Emerson’s Ranjit Rao and James Nyenhuis joined an engineering professional with a leading electrical power company to present Consistent and Optimal Combined Cycle Operations at the 2025 Ovation Users’ Group Conference. The case study highlighted a project to standardize the company’s automation platform, addressing increased operational flexibility for the electrical power market it served, as well as cybersecurity, networking, and efficiency. The project team’s focus was on ramp rate flexibility and improving operational and commercial performance. Here is their presentation abstract.

The increase of renewable assets on the power grid are requiring combined cycle plants to cycle and move load more frequently. This session discusses the benefits of integrating operations procedures directly into the control logic while enhancing control of the major processes of the unit through the use of the following:

• Advanced Bypass Control
• Advanced Drum Level Control
• Advanced Inlet Pressure Control
• Advanced Pump Protection
• Duct Burner AGC Control
• Procedure Integration

Learn how improvements in some or all of these areas of CC operation, when combined with consistency of startup and shutdown procedure integration, can greatly enhance performance and reliability while also increasing unit flexibility.

Before getting into how performance gains were achieved, they shared the project’s results. Gas turbine ramp rates increased from 10-12 MW/min to 24 MW/min. Duct burner ramp rates increased from 2-3 MW/min to 6 MW/min. The overall reliability of the unit was increased by improved steam temperature control and drum level control.

The Ovation Advanced Power Applications added to make this project successful included:

• Response Optimization
• Duct Burner Optimization
• Steam Header Blending Automation
• Steam Temperature Optimization
• Selective Catalytic Reduction (SCR) Optimization
• Advanced Drum Level Control
• Procedural Integration Automation

One limiting factor for improving ramp rate was the ability to maintain the hotwel levels that collected the turbine exhaust condensate. The condensers and hotwels were not changed in the project.

Here is a look at the suite of Ovation Advanced Applications for Combined Cycle operations that are available beyond traditional control strategies.

Advanced control strategies have several advantages over traditional proportional-integral-derivative (PID) control. PID requires complex balancing of these three components, and rule-of-thumb tuning is often used to achieve the control objectives. It’s challenging to achieve multiple goals with setpoint tracking compared to disturbance rejection. Higher-order processes, such as those around boilers, are approximate. Deadtime in the process can induce windup, which requires controller detuning. And finally, the control can outpace the actuator, resulting in a mismatch in windup.

Model predictive control (MPC) can help address complex interactions between control loops. Inputs for effective optimization include power block demand, power block load, main steam pressure, and an optional economic optimizer. Using these inputs, the Ovation MPC enhanced the automatic generation control (AGC) response by incorporating heat recovery steam generator (HRSG) heat transfer process response models. The load distribution logic interacts with the constraint management logic on the gas turbine side, featuring load-limiting ramp capabilities. It interacts with the duct burners’ side, managing constraints around the HRSG thermal stress limits.

The steam header blend system used advanced bypass control to enhance resilience in managing significant load rejections from the steam turbine. It supports an expanded pressure range and increased reliability in blending and unblending operations. Other improvements included:

• Better manages piping thermal conditions, reducing stress on critical bypass piping
• Bypass valve and attemperation sizing and actuation review via historical data
• Energy balance-driven attemperation control scheme
• Model predictive bypass pre-positioning on plant upsets
• Bypass system protection functions and leak mediation schemes
• Bypass attemperator control based on optimal temperature & enthalpy control for protection of condenser and piping per HEI and EPRI recommendations

For steam temperature optimization, MPC advanced control included disturbance models for key process changes and transient dynamics prediction, helping to maximize ramp rates.

Two other areas the presenters discussed were advanced drum level control and SCR optimization. Drum control is challenging due to dynamic and higher-order effects caused by variable drum geometries, pressure-based shrinkage and swelling effects, and equipment capability limits. These effects make MPC advanced control strategies more effective than PID for improving ramp rate performance and stability.

Learn more by reading this “Improve Combined Cycle Plant Reliability and Reduce Costs” document.