09 April 2001
Satellite SCADA for treatment plants
The technology is available, but is it cost effective?
Dedicated, satellite-based business communications using relatively small dish antennas, or very small aperture terminals (VSATs), are no longer only for governments and colossal corporations. An industry study predicted in 1990 that supervisory control and data acquisition (SCADA) services would become a market for VSAT technology by the mid-1990s. VSAT SCADA is now an important tool in the oil, pipeline, and electric utility markets, fulfilling the prediction.
VSAT has been successful in these industries, according to Satellite Communications contributor Phil Samuels, because the "electric, gas, and liquid transmission systems transit vast distances, often far from telecommunications and electrical power sources. Monitoring of these transmission facilities and the ability to take action to control their operations are important for inventory, regulatory, and safety reasons."
Following its successful entry into these markets, the VSAT industry is now looking for new SCADA applications. One market to attract its interest is water and wastewater treatment, currently about 0.5% of VSAT SCADA installations. A close look suggests VSAT might soon play an important role in water and wastewater plant SCADA.
A model installation
The VSAT SCADA system for a typical water or wastewater treatment plant with remote sites would have the following components:
- Eleven remote terminal units (RTUs), weatherized enclosures that contain the programmable logic controllers (PLCs) that collect data from the local field instruments. The receiver/transmitter boards and LAN interface cards for data transmission and protocol conversions between the PLC and VSAT are also inside the RTU.
- The VSAT, a 1.8-meter dish antenna mounted on a pole or building that communicates with the geostationary earth orbiter (GEO). VSAT transmissions to the GEO are usually in the 12-14 gigahertz range in the U.S. and 4-6 gigahertz range outside the U.S.
- A human-machine interface (HMI) computer, used by the plant operator to monitor and control processes at the RTU sites.
- A master PLC, located at the control room, that polls the remote VSAT units at the RTUs. The polling sequence sends the appropriate commands for transmitting and receiving data from the RTUs. The indoor unit processes communications commands and forwards them for transmission through the VSAT unit.
VSAT costs and benefits compared
The cost of the system described above was compared with the cost of dedicated telephone lines and spread-spectrum radios (SSRs), the most common communication technologies now used.
The end result was that the installation and hardware costs for VSATs far exceeded those of telephone and SSR systems. Capital costs are only part of the picture, however; operational costs play a role, too.
The benefits of VSAT begin to appear once operational factors are considered. Compared with leased telephone lines, for instance, monthly VSAT service charges are competitive when data rates are low. This is realistic for the water and wastewater industry because most remote sites have very small data requirements.
Mean time between failure and uptime are similar for the three technologies. However, system monitoring is the strongest operational feature of VSAT technology.
Hughes Network Systems and Nova-Net offer 24-hour, 7-days-a-week network monitoring from their Network Management Center. Don Rowe with Nova-Net said, "Very often, the customer is more concerned about the services that are provided, like monitoring of his network, the responsiveness of the network operators, how well they will work with the customer, etc. For example, it is our goal to call a customer about a detected event that might be a problem before his own system tells him that something might be amiss." This kind of customer service simply isn't available from SSR vendors and telephone companies.
The transmission speeds for these technologies are by no means "cutting edge." Data transfer rates are not critical in the water/ wastewater industry, however, because the monitored processes are rarely time sensitive. Therefore, the speeds shown for all three technologies are adequate.
The propagation delays are significantly higher for VSAT than for the other technologies because a transmission is required to travel more than 20,000 miles before reaching its destination. Like transmission speed, propagation delays of these magnitudes are insignificant in this particular industry. The water level in a tank or well, for example, doesn't change so rapidly that a pump delay of a few seconds is decisive.
Transmission range is where VSATs shine. If the remote sites are on the order of 20 30 miles away, then VSAT becomes an important communications technology. The fact that VSAT is not distance dependent and does not have transmission constraints allows this technology to go where SSRs and telephone lines cannot.
SCADA applicability limited
VSAT technology offers benefits to SCADA installations for only a limited number of casesthose where the operational benefits and savings can outweigh the capital costs. Two conditions that affect this are plant geography and the central monitoring service of the VSAT service provider.
SSR and telephone, for example, may not be options if the remote location is more than about 20 miles away or is unavailable to line-of-sight transmissions, such as when the RTUs are in a very desolate area not served by telephone or the terrain is very mountainous. VSATs are not susceptible to these difficulties and can even be specified to use solar power.
The proactive approach of most VSAT providers means they become a sort of extension of the plant's staff. This can mean operational savings, with less staff time devoted to system maintenance. The more VSAT SCADA sites a plant has, the greater the corresponding savings. IT
Figures and Graphics
Eric Silverman is an engineer with Camp Dresser & McKee, Inc. in Cambridge, Mass.