I have another slew questions: What exactly is the SCADA system? The PR guy at my local power department gave me a rather vague answer. Is the SCADA a single system or is it a collection of computers? If it is a collection of computer systems do all the systems have to remain fully functional for the SCADA to perform properly? And what would happen if the entire SCADA shut down in a plant?

-- Anonymous, December 02, 1998

Answers

Ben, I'm sure others who work with SCADA boards can answer you more specifically, but here is a definition:

"SCADA (Supervisory Control and Data Aquisition) systems allow utility personnel to monitor and control virtually any aspect of electrical or water distribution systems locally or at distant locations. Information is provided in a real-time environment that identifies problems as they occur and takes corrective action or alerts appropriate personnel when assistance is needed. A central control computer, called the Master Control Center, and a network of remote terminal units (RTU's) provide fast, accurate data from any site the user selects."

There is also information on what AM/FM (Automated Mapping/Facilities Management), GIS (Geographic Information Systems) and SCADA are at:

http://www.amfmintl.org/scope.html

-- Anonymous, December 02, 1998

Ben:

To add to Bonnie's description: Technically a SCADA/EMS system would cover monitoring and control of a utility's transmission and distribution system(wide geographic area where facilities are normally unstaffed). In larger systems it would also include study applications such as power flow, state estimation, economic dispatch, tie-line interchange and automatic generation control. SCADA/EMS systems keep the man in the control loop(the system dispatcher). Sensors are typically scanned in cycles of 4-30 seconds. These systems have a central master station, a low speed communications network(radio, microwave, fiber and leased circuits) and RTUs.

The system at the generation station is typically called a Distributed Control System(DCS). As the name implies the intelligence is usually distributed across the plant and tied together with a fiber LAN. DCS systems perform closed loop control of fast processes. Sensors are scanned from .01 to 1 second cycles. The operator responds to alarms and may adjust set points. He usually doesn't control the process directly.

My opinion is there is more risks with the DCS systems than with the SCADA systems. I manage one of each type of system and I will try to answer any followup questions that you might have.

Jim

-- Anonymous, December 03, 1998

Jim, If a PLC monitoring variables in some subsystem rolls over to a date of 01/01/1980 and the DCS rolls over to 01/01/1986, would this difference in the dates cause any problems to occur, such as the PLC getting shut down?

Ben

-- Anonymous, December 03, 1998

Ben, you might find the following real life example interesting. It's a question and answer from www.control.com's listserver archives from early 1996.

The person answering is recommending a strategy where the main server computer gets its time from an atomic clock, a downstream control PC gets its time from the server and then all the downstream PLCs are updated by the "golden" PC (those PCs aren't all that "golden" anymore). Also, guess what? Most atomic clocks only output a 2 digit year.

The way this system will behave is unknown. It could perform flawlessly, it might fall over, or it might just cause some minor glitches with error logging. Whomever is responsible for this system won't know until they test it.

--AJ

(ID's removed to protect the innocent -- and me...)

---------- Forwarded message ---------- Date: Fri, 8 Mar 1996 00:58:42 -0500 From: zzzz To: automation@control.com Subject: Re: Time Synchronization for a plant network

In a message dated 96-03-07 09:51:20 EST, Mr. X. wrote:

> I have a plant network consisting of several (about 15) Alen-Bradley >PLC's and two SCADA nodes (running InTouch) all communicating over DH+. > I would like to be able to Synchronize the time and date between ALL >nodes based on one node being the 'master' and all the other nodes updating >their info based on a broadcast message (or individual messages to each PLC) >....(quote clipped by Moderator)

Mr X., I would suggest that you start with having the PC's clock as being the "golden" time. Then sending this date/time info to the PLC's.

I don't use InTouch for SCADA, but I do use FactoryLink, and I imagine that you can read and write to a PLC address. I am also sure that you know the "S" addresses in the PLC for the date and time (S:18 = year, etc) (if you don't here they are:)

year = S:18 month = S:19 Day = S:20 Hour = S:21 Minute = S:22 Second = S:23

It should be easy to set up task in InTouch that sends the date and time to the PLC once a day. Then you could block transfer that updated date and time to the other PLC's on your DH. Or you could have InTouch send it to all the PLC's individually.

I don't know of anything that will send it to "all" the PLC's on the DH at once, like an automatic batch upload, so if you find anything out, pass it along.

One more thing, the main reason that I use the PC as the golden time, is that the PC's clock and date gets updated from the server every time it loges in on the lan. And because the lan updates it's time (daylights savings, standard time) automaticly, the PC's clock is always accurate. Also the Server dials up the atomic clock in Denver once a month to update the time. This keeps the seconds very accurate.

I update the PLC's clock once a day, because I have noticed that our PLC's clock isn't very accurate, it varys as much as 15-30 seconds a day. Anyone know if this is normal for an AB-PLC-5?

Good Luck, Mr Y.

-- Anonymous, December 03, 1998

Ben: We use mostly Modicon Plcs. They use a 2-digit year that will roll over to 00. Our applications don't use this System Word so it's not a problem. Our logs get time stamped at a higher level either the DCS System or an Intellution MMI node. The Intellution MMI had to be upgraded one revision level to fix a minor problem. The DCS system rolled over without a problem. We have not found any unexpected problems yet. But we will continue to look.

Jim

-- Anonymous, December 04, 1998