Cost Benefits of a Cycling Analysis on a Combined Cycle Unit

Every time a power plant is turned on and off, the gas turbine, heat recovery steam generator (HRSG), steam lines, steam turbines, and auxiliary components go through unavoidably large thermal and pressure stresses, which cause cumulative damage and incur future costs. The damage is compounded by fatigue and creep-fatigue interaction damage. These cycling-related costs and damages are attributable to current startup, shutdown, and rapid load following.

Combined cycle power plants have made rapid gains in market share and claimed a significant portion of the total generation portfolio over the past 10 or more years. With the recent market price trends for natural gas, they have become even more attractive for baseload and cycling generation. As major capital investments with strong earning potential, a focus on asset management is key to understanding and managing their total cost of operation and maintenance.

Many plants that were built in a competitive bidding market and designed for low initial costs and baseload operation are now being required to operate in load-following and on/off cycling modes. This change has created a need for quantifying true short- and long-term costs by determining the hidden costs of cycling damage and the losses associated with operating off the design point of a unit. An owner-operator or utility may find itself in financial distress by ignoring hidden or poorly understood costs of cycling damage as production costs spiral out of control.

This change in annual costs signals the end of what one might call the “honeymoon period.” To the uninformed, it may be an unexpected change from early life, reliable, low-cost cycling as the operating change results in cycling damage, and units begin to have failures as they reach fatigue stress limits. These may be surprise failures, since they are unseen in early plant life, and routine failures or forced outages increase costs dramatically. The original business model or pro forma is no longer valid as the costs increase and reliability decreases. The asset management mandate then is to prevent unexpected damage and understand the true cost of cycling for economic dispatch of the units in a competitive business model.

Whether cycling is required by the grid operator because of renewable integration or other factors, owners must be proactive about each manageable or variable part of the total asset and have: Personnel or training for understanding the impact of cycling operation.Procedures or updated operating processes and use of standardized “best practices operation.”Equipment upgrades for reliable, expected life when cycling.

All of these play a part in true asset management when cycling a unit or fleet of units so the transition to cycling operation goes smoothly at an expected cost with no surprises.

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