SCADA sewage control no day at the beach

InTech, Nov 2006 by Wendt, Curt, Murphy, Robert, Nelson, William

Florida's Orange County has 100 days of lightening per year, is hot and humid, gets heavy rain and wind, and has lots of vegetation. They all play havoc with radio systems.

Nonetheless, the county uses a new state-of-the-art radio-based SCADA to automate the monitoring and control of 600 sewage lift systems connecting to three wastewater treatment plants that has grown from 300 to 600 in a short time because of a burgeoning population.

In fact, the county's wastewater collection system is growing by almost 50 lift stations a year. Each additional lift station increases this management effort exponentially. The tremendous number of lift stations is a tracking and coordination challenge.

The SCADA system monitors each lift or pumping station through a radio communications system. Flow, wastewater levels, and pump-status data transmits to a central location where personnel can be dispatched to make repairs and correct problems.

Because of lightning concerns, the Remote Telemetry Units (RTUs) do not control the stations-the preexisting relay logic panels stayed in place for that purpose with a limited amount of control available for manual starting and stopping of pumps.

Design criteria for the new system included robustness, standard products, lightning tolerance, sunshield protection of field panels, resistance to Radio Frequency (RF) interference, and low recurring charges for phone service.

Overall, the radio approach has been successful and has saved thousands of dollars in dedicated line fees. However, this is not the tale of a trouble free installation, and next time we will do some things differently.

Radio band selection

Whenever possible, use a data-only radio band for SCADA radio systems. Previous attempts to operate a SCADA system for the Orange County Utilities (OCU) Water Division were unsuccessful.

OCU Water used two licensed frequencies in the 450 MHz range for SCADA data communications to 18 sites. Additions over the years required the use of repeater sites and a combination of series and parallel branches to achieve the required links. Failures at a sequential repeater site caused a loss of communications to sites located beyond that point. Because of this, the system was extremely vulnerable to interference, lightning strikes, thermal overload, and equipment failure.

Additionally, the radio equipment was custom made and required specialized training and test equipment for calibration and repair. Adjacent radio users include the voice designation of "land mobile," which includes taxi service, trucking dispatch, and the like. Interference came from adjacent frequency splatter and close proximity to transmitters as mobile users drove through the area. All these factors, coupled with the slow response of the communications (data bottleneck), made the former radio-based system less than desirable.

The OCU Water Division ultimately abandoned the 450 MHz radio system and moved its sites to ISDN phone lines. This was economical for the Water department because there were fewer than 20 remote sites.

The Waste Water division of OCU needed to select a radio band for their new SCADA system. The experience in the water division ruled out using the 450 MHz band. Adapting the OCU 800 MHz trunking radio system was a possibility ruled out early in the process because this system shares with other users and is slow.

There were no available licensed 928/952 MHz point-to-multipoint frequencies in the Orange County area. Spread spectrum radios in the 900 MHz band were the best design because of their ease of use and license free status.

Designed to tolerate interference well and operate in a data only band, there are no voice users, and they are widely available from a number of manufacturers. Because they operate using only 1 watt, properly locating the master radios is a critical issue and might have involved telephone leased-lines from the control room to distant locations. By using an innovative combination of radio, computer network, and telecommunications equipment, OCU's existing investment in micro-wave communications helped to reach distant master radio sites spread throughout the county. The remote radio diagnostic software that came with the radios has proved invaluable in tracking down radio problems.

Radio survey of the land

Our software radio survey between county radio towers and wastewater lift stations showed 30-ft poles would be high enough to enable communication to most, if not all, sites. It appeared that using 150-ft master radio towers and 30-ft poles at each end site would suffice for good radio reception through-out the county.

However, software surveys are only a rough determination that a radio path may exist; a field survey is required to confirm it. The field survey should happen early in the process, preferably during initial system design.

In this project, the field survey took place late in the project. Technicians temporarily installed five-watt 928 MHz transmitters and antennas with known gains at each master tower. Radio technicians at each remote location rotated a directional test antenna to maximize the intensity of the signal and to assure reception is through the main antenna lobe. Then they record the received signal strength and the GPS measurements.