PLC Selection in SCADA Systems
With the improvement of modern technologies, more and more PLC manufacturers are
providing different kinds of PLC’s for a variety of applications in various industries.
Most manufacturers now offer basic level, medium level and top level series of products
to meet different demands. This gives system integrators more choices to design
a system, but it creates also a challenge to choose the best fit for the many different
applications.
With more than 30 years of experience in the Oil & Gas industry, TRC has acquired
a wide range of expertise and a broad experience in selecting appropriate PLC products
to meet different requirements.
There are several criteria that need to be considered when selecting a PLC as controller
for a Remote Terminal Unit (RTU):
1) System structure. Some Systems are compact with CPU and IO integrated on one
board, these kinds of PLC Systems are used in smaller control systems and expandability
is limited. Other systems are modular with CPU module and other IO modules installed
separately, these kinds of PLCs are used in more complex systems with requirements
for expandability.
2) IO Capacity. This is an important factor in selecting PLC systems. Normally,
more field instruments means that more inputs and outputs are required, which will
lead to the need for higher-performance PLCs. The system integrator needs to be
familiar with all products categories of the major brands to select the best fit
for a specific set of customer’s requirements.
3) Reliability requirements. For critical applications, most customers in the Oil
& Gas industry would require a redundant system (redundant CPU, Power and even I/O),
this will ultimately lead you to the series that supports redundant systems. Siemens,
Rockwell and Schneider etc., all have their own redundant system configuration available.
4) Communication network. SCADA systems have a wide range of communication network
structures, depending on availability, speed, reliability requirements, and government
regulations. Major PLC brands offer serial communication protocols from MODBUS,
DNP 3, to Ethernet TCP/IP, depending on the field condition and communication backbone
configuration. The integrator can choose the product that meets the communication
requirements.
5) Environmental conditions. In extreme conditions such as high summer temperatures
and extreme cold in winter, standard PLC’s cannot meet this requirement. A special
PLC with a wide temperature range will be needed for such an environment. For other
environments with corrosive gas, anti-corrosive PLC modules with conformal coating
will be the appropriate choice.
6) Cost. Ultimately it comes down to choose the optimal cost-performance ratio for
each application. Especially in an open tender bidding process, making the correct
equipment selection will directly affect the outcome of the tender.