01 February 2005
From VHF to wireless Ethernet: Modernization without compromise.
By Jim Ralston
The evolution of Sussex County's water/wastewater SCADA system from a Motorola Intrac VHF communication network to a wireless Ethernet configuration is an ongoing project of significant modernization. Because of rapid expansion, the system needed an upgrade to modernize the pump station hardware and improve SCADA communications. Talking about an upgrade is one thing, actually doing it is quite another.
The total number of pump stations currently monitored is 250. Sussex County officials are now able to monitor the stations through a combination of T1 circuits, wireless Ethernet modems, and VHF serial modems. The new SCADA system architecture has improved efficiency and easily accommodates expansion.
Here are some of the challenges Sussex County faced and the steps they took to modernize their SCADA system and the technologies.
Sussex County is one of only three counties in Delaware. As a result, it is a substantial landmass of nearly 1,000 square miles.
"When the initiative began, Sussex was using a Motorola Intrac system for pump station and treatment plant alarming," said Steve Hudson, director of maintenance for the Sussex County Engineering Department and a proponent for the implementation of an Ethernet backbone. "This system was alarms only, no control. The 150 remote pump sites Sussex was operating at the time overwhelmed the Intrac equipment." At this point, Hudson said it was time to move into a full-blown SCADA architecture. Sussex County then hired the engineering firm of Whitman, Requardt and Associates, LLP to assist in finding a solution. Sussex County looked into microprocessor based and proprietary systems but decided that wireless Ethernet radios coupled with Modicon technology was the way to go. The spread spectrum technology would permit wireless Ethernet transmission to most pump stations. For locations with limited line-of-sight, they would continue to use the existing VHF serial radios.
Electrical distributor, United Electric, and the system integrator, Trijay Systems Inc., helped integrate the Modicon technology that would bring together wireless Ethernet connectivity. It was a tough decision bringing in a non-licensed frequency where interference from other systems could be a problem, but Sussex County felt license-free technologies offered performance advantages.
After considering several PLC architectures, the advantages of Ethernet became clear. Ethernet communications would support much faster data rates for dramatically improved scan times, real time diagnostics, and online PLC programming. Additionally, the open architecture would allow flexible system configurations and be compatible with wireless and high-speed leased line telephone services.
By installing Ethernet PLCs at every site, the move would create an Ethernet wide area network (WAN).
Trijay began working with Sussex County in 1998 converting the Motorola Intrac VHF system. Trijay handled the PLC programming, HMI development, and communications design. The original contract was to adapt the Intrac system before it experienced anticipated problems with the approach of Y2K. The first phase integrated Modicon PLCs with a VHF system on the 154 MHz band, good for long distances although a major caveat was the very slow data transfer rate. Sussex County also took into consideration that this type of radio system could run into bandwidth problems and the licensing was expensive. They brought over 20 of the existing 150 stations into this architecture.
Phase II began to introduce spread spectrum technology. Sussex County officials then contacted Data Linc-Group who gave a demonstration showing how wireless Ethernet technology could enhance the system architecture. Sussex County realized wireless Ethernet radios would enable them to run system diagnostics and human machine interface (HMI) access from any location.
Wireless Ethernet SCADA
The wireless system needed to be fast enough to support Ethernet packets, while facilitating long-range communications to the remote pump stations in a point-to-multipoint topology.
Additionally, the modems needed to be very reliable, secure, immune to interference, and operational in extreme temperatures. Ken Radley of Trijay Systems proposed frequency hopping, 902-928 MHz wireless Ethernet modems with high receiver sensitivity to meet project requirements. This technology supports data throughput rates of 100 Kbps, substantially higher than the 1200 bps VHF system. Currently there are 121 wireless Ethernet modems in operation. The polling rate is 15 to 20 times faster than a VHF radio.
Trijay and United Electric worked with Sussex County to ascertain potential problems with the terrain that might interfere with line-of-sight (LOS), a critical requirement for effective spread spectrum communication. Sites in wooded areas particularly were a challenge due to RF attenuation through the foliage. Sussex County is mostly flat, so the majority of the remotes were along the waterway or in housing developments that did not interfere with LOS. Trijay determined how to configure the wireless radios for store-and-forward operation in areas with terrain problems. The solution was to redirect one wireless radio to another identical unit about 100 yards away and use it as a repeater. In order to test an area, a wireless radio was set up, and then it made contact with the tower. This would determine whether or not problems with the LOS might occur.
Sussex County was divided into four areas that had large water processing plants to install a system and erect towers that enabled the chosen sites to poll concurrently from the remotes: Wolfe Neck (70 remotes), South Coastal Regional Water Facility (SCRWF, 100 remotes), Georgetown (20 remotes), and Inland Bay (100 remotes). SCRWF was designated as the backbone master PLC that retrieves information from the three sub-master PLCs. A high-speed T1 line connects the sub-masters to the treatment plant network. The four main towers and their remotes were installed one site at a time.
Users can remotely connect to the SCRWF plant, which feeds information from all the major sites. The software permits authorized technicians and plant managers to dial in from any location and have full network access.
The typical pump station has three analog inputs, 16 digital inputs, and eight digital outputs. Two pumps generally alternate control. Users view the analog inputs for monitoring the level of the wet well and the flow back to the plant. They use the digital inputs to see the status of the equipment, alarm condition, and whether the pump is running or not. They use the digital outputs during storm conditions to disable pumps or start them if necessary. They can also enable local manual control.
They place radios in locked freestanding NEMA-4 type enclosures. These enclosures sit above the well. Twelve inch, 3dB gain, omni-directional antennas are an average of 15 feet above ground.
In areas where there are foliage obstructions such as trees, they use 6dB gain, YAGI directional antennas. All of the four main plants have 9dB gain, omni antennas attached. Wolfe Neck has an antenna 100 feet above the plant, Inland Bay 320 feet, Georgetown 180 feet, and SCRWF 250 feet.
The main plant, SCRWF, monitors all of the information from the other three sub-master sites and is the only one of the four main plants that allows remote access to all of the sites. ProWORX NxT is the PLC programming software that provides remote PLC program viewing, program changes, and system troubleshooting over the wireless network.
At each main tower there is one Modicon Quantum PLC. SCRWF has two HMI stations, and each of the three other plants have one HMI station that run through a router at Georgetown. Each pump station has a Modicon Momentum PLC. The scan time for a master PLC to read and write to station is about two seconds. VHF takes about 30 or 40 seconds a station.
Sussex County is planning to add five new remotes in the next several months. The expansion will continue into the future utilizing their upgraded SCADA system architecture. The system has met all of Sussex County's expectations for performance and reliability. IC
BEHIND THE BYLINE
Jim Ralston is director of sales at Data-Linc Group in Bellevue, Wash. His e-mail is firstname.lastname@example.org.
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