POWER February 2011 - 35

INSTRUMENTATION & CONTROL
quicker response of the firing trim when
the override condition is no longer active.
The dispatch rate was set between 8
MW/min and 12 MW/min during this operational
period. Load varied from a maximum
of 630 MW to 390 MW. During the
last 8 hours, there was also a transition to a
much lower heating value coal, illustrating
satisfactory ramp rate performance over a
wide range of coal quality.
The peak north and south superheat temperatures
were closely controlled near the
setpoint throughout. Waterwall outlet temperature
variations were kept well within
alarm levels. All other steam temperature associated
control loops indicated responsive
control meeting the dispatch requirements.
There were several brief periods during
which the south superheat temperature fell
below the setpoint; those periods were consistent
with periods of low waterwall temperature.
These periods were caused by an
override controller limiting the firing trim
and providing protection from high temperatures
at the inlet to the desuperheat
sprays. Prior to configuring the flex tuning
of the override controllers, these superheat
temperature dips were excessive.
Based on the favorable control of final superheat
temperature and waterwall temperature,
the trend demonstrates that the ramp rate
control enhancements are equally beneficial
for both higher and lower heating value coals
spanning a wide range of load.
Performance Update
The original objective of this project was
to increase the ramp rate capability of both
units and reduce the impact of high-temperature-override
action on ramp rate. The
scope of the project was recently expanded
to apply the model predictive control system
to reduce the frequency and duration
of high-temperature-override incidents
during ramping.
The system changes have enhanced the
balance of steam temperatures throughout
the boiler, overcoming a particular challenge
in split-furnace boilers. The result has been
the near-elimination of high-temperatureoverride
incidents during ramping.
Further benefits have accrued from the
abt. power 1-3 . Feb 11:Layout 1 1/10/11 2:35 PM Page 1
system changes including these: sustainable
fast ramp rate capability from near minimum
load to near maximum load, the limitation of
transient high temperatures of internal boiler
components, and minimization of associated
thermal stresses. The upgrades have been
successfully implemented on both units.
The complex control strategies necessary
to protect boiler tubes and other high-temperature
equipment can have a deleterious
impact on unit ramp rate potential. Through a
combination of flex tuning and model predictive
control, the limitations were overcome at
Morgantown, allowing quadruple increases
in the unit ramp rate while protecting boiler
components from thermal stress.
This article is based on a paper presented
by the authors at the 15th Annual POWID/
EPRI Controls and Instrumentation Symposium.
Morgantown Generating Station was
the ISA POWID 2008 Facility of the Year. ■
-Donald Andrasik (don.andrasik@genon
.com) and John McNulty (john.mcnulty@
genon.com ) are senior DCS engineers for
GenOn Energy at Morgantown Generating
Station. John Gay (powermaxc@aol.com)
is president of Power Max Consulting Inc.
and is an ISA Fellow. Don Labbe (donald
.labbe@invensys.com) is a consulting
control engineer for Invensys Operations
Management. He is an ISA fellow and
director for ISA POWID.
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