COMMUNICATION INFRASTRUCTURE FOR SMART MICROGRIDS

A communication infrastructure interconnecting, almost, each and every entity, coming under all stakeholders, facilitating bi-directional information flow is essential for realizing a smart microgrid. This is the backbone system that has to meet the data transfer requirements of all various smart microgrid applications with extremely varying demands and does so with the capabilities of heterogeneous and interoperable communication architecture.

Need for Communication Architecture

Communication architecture is a framework that defines the role and functionality of each participating device in terms of message transfer and network organization to realize the data communication. There exists a variety of communication architectures and the choice or deployment depends on the application to be catered to (Table 3.15).

There is a growing consensus amongst smart grid stakeholders on features such as scalability, interoperability, availability, wide usage and reliability, latency, bandwidth, throughput and packet delivery ratio as the deciding factors on choice of communication technology for creating the network. Specifically, the smart grid activities include measurements of energy consumption and operational details, controlling energy provision, and monitoring and control of generation and distribution according to the power flow requirements. The communication relationships are not always peer-to-peer, but more of a multi-client multi-server nature. As seen from the previous two sections, each and every application has communication path defined by the respective standard and the hierarchical network levels are used to realize the message passing. As so many applications with widely varying operational styles and requirements exist in the same geographical area, the optimal communication network setup and message exchange is not a straightforward task; it needs a reliable and scalable communication architecture for progressive realization of the smart grid. Selection of the best-fit communication technology to implement the architecture is subject to many technical, legal and strategic restrictions.

TABLE 3.15

Features of Communication Network

Hierarchical Communication Architecture in Smart Grid

The smart grid communication network is composed of various smaller communication networks intended for realizing data exchange for a variety of applications that exist at various sectors of the grid and serve the stakeholders concerned (Figure 3.53).

1. HAN/BAN/IAN: HAN is a network of all smart devices in a home for smart operation of appliances. The smart devices include those which participate in DR or that can be remotely turned ON/OFF via commands, etc. Smart meter is by default a member of HAN network. BAN is a version of HAN that extends to a building and may have more than one smart meter each belonging to a different consumer. BAN also has smart devices other than smart meters which operate based on commands from respective home controller, building level controller or distribution utility. IAN is similar to a BAN in the context of an industry and may or may not have more than one smart meter and has a lot of smart devices. HAN/BAN/IAN forms the network of data collection and actuation points among the various consumer classes and is a critical element of the smart grid communication network. Both data collection and control are achieved through these networks which hierarchically take the bottom position; these networks are usually

FIGURE 3.53 Hierarchical communication network for smart microgrid.

fulfilling the last mile connectivity and these realize smart grid applications like AMI, DR, DSM, etc. PLCC is the widely used technology in many utilities and equally or even more preferred are the wireless counterparts in low data rate RF and other PAN, and wireless LAN standards.

• 2. FAN: FAN is the network of field devices distributed throughout the traditional transmission and distribution sectors, at substations and locations of installation of outdoor equipment like distribution transformers, for measurement, protection and control operations. This is a network in which end devices play a vital role in operation of the grid and is part of WAMPAC (sometimes in standalone microgrids and even in DER) realizing its capabilities by further getting integrated to NAN and WAN or to WAN directly. The network nodes act as source nodes in realizing WAMS sharing measurement data and WAPS and WACS sharing status data and also act as end actuators in realizing WAPS and WACS acting based on intelligence and commands from network operations and control centers. This network sometimes operates as a heterogeneous one with low bandwidth and low latency interconnecting the field devices to the local master; while highly reliable technologies like PLCC and fiber optics act as the back-up connecting FAN to NAN or WAN.
• 3. NAN: NAN is a network interconnecting smart meters and data concentrators and is often considered as the last mile communication link of the smart grid communication network. The number of smart meters under a concentrator varies and depends on the communication technology used and network topology employed. HAN/BAN/IAN are connected to NAN to share the data generated in one direction and to receive the commands and configurations in the other providing a gateway facility for the utility’s WAN to access the consumer’s devices. Typical NAN operation is within a communication range of Ю km supporting data rates in the range of 10-1000 kbit/s and employs mesh networks based on IEEE 802.1 Is, IEEE 802.15.4g and other PAN and wireless LAN technologies offering selfconfiguration, high speed, and reliability.
• 4. WAN: WAN implements a bi-directional backbone communication link across various sectors and forall the smart grid applications. Interconnections are provided between utility control centers, markets, service provider data centers, NAN and FAN for exchange of voluminous data in one direction and control and configuration commands in the other through high- bandwidth media. The WAN covers a range of 10-100 km and fiber optics forms the first choice for this network, being robust and having very high transmission capacity, although costly to deploy. Alternative technologies include WiMAX, broadband over PLCC, etc.
• 5. SCADA network: This involves networking among various devices in a substation or generation plant or control center for automation of the entity concerned. This has limited bandwidth requirements, but quite stringent latency and reliability demands. Typical choice of technology includes Ethernet, fiber optic, and wireless LAN.

Communication Messages in Smart Grid

The messages in smart grid communication network can be classified as monitoring/ status update, command/configuration and administrative at various levels of operation of the grid.

• 1. Monitoring/Status update messages: The very core feature that makes smart grid a reality is the capability of all smart grid applications to make intelligent decisions in real time. The prime necessity to achieve this is data. Smart grid at its various levels of operation generates humongous volumes of data which the resource-constrained nodes - distributed for data collection and control - cannot process and use for real-time decision making. There are some applications in smart grid, especially related to protection, where sensing, processing and action are done locally but local processing is not true for the major share of applications. Even in the case of protection, configuration and status, update messages need to flow in both directions. One major contributor to measurement data is a smart meter which is present at every point of consumption and measures the consumption data along with power quality data. These data are passed to the distribution utility centers, operation centers and data centers via several hierarchical layers of data concentrators depending on the area covered and metering standards in force. The number of intermediate devices is also dependent on network topology and choice of communication technology. The smart meter network typically uses HAN and NAN for achieving its connectivity and sometimes uses even WAN. Another major contributor to data monitoring is IED distributed in the grid for real-time measurement like PMU. These devices measure electrical signal parameters, do processing in terms of phasor computation, time tagging, etc., and include status messages of various components (like circuit breakers and relays) along with the data message. The PMU data flows via several hierarchical layers of PDC before it reaches the utility centers. The number and layers of PDC will vary based on the involved WAMS/WAPS standards. The data flow between these devices, utility operational centers, data centers and markets happen through FAN and WAN. The various SCADA networks deployed for automation at substations and power plants also do measurements. The data originating from the end devices pass through hierarchical layers of master nodes and gateway before it is passed on to utility centers. The number and levels of master nodes change based on the level and type of automation and the plant and associated standards. Even though the processing and actions needed for automation are done locally, the related data for real-time operation, control, analysis and storage are sent to the utility centers using LAN and WAN.
• 2. Command/Configuration messages: Upon receiving the information through the messages, the next step is initiation and execution of control actions within the time limits specified in the respective standards. The control signals are generated at different locations or levels of operational and control centers, and sometimes even from local field level masters. Another type of data that flows from the utility control centers to the individual devices is the configuration for operational specifications of all measurement, protection and actuation devices in the grid. Sometimes the configuration messages flow in the opposite direction (i.e., from device to the control centers specifying the current configuration of the device). The hierarchical layers in the path and the delivery time limits for each message are all specified in the respective standards. The major command and configuration message exchange happens between IEDs (like PMU) and utility centers which are critical for realizing operational capabilities of WACS and configuration of WAMPAC and is achieved using WAN and FAN. Another share of command and configuration data exchange is between the smart meters and utility centers realizing DA, DSM and DR capabilities; also, between SCADA systems of distributed generation and storage systems as well as utility centers to realize DER capabilities and achieved using WAN, NAN and LAN. Similar message exchange exists between SCADA systems of substations and power generation plants for certain control and configuration actions, and sometimes based on inputs from control and operation centers and this is achieved using WAN and LAN.
• 3. Administrative messages: There is a need for message exchange among entities such as customers, DER, substations, generating stations, control centers, markets and service providers which are essential for the necessary governance in executing the right operational strategy for the power grid. This type of message exchange does not have a stringent requirement on latency, but sometimes the exchange demands large bandwidth availability for huge amount of data transfer for analysis and post-mortem. Such messages exchanged between consumers and utility help in developing the consumer information system, sharing consent on DR, information on multiple tariff schemes, operational limits for any DG present, billing information, incentives, etc. AMI administrative message service also includes request for measurement of energy data, service restoration and disconnection, firmware update, etc. The control centers, substations, generating stations and markets exchange administrative messages for energy trading, analysis of operational performance, and service management.