Telecommunication Media Technologies for Distribution Automation Systems
By Dariush Shirmohammadi, Shir Consultants, and Belvin Louie, Pacific Gas & Electric
Distribution automation (DA) refers to the integration of measurement and control devices with telecommunication and computer systems for real-time monitoring and (automatic) control of distribution systems. DA is implemented at three levels–substation automation, feeder automation (primary automation) and customer interface automation (secondary automation).
Figure 1 demonstrates the general architecture of a modern DA system that would perform these functions. In this figure, data communication paths are indicated in bold. A review of both DA functions and architecture clearly indicates that data communication is a crucial function of a DA system. Data communication between the distribution control center (DCC) and remote units (RTU, PLC, IED, etc.), or WAN, will be discussed in further detail. This link supports the transmission of real-time data (analog and status) from remote units to the control center and transmission of control or configuration data from the control center to remote units.
In a well architectured DA system, the telecommunication system is designed with regard to the type, amount and source of transmitted data. For example, the telecommunication system designed for energy metering purposes could be simpler and slower than the one designed for the transmission of critical information, such as substation alarms.
DA Telecommunication Technologies
Generally, telecommunication technologies used for DA can be divided into two major categories–telecommunication media technologies and data transmission technologies. The former technology refers to the media used for data transmission between the DCC and remote units. The latter technology refers to the manner data is “prepared” for transmission. It includes technologies related to data formatting, modulation and multiplexing. In this report, the emphasis is on media technology.
There are many telecommunication media technologies in use or proposed for DA systems. For discussion purposes only, these technologies are classified under seven broad categories. These categories are independent metallic wire, distribution wire, telephone wire, public broadcast systems, ground radio, satellite radio and fiber-optic cable.
Independent Metallic Wire
Independent metallic wire, consisting of wires (twisted pair, concentric cable, etc.) installed between the DCC and the remote units, is one of the oldest media technologies used for DA communication. This technology is mainly used for an in-plant system where the distance between the DCC and remote unit is short. A typical example will be in a substation automation scheme where the DCC and remote units are both within the same substation. Depending on the type of wire used and the distances involved, data transmission rates of up to several million bits per second (bps) can be attained.
Electric wires have been used for data transmission since the 1920s, mainly at transmission level for voice and protective relaying data. Distribution wires have also been used for DA data communication purposes. Three distinct technologies have been deployed for data transmission over distribution wires–distribution line carrier (DLC), ripple control and zero crossing technique.
The telephone is a proven, highly matured communication technology, extensively used by utilities for a variety of applications. Low initial cost for locations with telephone access and universal availability of telephone lines make this communication attractive, especially for situations where short lead time for establishing communication is essential. The only action required is the installation (including the isolation and protection) of proper modems at the terminal points. Availability of cellular phones has added to the universality of this communication alternative.
Depending on the type of telephone circuit selected, the data transmission rate can be as high as desirable. The following are some of the different methods where telephone wires may be used for DA telecommunication purposes: leased circuit, dial-up access and scanned access system (derived channel). Telephone wire is typically used as the telecommunication media technology for substation and customer interface automation.
Public Broadcast Systems
Technically, public broadcast systems could be categorized under other telecommunication media technologies such as ground radio, satellite radio or metallic wires. However, since there are several unique features, they are classified in a separate category. This telecommunication media technology is mainly useful for one-way communication from DCC to remote units. Public radio (AM/FM) broadcast and cable television are public broadcast systems which have been used for DA systems.
Radio is becoming the most popular telecommunication media for feeder automation applications. Radio technology for DA may be divided into three distinct categories–VHF radio, UHF radio and microwave radio.
Radio waves in the frequency range between 30 to 300 MHz (VHF band) are generally used for voice communication. The prominent application of VHF radio has been for two-way land/mobile (TWLM) communication between dispatch centers and line crews. Due to near straight-line propagation properties in this frequency range, use of VHF radio may result in multiple communication “dead spots” (places where signal strength is too small) particularly in rugged terrain areas (mountains and valleys) and in places with tall buildings (downtown areas). These dead spots are mainly the result of obstructions and reflections.
In the United States, several frequency bands within VHF band are “unofficially” used by utilities for DA communication applications–mainly 3 kHz wide bands within 154 to 173 MHz frequency range. Data transmission rate for VHF radio may reach 1,200 bps. There is serious crowding around these frequency bands and hence their use for DA within urban areas is limited. The VHF radio frequency range is heavily regulated. Use of radio waves in the frequency between 300 MHz and 3 GHz (UHF band) are becoming popular for DA applications. Here again, the traditional common application of the UHF radio has been for TWLM communication between dispatch centers and line crews. Also similar to VHF frequency range, due to straight-line propagation properties in the UHF frequency range, use of UHF radio may also result in multiple communication dead spots in rugged terrain areas and in places with tall buildings. Obstructions and reflections are again the main cause.
Utilities have used frequency bands between 450 and 470 MHz and from 800 to 960 MHz for DA applications. The 450 to 470 MHz band systems are mostly older installations and offer data transmission rates of up to 2,400 bps. Here, too, there is serious crowding around these frequency bands and hence their use for DA within urban areas is limited. The UHF radio frequency range is heavily regulated as well.
Many modern DA communication radio systems operate in the 900 MHz frequency range, where the bands available are 12.5 and 25 kHz wide with data transmission rates of up to 9,600 bps. UHF radio systems are used for all DA applications. Most common applications are, however, for feeder and customer interface automation. Some of the DA specific radio systems operating in the 900 MHz frequency range are Generic Multiple Address System radio, Itron radio system, CellNet radio system, Metricom spread spectrum packet radio system and the GE/Ericsson trunked radio system. Utilizing mainly frequencies of 1 GHz and higher, microwave radio has been used for point-to-point communication carrying both voice and data. Microwave radio systems have been used for SCADA and protective relaying. However, due to high costs, their use for DA telecommunications is mainly limited for point-to-point transmission configuration when carrying DA data as only part of the data payload.
Satellites in geosynchronous orbit are commonly used for telecommunication purposes. These satellites have the advantage of looking stationary from stations on earth and hence do not require complex tracking systems for ground radios, allowing for continuous data transmission. Low earth orbit satellites are also becoming available for data communication purposes. These satellites move in relation to fixed stations on earth. Therefore, data transmission is not continuous, but data transmission with low orbit satellites can take place on a pre-scheduled basis. Frequency bandwidth on low orbit satellites can be leased on a reasonable basis. Combined with extremely good coverage that satellite radio systems provide, low orbit satellites can become popular for energy metering applications at ultra remote sites. Satellite communication is subject to time delays of up to seconds due to large distances involved for data transmission. The delays can have serious impact on interactive communications unless complex message buffers are installed at each satellite terminal. Satellite link is prone to eclipses and other sun effects. Data transmission can be lost for days due to such extraterrestrial effects.
Fiber-optic cables are becoming commonplace in utilities for voice as well as data transmission. Each fiber-optic cable system consists of three main components–transmitter, fiber-optic conductor and receiver. Fiber-optic cable systems offer numerous advantages ranging from extremely high data transmission rates to immunity from electromagnetic interference and lack of licensing requirements. Fiber-optic technology is cost effective for very high data transmission rates in a point-to-point configuration. However, for DA applications where the configuration is usually point-to-multipoint with modest data transmission speed requirements, cost effectiveness of fiber-optic cable systems becomes more difficult. The cost of a fiber-optic cable system depends on the cost of its components as well as the cost of installing the cable. Since cheaper multimode fibers with low cost LED transmitters are more than adequate for DA applications, the main cost becomes that of installing the cable. Due to the flexibility of fiber-optic cables, they can be placed in trenches (existing duct lines) or “attached” to overhead utility wires.
There are situations where high reliability and installation flexibility force the use of fiber-optic cables. This was the case for downtown San Francisco where radio solution was an impossibility due to tall buildings and underground remote units. Furthermore, the existing underground conduits had very limited space for adding metallic cables. In addition, there were concerns over faults and interference, and the reliability requirement of the DA system for downtown San Francisco was very high. The DA communication system set up in downtown San Francisco uses a fiber-optic cable system. This is a bi-directionally redundant loop system which allows continuous communication even if one transceiver or one section of fiber fails.
Hybrid Telecommunication Systems
Hybrid telecommunication systems comprising of two or more of the telecommunication media technologies previously mentioned are commonly used for DA applications in order to overcome the limitations of individual telecommunication technologies. Typically these hybrid systems use high transmission rate communication systems (e.g., leased line, fiberoptic cable or microwave radio) for point-to-point data transmission, usually from DCC to a remote location closer to a number of remote units. Communication from the remote location to remote units are then handled with the slower communication technologies such as DLC or UHF radio.
Future Trends in DA Telecommunication Technologies
In general, the future trends for DA telecommunication system technologies are moving toward an open systems standard. More and more telecommunication media technology developers are making their application program interfaces available to the marketplace in order to make it possible to interface their systems with other telecommunication systems. As far as specific automation functions are concerned, the general trend in feeder automation applications is toward radio systems, particularly in suburban and rural areas.
In addition to highly customized systems, such as CellNet, Itron, GE/Ericcson and Metricom, other more general radio systems are being seriously considered. These include cellular digital packet data radio technologies that are being developed for cellular telephony. Fiber-optic cables are becoming popular for substation automation or all automation functions in urban areas. Finally, public broadcast systems (e.g., cable TV) are becoming acceptable and at times the media of choice for customer interface automation application as well as for automation within the home (home automation).