Despite past reliability concerns, it’s fair to say that wireless networks have come into industry full force, particularly in harsh environments, in difficult-to-reach spots or in place of long cable runs. A utility in West Virginia faced all three of these criteria, and not only faced issues in how to monitor its assets, but also how to get wireless signals to cover the distances needed for signal communications.
To comply with Federal Aviation Administration (FAA) regulations after building a 150-mile power transmission line, the utility had to install steady-beacon obstruction lights to warn aircraft of the presence of high-voltage power lines. The operation of those lights also had to be continuously monitored.
The lights are on tall towers across rugged, mountainous terrain, making monitoring difficult. Some of the towers are virtually inaccessible, so sending a crew out to check the lights manually would have been difficult and expensive.
To address the issue, the utility turned to Sterling Engineering Solutions, an engineering sales and service company with expertise in applying wireless technologies. Sterling tackled the problem with a 900 MHz wireless system from Phoenix Contact.
Transmitting through tough terrain
The 500 kV transmission line runs in three corridors through mountains from Southwestern Pennsylvania to West Virginia to Northern Virginia. The new line was necessary to meet the demand for electricity in the Mid-Atlantic region, and to prevent overloading the grid. The line will forestall electrical problems on the regional grid, which might have resulted in blackouts, rolling blackouts and brownouts. The power line has 661 structures and cost nearly $1 billion to build.
The utility chose 500 kV as the operating voltage because all substations along the three transmission corridors contain 500 kV components. However, it used 765 kV line construction techniques to allow for future conversion if dictated by system needs. Upgrading operation to 765 kV at a later date will just entail adding the proper transformers and associated equipment at the substations.
“Building the power line wasn’t a challenge, but meeting the FAA requirement to monitor the steady-beacon obstruction lights was,” says Reid Garst, Sterling’s president. “No utility had ever built such an FAA light monitoring system on this scale before, and this system presented particular problems because of the mountainous terrain. The path of the transmission line did not allow for a typical master/multiple repeater topology, so many radios had to serve as repeaters for other radios down the line.
Sterling Engineering Solutions, based in Salem, Va., specializes in instrumentation, controls and communications for municipal, utility, industrial, military and manufacturing applications. Sterling’s expertise in applying wireless and licensed radio communications systems including 802.11, 900 MHz, VHF and UHF made it a good candidate to solve the tower transmission problem.
“In addition to supplying the communication and monitoring equipment for the project, Sterling assisted the owner and contractor in engineering the system, and also provided relevant documentation and drawings,” Garst says. “Engineering activities included software path analysis for determining locations for the repeaters.”
Sterling considered several wireless telemetry technologies including satellite, cellular and license-free radio. In the final analysis, license-free radios in the 900 MHz band provided the lowest cost of ownership.
The project uses Phoenix Contact Trusted Wireless Ethernet radios, Garst says. “These 900 MHz radios have expandable I/O modules, which map to Modbus TCP registers,” he explains. “The I/O modules monitor current delivered to each light, and send outputs to each of two lights on each tower to alternate their operation.”
The radios have onboard Modbus registers for received signal strength indication and the voltage to the lights is provided by the expandable I/O module. These registers are polled by a remote terminal unit (RTU). Variables from these registers are trended for monitoring operation of the overall system.
Sterling installed nearly 100 radios; there’s a radio on each of the 70 lit structures (towers with FAA lights), and the rest of the radios are used as repeaters. “Some radios communicate through as many as seven repeaters, and the system has 15 separate radio networks,” Garst says. “Some of those networks are connected to each other via fiber-optic ground wire cables run along the transmission line corridors.”
The lights on the towers don’t flash, instead two lights on each structure alternating operation daily so that each light operates for an equal amount of time over its life. Two RTUs installed at substations send signals to alternate operation of the lights. Each RTU polls its radios for operational data, and sends commands to each lit structure to determine which light of the pair will operate.
“If one light on a structure fails, the operations department notifies maintenance that repairs are needed,” Garst explains. “If both lights on a structure are out of service, the FAA is contacted to issue a Notice to Airmen.”
RTUs in the substations are connected via redundant serial Modbus interfaces to the SCADA system in each substation. They use two ports for redundancy.
Local paging systems operating at just over the 928 MHz frequency caused interference, so RF notch filters were installed at each radio to limit exposure to the interfering frequencies.
Solar power was used at all of the sites, so the radios had to operate reliably at voltages down to 12 VDC. The radios aren’t in air conditioned enclosures, so they are subject to extreme heat and cold and are rated accordingly to operate at temperatures of 0-65 °C.
Phoenix Contact also provided surge arrestors along with the antennas and associated cable systems.
Sterling provided on-site support including radio configuration and testing, system troubleshooting, and programming for RTUs, fiber devices and HMIs. The HMIs, manufactured by Red Lion Controls, are installed with the two RTUs to provide an overview of system status.
The system was in operation for two months prior to the official opening of the line in mid-2011. Since that time, the system has performed as designed, saving the utility money and time by eliminating the need to manually monitor each tower’s lighting system.