POWER June 2014 - 38

GAS-FIRED GENERATION
Managing the Changing Profile of
a Combined Cycle Plant
Whether it's market forces or other factors leading to a change in operating mode,
going from baseload to flexible operation places substantial new stresses on
a plant and its staff. Knowing what you're getting into is key to successfully
managing the transition.
Neva Espinoza, Bill Carson, and Rick Roberts
W
ith the growing need for operational
flexibility across the power
industry, the combined-cycle gas
turbine (CCGT) fleet is increasingly being
subjected to load-following and cyclic operation.
This change in operating mode is introducing
new types and higher rates of damage
and can result in reduced performance and
increased operation and maintenance (O&M)
and repair costs. To meet these challenges,
today's fleet is taking strategic and tactical
measures, and the Electric Power Research
Institute (EPRI) is compiling information on
effective approaches and best practices.
Changing Landscape
Over the past decade, the pattern of duty
modes and plant dispatch of CCGT plants
has changed. This change involves a general
shift for CCGT plants from baseload to flexible
operation with increased frequency and
level of cycling of these plants.
Flexible operation is broadly defined as
any mode of operation other than baseload,
and includes several specific types of duty
modes. Two-shifting is starting up and shutting
down a plant each day to meet load demand
during periods of high demand. Double
two-shifting is starting up and shutting down
a unit twice a day to match the early morning
and evening peaks in load demand. Load following
is operating online for more than 48
hours, with varying load throughout the day
as demand changes. Many times these plants
turn down to some minimum load when demand
is low.
Whatever the duty mode, flexible operation
typically involves more frequent startups,
more rapid ramping, low load operation,
and more frequent shutdowns or layup.
Factors contributing to this trend include
reduced overall demand following the economic
recession, competition, changes in
fuel prices, aging plants, demand for a more
reliable power grid, and stricter environmental
controls. In some regions, one key factor
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POWER | June 2014
is the increasing deployment of intermittent
renewable generation, such as solar and
wind, which is dispatched as " must-take " and
forces fossil plants to provide load-balancing
services. In this context, relatively small differences
in costs and reliability can make a
large difference in station ranking, leading to
many older, less-efficient CCGT plants being
required to load follow.
Effects of Flexible Operation
CCGT plants have less operating flexibility
than conventional steam plants, which can
be run down to 40% of rated output, while
CCGT plants have difficulty in getting below
60%. A further problem is the length of time
that it takes for the heat recovery steam generator
(HRSG) plant to achieve full output.
Hence, although a CCGT plant may be able
to produce power relatively quickly, it is not
really suitable for load-following until some
time after startup.
EPRI research has identified a number of
common damage mechanisms related to cycling.
Cycling load is associated with stresses
from varying temperatures and pressures,
which can trigger fatigue and fatigue-related
damage mechanisms.
Thermal Fatigue. The most common
problem resulting from cycling is thermal
fatigue damage, which manifests itself in the
form of material deformation, cracking of individual
components, or mechanical failure
of structures. This mechanism is caused by
the large temperature swings associated with
flexible operation.
Thermal Mechanical Fatigue. This
mechanism, which occurs in rotating compo1.
Wide range of challenges. This graphic details the many areas of potential cycling
damage in a combined-cycle plant. Courtesy: EPRI
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POWER June 2014

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