02-05-2013, 12:55 PM
An Introduction to SCADA Fundamentals and Implementation
An Introduction to SCADA.ppt (Size: 1.71 MB / Downloads: 450)
INTRODUCTION
These functions are performed by four kinds of SCADA components:
Sensors (either digital or analogue) and control relays that directly interface with the managed system.
Remote telemetry units (RTUs). These are small computerized units deployed in the field at specific sites and locations. RTUs serve as local collection points for gathering reports from sensors and delivering commands to control relays.
SCADA master units. These are larger computer consoles that serve as the central processor for the SCADA system. Master units provide a human interface to the system and automatically regulate the managed system in response to sensor inputs.
The communications network that connects the SCADA master unit to the RTUs in the field.
Data Acquisition
SCADA system needs to monitor hundreds or thousands of sensors.
Sensors measure:
Inputs and outputs e.g. water flowing into a reservoir (input), valve pressure as water is released from the reservoir (output).
Discrete inputs (or digital input) e.g. whether equipment is on or off, or tripwire alarms, like a power failure at a critical facility.
Analogue inputs: where exact measurement is important e.g. to detect continuous changes in a voltage or current input, to track fluid levels in tanks, voltage levels in batteries, temperature and other factors that can be measured in a continuous range of input.
For most analogue factors, there is a normal range defined by a bottom and top level e.g. temperature in a server room between 15 and 25 degrees Centigrade. If the temperature goes outside this range, it will trigger a threshold alarm.
In more advanced systems, there are four threshold alarms for analogue sensors, defining Major Under, Minor Under, Minor Over and Major Over alarms.
Data Communication
A communications network is required to monitor multiple systems from a central location.
TREND: put SCADA data on Ethernet and IP over SONET.
SECURITY: Keep data on closed LAN/WANs without exposing sensitive data to the open Internet.
Encode data in protocol format (use open, standard protocols and protocol mediation)
Sensors and control relays can’t generate or interpret protocol communication - a remote telemetry unit (RTU) is needed to provide an interface between the sensors and the SCADA network.
RTU encodes sensor inputs into protocol format and forwards them to the SCADA master;
RTU receives control commands in protocol format from the master and transmits electrical signals to the appropriate control relays.
Selection of SCADA Master
A SCADA master should display information in the most useful ways to human operators and intelligently regulate managed systems. It should
have flexible, programmable soft controls to respond to sensor inputs
allow programming for soft alarms (reports of complex events that track combinations of sensor inputs and date/time statements).
automatically page or email directly to repair technicians and provide detailed information display in plain English, with a complete description of what activity is happening and how to manage it.
have tools to filter out nuisance alarms (to prevents operators from loosing confidence and stop responding even to critical alarms)
support multiple backup masters, in separate locations (primary SCADA master fails, a second master on the network automatically takes over, with no interruption of monitoring and control functions)
Support multiple open protocols to safeguard the SCADA system against unplanned obsolescence.