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In the case of coal plants, units designed to be base-loaded and run continuously are now being turned on and off frequently. The result is more starts and longer periods of reduced load operation for these units, producing more thermal cycles and additional wear and tear on system components.
Studies show that periods with thermal cycles have a higher likelihood of producing unavailability than periods of normal, steady or stable operations.
Related to this is the impact on plant personnel. In the case of some European coal units, the number of starts has risen from 4 or 5 times a year to 40 or 50 times a year. For employees, this is equivalent to the challenge of driving a tractor-trailer truck through traffic, with constant braking and accelerating.
When a unit transitions from base-load operation to load following or intermediate duty, plant personnel must become far more flexible to meet dispatch or market requirements. This necessity can move plant personnel into unfamiliar territory, out of their comfort zone.
Beyond these considerations,
When a unit’s utilization changes, its maintenance programs must anticipate and adapt to changes in failure patterns. For example, a steam unit that experiences 50 starts per year will have a far different failure history and maintenance requirement for start-up valves than a unit that experiences only 5 starts per year.
Value of GADS analysis
To gain a better understanding of trends that might impact deployment, maintenance and operations strategies for units, GADS data as defined by NERC can be helpful. This information is used to support equipment reliability and availability analyses and decision-making. While primarily a North American reliability data reporting system, NERC opened the GADS database to generating companies outside North America in 2004. As a result, a number of European, Asian and South American generating companies are collecting and reporting GADS data directly to NERC.
European and North American plants have experienced opposite trends in EUF for coal unit boilers, coal unit balance of plant equipment and CCGT gas turbines (Figures 3–5). Considering that the design and manufacture of this equipment is similar, it is clear that carbon trading and renewables are driving these differences.
While general trends in large groups of plants cannot point to a specific course of action for any one unit, they can suggest influences that reliability professionals should take into account when doing their own planning. While carbon trading has not come to pass in the U.S., for example, some states have begun to introduce significant amounts of wind in the dispatch portfolio, which must be accommodated at the expense of existing generating units.
It is incumbent on reliability and maintenance engineers to use the best analysis tools and data at their disposal to prepare for these changes before they occur. Analysis tools are available from a variety of sources: those developed in-house at the generating company, engineering services or reliability and cost data analysis firms such as Solomon Associate, non-governmental organizations such as NERC, and research organizations such as EPRI. Modifications to operations and maintenance plans should be made based on facts and not presumptions; the best place to start is with the performance of units that have experienced similar transitions, followed by a review and modification of programs based on unit-specific data.