Nor-Cal_Controls-logo-Combination_Mark-CMYK with Tagline-2
solar pv panels and substations

Real Time Automation Controller (RTAC) Applications in Solar PV

  • News

A Real Time Automation Controller (RTAC) is a powerful multipurpose controller/communication device. RTACs can potentially fulfill many roles at a solar PV plant, including data concentration, protocol conversion, data telemetry and device control.


RTACs are manufactured by Schweitzer Engineering Laboratories (SEL), who also produce many other energy industry devices.


How are RTACs used at solar PV power plants?

Substation Use

RTACs are industrially hardened so they're suitable for substation environments. They are typically used in substations as data concentrators and/or protocol converters, mostly for communicating with the electrical metering and protection devices.

  • Data concentrators receive information from the various networked devices at the solar PV site and aggregate (or "concentrate") it in one place. The SCADA system or data telemetry system can then extract that data for real-time monitoring.
  • Protocol converters ensure that the networked devices can communicate properly. A protocol is a system of rules that allows two or more networked entities to communicate. Modbus and DNP3 are two of the most common protocols used in SCADA networks. SEL RTACs can support these popular protocols as well as many others.


RTACs can be used as power plant controllers (PPCs), with some limitations. (We will cover these later on in the article.) They are able to run and execute logic for nearly any application, sending out commands to control plant devices and regulate output. Some RTAC models can even serve as an HMI, providing an interface for operators to send commands.


Typically, the substation and SCADA functions are handled by two separate RTACs that have a single line of communication between them. However, these two functions can potentially be performed by the same RTAC.


Other Uses (i.e. RTU, telemetry, 3rd party off takers, RIG, etc.)

RTACs are excellent at facilitating data telemetry in and out of the solar PV site, and can even function as a Remote Intelligent Gateway (RIG). A RIG is a telemetry device that allows an ISO or utility to collect required data from the project site. It is a data aggregator that concentrates data on a local basis and sends it off to an ISO or utility. Many utilities use the SEL RTAC for data concentration.


The SEL RTAC has the capability to function as a California ISO (CAISO) RIG, with an encrypted telemetry connection.


Can an RTAC be used to support either a solar-plus-storage or a storage-only project?

Absolutely. As we covered earlier, RTACs support industry standard protocols like Modbus and DNP3. This makes them well-equipped to handle any type of storage-only, solar-only or solar-plus-storage project.


What are the various types of RTACs available and why would you use one versus another (i.e. 3505 versus 3530 versus 3355)?

SEL offers three different "levels" of RTACs:

If you've ever purchased a computer, you're familiar with comparing processing power and RAM between different models, and RTACs have a similar hierarchy. The 3555 has the fastest and most powerful processor and the most RAM, the 3505 has the least, and the 3530 is in the middle. You can see a comparison chart here.


The faster the processor and the greater the RAM capacity, the more plant devices the RTAC can handle. The 3505 can usually handle five to 10 devices, and the 3530 up to about 50 devices. The 3555 can handle many more than that, in the 150 to 200+ range, and it can also support up to 100,000 tags (data points).


Additionally the RTAC you choose will depend greatly on how many devices you will need it to communicate with, and how many tags you need to support.


There are also physical connection limitations to consider, in terms of how many devices you can aggregate. The 3505 has only four serial ports; the number of ports increases with the 3530 and 3555. If you need devices to communicate via a serial connection or need more individual physical connections, you may need to scale up your RTAC.


Does the RTAC come with software (i.e. logic development software, HMI software, alarm notification)?

All SEL RTACs come with no-cost logic development software that you can use to configure the logic or data concentration. SEL also offers an HMI software called Diagram Builder, which includes some alarming capabilities. It is important to note that the 3505 basic model does not have HMI capabilities, while the 3530 and 3555 do. The 3555 also has an integrated video port so there is no need for a separate computer to view the HMI locally.


How do the pieces work together to provide a complete system for a power plant? Can you only use certain components versus all?

That depends on the specific use-case for the RTAC at your plant. If you're only using the RTAC as a data concentrator and/or protocol converter at the substation, or as a logic-only device, you may not need the HMI. But if you're using the RTAC as a plant controller, you can add on the HMI as a visual and control interface.


Why do so many companies like the SEL RTAC?

SEL has been a trusted manufacturer in the protection and metering space for decades. They began on the protection device side, creating products to protect breakers, transformers, capacitor banks, etc. This requires their hardware to be hardened and extremely reliable, because there's so much at stake. When they introduced their RTACs, they already had that reputation for quality.


It is rare to have a failure with an SEL RTAC, but if you do, their support is incredible. They respond very quickly, and if you need a replacement, most of the time it’s no questions asked—they'll send a replacement unit.


Are there any limitations or considerations when using an RTAC for a solar or storage power plant?

Yes. As we covered earlier, the processing power, speed and hardware will determine how many devices—and their corresponding data points/tags—the RTAC can handle. You can run into issues with response time if you overload your RTAC with too many devices and/or data tags. Staying conscious of these limitations is critical to consider while integrating into a project.


The logic that runs on an RTAC is fairly powerful, but it does have some limitations in terms of its control capabilities. There are some situations where an RTAC does not completely fill advanced control requirements, and in those cases we recommend using a hardware-based PLC, like a GE PLC, or when you need extremely fast response control.


As a whole, however, RTACs are a powerful and useful multipurpose tool in the automation space.


How can Nor-Cal help with your solar PV automation needs?

We’ve been using RTACs for many years in our SCADA systems, including SEL RTACs. We can recommend the best model for your system requirements and handle the configuration, including any logic development and HMI capabilities you need. If you intend to use an RTAC as a CAISO RIG, we have extensive experience in this area and can help you comply with CAISO guidelines.


Schedule a call with us today.


Troy Morlan

Written by Troy Morlan