(560h) Dynamic Modeling and Control of a Natural Gas Combined Cycle (NGCC) Power Plant with a Damage Model

As the penetration of intermittent renewable energy sources into the electric grid keeps increasing, natural gas combined cycle (NGCC) plants are expected to follow their load much more frequently than in the past. While cycling operation helps to improve the viability of a power plant, it can lead to its lifetime consumption due to creep, fatigue and corrosion damages in the long term ^[1]. With this motivation, a dynamic model of a NGCC plant coupled with a lifetime estimation model is developed. Advanced model-based controllers are developed to optimize the transient operation under different scenarios while maintaining acceptable component life.

A high-fidelity dynamic model of a NGCC plant is developed with rigorous equipment level models to capture the impact of load-following on the equipment health and life. A thermo-hydraulic model of the heat recovery steam generator (HRSG) is developed and used to obtain the optimal design of various sections of the HRSG including the number of passes, rows of tubes and tube numbers, dimensions of the tubes, and fin geometry. A stage-by-stage model of the steam turbine is developed to estimate the performance of the triple-pressure steam turbine with multiple steam addition and extraction points under nominal and off-design operations.

The high pressure superheater, intermediate pressure reheater and high pressure steam drum are the most stressed components during load variations. Both creep and thermo-mechanical fatigue damage models are developed for the superheater/reheater. A fatigue damage model is developed for the steam drum. The creep assessment is conducted based on the R5 British procedure and the fatigue lifetime calculation is computed according to the EN 13345 Standard and UNI EN 12952^[2-4]. The Rainflow cycle counting method is used to determine stress ranges and cycles. According to the overall stress amplitude, the lifetime consumption of each cycle is estimated.

A model predictive controller (MPC) is developed for fast load following while keeping the stress within admissible bounds. Our study shows that a model based approach can help to accomplish fast ramp rates while maintaining acceptable component life.

[1] Benato, A., et al. "LTE: A procedure to predict power plants dynamic behaviour and components lifetime reduction during transient operation." Applied energy 162 (2016): 880-891.

[2] British Energy. R5 Assessment procedure for the high temperature response of structures, Issue 3. British Energy; Gloucester, UK; 2003.

[3] European Committee for Standardization. EN 13345 part 3 unfired pressure vessels, clause 17; simplified assessment of fatigue life, and clause 18; detailed assessment of fatigue life; 2002.

[4] UNI – Ente Nazionale di Normazione. UNI EN 12952-5:2011. Water-tube Boilers Standards; 2011.