- By Josh Eastburn
- September 09, 2021
- Case Study
Pennsylvania-based engineering services company RDI Controls designed distributed control systems to retrofit three turbine peaking plants with Opto 22’s groov EPIC and SNAP PAC controllers. This article was originally published in the Automation 2021: Control Systems Ebook.
In regional power generation systems, combustion turbines have become a mainstay recently. They work by compressing and accelerating air to high speeds, then injecting fuel to create a mix that burns at thousands of degrees. The combustion reaction drives the vanes of the turbine, which in turn, drive an electric generator.
These powerful, complex machines are used in combined cycle configurations for baseload generation, whereby the waste heat output from the turbine drives other power-generating systems. They are also used in simple cycle arrangements for so-called peaking plants, which handle mid- to high-demand power fluctuations.
Pennsylvania-based engineering services company RDI Controls provides fully engineered retrofit packages for industrial and aeroderivative turbine control systems, including demolition, installation, control valve and field device integration, water injection, nitrous oxide (NOx) emission mitigation, generator protection, and auxiliary/balance of plant (BOP) controls.
One of RDI’s customers invited the firm to bid on a project to retrofit three turbine peaking plants, each equipped with Westinghouse W301 and GE Frame 5LA combustion turbines in a simple cycle configuration with each operating in the 20 to 30 MW range. The previous iteration of the control system was designed with a pair of Allen-Bradley ControlLogix PLCs for each of the six turbines, but due to repeated control and maintenance issues, the customer needed a betterengineered solution that was easier to troubleshoot and more reliable.
- Peaking plants trade off efficiency for responsiveness, generating significant waste heat and losing work to turbine operation rather than power generation. To minimize these losses and reduce stress on the system, it was critical for RDI to keep the turbines running within tight tolerances during startup and operation.
- High-speed, rotating machinery requires a great deal of safety consideration as well. RDI’s solution must include fail-safes to handle situations such as load rejections, which occur when there is a sudden decrease in demand and can cause a turbine to accelerate beyond capacity and break apart.
- Although each turbine would have its own control system, every plant housed a pair of turbines, which used common subsystems that would need to be integrated into the primary controls. These included ammonia control systems and electric starter motor controls.
- For speed, cost, and error-reduction, RDI aimed to leave as much of the existing field wiring in place as possible.
To satisfy these objectives, RDI designed a distributed control system (DCS) at each site using a combination of Opto 22’s groov EPIC and SNAP PAC controllers. Lou Bertha, principal engineer at RDI Controls, said it was this same kind of experience that first led him to try Opto 22. “At the time, Allen-Bradley was the biggest contender in industrial turbine control, but it was expensive and kludgey. From a flexibility standpoint, for my money, Opto 22 gave me more bang for my buck. I was able to do just about anything you could do with A-B at a fraction of the cost.”
More than 20 years later, RDI Controls has carried out over 160 retrofit and design projects on a variety of turbines from brands like GE, Westinghouse, Pratt-Whitney, and Rolls Royce using control systems from Allen-Bradley, ABB/Bailey, and Westinghouse/Emerson. But Opto 22 continues to be Lou’s supplier of choice, and his ability to deliver affordable, high-performing systems continues to win him business.
In recent years, Opto 22 has expanded its range of edge-oriented control options, which embed high-level automation functions like data transformation and distribution, database transactions, and manufacturing execution systems (MES) at the controller level. This change helps to simplify control systems and bridge (operational technology (OT) and information technology (IT) networks. To support this level of integration, edge controllers also provide more processing power, storage, and connectivity than traditional PLCs or PACs.
Using Opto 22’s groov EPIC edge programmable industrial controller, as well as older SNAP PAC controllers, RDI easily outbid the competition. It designed a distributed system that incorporated highspeed PID control, secondary control and networking, and embedded third-party communication.
At the peaking plants, the controllers integrate all functions of the W301, F5LA, and shared subsystems. Subsystems are integrated using native peer-to-peer communication and through
embedded OPC UA communication. Additionally, each site reports to an external historian through a secure PAC gateway and integrates with a Citect SCADA HMI network, which RDI also designed.
Distributed control architecture. One EPIC provides primary governor control for each turbine, managing fuel mix, combustion, and shaft speed, with one PAC providing backup control and overspeed protection. An additional EPIC handles sequencer control, including the startup of auxiliary systems, process monitoring and coordination, and alarming, with a second PAC integrating high-density thermocouple sensing for temperature limiting and ramping. Finally, RDI uses the dual Ethernet interfaces on each controller to set up redundant network connections.
This distributed control architecture provides RDI with inherent fail-safes. Since all four controllers are peers on the network, not remote input/output (I/O), each has the ability to interlock the system in the event of a partial loss of control or network connectivity.
“Network redundancy is critical,” said Lou Bertha. “We can’t let the system fail for the sake of a bad switch. But in the event of network issues, each controller has the ability to trip the system as needed while maintaining its own control functionality, for example, lube oil operation, fan control, monitoring, etc.
“I can say that [full controller redundancy] doesn’t matter since the control is at the I/O level. If we are isolated from the network, we can continue running, and if we lose the I/O, redundancy won’t matter anyway.”
As with remote I/O, distributing control and I/O across multiple controllers also allowed RDI to reduce costs and minimize rewiring. High-density and specialty I/O was placed on lower-cost PACs and then located close to the equipment, rather than requiring long runs back to the primary controllers.
In Lou’s assessment, “The existing hardware was susceptible to damage if moved and the drawings were out of date, so we avoided rewiring altogether by dropping controllers into existing panels” instead of building new panels or attempting to marshal field wiring to a smaller number of controllers.
Fortunately, with the EPICs as primary controllers, performance is not an issue. Powered by quad-core ARM processors, Lou said “the horsepower is amazing.” Proportional-integral-derivative (PID) control functionality is used for speed, load, startup, and temperature control with the speed control loop executing at 50 ms or less. With that speed, RDI was easily able to maintain the turbine/generator speed at 3,600 RPM to within ± 1 RPM.
Subsystem integration. To interface with the shared subsystems at each of the three power generation sites, RDI leveraged a mix of hardware and software communication options included in Opto 22’s EPIC and PAC controllers. With these embedded tools, there’s no need for RDI to maintain a separate communication server. The EPIC sequence controllers independently coordinate data passing between the starting resistor controller (SRC), ammonia system, the other turbine controllers, and remote I/O for both native and non-native communication.
For example, the SRC, a kind of electric starting motor, is controlled by an Allen-Bradley PLC, which adjusts the output of the SRC in response to the turbine speed. It needs to get that information from the Opto 22 network, and normally would require an external protocol gateway or OPC server to make that happen. Instead, RDI uses Inductive Automation’s Ignition Edge communication platform, which is built into groov EPIC.
Ignition includes its own OPC UA server and a suite of native protocol drivers for common industrial devices, creating a bridge between the EPICs and the PLC. It consumes A-B tags and exposes them as OPC UA tags, which can be linked directly to the control engine and other applications running on the EPIC.
Compared to passing data via an HMI/PC interface, this approach provides a fully independent link between the systems to ensure it maintains operation in the event of an HMI/PC failure. Lou Bertha said, “If you haven’t used this on a project, it’s fantastic. I could shut down the HMI and it will keep running. If I were using the HMI to pass tags around, as is common, I would have been in trouble.”
RDI uses the native OptoMMP protocol to communicate with the other subsystems. Turbine flow rate and temperature are passed to the ammonia system, which RDI had previously retrofitted using Opto 22 SNAP PACs to control the amount that is injected into turbine exhaust gas. Another PAC interfaces with Modbus serial devices to communicate voltage and protective relay statuses. And a pair of PACs create a non-routable interface for the external OSI PI historian. One PAC is linked to the control network, gathers data, and passes it to another PAC on the historian’s network. The dual independent Ethernet ports on each PAC allow RDI to segment the networks so that data can only flow out, not in.
“We definitely had a competitive advantage price-wise,” said Lou Bertha. “We redid everything, including demo and install, and still came in 30 to 40% cheaper than competing bids.” With Opto 22, RDI won the peaking plant contract and then completed the project in only two weeks.
Besides lower material costs, Lou attributes his advantage with Opto 22 to three factors:
- Control at the I/O level: Opto 22 automation systems leverage intelligent I/O networks to distribute functions that, in a traditional remote I/O system, would typically be handled by a central controller. This architecture ensures that systems can continue to run even with partial impairment.
- Flexibility: groov EPIC and SNAP PAC are designed with embedded hardware and software tools—multiple networking options, communication servers, data processing functions—that provide engineers with many options for designing and scaling systems.
- Openness: Opto 22 products are built on open standards and technologies, making it easy to integrate with 3rd-party devices and external systems.
By leveraging these capabilities, RDI was able to meet the project’s objectives for performance and reliability. With no external dependencies or expensive server licensing, the system was also straightforward to design and simple to maintain.
“With Opto 22,” Lou said, “we can do more and my customers are happy. It’s easy. It’s clean. The system just runs. If people don’t have to think about it, they are happy.”
This article was originally published in the Automation 2021: Control Systems Ebook.
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