effectiveness of OPC for longer distances in Power Plant


Thread Starter

Gaurav Dhiman

We are into building a Coal based Thermal Power Plant. We'll be having a centralised Distributed Control System for Boiler, Turbine and auxillaries. We'll have offsites Coal Handling, Ash Handling, Raw water Treatment etc. within One KM. Besides this, there is River water intake house and Water Reservior at a distance of 6 KM and 4 KM respectively. We plan to have independent redundant PLC's with HMI's for respective offsites.

How effective is the Control and Monitoring from DCS to respective PLC's by using OPC communication protocol. Are we having this type of setup anywhere, Power Plant or somewhere. How much is the Input/Output count. What could be the best Control System (DCS) for this. Can you suggest some better practice or effectiveness in the long run.

thanks & regards
OPC isn't a protocol. OPC is a type of software interface to a driver for the actual protocol. The actual protocol will be something such as Modbus, Profibus, FF, etc. Your DCS will interface to the protocol driver(s) using OPC, and the protocol driver(s) will talk to the PLCs using whatever protocols the PLCs have built into them.

The PLCs have no idea what sort of driver interface your DCS uses. If your DCS supports a different protocol driver interface, you could use that instead of OPC and the PLCs wouldn't know the difference.

You need to look at what protocols the PLCs themselves are capable of using. Many PLC brands have options for transmitting data over longer distances. Also, if you plan on having redundant PLCs, you might want to look into redundant communications as well.
The 'protocol' OPC uses would be Microsoft COM/DCOM. While OPC (i.e. DCOM) can be used over a remote link, it is generally not advised due to issues with firewalls, open ports, timeouts, etc.

There are two main options for using OPC in remote architectures.
1) Use an OPC server at the central site and speak a native telemetry based protocol to the PLC. You should choose an OPC server that is specifically designed to be used in a telemetry set-up.

2) Another option is to use OPC Tunelling technology that allows OPC to be used securely and reliably in telemetry systems.

Since you mention redundant HMIs at the offsites, I assume there will be a Windows PC at the remote site, rather than just the PLC hardware? Is the redundancy scheme designed to maximize up-time communications with the offsites, or do you also need to consider guaranteed data delivery of the offsite data to a central historian or other system?

Here are a few whitepapers that talk about the above subjects in more detail.

Ensuring OPC Connectivity in Mission Critical Applications (http://www.matrikonopc.com/downloads/256/whitepapers/index.aspx)

Deploying OPC in Telemetry SCADA Systems (http://www.matrikonopc.com/downloads/551/whitepapers/index.aspx)

OPC Tunnelling Increases Data Availability (http://www.matrikonopc.com/downloads/158/whitepapers/index.aspx)

OPC Redundancy – Power of Prevention (http://www.matrikonopc.com/downloads/257/whitepapers/index.aspx)

If you have any questions, should get in contact with one of the OPC System Architects at MatrikonOPC, and they can help you work through the various options that best fit your needs.

Eric Murphy - OPC Exchange - http://blog.matrikonopc.com
The first question is, if there is a relyable TCP/IP connection available over this distance or if you will have only a dial up connection over GPRS for example.

In case of GPRS you will probably need something like.

In case of a relyable TCP/IP connection you are more free to do what you want.

With OPC you must decide if to use the tradional COM/DCOM based method or the newer method based on web services (OPC XML-DA/OPC UA). I would prefer web services, because they are more network friendly and are not tied to windows.

If you only have a COM/DCOM based server you should use a OPC XML-DA gateway like, http://www.opcconnect.com/xml.php, for better integration into the TCP/IP network.

Since you are talking about redundant HMI's, i point you to our open source project, http://pvbrowser.org

With pvbrowser the HMI is implemented within server(s) that are located within the field (any distance away from the operator). Now the operator uses our pvbrowser client to connect to such a HMI. These HMI's can be surfed like you do within the internet.

Thus you have a redundant system. You could implement your data aquisition locally (as close to the plant as possible) with fieldbuses based on RS485 (Modbus/Profibus/CAN/...). Then you only need a TCP/IP connection between these subsystems and the place the operators are located.

Also see the principle of pvbrowser:
(View that PDF as a slide show)
OPC is widely used and is very good at providing an HMI link to remote sites.

OPC is an ethernet based protocol. If your ethernet links betwen your remote sites and your central control room are good, then OPC will do just fine. You should consider installing your own private ethernet, not relying on a general internet VPN connection from a telecom provider as these are not usually considered secure enough and your plant data doesn't get the priority it requires.

ABB's 800xA system offers a product called "Multi System Integration" which allows individual HMI's on-site plus a centralised HMI and has good references in the Power Generation market. But don't worry there's a ton of other vendors out there who can do what you require.

If you cannot get reliable ethernet comms to your remote sites, then you will need to look at some of the other ideas posted here which are adapted to intermittent remote communications.


Fred Loveless

As you have already found out from the other posts OPC is not a communications protocol it is a way to transfer data between applications in a client server application.

If you hae good TCP/IP connections and all of your PC's are in the same domain or workgroup then you could have servers running at your remote locations and connect the OPC Client applicaiton to them. You have some fairly long runs and unless you are using fiber you will probably hate to do this wirelessly.

I review a lot of systems for customers, many of them similar to yours. The more acceptable procedure is to have PLC's at the remote locations, use radio Ethernet modems to connect to them. On the DCS control side you can have an OPC Server that is communication via the PLC's native protocols to the PLCS and upating the DCS. Like wise you can alse control the remote units this way.

Hopefully all the responses will provide you with the answers you need.

Fred Loveless
Senior Application Engineer
Kepware Technologies