Evaluating Wireless Technologies for Distribution Automation

By Scott Harris

Several wireless communications options currently exist for automating equipment located down-line from the substation, such as capacitor banks, reclosers, line switches, fault locators and voltage regulators. The advantages to automating this remote equipment are beginning to outweigh the costs of the solutions.

Distribution automation with wireless communications enables a defensive strategy or preventive maintenance solution that can help improve operational efficiency. Utilities can automate equipment that in the past has been managed passively by annual physical inspections-such as fixed capacitor banks-or defensively by emergency dispatch of service personnel as problems arise-such as tracking down fault locators during power outages.

Available Technologies

There are at least five capable technologies currently available for wireless communications in the electric utility industry: cellular control channel, cellular digital packet data (CDPD), paging, spread spectrum radio and satellite. All are radio transmission technologies with pros and cons depending on the automation application. Each of the technologies taken independently may require some integration with the utility equipment targeted for automation. They often require a wireless radio, field device or remote terminal unit (RTU), and software for control and monitoring to be purchased and installed. Off-the-shelf integrated products also are available and can offer immediate automation solutions for less than $1,000.

Cellular Control Channel

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Cellular control channel communications utilize bandwidth for data communication on the existing cellular network. This “non-voice” channel has an abundance of excess capacity from the major cellular carriers, who have licensed its use to two companies, Aeris.net and Cellemetry. Aeris.net and Cellemetry make this bandwidth available throughout North America to telemetry developers who produce solutions for communicating with remote equipment. Ideal applications for the control channel include alarm-based events, such as providing notification when a line switch fails, and monitoring and control applications where relatively small amounts of data are sent infrequently.

Pros:

  • Nationwide Coverage: Provides the most extensive coverage of all wireless options, with near seamless coverage in the United States, Canada and Mexico. Aeris.net and Cellemetry are in the initial deployment stages in South America. The services operate anywhere Advanced Mobile Phone Service (AMPS) is available, include automatic roaming, and do not require individual cellular accounts for each device.
  • Low Cost: As little as $5 per month makes it the lowest cost solution available. Since it leverages the unused cellular radio network, users are spared the burden of building, maintaining and deploying an infrastructure.
  • Two-way Communication: Data is bi-directional. Field units can communicate alarms or receive commands with acknowledgement of the command.
  • Near-real-time Connectivity: Data packets require only seconds to transmit from a field device to the customer.
  • TCP/IP (Internet) Connection: Field devices can be accessed through a TCP/IP-based connection.

Cons:

  • Data Size: Data packets are limited to eight digits (Cellemetry) or 25 digits (Aeris), so the technology is best suited to applications that send small amounts of data.
  • Coverage: Coverage is nearly 98 percent of the U.S. population, but some geographical areas do not have cellular service.
  • Modem: The wireless radio is not a modem that accepts “AT” commands.
  • Analog Service: The control channel works on the first generation (1G) of the cellular network which is not as efficient in compressing data as the digital service (second generation, or 2G). However, since it is analog service, it is far more widespread than digital service. Aeris.net has announced it is rolling out VBurst, which will operate under both analog and digital networks and increase the data packet size to 1,024 bytes.

CDPD

Cellular Digital Packet Data (CDPD) is a wireless data transmission technology that works by sending data packets through the cellular network during the “dead time.” CDPD is “always on” technology, which means users are connected at all times with their own static IP addresses. With a wireless modem that supports CDPD, communication speed is typically 9,600 baud with service available from the cellular providers in major metropolitan areas. CDPD is well-suited for public safety, Internet access and telemetry applications that require large amounts of frequently sent data.

Pros:

  • Always on: The operation of CDPD devices supports near-real-time transmission and reception of data.
  • Airtime: CDPD users are not charged for airtime. Users are charged by the volume of data transmitted. Each carrier offers a range of rate plans that can vary greatly from one carrier to another.
  • Standard Modem: The unit functions as a standard modem that accepts “AT” commands.
  • Near-real-time Connectivity: Data packets require only seconds to transmit from a field device to the customer.
  • TCP/IP-based: You can access your field devices through a TCP/IP-based connection.
  • Reliable and Secure: Accurate. Utilizes latest encryption technology to keep transmissions safe from eavesdroppers.

Cons:

  • Limited Coverage: Coverage is limited, typically to areas near major metropolitan areas.
  • High Cost: This is a higher cost solution for telemetry events and alarms. Standard phone lines can offer more flexibility in many cases.
  • Cellular Capacity: Since CDPD works over the cellular voice network during “dead times” and since voice traffic has priority, the technology is susceptible to limits on capacity availability.

Paging

The paging network, like the cellular network, is based on a private radio network. Paging service packages are available for one-way paging, two-way paging and telemetry paging services. Motorola popularized paging with FLEX (one-way) and ReFLEX (two-way). They also have a solution for telemetry called CreataLink, which utilizes a two-way data transceiver. Other major paging vendors have similar solutions. One-way paging applications are well-suited for applications that do not require acknowledgment or status reports from the field unit, while two-way paging enables bi-directional communication.

Pros:

  • Cost Effective: Competition in the paging market keeps cost low. One-way paging prices are about $.02 per page, and two-way paging costs are about $.12 per page. A base monthly fee can vary between $7 and $30 per month, depending on the particular plan.
  • Mature Network: Many companies are offering service plans. Cellular providers also are entering this market and providing good options for their customers.
  • Device Availability: Several devices are available for integrating into field units for telemetry applications, such as Motorola’s CreataLink.

Cons:

  • Latency: The paging network can delay messages several minutes because of store-and-forward technology. Applications that need immediate response may not be able to utilize this solution.
  • Coverage: One-way paging coverage is fairly widespread, while two-way paging coverage is limited to areas near major metropolitan areas.

Spread Spectrum Radio

Spread spectrum devices operate in the 900 MHz spectrum courtesy of Part 15, an FCC regulation that exempts devices and users from licensing requirements provided they meet two conditions: They must create no interference, and they must tolerate any interference. Devices operating in unlicensed spectrums use frequency-hopping techniques to meet FCC guidelines and minimize interference. A radio receiver can communicate at speeds of 4,800 baud for up to two miles, provided there is line-of-site. Private radio is best suited for applications that require immediate and frequent reporting of information and where construction costs are not a significant issue. Deployment of a private radio system requires an infrastructure of radio transmission towers and deployment of radio receivers.

Pros:

  • No Airtime: Once the radio infrastructure is deployed, receiver radios can be deployed anywhere within approximately two miles of the transmission towers, as long as they have line-of-site. Airtime for sending data is unlimited.
  • Two-way Communication: Receiver radios function like a modem. They accept “AT” commands, send data bi-directionally.
  • Good Reception: Communication is very reliable, both in buildings and outdoors.
  • Flexibility: It is easy to add or expand remote sites as customer bases increase and service areas grow.

Cons:

  • Cost: The cost and time to build a transmission system is significant and requires resources for continued maintenance.
  • Unlicensed Frequency: Unlicensed applications have requirements from the FCC for hopping between frequencies as the transmission takes place. As a result, frequent retransmission of data may be required. As more devices communicate in the 900 MHz area, more frequent hopping may be required, due to increased noise and high use of the frequency. A frequency can be licensed from the FCC for less than $1,000.
  • Coverage: While coverage is good, it is restricted to line-of-site.

Satellite

Satellite technology is available for telemetry applications through geostationary earth orbit (GEO), middle earth orbit (MEO) and low earth orbit (LEO) satellites. All satellite systems offer extensive geographic coverage, but provide little or no building penetration. Satellite radio manufacturers such as Qualcomm, Orbcomm and others produce radios that can be deployed for telemetry applications, ranging in price from $1,000 to $1,500 with universal coverage. Satellite technology is a good fit for mobile asset management, such as tracking rail cars.

Pros:

  • Broad Coverage: Best coverage for outdoor applications.
  • Data Communication: The speed of uplink communication is 9,600 baud and should be increasing soon. It is a good solution for two-way mobile data communication.
  • Airtime: Airtime is fairly reasonable with plans starting around $20 per month.

Cons:

  • Coverage: No penetration within buildings, even a metal shed or pad enclosure.
  • Radio Cost: Transceivers are expensive, ranging in price from $1,000 to $1,500.
  • Modem: The wireless radio is not a modem that accepts “AT” commands.
  • Unstable Industry: The satellite industry has been hit hard, with many vendors filing for bankruptcy over the past few years. Lack of building penetration has been a contributing factor.

Wireless Automation Recommendations

With the number of communications options available, potential users should thoroughly understand their needs before choosing a solution for their automation projects. In summary, the wireless automation recommendations are:

Control Channel: This is the least costly of all the solutions and is the best fit for reporting alarms and small amounts of data with monitoring and control. Control channel communications can be used for such applications as capacitor banks, reclosers, line switches, fault locators and voltage regulators.

CDPD: When large amounts of data must be sent frequently, such as with data logs or DNP data points, CDPD is a good possibility. Users should make sure though, that the application is within the coverage area. Good application targets are those that currently require a technician to take a laptop to the equipment and download a data log on a routine basis.

Paging: Load management applications, such as sending commands to reduce load, are an excellent fit for one-way paging applications. Two-way paging applications have limited coverage, but can work well for automation projects that do not require immediate notification, such as reporting interval voltage usage.

Spread Spectrum Radio: Building a radio transmission network is expensive. But once it is in place, deploying radio receivers adds little incremental cost, and air time is included. For users who have hundreds or thousands of devices to automate in areas with good line-of-site, a spread spectrum radio is the recommended solution.

Satellite: This is a good solution for automation in hard-to-reach locations, such as the base of a dam or other remote location. Satellite technology is the only solution that will get communication service to very remote equipment. n

Scott Harris is vice president of business development for Telemetric, a wireless telemetry solution provider. You can contact him at (208) 658-1292 or sharris@telemetric.net. Telemetric’s Web site is located at www.telemetric.net. Telemetric’s service utilizes technology from Aeris.net.


PG&E Tests Control Channel Communications

Pacific Gas and Electric Co. (PG&E) is in the early stages of testing digital control channel technology. They are installing a small number of units in substations and other facilities. Two primary applications are:

Station Alarms: A number of PG&E’s substations have the ability to transmit a simple alarm indicating that there is a problem at the station. In the past, PG&E has used leased phone lines or licensed dispatch radio frequency technology to report these alarms, but the phone lines are expensive and the radio equipment is, in many cases, older and expensive to repair. They believe digital control channel units may be an excellent fit for this application.

Outage Notification: PG&E is always searching for information that could speed up their knowledge of when and where an outage has occurred. They are investigating digital control channel technology as a way to extend their view of the system farther down the distribution line. Digital control channel units could be installed on the downstream side of reclosing devices to watch for momentary and sustained outages and provide reliable feedback.

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