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RTU vs. PLC on a City-wide Scale

A letter from a reader involved in the development of a new city in Qatar (yes, you read that right—a new city) highlights the ongoing control system design dilemma of when to use a remote terminal unit or programmable logic controller.

Promotional image for the Lusail City development.
Promotional image for the Lusail City development.

I recently received a letter from a reader who happens to be the senior electrical and instruments design manager for the Lusail Development Project in Doha, Qatar. Lusail is a northern extension of Doha. This project is extremely interesting in that it literally involves the creation of a new city for nearly 200,000 people. You can see a video outlining the growth in Qatar as well as information about the Lusail project at the bottom of this article.

In the letter, the reader referenced having read an earlier blog post of mine about RTUs and PLCs which lead him to ask for advice on this question: One of our consultants responsible for the design of a district (in Lusail) is considering the use of RTUs for car parks, road tunnels, water district metering, street lighting control feeder pillars, etc. However our project management company insists on using PLC panels. Practically, both work. What is the difference really, between the two, and which one should we use?

He added that “every district, building, utility tunnel, car park, pump station, electrical substation, etc. in Lusail will have its own individual control system. But all will be integrated into the Lusail Command Control Centre. However, there will be SCADA stations for some locations such as car parks and utility tunnels.”

To help answer his question, I reached out to several industry contacts that supply both RTUs and PLCs to get advice for such a big project. Here’s what those who responded had to say.

Feature/Functionality Differences
“Since a small PLC and an RTU can both collect data from sensors, run logic, change outputs and send messages,” says Mike Foley, director of SCADA products for Invensys, “let’s focus on the differences.”

Foley says that when it comes to power requirements, PLCs are typically designed to work with industrial power sources (+24V dc and 85-264V ac). However, RTUs, because they are designed to operate remotely, can be located in areas where the nearest conventional power source is hundreds of miles away.

“They (RTUs) are designed to cover these same voltages (as PLCs), but can receive input power from low-power sources, like solar cells and batteries as well as high voltage dc sources, including sources greater than 100V dc in utility applications,” Foley says. “This power may be simplex or redundant (supporting applications that require battery backup power) for operations that are critical even when system power is lost. If power isn’t an issue in your application, either path will get you there. Otherwise, an RTU might be what you need.”

In terms of communications, PLCs are designed to communicate using embedded protocols. They can also use commercial or industrial converters to talk across many additional protocols. The bottom line with PLCs is that end users are counting on the communication channels to work.

RTUs, on the other hand, are designed to compensate for the fact that communications are expected to fail, according to Foley. “RTUs tend to have massive onboard memory capabilities to store incoming, time-stamped data. Once communications resume after a failure, this data can be forwarded (store and forward) to the MTU, where the data history from all of the RTUs can be reassembled as if communications were never lost. The time stamping and reassembling the sequence of events can not only be used to keep an accurate history, but also to diagnose problems, such as power grid failures, because the data is time stamped at the hardware level and accurate in the millisecond range,” he says, adding that “PLCs typically are not designed for this type of operation.”

Foleys points out that another critical difference between RTUs and PLCs is their operational availability, i.e., the percentage of time the controller is operating.

“Although PLCs can be used redundantly, many RTUs are designed for redundancy, so that if a point—or an entire I/O module—fails, the module can be hot swapped and operations can continue,” Foley says. “This may also be true for the processor, so that when one processor fails, the other one takes over, allowing operations to continue until maintenance can occur. This is a nice feature when the RTU is around the corner, but it’s critical when the RTU is on a pipeline that requires a helicopter to reach.”

Application Considerations
RTUs are typically used for applications where special requirements must be addressed that a PLC cannot fulfill, such as data collection and minor control for wellheads in the desert or for electrical substations, according to Hans Sjodahl, technical sales support manager for ABB Automation. He adds that using RTUs in an application like the Lusail project, engineers on the project should look to low-power devices, as solar power will likely be used, and these devices should have a compact design, be able to support a combination of several I/O types with few signals, and meet temp specs up to 70 degrees C.

Sjodahl adds that numerous available RTU models “support redundancy and many telecom protocols such as DNP 3.0 and IEC 670-5-104.”

For applications in this kind of environment requiring control functions and use of Foundation Fieldbus as well as connection to electrical equipment using Profinet or IEC 61850, Sjodahl says these requirements can be addressed by PLCs or small distributed control systems (DCS).

“For the utility application referred to in the Lusail project, we would prefer to use a PLC or DCS solution, as these options have all the interfaces needed at a good price due to larger production volumes compared to a RTU solution,” adds Sjodahl.

Judging from Foley’s and Sjodahl’s comments, it’s apparent that no clear cut answers exist for the RTU vs. PLC question. Steve Garbrecht, vice president of software marketing for Invensys sums up the predicament nicely: “As far as one versus the other, it is hard to tell these days. It is a little bit of a religious subject with many users. Both RTUs and PLCs log data, have some control capability and can speak over networks. But RTUs are usually a little more purpose-built for harsh environments and also for polling over geographic networks. They can also be used in low-power environments and might also have more specialized SCADA protocols like DNP3. All that said, many facilities management specialists I speak with swear by PLCs, which are used extensively in water and wastewater SCADA.”

Grabrecht adds that many PLCs are also less expensive than RTUs.

As for the reader whose question sparked this whole discussion, he wrote me to say that, based on the advice he has gotten from Automation World in addition to his own research on the topic, he is suggesting the use of PLCs within the car parks, utility tunnel and pump stations in Lusail. However, he will recommend that RTUs be used in the road tunnels.

See a video overview of development in Qatar and the Lusail project below. The Lusail project coverage starts at 2:50.  Plus, it’s narrated by Richard Quest from CNN — he’s always entertaining!

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