POWER June 2011 - 40

INSTRUMENTATION & CONTROL
minute (automatic), according to the Mark
VI system specifications.
When the load on each CT is increased such
that the measured exhaust temperature reaches
the prescribed baseload temperature control
limit, the CT output is then said to be operating
at baseload conditions. Once at baseload,
the CTs operate on the baseload inlet guide
vane/temperature control curve. Under certain
ambient and operating conditions, the CT may
be limited by this control curve even though
expected baseload output is reached-a notunusual
scenario under high ambient temperatures.
The power demand signal is directed to
the Mark VI CT digital controls from the plant
distributed control system (DCS) to produce
the required electricity.
Table 1. Plant ramp rate constraints.
The speed at which a power block's output can
be increased or decreased is limited by equipment
design. For example, a minimum value
of steam flow to the block 1 steam turbine
must be available before the desired combustion
turbine MW ramp rate can be achieved.
Source: GE Power
Operating constraint
ST 1 inlet flow
ST 2 inlet flow
HP steam flow 1
HP steam flow 2
HP steam flow 3
HP steam flow 4
Condenser pressure 1
Condenser pressure 2
ST 1 HP header pressure
ST 2 HP header pressure
Train 1 NOx
Train 2 NOx
Train 3 NOx
Train 4 NOx
Train 1 CO
Train 2 CO
Train 3 CO
Train 4 CO
CT1 TREF
CT2 TREF
CT3 TREF
CT4 TREF
CT1 exhaust pressure
CT2 exhaust pressure
CT3 exhaust pressure
CT4 exhaust pressure
Type
Min
Min
Min
Min
Min
Min
Max
Max
Min/Max
Min/Max
Max
Max
Max
Max
Max
Max
Max
Max
Min
Min
Min
Min
Max
Max
Max
Max
Notes: ST = steam turbine, CT = combustion turbine, HP
= high pressure, TREF = CT reference firing temperature.
40
The updated CT control strategy is a remarkable
improvement. The MVC controller
sends supervisory megawatt (MW) setpoints
to the DCS for the operating CTs. In turn,
the DCS sends a setpoint signal directly to
the Mark VI CT controller, just as before.
Modified control logic in the DCS allows the
operator to select " computer Mode " for the
individual CTs, which enables MVC control.
computer mode is an added operating mode
now programmed in the DCS in addition to
the " baseload " and " preselect " modes discussed
above.
MVC takes control of the CT MW setpoint
when the operator selects computer mode
and the Mark VI remote control is enabled.
The MVC is only used when the CTs are operating
and is secured before CT shutdown.
During start-up, the MVC has high and
low MW and maximum MW ramp rate limits
defined for individual CTs. A feedback signal
from the Mark VI indicates when baseload
conditions have been reached. That signal is
used in the MVC controller to set the high
limit of the controller. The MVC high limit
is set as the current output of the CT plus
three times the maximum ramp rate (that is,
9 MW) until the feedback signal indicates
that baseload (or maximum output based on
operating conditions) has been reached.
When baseload has been reached, the high
limit is set at current output plus 0.5 MW.
This 0.5-MW margin allows for the normal
oscillation of the CT output and for maintaining
the turbine at stable baseload condition.
In this way, the MVC optimally controls the
online CT MW setpoints to meet the K-Power
dispatch commitment. Table 1 summarizes
many of the other plant design constraints
in the MVC that will limit the power block
ramp rates.
MVC System Architecture
Figure 2 depicts the communication interface
between the K-Power DCS and the GE Bently
Nevada EfficiencyMap/CLOC (closed-loop
optimal control) computers. EfficiencyMap
is plant optimization and performance monitoring
software designed to measure, predict,
and track plant performance. CLOC is a supervisory
control and optimization system
for complex, dynamic processes.
The MVC software and plant historian
are installed on one computer; EfficiencyMap
is installed on a second computer.
The plant historian, hosted on the CLOC
computer, communicates with the plant's
Honeywell DCS OPC server via the historian-OPC
interface. The CLOC system
does not communicate directly with the
turbine controllers. All communications
pass through the DCS OPC server, and the
necessary information (such as operating
parameters and setpoints) is communicated
to and from the CT controllers to the DCS
via OPC protocol. To facilitate receiving
incoming data, the Honeywell DCS has additional
logic programmed to ensure that the
incoming data is permitted and will not adversely
affect the control system.
Scheduling and Planning
The system also includes an off-line GateCycle
" what-if " tool. This tool, in the form of an
Excel spreadsheet, is used by plant personnel
for what-if parametric studies. It imple2.
MVC data communications at K-Power. Source: GE Power
Mark VI
CT#1
i-Historian
System 1
OPC data (vibration) to
i-Historian using DCOM
OPC data (HRSG, BOP) from
Honeywell OPC server to
i-Historian using DCOM
OPC data (HRSG, BOP)
from Honeywell OPC
server to CLOC
Honeywell DCS
OPC server
Calculated data to Honeywell
OPC server. CLOC is client and
can write data to OPC server
CLOC
EfficiencyMap
Mark VI
ST#2
Kepware Modbus to OPC server
to collect Mark VI data from
i-Historian Cimplicity HMI
www.powermag.com
Calculated data
from/to CLOC
POWER | June 2011
Modbus TCP/IP
Mark VI
CT#2
EGD
Mark VI
ST#1
Mark VI
CT#3
Mark VI
CT#4
http://www.powermag.com
POWER June 2011

Table of Contents for the Digital Edition of POWER June 2011
