An uninterrupted and quality power supply is the fundamental requirement of a modern society. As the demand for power increases, continuous expansions can be seen in power system infrastructure as well. Communication is an enabling technology that plays a significant role in the modernisation and atomisation of power systems. Without an effective communication system, the control and protection of large and complex power networks is not possible.
Communication is of two types: voice and data. This can be further classified into the analog and digital categories. The transport medium used for communicating voice or data information can be wired, wireless or hybrid.
No single communication technology is best suited for all the needs of a power system. However, ideally, it should have the ability to exchange data in real time and in difficult situations, and be able to restore interrupted links quickly. Confidentiality, data integrity and availability, cost-effectiveness, ease of operations and maintenance (O&M), etc. are the other key features of an ideal communication system. The type, source, amount, frequency and delivery requirements of the data/voice to be transmitted are also important considerations that should be taken into account. Moreover, as such communication systems have a bandwidth limit, there should be enough bandwidth along each path for meeting data requirements. A good margin allows for future growth and increased system flexibility.
System costs must be taken into account while deciding upon a communication technology as they could become an impediment. Organisations should evaluate upfront costs as well as lifetime O&M costs and then seek the trade-off between cost and overall performance.
Types of communication technologies
Pilot cable network
Although rarely used these days, pilot cable networks were the only means of wired communication till the beginning of the 1990s. These networks used a dedicated copper cable for communication and were utilised for protection signalling, data communications for telemetry, tele-control and supervisory control and data acquisition (SCADA) systems.
A power line carrier communication (PLCC) system operates by sending a modulated carrier signal to the transmission lines. The basic elements of a PLCC system include the transmission lines, which present a channel for the transmission of carrier energy; tuning, blocking and coupling equipment; and transmitters, receivers and relays. Power lines are used for point-to-point communication, and various kinds of terminal equipment are used to send or receive data/voice. PLCC is used for voice communication, tele-control, tele-protection, and for receiving SCADA data from remote terminal units. A disadvantage of this technology is that it operates on a limited bandwidth (4 kHz) and can get affected by extreme weather conditions.
A public switched telephone network (PSTN) uses circuit switching technology and is the most common kind of data and voice communication system. In this, dial-up and dedicated leased line telephone networks are generally used for upgrading and automating power systems. PSTN operates in full duplex mode, which allows simultaneous message exchange in both directions. It consists of two simplex channels: a forward channel and a reverse channel, which link the same points.
Multiprotocol label switching (MPLS) is a technique that is used to establish a secure communication link between cities by using a private cloud. Its key feature is that multiple protocols (internet protocol, asynchronous transport mode, frame relay, tunnelling protocols) can be used between the source and destination. However, its downside is that end-to-end convergence is controlled primarily by the service provider. Reliance Communications (RCOM), Tata Communications and Bharti Airtel are some of the MPLS providers in India.
Leased line circuits
Leased line circuits are dedicated links that interconnect important nodal centres and sites. Point-to-point data circuits transmit a considerable volume of data between two fixed terminals located in the same city or in different cities. The application finds use in primary or backup links for critical data circuits. Bharat Sanchar Nigam Limited, RCOM, and Tata Teleservices Limited are the key leased line circuit service providers in India.
Optical fibre communication is a method of transmitting information from one place to another by sending pulses of light through an optical fibre. Optical fibres have greater resistance to electromagnetic noise like radios, motors or other nearby cables as well as comparatively low maintenance costs.
Very high frequency (VHF) systems are ideal for establishing quick communication links and have frequencies ranging from 30 MHz to 300 MHz. Multiple channels are used in each frequency slot and the system operates in half-duplex mode, which can be considered a single physical channel in which the direction can be reversed. Messages may flow in two directions, but never at the same time. The system is used for voice communication from stations and mobile vans.
Microwave systems are broadband communication systems with a frequency range of 1-30 GHz. They are dedicated to SCADA data and express voice communication, and have allocated frequencies of 2.41 GHz and 2.3 GHz. Microwave radio transmission is commonly used in point-to-point communication systems on the surface of the earth as well as in satellite communications and deep space radio communications.
Very small aperture terminals (VSATs) are two-way communication systems between stations through satellites. They are used for long distance communication. Each end-user is interconnected with the hub station via a satellite, forming a star topology. The entire network’s operations are
controlled by the hub. A VSAT has near-universal coverage and high reliability. However, high costs, transmission delays and blackout periods due to eclipses are its downsides.
Synchronous digital hierarchy (SDH) is a standardised multiplexing protocol that transfers multiple digital bit streams over optical fibre using lasers or light emitting diodes. SDH multiplex rates vary from STM-1 to STM-16, where STM-1 provides data rates in the range of 155 Mbps. This technology is used for voice communications, SCADA and protection systems.
GSM is an open, digital cellular technology used for transmitting mobile voice and data services. It deploys a mobile service switching centre, base station controller, base transceiver station, and mobile station with a subscriber identity module.
ZigBee is an IEEE 802.15.4-based specification used for creating home area networks from small, low-power digital radios. It covers distances ran-ging from 10 to 100 metres (line of sight), depending on power output and environmental characteristics. The key advantages of this technology are its low cost, low power consumption, the inexpensive nature of consumer devices, and a self-organising, secure and reliable mesh network that can support a large number of users. However, it does not penetrate structures well and has low data rates. Moreover, developers wishing to use the application must join ZigBee Alliance.
RF mesh networks
A radio frequency (RF) mesh network is a communication network made up of radio nodes organised in a mesh topology. A unique pair of dedicated radios is used on each end of the link and a unique frequency is used for each wireless hop, thus creating a dedicated carrier sense multiple access collision domain. Among the various advantages of an RF mesh network is its low one-time cost, maximum performance without bandwidth degradation, and low latency. However, the lack of system integrators and interoperability, along with the low data rates, are its downsides.
Voice over internet protocol (VoIP) allows users to make voice calls using a broadband internet connection instead of a regular phone line. No extra costs are incurred for cabling and there are no extra phone bills for interconnected long distance offices. Conference calling, call forwarding, automatic redial, and caller identification are the key applications offered by this technology.
As a result of the steady growth in power infrastructure, there has been an increase in the complexity, uncertainty and unpredictability of power systems. Communication technologies have a wider role to play in tackling issues related to generation scheduling; high consumer expectations (demand for quality power, uninterrupted power, quick response and redressal of issues); distribution network augmentation; complex load management; compliance with state and central regulatory requirements; and adherence to commercial requirements (reading a million meters every month, generating and despatching bills on time, ensuring timely payments and matching payments, and settling disputes). There is a need for a smart and intelligent power system network (smart grid) that uses efficient communication technologies to integrate all users, deploys advanced information and control systems, saves energy, reduces costs, and increases reliability and transparency.
Based on a presentation by Sumit Poddar, Deputy General Manager, Corporate Information Systems, CESC Limited, at a recent Power Line conference