wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

58-2 Industrial Communication Systems
Power system control requires high integrity of both data reporting and control activation. The
operator must be able to trust the accuracy of the information reported to the control center in order to
quickly deduce any required responses. The system must correctly respond to any operator command
and only activate exactly the equipment that is requested by the operator: Inadvertent operation of other
devices cannot be tolerated and constitutes a safety risk to personnel and equipment. If any aspect of a
control command fails a validation step, the command is discarded rather than allowing the possibility
of incorrect execution of a command request.
When faults occur in electric power systems, automated protection equipment installed in substations
typically act to identify the fault and take measures to minimize damage to the system. They take actions
such as automatically disconnecting or isolating the faulty section of the network. The actions of this
protection equipment are rapid and are tuned by carefully engineering a complex set of parameters that
affect their operation. To verify the correct operation of this equipment and to facilitate the correct tuning
of the parameters, the exact timing of changes of the data being monitored by the protection equipment
and the responses of the protection equipment are important to facilitate the postmortem analysis of any
fault and the corresponding operation of the equipment. This data gathering requires accurate collection
of the time of the changes with resolution in the range from milliseconds down to microseconds and can
involve capture of the power system waveforms for voltage and current at various points in the system. The
correlation of data collected from different places in the power network requires close synchronization of
the clocks used to time-stamp the data. The reporting of the time-stamped data to a system that archives
this information is sometimes called “sequence of events” reporting or SOE.
Electric power SCADA systems are among the earliest applications for wide-scale distributed telemetry.
The primary plant in generation stations and substations has a long service life (typically 30 or more
years). The associated control equipment and communications systems typically have commensurate,
though shorter, life spans in the order of 10 years. Thus, there are many SCADA systems operating over
relatively old communication systems or systems with old architectures. These communication systems
often have limited bandwidth and may have relatively high error rates when compared with those used
in other applications such as Information Technology or factory automation. Many installed systems
operate over serial data links with data rates in the range 300–9600 baud. Even where the equipment can
be updated, it is not always feasible to significantly modify the available bandwidth without replacing
the entire communication system. When upgrades occur, modern networking technologies are sometimes,
but not always, adopted. For some applications, traditional serial data systems are more efficient
or more cost-effective. This can be especially the case where it is necessary to provide access to a number
of field devices over a shared media such as a limited-bandwidth radio channel.
The requirements for rapid data update and limited bandwidth availability dictate a need for efficient utilization
of the available bandwidth. The data and command integrity requirements are satisfied by mechanisms
that verify and validate data and commands. The accurate time-stamping of data requires that SOE
information must be supported and that there is a mechanism to accurately synchronize the time across
multiple widely dispersed devices. Some of these requirements are inherently contradictory (e.g., short
latency and low bandwidth) and consequently a compromise must be achieved when addressing them.
Traditionally, these needs were met by vendors providing a variety of proprietary protocols and communication
interfaces that specifically address these requirements. In more recent times, a number of
open standards have been developed and widely adopted for this purpose.
Modern SCADA systems almost universally adopt standard interfaces from the telecommunication
industries such as V.24/V.28 (RS-232) or V.11 (RS-422/RS-485) interfaces for serial or modem interfaces and
any of the IEEE 802 interfaces for Ethernet (usually TCP/IP), with 100BaseT and 100BaseFX now being common.
These interfaces are used with SCADA-specific protocols: In the electric power industry, the most commonly
used SCADA protocols are IEC 60870-5-101 or IEC 60870-5-104 (dominant in Europe) and DNP3
(common in English-speaking countries). These two families of protocols are described further below.
Some other industries (e.g., rail transportation) have similar requirements for SCADA to those
found in the electric power industry (e.g., traction power control) and adopt the same SCADA
© 2011 by Taylor and Francis Group, LLC