04-06-2013, 12:03 PM
The Fundamentals of SCADA
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INTRODUCTION
A Supervisory Control and Data Acquisition (SCADA) system is a widely distributed
computerized system primarily used to remotely control and monitor the condition of
field-based assets from a central location. Field-based assets include wells, pump stations,
valves, treatment plants, tanks, and reservoirs.
For a water distribution network, the common objectives of a SCADA system are to
do the following:
• Monitor the system
• Obtain control over the system and ensure that required performance is
always achieved
• Reduce operational staffing levels through automation or by operating a system
from a single central location
• Store data on the behavior of a system and therefore achieve full compliance
with mandatory reporting requirements for any regulatory agency
• Provide information on the performance of the system and establish effective
asset management procedures for the system
• Establish efficient operation of the system by minimizing the need for routine
visits to remote sites and potentially reduce power consumption during
pumping operations through operational optimization
• Provide a control system that will enable operating objectives to be set and
achieved
COMPONENTS OF A SCADA SYSTEM
SCADA encompasses the transfer of data between a SCADA central host computer
and a number of remote sites (Remote Terminal Units or RTUs), and the central host
and the operator terminals. Figure 1 shows a generic SCADA system that employs
some form of data multiplexing (MUXs) between the central host and the RTUs.
These multiplexers serve to route data to and from a number of RTUs on a local network,
while using one or very few physical links on a Wide Area Network (WAN)
backbone to pass data back to the central host computer.
Field Data Interface Devices
Field data interface devices form the “eyes and ears” of a SCADA system. Devices
such as reservoir level meters, water flow meters, valve position transmitters, temperature
transmitters, power consumption meters, and pressure meters all provide information
that can tell an experienced operator how well a water distribution system is
performing. In addition, equipment such as electric valve actuators, motor control
switchboards, and electronic chemical dosing facilities can be used to form the
“hands” of the SCADA system and assist in automating the process of distributing
water.
However, before any automation or remote monitoring can be achieved, the information
that is passed to and from the field data interface devices must be converted to a
form that is compatible with the language of the SCADA system. To achieve this,
some form of electronic field data interface is required.
The Central Host Computer
The central host computer or master station is most often a single computer or a network
of computer servers that provide a man-machine operator interface to the
SCADA system. The computers process the information received from and sent to the
RTU sites and present it to human operators in a form that the operators can work
with. Operator terminals are connected to the central host computer by a computer
network so that the viewing screens and associated data can be displayed for the operators.
Recent SCADA systems are able to offer high resolution computer graphics to
display a graphical user interface or mimic screen of the site or water supply network
in question. Figure 8 shows the types of display screens offered by most systems.
ERRORS AND ACCURACY ISSUES
As previously discussed, SCADA systems for water distribution systems generally
use low bandwidth communication channels. Data from the field may therefore need
to be compressed by the field devices before being transmitted to the central host to
avoid overtaxing the capacity of the communications media to transmit information.
The result may be that data from the field include some form of error that must be
considered before analysis.