The municipal heat distribution network for Lodz, Poland, a city of about one million citizens, is supplied by three heating plants with a total thermal output of 2,560 MW. The optimal utilization of the plants requires a control system that can work remotely across common areas. As the system is distributed across a large geographic area spanning about 800 kilometers of piping, safe communication among the automation nodes is critical. Additionally, because of the great number of nodes in the system (approximately 7,000), an appropriate system architecture needed to be developed through a detailed analysis of availability, throughput and connectivity standards.
CAS, a software developer and services integrator based in Lodz, selected OPC as the communication engine for the project, coupled with a Very High Frequency (VHF) radio network. Because of the propagation limits, CAS had to use several central base station locations. The biggest challenge was to synchronize transmission on one common frequency and provide the appropriate level of redundancy and throughput.
The automation nodes, which are programmable logic controllers (PLCs), are equipped with radio transceivers and connected to a single OPC server—the CommServer 2.5 from CAS. This fully configurable OPC server provides multi-protocol, multi-medium and multi-channel redundant access to the process control device data. To ensure short response times and effective utilization of the communication channel’s throughput, unique scheduling and scan-on-demand algorithms were implemented.
CommServer can use many protocols simultaneously and provide redundant links. Any user can define logical sets of devices (segments) with common addresses or scanning rules. New protocols can be added easily via plug-ins. CommServer supports various communication media simul-taneously, including copper and fiber-optic wires, digital radio, ISDN, GSM/GPRS, satellite and Internet.
Redundancy on demand
As well, CommServer offers a unique algorithm for scheduling the data cache update, based on multithreaded technology. The unique scanning-on-demand algorithm manages dynamic priority allocation by using common scanning rules defined for tags grouped into sets. The radio base station locations were chosen to allow communication with the most important nodes. For these nodes, redundant communication paths were configured. Failure of one base station causes the immediate use of a redundant one. All redundant communication paths are periodically checked to verify their availability.
The control process requires the transfer of process variables between nodes and from the power plants’ local control subsystems. This was fulfilled using OPCClientTransporter Rel 1.0. software from CAS. The software allows the definition of transactions for groups of tags located on different OPC servers. Each transaction can process selected input tags to calculate the current value of an output tag—e.g. scaling, addition, multiplication—assigning constants and not a value. It is also possible to define a transaction by archiving data in Structure Query Language (SQL) databases.
When controlling the energy streams distribution produced by the plants (hot water with flow of up to 7,000 m3/h), all components of the system must be monitored in real time, including the communication network. The software provides diagnostic and statistics information via OPC tags, and can be monitored by Human-Machine Interface (HMI) and Supervisory Control and Data Acquisition (SCADA) stations.
Such systems cannot work alone. Thanks to its open and coherent infrastructure, the remote control system was seamlessly integrated with the following systems: power consumption prediction, geographical information, remote control of water main pumping stations and power plant monitoring systems. All of the systems share a wall-mounted visualization system to provide ergonomic human process interface.
Using OPC as the core technology, CAS has proven that the obtained structure is open, robust (because of built-in redundancy) and allows a straightforward integration process.
For more information about the CAS server, visit www.cas.com.pl/index.php?p=zakres_produkty&sp=comm_server〈=en
For more about the CAS software development services, visit www.cas.com.pl/index.php?p=zakres&sp=oprogramowanie〈=en
