. Smart grid is a multi-faceted solution to the challenges faced by conventional power system. It represents a shift towards a more flexible grid topology that encourages two-way power flow between the grid and small-scale distributed power generating resources encourages increased flow of information between all entities that are part of grid for better observability and control encourages increased cooperation between consumers and utilities to reduce peak loads and optimize load flow encourages optimized use of resources at high efficiency rendering cost effective operation. In simple terms, smart grid represents a dynamic network (similar to internet) that has all small and big grid entities of generation, transmission and distribution infrastructure, right from generators to end consumers, tied together. With use of advanced information and communication technologies (ICT) and automation systems these entities on the grid will be able to communicate within and dynamically manage power flow, thereby rendering more efficient, reliable and transparent power system. To achieve these goals, compared to the infrastructure of existing power system grid, smart grid will have few more additional components as listed below. 1. New grid components related to energy generation and storage of different capacities. For example distributed and small scale energy generators like solar photovoltaic cells, domestic and industrial energy storage units, etc 2. Sensing and controlling devices that will be responsible for gathering and forwarding information from the physical layer (load end) to central 74

3 system and also executing desired functionality on basis of control commands received. For example, intelligent energy monitoring devices, smart meters etc. 3. Communication infrastructure that will be responsible for transferring massive amount of information over varying distances over the network. 4. Automation and IT backend in terms of high-end servers, middle ware, data storage and data management systems to process and manage data coming from grid. 5. Advance analytic applications that allow utilities i.e. grid operators and business executives, to analyze and extract useful information from grid as required. Thus, smart grid is a multi-domain project and has opened up directions of research in all domains of engineering. One of the major domains from these is ICT and automation system that is considered as a back bone of smart grid. This is popularly refereed as Advance Metering Infrastructure (AMI). Details of AMI including its functionality, architecture as well as possible technology solutions are discussed in following part of this chapter. 5.1 AMI system architecture and components AMI is a collective term to describe the whole infrastructure and applications of smart grid related to communication and system automation. This includes infrastructure like smart meters, communication links and central control centers and also applications for gathering, transferring and analyzing energy related information in real time. In the present section architecture of AMI and its primary functionality are discussed AMI architecture Typical architecture referring main components of AMI is shown in Figure 5.1 and discussed as follows [79]. 75

4 Figure 5.1: AMI architecture. Electricity meter and communication hub This element of AMI is generally present at consumer premises and is also referred as smart meters. It performs two basic functions [80]. First is to measure and record electrical energy consumed and/or produced along with other energy related information like load profile, power quality analysis, etc. and to provide information to consumer like energy usage, billing details, prepayment options, tariff tables, etc. Second function is to act as a communication hub providing communication interface between in-house network (called Home area network) and external AMI network. There can be more than one form in which this element may exist. In new installations a single smart meter performing both the functions of metering and communication can be used. In existing installations where already an electricity meter capable of satisfying metering needs of AMI exist, an add-on module that acts as a communication hub can be used. Smart meter or communication hub, used as an add-on unit, is an intelligent device that is capable of data processing, data storage and data communication. Smart meter or add-on communication hub may be directly connected to the central system or may be connected through data concentrator unit 76

5 (DCU). In either of the cases, communication hub operates primarily as a proxy gateway. Communication hub buffers the periodic data received from connected devices of HAN and forwards it to higher level on demand or as scheduled. Similarly, commands received from higher level are buffered before being delivered to connected device. This mode of working is also necessary as most of the devices of HAN are generally battery operated and hence their always on status may not be always possible. Communication hub may also have a facility of local interface. This facilitates local operation and maintenance (O&M) of smart meter. Data Concentrator Unit This element of AMI acts as an intermediate element between smart meter and central system. Smart meter being the tail element of the network present at consumer premises it is less likely to be directly connected to the central system. Generally number of smart meters present in a neighboring area communicates to central system through a DCU. Thus DCU is identified as an element of NAN. It is an intelligent device with primary function of managing two way data exchange. It will collect and manage information received from various smart meters and forward it to central system and also transfer commands or information received from central system to smart meters. Some of the typical functions that a DCU may perform are as follows [81]. Automatically discover meters, other grid devices and topology changes giving exact picture of asset location to central system. Collect data from each meter connected and report to central system. Data may include information like energy consumption, load profiles and power quality measurements. Monitor and report tampering. Uploads tariff tables and configuration settings to smart meters and other grid devices as received from central system. Broadcast information like demand response and load shedding to inform consumers. 77

6 Central system and Legacy system Central system acts as a central server responsible for management of all information and data related to smart metering. It is also responsible for the configuration and control of all system components and responding to all events and alarms over the network. It is possible that central system may delegate part of its operation to DCU or smart meter so that some operations may be performed locally at lower level of network structure. This is generally referred as distributed computing and control. Legacy system represents the commercial or technical system of the grid operator. It is responsible for the management of business processes such as meter registrations, remote meter reading, tariff adjustments, remote connect/disconnect, billing, outage managements, customer care, etc. Legacy system is purely to support operational and business processes and operates independent of type metering infrastructure and communication technologies involved in the network. Central system will execute the received request from the legacy system over the network and also conveys back the response received from meters thus completing the business process. Local operation and maintenance (O&M) devices and External devices Local O & M devices are portable devices that may be used by system operators to locally configure, operate and maintain various elements over the network. This is particularly useful at the time of installations and later to perform maintenance or reconfiguration if not possible remotely by central system. This facility may also help to retrieve meter data as redundant measure in case of sustained communication failure. External devices refer to auxiliary equipments that may be optionally connected to the DCU and utilizes network facilities to support objectives of AMI. For example this may include electrical substation automation system like SCADA. These devices measure and monitor data over the power system and relay this information to concerned entities. For example, in case of temporary overloading of a particular power feeder in a substation, the 78

7 information could be sent to the central system as well as smart meters to actively manage customer load in the short term to smooth-out the problem. End consumer devices End consumer devices are auxiliary equipments connected to the smart meter that enables the consumer to interact with smart meter and/or load devices within consumer s premises. A simple example of this can be display unit that gives details of consumption, current tariffs etc. On the other hand, a sophisticated example can be an energy management system. Energy management system handles HAN of major electrical loads and/or sources present in the consumer premises. It allows consumer to monitor, analyze, customize and control behavior of these devices in real time. Such systems allows consumer to play a proactive role in energy management through participation in various demand management programs proposed by utilities AMI system functionality AMI is aimed to increase the level of observability and controllability of power system. Scope of functions that AMI can support is quite large. Some of the primary functions are listed as follows [79], [82]. Accommodate all energy generations and storage options in the grid. Meter registration to incorporate new meters in the grid. Automatic adaptation to grid changes. Remote meter reading (cyclic and on demand) for the purpose like billing. Remote tariff programming for updating parameters related to tariff, calendar, contract period etc. Remote access to system elements over the grid and remote firmware updation. Management of alarm and event over the grid. Anticipate and respond to system disturbances showing self-healing characteristics. Fraud detection. 79

8 Remote programming and gathering of load profile for energy management. Demand response facility to connect and disconnect load on predefined variable load settings for load management. Power quality management. Operate resiliently against physical and cyber attacks. Give real time information to consumer about their energy consumption, energy pricing, etc. and enable them to efficiently control their energy consumption pattern and thus energy budget. Local and remote device management at consumer end. 5.2 DLMS/COSEM standards AMI is one of the essential steps towards realization of smart grid. However, one of the major factors of paramount importance in success of AMI implementation is the communication protocol adopted. In conventional power system there is no precise definition about the communication protocol to be used. International standards that exist and adopted are generally at the level of substation automation and are not sufficient to answer the scope of AMI [83]. The technology that exists is dominated majorly by proprietary protocols as defined by the manufacturer or the solution provider. Generally, such proprietary protocols are developed considering the needs of current project and budget allotted and issues like scalability and interoperability are not sufficiently emphasized. When AMI aims to integrate all grid elements, interoperability of elements working on number of proprietary protocols presents a serious concern. This created a need for open protocol standard for communication in AMI and this is answered in terms of DLMS/COSEM standards. DLMS, which refers to Device Language Message Specification, is a suite of open standards developed and maintained by the DLMS User Association [84]. It defines the common international approach to data formatting and messaging for metering equipments over AMI. COSEM, which 80

9 refers to Companion Specification for Energy Metering, is a set of rules for modeling meter data and is a part of overall DLMS standards [85]. The DLMS/COSEM standard suite has been developed based on two strong and proven concepts: object modeling of application data and the Open Systems Interconnection (OSI) model. This allows covering the widest possible range of applications and communication media. These standards are officially endorsed and registered by the International Electrotechnical Commission (IEC) under IEC [86]. DLMS/COSEM standard suit is published as set of colored books details of which is shown in Table 5.1 along with corresponding equivalent IEC standards. These standards are discussed as follows. Table 5.1 DLMS/COSEM standards and its equivalent IEC standards. DLMS User IEC Standards about Association Blue Book IEC IEC COSEM meter object model and object identification system (OBIS) Green Book IEC IEC Architecture and protocol to transport the model IEC IEC IEC Yellow Book -- Conformation testing process White Book -- Glossary of DLMS/COSEM terms Blue Book This book specifies standards to describe object model and object identification system for a physical device. Details of the standards are as follows. IEC : This part of the standard suits specifies overall structure of the identification system and mapping of all data items in metering domain to their unique identification codes. This is defined by object identification system (OBIS) [87]. OBIS covers identification of metering data of energy types other than electrical also like gas, heat, etc. Further, it not only covers identification of measurement values but also abstract values used for configuration or 81

10 obtaining information about the behavior of metering equipment. As per OBIS each identification code consists of a six-number sequence with each number represented by value between 0 and 255 that refers to value in a group. Thus a complete OBIS code consist of a number sequence of the form A.B.C.D.E.F. Value in each group A to F signifies specific metering information like energy type, measurement channels, quantity being measured, algorithm used in measurement, billing period, etc. For example, group A defines energy type e.g. 1 for electricity, 7 for gas. With A as 1, group C defines various electricity related objects e.g. 1 for active power, 5 for reactive power, etc. IEC : This part of the standard suits specifies a model of a meter as it is seen through its communication interface(s) [88]. It defines generic building blocks using object-oriented methods in form of interface classes. Interface classes represent the classes or blueprints for objects that display similar properties (attributes) and methods. The specification defines a large number of interface classes to define different kind of information. For example interface class 1 models instantaneous quantities e.g. billing period counter, interface class 3 models quantities like energy that requires information in terms of value, units and scaling factor, interface class 7 models historic values like load profiles, etc. In addition to classes representing the metered data, there are also large number of classes for abstract information like meter clock, communication profile, etc. Modeling of a physical device (e.g. metering equipment) as per the object model and OBIS as discussed above can be understood as follows. As shown in Figure 5.2 a physical device would be modeled as if containing one or more logical devices. Each logical device may represent specific functionality of a meter e.g. a multi-utility meter may have electrical meter as one logical device and gas meter as another logical device. Each physical device has by default one logical device called Management logical device. This logical device contains information of all other logical device present in physical device. Each logical device consists of one or more objects. Objects simply represents piece of structured information about a quantity (may be physical or abstract). Each object is identified by its logical name that is 82

11 defined as per OBIS code defined by IEC Similar objects in a logical device are grouped into a common interface class defined by IEC Each logical device contains by default one object of class Association that has a predefined OBIS code of This object contains list of all objects present in the logical device along with details of their OBIS code and interface class. Management logical device also contains one default object with predefined OBIS code that contains list of all logical devices present in a physical device along with details of their name and address. Figure 5.2: COSEM model of a metering equipment. Green Book This book specifies communication profile for various communication media and the protocol layers of these communication profiles. It explains how to map data in interface model to protocol data units and transport it through the communication channel. DLMS/COSEM communication profile [89] is shown in Figure 5.3. DLMS/COSEM communication profile is based on Open system interconnection (OSI) model; however 7 layers of OSI are collapsed into primarily 3 or 4 layer structure. As shown in Figure 5.3, DLMS/COSEM standards specify more than one communication profile each with common COSEM application layer. A single device may support more than one communication profiles so as to allow data exchange using various communication media. Various communication profiles are listed as follows. 83

12 Figure 5.3: DLMS/COSEM communication profile. Three layer connection oriented High-level Data Link Control (HDLC) based communication profile: This comprises of COSEM application layer, HDLC based data link layer and the physical layer for connection oriented asynchronous data exchange. It supports data exchange via a local optical or electrical port, leased lines and Public switched telephone network (PSTN) or the GSM network. This communication profile has been covered under IEC and IEC IEC specifies use of physical layer in three layer connection oriented communication for the purpose of direct local data exchange particularly for hand-held units used for local operation and maintenance [90]. On the other hand, IEC specifies use of physical layer for data exchange through PSTN [91]. 84

13 TCP-UDP/IP based communication profile: This profile supports data exchange using the internet over various physical media like Ethernet, ISDN, GPRS, UMTS, PSTN/GSM using PPP, etc. In these profiles the COSEM application layer is supported by the COSEM transport layer(s), comprising a wrapper and the Internet TCP (connection oriented) or UDP (connection less) protocol. Lower layers can be selected according to the media to be used as the TCP-UDP layers hide their particularities. This communication profile has been covered under IEC that specifies the transport layers for COSEM communication profiles for use on IPv4 networks [92]. S-FSK (Spread Frequency Shift Keying) PLC based communication profiles: This profile supports data exchange via power lines using S-FSK modulation. In this profile, either the COSEM application layer is supported by connectionless Logical link control (LLC) sub-layer and the Medium access control (MAC) sub-layer, or the COSEM application layer is supported by connection oriented LLC sub-layer using the data link layer based on the HDLC protocol and MAC sub-layer. The second option has been covered under IEC [93]. Figure 5.4: COSEM application layer on top of various lower layer protocol stack. 85

14 In all the three communication profiles discussed above uses COSEM application layer. COSEM application layer is structured on client-server paradigm. Metering equipment plays the role of the server and the data collection system like DCU or central system plays the role of a client. Here communication takes place between Applications process (AP) of client and server in form of request and response mechanism. In case of events or alarms, server can also execute an unsolicited service to notify clients. COSEM application layer consists of a Application service object (ASO) that in turn consist of a normal Application control service element (ACSE) and a COSEM specific element called Extended DLMS application service element (xdlms_ase). xdlms_ase is responsible for providing services related to COSEM interface objects. For example, it is responsible for exchanging meter information modeled as object interface classes and named by OBSI codes. Because of presence of xdlms_ase other protocol layers become independent of COSEM model and COSEM application layer can be placed on the top of a wide variety of lower protocol layer stacks as shown in Figure 5.4. Specifications of COSEM application layer has been covered under IEC [94]. Yellow Book This book specifies standardized conformation tests for Implementation under test (IUT) designed as per DLMS/COSEM specifications. The objective of the conformance testing is to establish whether the IUT conforms relevant specifications thereby indicating its capability of interworking with other similar devices [95]. White Book This book is glossary of important terms used in DLMS/COSEM specifications and IEC series of standards [96]. 86

15 5.3 Communication technologies for AMI AMI architecture as discussed in Section 5.1 can be divided into 3 segments from the point of view of communication needs viz. HAN, NAN and wide area network (WAN). WAN represents long distance communication e.g. between central system and DCU. NAN manages information over a neighboring area and acts as interface between WAN and HAN. HAN extends the communication facility up to the endpoints i.e. within consumer premises. While there is no doubt that communication technology is the key enabler for smart grid, there are number of challenges that are required to be looked at while selecting communication options. Some of the major challenges are discussed as follows [97-99]. 1. AMI consists of large number of interconnected components related to generation, transmission, distribution and utilization system that are required to communicate. Thus it can be envisaged that the communication technologies involved in AMI will be of heterogeneous in nature. In such a situation to meet goals of smart grid it is necessary that components in AMI communicate seamlessly bridging different standards, technologies and manufacturers. Thus one of the major requirements of communication networks in AMI is interoperability and support for coexistence of multiple technologies and standards. 2. Smart grid applications, components and participants are expected to grow with time. Hence, it is essential that communication solutions used in AMI should be conveniently scalable to support this. 3. Lot of investments has been made on power system in existing form both by utility companies and consumers. Hence, technology solutions in AMI should act as an overlay to the existing system where ever possible. 4. Communication networks in AMI should have self-organizing capabilities so that it can support functions such as communication 87

16 resource discovery, negotiation and collaborations between network nodes, connection establishment and its maintenance, etc. 5. Communication over AMI should be secured enough for utility companies as well as consumer to trust the data. Because of the scale and deployment complexity of AMI it can be envisaged that communication network in AMI may rely on existing public networks such as cellular and wired technologies. In such a scenario security of data over AMI becomes an issue of paramount importance. 6. In AMI smart meters at consumer premises are expected to periodically provide accurate energy related information to the utility companies. The data thus obtained from consumer reveals a wealth of information that can be used for purposes beyond energy efficiency and thus it gives rise to challenges related to data privacy. 7. For different category of data communication, network should be able to support different quality of service (QoS) profiles in terms of transmission latency, bandwidth, reliability, etc. For example data communication related to monthly bill caries low priority compared to information related to some event generated due to abnormal behavior over the grid. 8. Installation of AMI extends from residential to commercial properties in urban, suburban and rural areas and hence communication networks should be able to provide coverage over very wide and diverse geographical regions. Thus issues involved in design of communication networks in AMI are highly demanding and intertwined. Needs from the communication network are beyond the scope of some proprietary standards and/or single technology. Hence, the solution has to be of heterogeneous type with inclusion of both proprietary as well as off-the-shelf technologies. This has been exemplified in the following discussion. For AMI architecture discussed in Section 5.1, 88

17 possible communication technologies that can be used for interface between various elements are shown in Table 5.2 and discussed below. Table 5.2 Communication technologies for various AMI interfaces. Interface tag in Figure 5.1 Technology type Proposed technology and lower layer protocol I1 Wired PLC IEC I2, I3 Wireless and wide area GPRS,3G,4G UMTS,TCP-UDP/IP I4,I5, I7 Wireless and local area ZigBee, Wi-fi, Bluetooth IEEE IEEE IEEE I6 Wireless and local area ZigBee, Wi-Fi, Substation Automation IEEE IEEE IEC IEEE Standards IEEE is a set of IEEE standards that govern wireless networking transmission methods [100]. They are commonly used today in their b, g, and n versions to provide indoor or in-campus wireless local area network (WLAN) and home area network (HAN). They are popularly referred as WiFi. They can be used in AMI for HAN and home automation. Use of these standards supports design of low cost application devices to be used at consumer end. Use of this however is up to 100m and security issues arising due to multiple networks operating in the same locations has to be resolved. IEEE s is amendment to IEEE for mesh networking in WLAN, popularly known as wireless mesh network (WMN). This consists of radio nodes organized in a mesh topology. In AMI this can be an option for defining how wireless devices can interconnect to create a WLAN mesh network, which may be used for static topologies and ad-hoc networks. This can act as an AMI backhaul particularly at distribution end supporting 89

18 automation, demand response and remote monitoring. It is easily scalable and allows improved coverage around obstacles, node failures and path degradation. IEC IEC is a standard for low-speed reliable power line communications. It is also known as S-FSK (spread frequency shift keying). A typical PLC system in AMI may consist of a backbone-coupled DCU close to a MV/LV transformer. All traffic on the line is initiated by the DCU, which acts on behalf of central system. More recent narrowband PLC technology include sophisticated techniques such as OFDM (orthogonal frequency-division multiplexing) to provide higher data rates, and to target broadband solutions operating in the 1-30 MHz band [101]. Further, installation of filters highly improves SNR ratios. Despite the difficulties, PLC technologies are at a clear advantage for utility companies as no separate communication channel is required and it can prove to be relatively cheaper [102]. ZigBee ZigBee is a low-cost, low-power communication standard maintained and published by ZigBee Alliance and is suitable particularly for personal area network. It is based on IEEE 802 standard and works in industrial, scientific and medical (ISM) radio bands. One of the important advantages of ZigBee is that it supports mesh-networking. This provides high reliability and more extensive range. For AMI, ZigBee is very suitable for realizing HAN that includes interface between smart meter and other elements like multi-utility meter, local O&M device, end customer device etc. Currently, under ZigBee Smart Energy profile [103], number of agencies is jointly working to develop a standard for interoperable products that monitor, control, inform and automate the delivery and use of energy to support goals of smart grid. Bluetooth Bluetooth technology is one of the very popular short-range communication technologies in applications related to mobile phones, computers, medical devices and home entertainment products. Like ZigBee it is also based on 90

19 IEEE 802 standard and works in industrial, scientific and medical (ISM) radio bands. Bluetooth low energy (BLE), that is subset of the latest Core version, Bluetooth v4.0, is designed to support applications that require low power wireless connectivity. BLE technology can be used in HAN for wireless connectivity between energy sensors to smart meters [104]. One of the major advantages of this technology is presence of many other Bluetooth devices in home with BLE based smart grid applications can be seamlessly connected. IEC IEC [105] is primarily designed for intra-substation communication for substation automation. The standard defines the application layer and is thus independent of the underlying communication medium. All services and models are designed in an abstract form called ACSI (abstract communication service interface) which then can be mapped to protocols such as MMS (manufacturing message specification) and TCP/IP over Ethernet. Typical use of this standard in AMI is for interface between DCU and External devices e.g. a SCADA system. Cellular Technologies For data communication over a wide geographic area (WAN) cellular technologies are one of the best available options. With evolution of cellular technology from 2G to 3G and presently towards 4G it has became possible to achieve higher data rates, better security and wide coverage [106]. Scalability is another important advantage that these technologies provide. These technologies can be used for interface between DCU and central system or where smart meter is directly connected to Central system. Because of continuous rapid growth in this domain the major concern in use of these technologies is their life span. 91