SCADA Systems ‘Dampen’ Infrastructure Problems

KEY WORDS

Process control & instrumentation

SCADA systems

Remote terminal unit

Programmable logic controller

Networks

Sidebars: New Control Strategies

The southern tip of California’s Marin county is separated from the city of San Francisco by the most famous mile-long strip of steel and concrete in the world. Yet, within a 10-minute walk of the Golden Gate’s Marin terminus begins a county-wide network of California Wildlife Service signs warning against mountain lion attacks. Marin county’s geography is rugged!

Precipitous hills covered with dense scrub growth and isolated population centers typify this picturesque but geologically lumpy and unstable part of the world. And it is here—with an advanced supervisory control and data acquisition (SCADA) system that would even make the Mir astronauts sleep more soundly—that the Marin Municipal Water District (MMWD, Corte Madera, Calif.) now reliably provides its client population of 180,000 with potable water.

Prior to the implementation of the new SCADA system, MMWD’s 104 pumping stations, 139 storage tanks (ranging in size from 20,000 to 5,000,000 gal.), and myriad control valves scattered across the district’s 147 sq miles were largely monitored only by scheduled inspections or “emergencies”—such as gushing ruptured mains, land erosion from tank or reservoir overflow, and customer complaints of having no water. Mechanical failures and aging infrastructure were aggravated by occasional excessive rains, often accompanied by mud slides and earthquakes, and by a poorly performing system control.

MMWD’s previous control system had evolved since the 1960’s, and consisted of tone-telemetry system that used leased telephone lines to allow the district to remotely monitor tank levels using pulse-duration signals and start pumps using discrete signals. Initially, the manual start/stop pump system automatically controlled each pump station based on tank levels. The original computer system occupied three 6-ft high, 19-in racks with a “whopping” 64K memory. To increase reliability and facilitate service, this computer was soon replaced with a system of programmable logic controllers that duplicated the computer’s functions.

MMWD experimented with installing remote PLCs at a few pump/tank facilities that communicated digitally back to the master PLC. However, the excessive cost of leased lines and high equipment failure rate soon overcame any gains from the evolved control system. The old system’s maintenance still relied heavily on scheduled visits and “field alarms” (i.e., phone calls from irate customers). The necessity to accurately control and document system performance provided motivation for MMWD to implement the state-of-the-art SCADA system now in place.

Work begins

Although the new system’s control philosophy was simple (provide control of the booster pumps that lift water to hilltop storage tanks as needed), system integration was not. In addition to the new hardware and software installation, MMWD also saw the opportunity to develop new control strategies to fine tune water system operation. See accompanying sidebar.

The new SCADA system, based on UNIX-based OASyS software, version 6.0, developed by Valmet Automation (Houston, Tex.), runs on a DEC Alpha computer. The software incorporates object-based programming and three-dimensional data visualization. MMWD built and linked its displays from AutoCAD files and, more importantly in this installation, included drawing formats of geographical information system vendors.

The human-machine interface console consists of four CRT displays and an overhead projector. These devices provide a look into the system that monitors and controls 6,000 “hard” and “soft” I/O points spread over 200 remote tank/pump sites and 174 remote terminal units (RTUs). MMWD uses a PC node for the PLC ladder-logic software. Control executions from the command site include both pump mode and setpoint changes. Data such as tank levels, system flow rates and pressures, and alarm events are stored on the system’s hard drive for up to 6 months before being archived to a CD-ROM disk.

Power for all sites needing monitoring was not always readily available. Approximately 20 MMWD water storage tanks were remote enough to require a solar-powered electrical supply with battery-backup for radio modems and on-site RTUs. Despite the rugged terrain, the remaining sites have utility power.

MMWD’s system uses low-cost Schneider Automation (North Andover, Mass.) “Micro” 612 PLCs as remote terminal units at each site. These incorporate 56 kbit/sec, digital lease-line modems, and 9.6 kbit/sec digital radios. MMWD has its own frequency for radio communications, using MDS radios with multiple address 900 MHz for data only. In cases where a line of sight does not exist between controlled pump/tank installation and one of the radio system’s three repeater stations, leased digital telephone lines are used.

In addition to handling MMWD’s water transmission and distribution, the new system also monitors four water-treatment plants. Controlled and monitored by similar “smart” hardware, the plants are fully integrated into the water district’s operational control system, enhancing operation of the entire water distribution system.

Project implementation was facilitated through teamwork between MMWD and its prime contractor, Valmet Automation’s Calgary, Canada office. Cal Tech Controls (Livermore, Calif.) and DST Controls (Benicia, Calif.) provided project management, hardware fabrication and integration, and back-up service for the HMI and RTU portions of the project.

The clear winners in this upgrade have been MMWD’s customers. Improving system control provides both uninterrupted water service and system flexibility to respond more quickly in case of a malfunction.

New Control Strategies

As part of the SCADA system implementation, Marin Municipal Water District (MMWD) took a closer look at its control strategies. Along with equipment and software upgrades, it was decided some new strategies were in order. Included as a result of the upgrade was closed-loop control on valves, alternate control-mode capability for pump stations, optimum pump selection, and pressure surge data collection.

Key control valve loops within the system were modified to allow local closed-loop control. An operator can enter criteria governing local control conditions via the central console. Flow and/or pressure parameters are then downloaded to the RTU at the pump or reservoir station. Control valves can then be reconfigured for either rate-of-flow or pressure-based control by the click of a mouse.

MMWD was also concerned that, because the new system was centrally controlled, it may have communication problems with pump lift systems during the winter storm season. For the system to function properly, the SCADA system must communicate with the pump and its receiving tank. To solve this problem, an RTU subroutine known as Alternate Control Mode (ACM), was developed.

If communication is lost with either the pump or its receiving tank, the ACM subroutine would be automatically initiated after a predetermined period of time. Once in this mode, the RTU will not restart the pump until its discharge pressure drops to a preset value. Once restarted, the pump will only deliver a preset volume of water. This process will continue until communication has been restored and regular operation can be resumed.

Pump selection/data collection

In addition to monitoring and controlling “time-of-day” pump-motor energy use, additional savings were obtained by programming the RTUs to run the most efficient pump—there are several in each stations—based on capacity required. High demand conditions automatically switch pump operation to the unit that can efficiently meet the volume requirement.

In order to more tightly monitor pressure surges in the system, MMWD needed to retrieve finer resolution data than could be provided by the system’s 15-sec scan time. To facilitate retrieving high resolution data from this sprawling control system, the user resorted to some “creative” PLC programming. The solution was to take the PLC registers and place them in a “rolling table” (FIFO) configuration with a time difference between each register as low as 10 msec. Once a pressure surge occurred, the registers were frozen at their the pre-event data content. At this time, the PLC starts to fill an additional register with post-event data. Once this table fills, all 200 registers are downloaded to the SCADA system.

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