Communication Infrastructure for Smart Microgrids

P. Sivraj

THE NEED TO TURN "SMART"

All major innovations paving the way for a smarter world were either directly or indirectly connected to developments in the industrial sector. When we look at industrial revolutions, historically, they address major technological advancements in the field of manufacturing and associated industries. With each incremental phase of industrial revolution, the changes and modernization became more and more cyber and less physical leading to cyber-physical systems where the cyber system is an essential feature that adds smartness to all physical systems and services. As a result, transformations today are more on the digital side in industries, treating production and operation as services of smart systems revealing new business models and solutions. Such transformations bringing in smart systems are not limited to industrial systems but extend to almost all application domains in the real world. The transformation of various domains and services into smart systems is more of a natural evolutionary process than a forced integration of smartness into these systems (Figure 3.1).

A power system network is an infrastructural system designed, setup and maintained for delivery of electrical energy from electricity generating stations at different locations to different types of consumers who are geographically distributed. A close understanding of the power system evolution over the last century will reveal the slow and steady way in which smartness has crept into the various sectors of the network. The need to shift to smart grids has mainly been two-fold - namely, natural and progressive evolution and pressing demand for improved and innovative services from all stakeholders. All bulk generation plants are being automated with SCADA systems reducing the need for human intervention in managing the operation of generation plants. This automation is also expected to fulfill the automated control of generation and grid integration

FIGURE 3.1 Smart services leading to smart infrastructure.

based on demand and grid status. The transmission sector has substations distributed at different geographic locations and overhead transmission lines to transfer electrical energy from one location to another. The modernization and automation in the transmission sector involves automating the operation of these substations and also enabling connectivity among these substations and control centers. Another aspect of automation is the real-time monitoring of the power transmission network for optimal control and protection. The same needs of substation automation and grid monitoring, control and protection are essential in the power distribution network, also.

Generation, which used to be primarily centralized and bulk, has been transformed into a mixture of centralized and distributed, with options for distributed energy storage, too. Also, the emphasis and focus of the new technologies are on the development of more distributed generation plants integrating electrical energy from renewable resources, thereby helping to realize the concept of microgrids. With the advent of technology that can enable fruitful and seamless integration of distributed generation with the grid, consumers are becoming prosumers by having captive RE power plants on one hand and exploring import-export transactions with the grid on the other hand.

The metamorphosis of the electric power system is also being shaped by the new operational features and management structure of the evolving hardware infrastructure. Whereas in the past, power flow was unidirectional and power system operation was monopolistic, the bidirectional power flow in the networked lines as well as participatory operation of the business has necessitated active communication in the system, among the nodes of the network and between the utility, operator and con(pro)sumer. Such communication involves data exchange, handshakes, switching commands, permissions, bills, bids, etc (Figure 3.2).

All these changes clearly bring out real-time data collection, processing, message passage and decision-making capabilities as unavoidable requirements to realize the

FIGURE 3.2 Traditional power grid vs smart grid.

vision of a smart grid. More than a simple networked system the communication network added to power system is expected to bring in required intelligence at all levels of operation. The smartness envisaged for the future grid is expected to convert the conventional power delivery system to a reliable, uninterrupted, affordable and universal system.

COMMUNICATION TECHNOLOGIES AND STANDARDS

Communication technology or information and communication technologies (ICT) refers to all software and hardware modules that are used in a system to communicate information or exchange data. A communication network is a collection of nodes or network elements that can interact with each other for information exchange over a shared resource such as wired or wireless medium. Realization of a network for data exchange in any application domain happens only with the help of communication technologies. Various network elements can be interconnected using the same communication technology to form a homogeneous communication network or by different communication technologies to form a heterogeneous communication network depending on the communication requirements of the applications. Depending on the number of member nodes, area covered, amount and rate of data transfer, the network can be classified typically into local area network (LAN), metropolitan area network (MAN) and wide area network (WAN). The ICT is an umbrella term that includes communication over any medium and the various services associated with it (Figure 3.3).

FIGURE 3.3 Smart infrastructure with heterogeneous communication.

There exist a large variety of wired and wireless communication technologies that can be used for interconnecting various elements of the smart power system network to create an ICT network for message exchange. A few technologies that fit the purpose are listed as follows:

1. Wired

a. Fiber optic communication

b. Ethernet

c. Power line carrier communication

2. Wireless

a. GPRS/LTE

b. Wi-Fi

c. WiMAX

d. ZigBee

e. Bluetooth

f. Cognitive radio

g. Wavenis

h. HomePlug

i. 6L0WPAN

j. Z-Wave

k. Wireless HART

l. Insteon

m. Wireless M-Bus

With the advent of new communication technologies and improvements in the existing ones, ICT is turning out to be the backbone of automation in power systems, paving the way to the realization of the smart grid. As power system automation demands large amounts of message passing among the widely distributed nodes in real time, the challenge involved in making a power system “smart” is manifold. The data that is generated by distributed devices must be communicated to multiple nodes at different hierarchical levels of operation to meet the large spectrum of applications with diverse requirements in the new grid. The choice of communication technology is not easy, as there are many technologies that can potentially suit the requirements of a given application and therefore the optimized selection needs a careful performance analysis. As the communication requirements of applications across different sectors and hierarchical levels of smart grid are largely diverse, the end-to-end communication architecture in a smart grid system will be a heterogeneous one. It poses a serious challenge to ICT integration within power systems (Figure 3.4).

This challenge is addressed by means of standards. The standards of a particular communication technology define its features, including performance and operational aspects, in establishing interconnection of multiple devices to form a network. The standards of various smart power system applications define the respective communication requirements for message exchange in terms of message sequence, type, rate, participating members and other aspects. A few of the many available standards are listed as follows:

• 1. IEEE 1646-2004
• 2. IEEE 1547.3-2007
• 3. IEC 61850-90-5
• 4. IEEE C37.118.2
• 5. IEEE 1815-2012
• 6. DLMS/COSEM - IEC 62056
• 7. IEEE 2030

FIGURE 3.4 ICT interconnecting various stakeholders of power grid (based on NIST model).

Wired Communication Technologies

Wired communication technologies use a wired physical medium for exchange of data from one network element to another. Use of hardwire connection provides better security and minimal interference along with additional benefits like large bandwidth, better data rate and range. High installation and maintenance cost, right of way issues to lay the cables and network scalability are some of the disadvantages of wired medium. Some examples of typical wired communication systems are telephone, ethernet, cable television network, and fiber optic communication. There are various standards and protocols that govern the data communication over wired medium. These protocols and standards define the operational measures and specifications of medium for data transmission among multiple nodes. Most commonly used media for wired communication include coaxial cable, twisted pair copper cable, fiber optic cable and power line.

Ethernet-IEEE 802.3

Ethernet represents a variety of data communication techniques over wired medium commonly used for computer network communication in LAN, MAN and WAN. Ethernet was developed by Xerox PARC in 1970s, commercialized as a technology in 1980 and standardized by Institute of Electrical and Electronics Engineers (IEEE) in 1983 as IEEE 802.3. Ethernet has since undergone a lot of change to accommodate new features - such as, larger bit rate, higher number of nodes, improved range of communication, etc., and is now widely used in all forms of automation having wired communication. Ethernet technology first started using coaxial cable as the shared physical medium and later shifted to twisted pair copper cable. Many variants of ethernet like ethernet over power line and fiber optics exist for long distance and large bandwidth communication (Table 3.1).

TABLE 3.1 Types of Ethernet

Devices using ethernet for data communication divide larger data streams into smaller units called frames. Each of such frames will have a set of header control bytes like source and destination addresses followed by data payload and error checking data as tail end bytes of the frame. The end-to-end operation on ethernet for data communication is governed by the open systems interconnection (ISO-OSI) framework model of the International Organization for Standardization and later by the transmission control protocol and internet protocol (TCP/IP) model.

Coaxial cable: Coaxial cable has a central copper conductor that carries the data in the form of high frequency signals and usually has three more layers of insulation and shielding around it. The first layer is a dielectric insulator, typically PVC, and around this is the second layer of a metallic wrapping which acts as the second conductor for completing the circuit. This whole arrangement is covered in an insulating sheath or plastic cover. There are various standards that govern the choice and development of coaxial cables for real world applications (Figure 3.5).

Twisted pair copper cable: Twisted pair copper cable has two insulated copper wires of the same circuit twisted together which acts as conductors for data transmission. This twisting of wires of the same circuit will reduce electrical interference, noise from adjacent pairs and crosstalk besides improving electromagnetic compatibility. There are two variants of twisted pair copper cables - shielded and unshielded. Each conductor in the unshielded type has an insulation and then the complete set is wrapped by a sheath. A metal foil shielding is provided in the other type for individual or small group of twisted pairs and a collection of such pairs are wrapped around with braided mesh or metal foil or both (Figure 3.6).