SCADA Systems 'Dampen' Infrastructure Problems
Upgraded system provides reliable control
and data acquisition amid rough terrain, earthquakes, mudslides, and
mountain lions. |
Jon Mandell and Bob Cook, Marin Muncipal Water
District;
Brenda Riconscente, Cal Tech Controls;
Read Hayward, DST
Controls
Keywords:
Process
control & instrumentation
Batch
control
Programmable logic
controllers
Level control
Flow
control
|
|
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
147sq 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.
| 
651
Stone Road, Benicia, CA 94510