U.S. patent number 3,928,972 [Application Number 05/331,738] was granted by the patent office on 1975-12-30 for system and method for improved steam turbine operation.
This patent grant is currently assigned to Westinghouse Electric Corporation. Invention is credited to Robert L. Osborne.
United States Patent |
3,928,972 |
Osborne |
December 30, 1975 |
System and method for improved steam turbine operation
Abstract
An electric power plant steam turbine system with programmed
digital computer control in which excessive rotor stress and strain
are prevented by developing control signals based upon a comparison
between present calculated heat flow and a reference heat flow, the
turbine operation being controlled by the control signals so that
strain is maintained substantially at the maximum allowable
value.
Inventors: |
Osborne; Robert L.
(Wallingford, PA) |
Assignee: |
Westinghouse Electric
Corporation (Pittsburgh, PA)
|
Family
ID: |
23295173 |
Appl.
No.: |
05/331,738 |
Filed: |
February 13, 1973 |
Current U.S.
Class: |
60/646;
60/660 |
Current CPC
Class: |
F01K
7/24 (20130101); F01D 19/02 (20130101) |
Current International
Class: |
F01D
19/00 (20060101); F01K 7/24 (20060101); F01D
19/02 (20060101); F01K 7/00 (20060101); F01k
013/02 () |
Field of
Search: |
;60/646,657,660 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Schwadron; Martin P.
Assistant Examiner: Ostrager; Allen M.
Attorney, Agent or Firm: Possessky; E. F.
Claims
I claim:
1. An improved steam turbine system comprising:
a. A steam turbine having a portion subject to thermal stress when
said turbine is in operation;
b. means for generating a heat flow representation of present heat
flow to said portion from a predetermined region in heat transfer
relation to said portion; and
c. means for controlling operation of said steam turbine as a
function of said heat flow representation, whereby said thermal
stress is controlled.
2. The improved steam turbine system as described in claim 1,
wherein said turbine portion is the turbine rotor, and said heat
flow representation represeents heat flow to the surface of said
rotor.
3. The improved steam turbine system as described in claim 2,
comprising means for generating a difference representation
representing the difference between a reference heat flow and said
rotor heat flow, and means for generating a control signal as a
function of said difference representation, and wherein said
controlling means controls operation of said steam turbine as a
function of said control signal.
4. The improved steam turbine system as described in claim 3
comprising means for detecting steam temperature and steam pressure
in a predetermined turbine region in heat transfer relation with
said rotor and for generating therefrom a steam temperature signal
and a steam pressure signal, and wherein said means for generating
a representation of heat flow to said rotor performs the function
of calculating heat flow to the rotor on the basis of said steam
temperature and steam pressure signals.
5. The improved steam turbine system as described in claim 4,
comprising means for determining rotor speed and generating a rotor
speed signal, and wherein said means for generating said heat flow
representation makes said heat flow calculation as a function of
rotor speed when said system is controlling the speed of said
turbine.
6. The improved steam turbine system as described in claim 5,
comprising means for determining steam flow in said region and
generating a steam flow signal, and wherein said means for
generating a heat flow representation makes said heat flow
calculation as a function of said steam flow signal when said
system is controlling turbine load.
7. The improved steam turbine system as described in claim 3,
wherein said means for generating a control signal incorporates
means for limiting the rate of increase of said heat flow
difference representation, and means for limiting said control
signal as a function of present rotor surface strain and predicted
rotor surface strain.
8. The improved steam turbine system as described in claim 5,
wherein said means for generating a control signal generates a
speed control signal and said means for controlling operation
controls turbine speed when the system is in speed control
operation.
9. The improved steam turbine system as described in claim 6,
wherein said means for generating a control signal generates a
steam flow signal and said means for controlling operation controls
turbine steam flow when said turbine system is in load control
operation.
10. The improved steam turbine system as described in claim 1,
wherein said steam turbine has a plurality of portions subject to
thermal stress, and comprising a plurality of means for generating
representations of heat flow to respective ones of said portions
and means for determining a controlling one of said heat flow
representations as a function of which said steam turbine is
controlled.
11. The improved steam turbine system as described in claim 10,
wherein one of said additional portions is the turbine casing.
12. The improved steam turbine system as described in claim 1,
wherein said turbine portion is the turbine casing, and said heat
flow representation represents heat flow to said casing.
13. A control system for a steam turbine comprising:
a. means for generating a representation of present time rate of
change of temperature at the turbine rotor due to heat flow to said
rotor from a predetermined region in heat transfer relation
thereto;
b. means for controlling steam flow to said turbine as a function
of said heat flow representation.
14. The control system for a steam turbine as described in claim
13, wherein said means for controlling said steam flow controls
said steam flow so as to change turbine speed.
15. The control system for a steam turbine as described in claim
13, wherein said means for controlling steam flow controls said
steam flow so as to change the load carried by said steam
turbine.
16. The control system for a steam turbine as described in claim
14, wherein said means for controlling steam flow includes steam
valve means positioned to determine steam flow through said turbine
so as to control turbine speed.
17. The control system for a steam turbine as described in claim
15, wherein said means for controlling steam flow includes steam
valve means positioned to determine steam flow so as to control
load delivered by the turbine.
18. A control system for controlling the operation of a steam
turbine, comprising:
a. means for determining the temperature difference between the
steam temperature in a predetermined turbine region in heat
transfer relation with the turbine rotor surface and the
temperature of such turbine rotor surface;
b. means for determining the rotor speed and steam flow through the
predetermined region;
c. means for generating a turbine operating representation as a
function of said temperature difference, rotor speed and steam
flow; and
d. steam valve means operated by said operating representation for
controlling steam flow to said turbine as a function of said
operating representation.
19. The control system as described in claim 18 wherein said
operating representation is a function of heat flow to the rotor
surface, and comprising means for generating a heat flow limit
representation as a function of a predetermined turbine thermal
condition and for limiting the operation of said valve means in
accordance with said limit.
20. The control system as described in claim 18, comprising a
general purpose programmed digital computer which performs the
following functions:
a. generating a representation of the heat transfer coefficient at
said rotor surface;
b. generating a representation of present heat flow to said rotor
surface as a function of said heat transfer coefficient and said
temperature difference;
c. generating a representation of the heat flow difference between
a reference heat flow corresponding to maximum heat flow for
allowable rotor strain and present heat flow; and
d. generating said operating representation as a function of said
difference representation.
21. The control system as described in claim 20, wherein said
operating representation is a speed representation.
22. The control system as described in claim 20, wherein said
operating representation is a load representation.
23. An improved method for operating a steam turbine, which turbine
when operating has one or more portions thereof subject to thermal
stress, comprising:
a. generating a heat flow signal representing present heat flow to
one of said portions from a predetermined region separate from and
in heat transfer relation to said one portion;
b. generating a control signal as a function of said heat flow
signal; and
c. controlling the operation of said steam turbine as a function of
said control signal, thereby controlling thermal stress in said one
portion.
24. The improved method for operating a steam turbine as described
in claim 23, wherein said one portion is the turbine rotor, and the
step of generating said control signal comprises generating a
difference signal representing the difference between a reference
heat flow to the rotor corresponding to maximum allowable rotor
strain and the heat flow to the rotor.
25. The improved method for operating a steam turbine as described
in claim 24 wherein the step of generating said heat flow signal
comprises determining steam temperature and pressure in a
predetermined turbine region in heat transfer relation with the
surface of said rotor; calculating heat flow to the rotor as a
function of said determined steam temperature and pressure; and
controlling turbine operation as a function of said calculated
rotor heat flow.
26. The improved method for operating a steam turbine as described
in claim 25, comprising determining rotor speed and calculating
heat flow to the rotor as a function also of rotor speed, and
controlling turbine speed as a function of the calculated rotor
heat flow.
27. The improved method for operating a steam turbine as described
in claim 25, comprising determining steam flow in said turbine
region and calculating heat flow to the rotor as a function of
steam flow, and controlling turbine load as a function of the
calculated rotor heat flow.
28. The method for operating a steam turbine as described in claim
25, comprising limiting said control signal as a function of stress
at other portions of said turbine.
29. The improved method for operating a steam turbine as described
in claim 23, comprising generating a plurality of heat flow signals
representing heat flow to a plurality of respective turbine
portions subject to heat stress; selecting a limiting one of said
heat flow signals; and generating said control signal as a function
of said selected heat flow signal.
30. The improved method for operating a steam turbine as described
in claim 23, wherein said one portion is the turbine casing.
31. The improved method for operating a steam turbine as described
in claim 29, wherein one of said turbine portions is the turbine
casing.
32. The improved method for operating a steam turbine as described
in claim 23, further comprising limit signals representing rotor
surface strain, accumulated fatigue damage, bore loadings, and
casing strain, and limiting said control signal on the basis of one
of said limit signals.
33. A method for operating a steam turbine comprising:
a. determining steam temperature in a predetermined turbine region
separate from and in heat transfer relation with a preselected
turbine rotor portion;
b. generating a representation of present heat flow from said
region to said turbine rotor portion as a predetermined function of
said determined steam temperature; and
c. controlling the steam turbine speed as a function of said heat
flow representation and continuously controlling the heat flow to
said rotor substantially corresponding to maximum allowable rotor
strain.
34. The method as described in claim 33, wherein the step of
controlling the steam turbine speed comprises:
a. generating a difference signal representing the difference
between a reference heat flow corresponding to maximum allowable
rotor strain, and heat flow to the rotor; and
b. generating a control signal as a function of said difference
signal, and controlling said steam turbine speed by said control
signal.
35. The method as described in claim 34 further comprising
generating limit signals representing limiting values of conditions
selected from the group consisting of rotor surface strain, bore
strain and casing strain, and limiting said control signal on the
basis of one of said limit signals.
36. A method for controlling a steam turbine comprising:
a. generating a representation of current heat flow to the turbine
rotor from a predetermined region in heat transfer relation to said
rotor; and
b. continuously controlling steam flow to said turbine as a
function of said current heat flow representation.
37. The method for control of a steam turbine as described in claim
36, comprising controlling said steam flow so as to change turbine
speed.
38. The method for control of a steam turbine as described in claim
36, comprising controlling said steam flow so as to change the load
carried by said steam turbine.
39. The method of controlling a steam turbine as described in claim
36, wherein said heat flow representation is generated as a
function of the impulse chamber steam temperature (T.sub.I), the
rotor surface temperature (T.sub.S), the heat transfer coefficient
at the rotor surface (H), and the rotor surface area (A).
40. The method of controlling a steam turbine as described in claim
39, wherein H is determined as a function of rotor speed and steam
flow past the rotor.
41. The method of controlling a steam turbine as described in claim
40, comprising determining at least one other turbine thermal
condition, and limiting said steam flow to said turbine when said
thermal condition exceeds a predetermined limit.
42. The method of controlling a steam turbine as described in claim
41, wherein said thermal condition is rotor surface strain.
43. The method of controlling a steam turbine as described in claim
41, wherein said thermal condition is casing strain.
44. A method for controlling steam turbine operation using a
digital computer comprising:
a. determining the temperature difference between the steam
temperature in a predetermined turbine region in heat transfer
relation with the turbine rotor surface and the temperature of such
turbine rotor surface;
b. determining the rotor speed and steam flow through said
predetermined region;
c. generating, with a general purpose programmed digital computer,
a turbine operating representation as a function of said
temperature difference, rotor speed and steam flow; and
d. controlling steam flow to said turbine as a function of said
operating representation.
45. The digital computer control method as described in claim 44
wherein said operating representation is a function of heat flow to
the rotor surface, and further comprising the step of generating a
heat flow limit representation as a function of a predetermined
turbine thermal condition and limiting said turbine steam flow in
accordance with said heat flow limit.
46. The digital computer control method as described in claim 44,
comprising performing the following steps with said general purpose
programmed digital computer:
a. generating a representation of the heat transfer coefficient at
said rotor surface;
b. generating a representation of present heat flow to said rotor
surface as a function of said heat transfer coefficient and said
temperature difference;
c. generating a representation of the heat flow difference between
a reference heat flow corresponding to maximum heat flow for
allowable rotor strain, and present heat flow; and
d. generating said operating representation as a function of said
difference representation.
47. The digital computer control method as described in claim 46,
wherein said operating representation is a speed
representation.
48. A digital computer control method as described in claim 46,
wherein said operating representation is a load representation.
49. An improved steam turbine system comprising:
a. a steam turbine having a portion subject to thermal stress when
said turbine is in operation;
b. means for generating a first representation of a first
predetermined temperature condition at said portion;
c. means for generating a second representation of a second
predetermined temperature condition at a predetermined temperature
region in heat transfer relation with said portion;
d. means for utilizing said first and second representations for
generating a representation of heat flow to said portion as a
function of said first and second temperature conditions; and
e. means for continuously controlling operation of said steam
turbine as a function of said heat flow representation, whereby
said thermal stress is controlled.
50. An improved steam turbine system comprising:
a. a steam turbine having a portion subject to thermal stress when
said turbine is in operation;
b. means for generating a first representation of a first
predetermined temperature condition at said portion;
c. means for generating a second representation of a second
predetermined temperature condition at a predetermined temperature
region in heat transfer relation with said portion;
d. means utilizing said first and second representations for
generating a representation of the rate of change of said first
predetermined temperature condition; and
e. means for controlling operation of said steam turbine as a
function of said rate of change representation, whereby said
thermal stress is controlled.
Description
CROSS-REFERENCE TO RELATED APPLICATION
System and Method for Operating a Steam Turbine and an Electric
Power Generating Plant, by T. C. Giras and M. E. Birnbaum, Ser. No.
722,779, filed Apr. 19, 1968, now abandoned, and continued as Ser.
No. 124,993 on Mar. 16, 1971, and assigned to the present
assignee.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to elastic fluid turbine systems and
more particularly to systems and methods for controlling the
dynamic operation of steam turbines as a function of heat flow.
2. Description of the Prior Art
This invention constitutes an improved system and method for steam
turbine operation, and as such provides a modification of, and
improvement over, the system described in U.S. Pat. No. 3,558,265,
"System and Method for Providing Steam Turbine Operation with
Improved Dynamics," by William R. Berry, and assigned to the
present assignee. This reference patent, hereinafter referred to as
"the Berry patent," provides a thorough disclosure of the
background of the steam turbine art, and particularly of the
effects of thermal loading on permissable turbine operation, and is
incorporated by reference for the purposes of indicating the
background of this invention and illustrating the state of the art
upon which this invention improves. Specific reference is made at
this point to the section of the Berry patent titled "Background of
the Invention," Col. 1, line 10 to Col. 3, line 17, as well as the
discussion of rotor thermal stress and plastic strain analysis and
its connection with turbine dynamics which appears from Col. 9,
line 36 through Col. 12, line 49.
The Berry patent discloses an improved method of determining
present rotor stress as a function of monitored turbine impulse
chamber steam temperature, comparing the present stress with a
predetermined stress limit, and deriving a control signal from such
comparison, by which inlet steam flow is controlled. In the system
described, limits of impulse chamber steam temperature may be
further controlled by considerations of bore loading or casing
strain. The effects of thermal expansion and contraction on
respective regions of the turbine are thus controlled as a function
of calculated present stress at such regions, which calculations
are based upon the monitored inlet steam condition, centrifugal
force loadings, and other input variables.
Another specific prior art example of programmed turbine control
based upon considerations of present stress is disclosed in the
patent to Zwicky, U.S. Pat. No. 3,446,224, issued May 27, 1969. The
Zwicky patent discloses another method of calculating present
surface stress and bore stress from measured steam temperature and
rotor speed. Comparisons between calculated values of present
stress and limit values are made for respective turbine regions,
and the lowest margin is gated to control the turbine.
In the above prior art examples, turbine operation is controlled as
a function of the real time calculated stress condition of the
rotor, which calculation is based upon measured steam temperature.
The calculated condition is compared with a limit which may be a
predetermined limit or a future predicted stress limit calculated
on the basis of the turbine's temperature history. Such prior art
techniques provide feedback control directed to dynamic loading
and/or speed changing without exceeding allowable stress
conditions. They are premised on present calculations of stress and
not on any variable which is determinative of future stress. Such
prior art systems place limits on the turbine operation without
commanding that the desired changes be accomplished by the shortest
possible sequence within such limits. For example, when turbine
speed or acceleration is varied due to a control signal derived
from a present calculation of rotor stress, the change in rotor
stress due to the operating change is necessarily delayed due to
thermal energy storage in the rotor metal. The lag between changes
in temperature, at any given region of the turbine which is being
monitored, and the turbine operation which is being controlled in
response to present stress calculations, necessarily results in
less than optimum control, even though the feedback system is fast
and accurate. This lagging effect has been observed to lead to some
oscillation tendency, and thus variations of thermally induced
strain in certain turbine regions.
It is well understood that, no matter how efficient and responsive
are the building blocks of a control system, the overall
responsiveness is limited by the manner in which the information
content of the input variables is utilized. There remains a clear
need for a control system having the capacity to determine, from
temperature measurements in respective regions of the turbine (such
as the rotor), not only present turbine stress in relation to
stress limits, but a more primary condition which relates to and is
determinative of future changes in stress, and for a system having
the capacity to control such primary condition so that quicker and
more accurate turbine operation can be obtained while minimizing
the oscillations and errors inherent in the prior art systems.
SUMMARY OF THE INVENTION
In accordance with the broad principles of this invention, there is
provided means for determining the temperature of turbine steam in
at least one predetermined steam flow region associated in direct
heat transfer relation with a predetermined turbine rotor portion.
Improved control of the turbine is provided by combining the steam
temperature determining means with means for determining a real
time representation of heat flow to the rotor portion. The
calculated heat flow representation is compared with a reference
heat flow representation, which reference is selected as
corresponding to maximum allowable rotor strain, and a comparison
signal thus derived is processed by control signal means to provide
a control signal for maintaining the turbine speed or load in such
a way as to optimize the heat flow to the rotor at the value which
maintains rotor strain substantially at the maximum allowable
limit. Since heat flow to the rotor is a direct determinant of
future rotor temperature, and consequently rotor strain, the system
of this invention provides an advantage over previously used
systems by immediately modifying the turbine operation to maintain
rotor strain substantially equal to the maximum allowable value, as
contrasted to "present stress" control wherein a differential
between present rotor strain and the maximum allowable value of
rotor strain is the basis for limiting turbine operation.
In another form of this invention, the steam temperature is
determined in one or more predetermined regions, the heat flow to
the rotor surface and present rotor surface strain are determined,
and a limit is placed on the rate at which the turbine operating
level is changed in order to limit the extent of the rotor strain
while meeting end controlled variable demand. In electric power
plant turbines, rapid starting of the turbine is accomplished by
directing operation toward steady maximum allowable rotor surface
strain during startup while limiting rotor strain below a maximum
limit. Operation during load transients is also controlled to
achieve rapid load change under conditions of maximum allowable
rotor strain.
In its preferred form, the system of this invention utilizes a
general purpose programmed digital computer for determining end
variable control actions during transient and steady state
operation with dynamic constraints computed as a function of heat
flow in one or more predetermined steam flow regions. The method of
this invention may be used in conjunction with the method of
operation disclosed in the Berry patent for providing control
limits based upon rotor strain, bore loading and casing strain
whereby, when any of such limits is exceeded, operation of the heat
flow control provided by the invention can be over-ridden.
It is therefore an object of this invention to provide a novel
method and system of steam turbine operation with greater accuracy,
efficiency and economy, and improved performance
characteristics.
It is a more specific object of this invention to provide a novel
steam turbine system and method of operating same with real time
control of heat flow to provide changes in turbine operation more
responsive to present and future thermal stress and strain in
monitored turbine regions so as to provide better turbine control
resulting in more efficient electric power generation, and enabling
more efficient turbine operation within dynamic constraints.
It is another object of this invention to provide automatic means
for generating a representation of heat flow to at least one
turbine region, and to control turbine operation as a function of
such heat flow representation in a manner so as to maintain such
heat flow at a level corresponding to maximum allowable
temperature-induced (thermal) stress in such region.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a schematic diagram of a large electric power plant
steam turbine supplied with steam by a steam generating system and
operated in association with certain sensor and control devices in
accordance with the principles of the invention.
FIG. 2 shows a schematic diagram of a programmed digital computer
control system operable with the steam turbine and its associated
devices shown in FIG. 1 in accordance with the principles of the
invention.
FIG. 3 shows an enlarged portion of a longitudinal section through
a high pressure section of the steam turbine of FIG. 1 and certain
sensor devices placed therein.
FIG. 4 shows a control logic flow diagram employed in part of an
overall programming system which operates the computer of FIG. 2 to
control turbine operation in accordance with the principles of the
invention.
FIG. 5 shows a more detailed flow diagram of a portion of the
diagram of FIG. 4.
FIG. 6 shows a simplified block diagram illustrating the alternate
procedure of calculating a plurality of differential heat flow
representations corresponding to different turbine regions, and
determining the lowest of same, which low representation is used to
develop the system control signal.
DESCRIPTION OF THE PREFERRED EMBODIMENT
More specifically, there is shown in FIG. 1 a large single reheat
steam turbine 10 constructed in a wellknown manner and operated and
controlled in accordance with the principles of the invention as
part of a fossil fuel fired electric power plant 12. Other types of
steam turbines, such as extraction turbines, reactor turbines, back
pressure turbines, etc. can also be controlled in accordance with
the principles of the invention.
The turbine 10 is provided with a single output shaft 14 which
drives a conventional large alternating current generator 16 to
produce three phase (or other phase) electric power as measured by
a conventional power detector 18. Typically, the generator 16 is
connected (not shown) through one or more breakers (not shown) per
phase to a large electric power network and when so connected
causes the turbogenerator arrangement to operate at synchronous
speed under steady state conditions. Under transient electric load
change conditions, system frequency may be affected and conforming
turbogenerator speed changes would result. At synchronism, power
contribution of the generator 16 to the network is normally
determined by the turbine steam flow which in this instance is
supplied to the turbine 10 at substantially constant throttle
pressure.
In this case, the turbine 10 is of the multistage axial flow type
and includes a high pressure section 20, an intermediate pressure
section 22 and a low pressure section 24. Each of these turbine
sections may include a plurality of expansion stages provided by
stationary vanes and an interacting bladed rotor connected to the
shaft 14. In other applications, turbines operated in accordance
with the present invention can have other forms with more or fewer
sections tandemly connected to one shaft or compoundly coupled to
more than one shaft.
The constant throttle pressure steam for driving the turbine 10 is
developed by a steam generating system 26 which is provided in the
form of a conventional drum type boiler operated by fossil fuel
such as pulverized coal or natural gas. From a generalized
standpoint, the present invention can also be applied to steam
turbines associated with other types of steam generating systems
such as nuclear reactor and once through boiler systems.
The turbine 10 in this instance is further of the double ended
steam chest type, and turbine inlet steam flow is directed through
a plurality of throttle valves and a plurality of governor valves
designated as inlet valves 25. Generally, the double ended steam
chest type and other steam chest types such as the single ended
steam chest type or the end bar lift type may involve varying
numbers and/or arrangements of throttle valves. More detailed
description on a particular throttle and governor valve arrangement
is presented in the aforementioned Birnbaum and Giras copending
application.
The preferred turbine startup method is to (1) raise the turbine
speed from the turning gear speed of about 2 r.p.m. to about 80
percent of the synchronous speed under throttle valve control and
then (2) transfer to governor valve control and raise the turbine
speed to the synchronous value, close the power system breaker(s)
and meet the load demand. On shut-down, similar but reverse
practices are involved. Other transfer practices can be employed,
but it is unlikely that transfer would ever be made at a loading
point about 40 percent rated loading because of throttling
efficiency considerations.
After the steam has coursed past the first stage impulse blading to
the last stage reaction blading of the high pressure section 20, it
is directed to a reheater system 28 which is associated with the
boiler 26. In practice, the reheater system 28 might typically
include a pair of parallel connected reheaters coupled to the
boiler 26 in heat transfer relation as indicated by the reference
character 29 and associated with opposite sides of the turbine
casing.
With a raised enthalpy level, the reheated steam flows from the
reheater system 28 through the intermediate pressure turbine
section 22 and the low pressure turbine section 24. From the
latter, the vitiated steam is exhausted to a condenser 32 from
which water flow is directed (not indicated) back to the boiler 26.
To control the flow of reheat steam, reheat valves 33 are provided
and these include one or more normally open check or stop valves
and one or more intercept valves operable to provide reheat steam
flow cutback modulation under turbine overspeed conditions.
In the typical fossil fuel drum type boiler steam generating
system, the boiler control system controls boiler operations so
that steam throttle pressure is held substantially constant. A
throttle pressure detector 38 of suitable conventional design
measures the steam throttle pressure to provide assurance of
substantially constant throttle pressure supply, and, if desired as
a programmed computer protective system override control function,
turbine control action can be adapted to throttle pressure control
as well as or in place of speed and/or load control if the throttle
pressure falls outside predetermined constraining safety and
turbine condensation protection limits. An impulse chamber steam
pressure detector 40 develops signals for use in programmed
computer control of turbine load and ultimately power plant
electrical load.
Respective hydraulically operated valve actuators indicated by the
reference character 42 are provided for the throttle and governor
inlet valves 25. Hydraulically operated actuators indicated by the
reference characters 44 are also provided for the reheat stop and
intercept valves 33. A computer sequenced and monitored high
pressure fluid supply 46 provides the controlling fluid for
actuator operation of the valves 25 and 33. A computer supervised
lubricating oil system (not shown) is separately provided for
turbine plant lubricating requirements.
The respective actuators 42 and 44 are of conventional
construction, and the actuators 42 and the actuators 44 associated
with the intercept valves are operated by respective stabilizing
position controls indicated by the reference characters 48 and 50.
These controls each include a conventional position error feedback
operated analog controller (not indicated) which drives a suitable
known actuator servo valve (not indicated) in the well-known
manner. Reheat intercept valve position control is imposed
typically only when reheat steam flow cutback modulation is
required. Stop valve operation requires no feedback position
control and instead is manually or computer directed with
conventional trip or other suitable emergency operation.
Since turbine power is proportional to steam flow under the assumed
controlled condition of substantially constant steam throttle
pressure, steam valving position is controlled to produce control
over steam flow as an intermediate variable and over turbine speed
and/or load as an end controlled variable(s). Actuator operation
provides the steam valve positioning, and respective valve position
detectors PDIV and PDRV are provided to generate respective valve
position feedback signals for developing position error signals to
be applied to the respective position controls 48 and 50. The
position detectors are provided in suitable conventional form, for
example they can make conventional use of linear variable
differential transformer operation in generating negative position
feedback signals for algebraic summing with respective position
setpoint signals SP in developing the respective input position
error signals.
The combined position control, hydraulic actuator, valve position
detector element and other miscellaneous devices (not shown) form a
local hydraulic-electrical analog valve position control loop for
each throttle and governor inlet steam valve. The position
setpoints SP are computer determined and supplied to the respective
local loops and updated on a periodic basis. Setpoints SP are also
determined for the intercept valve controls. A more complete
general background description of electrohydraulic steam valve
positioning and hydraulic fluid supply systems for valve actuation
is presented in the aforementioned Birnbaum and Noyes paper.
A speed detector 52 is provided to determine the turbine shaft
speed for speed control, for centrifugal stress determination and
turbine constraint operation, for frequency participation control
purposes, and preferably also for rotor surface heat transfer
conductance computation associated with rotor thermal strain
control. The speed detector 52 can for example be in the form of a
reluctance pickup (not shown) magnetically coupled to a notched
wheel (not shown) on the turbogenerator shaft 14. The process
sensor equipment further includes an impulse chamber steam
temperature detector 54 and casing temperature detectors 56, all of
which are employed in programmed computer loading and thermal
strain determination as subsequently described more fully. Analog
and/or pulse signals produced by the speed detector 52, the power
detector 18, the pressure detectors 38 and 40, the temperature
detectors 54 and 56, the valve position detectors PDIV and PDRV and
other sensors (not specifically shown) and status contacts (not
specifically shown) are all applied to a digital computer control
system 60 (FIG. 2) which provides turbine steady state and
transient operation control on an on line real time basis and
further provides system monitoring, sequencing, supervising,
alarming, display and logging functions.
The programmed digital computer control system 60 operates the
turbine 10 with improved dynamic performance characteristics, and
can include conventional hardware in the form of a central
processor 62 and associated input/output interfacing equipment such
as that sold by Westinghouse Electric Corporation and described in
detail in "Westinghouse Engineer," May, 1970, Volume 30, No. 3,
pages 88 through 93. As will be apparent from the description
hereinbelow, the control system of this invention may utilize, for
performing the indicated calculations, any general purpose
programmable computer having real time capability, in combination
with the control apparatus illustrated in FIG. 1 and the required
interface equipment, or equivalents thereof, as illustrated in FIG.
2. Also, it is to be understood that special purpose analog
computer apparatus may be utilized for making the specific
calculations required to practice this invention in controlling the
operation of any particular turbine.
The interfacing equipment for the computer processor 62 includes a
conventional contact closure input system 64 which scans contact or
other similar signals representing the status of various plant and
equipment conditions. Such contacts are generally indicated by the
reference character 66 and might typically be contacts of mercury
wetted relays (not shown) which are operated by energization
circuits (not shown) capable of sensing the predetermined
conditions associated with the various system devices. Status
contact data is used in interlock logic functioning in control or
other programs, protection and alarm system functioning, programmed
monitoring and logging and demand logging, functioning of a
computer executed manual supervisory control 68, etc.
The contact closure input system 64 also accepts digital load
reference signals as indicated by the reference character 70. The
load reference 70 can be manually set or it can be automatically
supplied as by an economic dispatch computer (not shown). In the
load control mode of operation, the load reference 70 defines the
desired megawatt generating level and the computer control system
60 operates the turbine 10 to supply the power generation
demand.
Input interfacing is also provided by a conventional analog input
system 72 which samples analog signals from the plant 12 at a
predetermined rate such as 15 points per second for each analog
channel input and converts the signal samples to digital values for
computer entry. The analog signals are generated by the power
detector 18, the impulse pressure detector 40, the valve position
detectors PDIV and PDRV, the temperature detectors 54 and 56, and
miscellaneous analog sensors 74 such as the throttle pressure
detector 38 (not specifically shown in FIG. 2), various steam flow
detectors, other steam temperature detectors, miscellaneous
equipment operating temperature detectors, generator hydrogen
coolant pressure and temperature detectors, etc. A conventional
pulse input system 76 provides for computer entry of pulse type
detector signals such as those generated by the speed detector 52.
The computer counterparts of the analog and pulse input signals are
used in control program execution, protection and alarm system
functioning, programmed and demand logging, etc.
Information input and output devices provide for computer entry and
output of coded and noncoded information. These devices include a
conventional tape reader and printer system 78 which is used for
various purposes including for example program entry into the
central processor core memory. A conventional teletypewriter system
80 is also provided and it is used for purposes including for
example logging printouts as indicated by the reference character
82. Alphanumeric and/or other types of displays 81, 83 and 85 are
used to communicate current rotor strain, accumulated rotor strain
fatigue, and other information.
A conventional interrupt system 84 is provided with suitable
hardware and circuitry for controlling the input and output
transfer of information between the computer processor 62 and the
slower input/output equipment. Thus, an interrupt signal is applied
to the processor 62 when an input is ready for entry or when an
output transfer has been completed. In general, the central
processor 62 acts on interrupts in accordance with a conventional
executive program. In some cases, particular interrupts are
acknowledged and operated upon without executive priority
limitations.
Output interfacing is provided for the computer by means of a
conventional contact closure output system 86 which operates in
conjunction with a conventional analog output system 88 and with a
valve position control output system 90. A manual control 92 is
coupled to the valve position control output system and is operable
therewith to provide manual turbine control during computer
shutdown and other desired time periods.
Certain computer digital outputs are applied directly in effecting
program determined and contact controlled control actions of
equipment including the high pressure valve fluid and lubrication
systems as indicated by the reference character 87, alarm devices
94 such as buzzers and displays, and predetermined plant auxiliary
devices and systems 96 such as the generator hydrogen coolant
system. Computer digital information outputs are similarly applied
directly to the tape printer and the teletypewriter system 80 and
the display devices 81, 83 and 85.
Other computer digital output signals are first converted to analog
signals through functioning of the analog output system 88 and the
valve position control output system 90. The analog signals are
then applied to the auxiliary devices and systems 96, the fluid and
lubrication systems 87 and the valve controls 48 and 50 in
effecting program determined control actions. The respective
signals applied to the steam valve controls 48 and 50 are the valve
position setpoint signals SP to which reference has previously been
made.
Reference is made to FIG. 3 for a detailed showing of the more
significant portions of an illustrative structural arrangement for
the turbine high pressure section 20 and for the preferred turbine
temperature sensor arrangement associated therewith. The turbine
high pressure section 20 includes a casing or cylinder wall 100
within which a rotor 102 is supported for rotation. Casing strain
at predetermined casing locations is based on conventional outer
and inner wall temperature thermocouple probes 104 and 106 which
form a part of the casing temperature detectors 56.
A suitable steam temperature sensor (not specifically shown but
included as a part of the analog sensors 78) can also be employed
in the intermediate pressure section 22, such as in the inlet steam
pipe but preferably in the IP inlet steam chamber (not shown). IP
steam temperature data is used in the computation of rotor bore
thermal stress in the intermediate pressure section 22.
Steam enters the turbine 10 through a plurality of peripherally
disposed inlets 108 and associated nozzle blocks 105, and the steam
is directed through a velocity compounded impulse control stage
including two rows of rotor impulse blades 107 and 109 and one row
of stationary blades 111 into an impulse chamber 110. As indicated
by the flow arrows, the steam then reverses its flow direction and
passes through reaction blading 112 in the successive stages of the
high pressure section. A conventional thermocouple probe 114 is
appropriately supported by the casing 100 to measure the impulse
chamber steam temperature.
Referring now to FIGS. 4 and 5, there are shown flow diagrams
representing the manner of calculating the control signals used in
the system of this invention. The operations indicated as carried
out in FIG. 4, as well as the more specific program steps charted
in FIG. 5, constitute, for the preferred embodiment where a
programmed digital computer is utilized, a sub-program which is a
portion of a programming system employed to operate the computer
system 60. Reference is made to the programming system disclosed in
the Berry patent starting at column 12, line 50, and continuing to
column 13, line 35, for an example of such a larger programming
system. The improvement of this invention over the system of the
Berry patent is focused in the use of the heat flow control
sub-program of FIGS. 4 and 5, independent of or in conjunction with
the sub-program of the Berry patent which derives limits of turbine
operation as a function of rotor strain, casing strain and bore
strain. It is to be understood that all or any specific portion of
the functional operations illustrated in FIGS. 4 and 5 may be
carried out by special purpose digital or analog calculating means
or equivalent apparatus which provides the necessary real time
capability. For operation with digital computer means, the
Westinghouse W-2500 has the requisite capacity and is suitable for
use as the central processor 62. In other cases, the Westinghouse
Digital Electro-Hydraulic (DEH) Control System for large steam
turbine generators may be utilized in practicing this
invention.
Table 1 set forth below gives definitions for the symbols used in
the flow charts of FIGS. 4 and 5. It is to be noted that some of
the arithmetic operations are represented by equivalent Fortran
symbols.
TABLE 1 ______________________________________ Q.sub.R Rotor heat
flow limit, i.e., heat flow for maximum allowable rotor strain. Q
heat flow; Q = (T.sub.I - T.sub.S) * H * A, for rotor. DQ Q.sub.R -
Q T.sub.S rotor surface temp. = f (W.sub.S, T.sub.I, P.sub.I) H
heat transfer coefficient = f (W.sub.S, SF) A rotor surface area
T.sub.I first stage temp. (measured) T rotor volume average temp.
(defined in the Berry patent) SF Steam Flow Rate (measured) W.sub.S
shaft speed (measured) P.sub.I first stage pressure (measured) DTO
T.sub.S - T, a measure of rotor surface strain DTP predicted DTO =
DTO + DDTO, where DDTO = d(DTO)/dt, extrapolated rate of change of
DTO. W.sub.R speed reference NR speed control signal g gain
constants DNR d(NR)/dt, derivative of NR LDNR limited DNR GDNR
gated DNR HRL high rate limit LRL low rate limit DPTLIM limit of
DPT DTOLIM limit of DTO INC increment signal INT integrated
increment signal NDL speed deceleration reference LR load control
signal DLR derivative of LR LDLR limited DLR GDLR gated DLR LDL
load deceleration reference L.sub.S load (measured) L.sub.R load
reference T period between calculations
______________________________________
Referring first to FIG. 4, block 310 represents the collective
steps of calculating representations of DTO, DTP, Q, T.sub.S, and
T. The input variables to block 310 are obtained as illustrated in
FIGS. 1-3, with W.sub.S, T.sub.I, SF and P.sub.I being continuously
monitored variables, while A is a constant which is stored and read
(as by the tape reader 78) when called for by the sub-program.
Reference is made to the Berry patent for a discussion of the
calculations of T and T.sub.S. DTO is a measure of rotor surface
strain, and DTP is a predicted value of DTO obtained by adding the
first derivative of DTO to DTO.
In the preferred embodiment of this invention, the representation
of Q, as calculated at block 310, is a representation of heat flow
to the rotor surface. However, as set forth more fully hereinbelow,
Q may represent calculated heat flow to other regions of the
turbine, such as the rotor bore, or casing walls. The significance
of the calculated heat flow term is that it represents not merely a
present temperature condition at a given region, but represents the
rate at which temperature and strain itself at such region is
changing. In the case of heat flow to the rotor, Q = (T.sub.I -
T.sub.S) * H * A, where H represents the heat transfer coefficient
at the rotor surface, and takes into account surface film and all
other considerations affecting heat transfer from the steam to the
rotor surface. Reference is made to the Berry patent for a
discussion of the derivation of the formula for the heat transfer
coefficient. As is seen, Q is a direct function of H, which in turn
is a function of W.sub.S and SF, measured quantities. Under startup
conditions, H is primarily a function of W.sub.S, and under load
control conditions, H is primarily a function of SF. By controlling
speed at startup, Q is modulated, and when the turbine is under
load control, Q is modulated by controlling steam flow. Thus, this
system entails control of turbine operation so as to control heat
flow, with heat flow being maintained at a level corresponding to
optimum turbine performance.
At block 320, the computer means carries out the function of
calculating the difference between Q.sub.R and Q, representing the
difference between the heat flow limit corresponding to maximum
allowable rotor strain and the present heat flow. Q.sub.R is the
permissible heat flow for maximum stress under normal transient
conditions. While generally treated as a constant, it is to be
noted that Q.sub.R may be periodically recalculated to take into
account the mode of control, centrifugal force loadings, or other
considerations as noted below. An example of a normal transient
condition is the condition of startup, or acceleration, where the
temperature gradient through the rotor is substantially a constant,
and under which conditions the heat transfer coefficient changes
with speed in a manner such that changing speed causes a
corresponding change in heat flow. A similar "steady state"
transient condition exists where, at synchronous speed, the turbine
is called upon to deliver an increasing load at a constant rate of
increase, i.e., a ramp increase.
The difference representation Q.sub.R - Q, and referred to as DQ,
is operated upon as shown in parallel blocks 330 and 351, by
multiplication and integration respectively, and summed at 332 to
produce a signal which is representative of the first derivative,
designated as DNR, of the speed signal NR. The DNR signal is
limited in step 350 by a high rate limit HRL and a low rate LRL, as
discussed more fully hereinbelow in connection with FIG. 5. The DNR
signal is gated, as shown at block 360, and is either passed or not
passed corresponding to limit controls derived from the DTO and DTP
values, to produce a gated DNR signal referred to as GDNR.
Referring back to the output of block 310, where are indicated the
steps of calculating DTO and DTP signals, the DTO signal is
operated upon in block 366 to determine the comparison of DTO and
DTOLIM, a predetermined limit of DTO. The DTOLIM signal represents
the maximum value of calculated DTO permissible to maintain
operation without rotor strain, and corresponds to the maximum
permissible present value of rotor strain. Thus, while the control
signal is being calculated in terms of heat flow, the comparison in
block 366 comprises a direct check upon present strain. If DTO is
greater than DTOLIM, a signal is derived, as indicated in block
362, to cause the gating operation as illustrated in block 360 to
cause the GDNR signal to be zero, i.e., the DNR signal is
multiplied by zero. If DTO is found to be less than DTOLIM, a
further comparison is made at block 364, between DTP and DTPLIM.
DTPLIM represents a limiting value of DTP, or the maximum value of
predicted rotor strain. Similarly, if DTP is found to be greater
than DTPLIM, a signal is developed at blocks 360 and 362 to cause
the DNR signal to be reduced to zero, i.e., make GDNR equal to
zero. If DTP is found to be less than or equal to DTPLIM, the DNR
signal is gated straight through, i.e., multiplied by one, such
that DGNR equals DNR.
The GDNR signal, being a gated representation of the first
derivative of the speed control signal, is operated upon as shown
in block 370. In the preferred embodiment, this operation comprises
the step of integrating the GDNR signal to obtain an NR signal
representing the speed control signal.
The speed control signal, as generated in the manner described,
represents the basic command for controlling the speed of the
turbine, as during startup, in order to bring turbine operation to
the desired (synchronous) speed in the quickest time and
substantially at but not exceeding rotor strain limits. During a
period of speed change, heat transfer from the impulse chamber
steam to the rotor surface is assumed to be proportional to speed
(since the heat transfer coefficient changes primarily as a
function of speed), and thus in controlling speed the heat flow to
the rotor is accordingly controlled. Further, and most importantly,
since the calculated value of heat flow (Q) is a determinant of
future strain, the turbine operation is controlled not merely to
keep the present operation within safe limits (as done through the
comparisons 364 and 366), but also to command changes in speed so
as to optimally maintain operation at the desired stress limits.
Stating it another way, the difference between operation under this
invention and operation under prior art control systems is that of
commanding operation to be maintained at maximum strain limits, as
opposed to constraining operation within such limits.
It is to be understood that alternate methods may be employed to
calculate the speed control representation NR on the basis of heat
flow Q. Other processing or limiting steps may be utilized in
deriving the NR signal from the Q signal so long as loop stability
is maintained, and be within the spirit and scope of this
invention, the important feature being that the ultimately derived
control signal be based upon the calculation of heat flow.
In actual practice, the control signal NR may be further operated
upon at speed/load control block 382, in order to develop a desired
valve position signal. For example, when in the speed control mode
of operation, the NR signal may be processed as a function of
W.sub.S and/or W.sub.R, and when in the load control mode of
operation, the load control signal LR may be further processed as a
function of LS and/or LR. The processed signal is then operated
upon at block 386 to determine digital output valve position values
which are transmitted to valve position control 90. In addition,
the digital output valve position values may be modified by
calculated maximum value signals from block 388, which calculations
are referred to further hereinbelow.
Referring now to FIG. 5, there is shown a flow chart for a specific
computer subroutine for calculating the speed reference signal NR
as a function of heat flow. The start of the subroutine is
understood to be initiated periodically when the control
calculations are carried out by digital computer, and at a rate
sufficient to maintain real time control. Of course, if the
computer means is comprised of analog circuitry, the calculations
are performed continuously.
The subroutine is started at 401, and a determination is made as to
whether the system is in speed or load control. H, calculated at
405, is primarily a function of the rate at which the steam passes
relative to the rotor. Under startup conditions, speed is
relatively low, and the change in speed comprises substantially all
of the relative change, such that steam flow itself is not a
significant factor. Thus, H is presumed to be primarily a function
of W.sub.S (measured at 52). For example, the formula H = k *
(W.sub.S + 1000), where k is a true constant, has been found to
represent H under startup control. Conversely, under load control,
the rotor speed is fixed at the synchronous value, and H is
presumed to be primarily a function of SF (measured at 74).
After calculation of H, the calculation Q = (T.sub.I -T.sub.S) * H
* A is performed at step 410. As indicated previously, the T.sub.I
variable is measured at the impulse chamber (see FIG. 3) and the
value thus monitored by detector 54 is introduced through the
analog input system 72. For digital computer processing, the analog
signal is converted into digital form.
In the remainder of the flow diagram of FIG. 5, it is assumed, for
illustration only, that the system is in speed control. In the
preferred embodiment, the value of Q is next subtracted from
Q.sub.R, as shown at block 320, to obtain a reference signal DQ,
representing the difference between the value of heat flow at which
turbine operation is maintained at maximum heat stress, and the
present calculated heat flow. The DQ signal is next multiplied, as
shown at block 330, by a gain function g.sub.l, to derive the
signal designated as DNR(P), being the proportional component of
DNR.
There is also shown in FIG. 5, within the section outlined by a
dashed line, a parallel path for obtaining DNR(I), the integral
component of DNR. This path comprises determining, as shown at 421,
whether the value of Q.sub.R - Q is greater than or equal to zero,
or less than zero. If the former is the case, a positive
incremental signal INC, having a constant value of +V, is developed
at 423. If Q.sub.R - Q is less than zero, a negative incremental
signal having a constant value of -V is developed as shown at 425.
The incremental signal, whether plus or minus, and designated INC,
is then integrated by the trapezoidal function shown in block 340,
Whereby the DNR signal is generated. To account for integrator
windup in block 340, a limiting function (not shown) may also be
incorporated. The DNR(P) and DNR(I) signals are summed at 332 to
provide the DNR signal.
The DNR signal is limited by the functions illustrated within block
350. DNR is first compared with the high rate limit HRL, as shown
at 431, representing a maximum allowable rate of increase of speed.
If DNR is greater than or equal to HRL, a limited DNR signal
designated as LDNR is derived which is equal to HRL, as shown at
434. If DNR is less than HRL, or within the high rate limit, the
DNR signal is then compared with the low rate limit signal LRL, as
shown at 433. If DRL is equal to or lower than LRL a minimum
allowable rate of increase of speed, LDNR is set equal to LRL, as
shown at 437. If DNR is greater than LRL (as well as being less
than HRL), LDNR is set equal to DNR.
The subroutine next performs the constraint checks indicated at
364, 366 in FIG. 4. As shown at 440 in FIG. 5, DTO is examined to
see if it is less than DTOLIM, and DTP is examined to see if it is
less than DTPLIM. If these conditions are met, the LDNR signal is
integrated, by the trapezoidal integration function indicated at
450, similarly to the step illustrated at 340, to derive the final
NR signal. If the condition as shown at 440 is not met, the old
value of NR, i.e., NR (t - 1), is provided as the NR signal.
The discussion of the preferred embodiment as set forth above has
illustrated the practice of this invention in controlling turbine
operation during startup, during which the control signal which is
derived is used to position steam inlet valves so as to control
turbine speed, the controlled speed in turn controlling heat flow
in such a way as to cause operation to be substantially at the
limit of allowable rotor stress. In practice, valve position
control 90 controls the throttle valve (on-off), or valves, during
the startup phase until the turbine speed reaches a predetermined
speed short of full speed, e.g., about 3,000 r.p.m. for a
synchronous speed of 3,600 r.p.m. At this point, control is
transferred to the control or governor valve, or valves, which
modulate steam input to any value between full off and full on, for
bringing speed up to the full synchronous value.
It is also to be noted, as indicated previously, that this
invention can be practiced in controlling operation changes in
response to different load demands made upon the turbine. After the
turbine has obtained synchronous speed, where it is maintained, an
increased demand for load presents a "steady state" transient
condition calling for increased turbine output, similar to the
startup condition except that while speed is maintained constant,
load output (MW) is increased. In this mode of operation, the
Q.sub.R reference signal which is inputted at step 320 may be the
same value, or may be another value chosen for load mode operation.
The difference signal, DQ, represents the difference between
calculated (present) heat flow and the reference heat flow under
load conditions, which DQ signal is operated upon in the same
manner as when in the startup mode. At step 350, the limits HRL and
LRL, stored in memory, represent maximum rates of increase of load.
Similarly, the value of DQ after having been multiplied by the gain
function at block 330, is referred to as DLR; the limited value of
DLR is referred to as LDLR; and the gated value of LDLR is
designated as GDLR. The final signal generated after the integrate
step at 370 is referred to as LR, the load control reference
signal. The above signals, which are developed under load control
operation, are shown in parentheses in FIG. 4.
The basic technique of this invention may also be applied to casing
strain and/or bore strain, by calculating (at block 310-A) heat
flow to the casing (Q.sub.C) and heat flow to the rotor bore
(designated Q.sub.B). As shown in FIG. 6, each of these values may
be compared (at 320-A) with a corresponding reference (Q.sub.RC,
Q.sub.RD, etc.) and respective values of DQ (normalized) may be
obtained which are compared at 465 to determine the low limit DQ
representation. The lowest value representation of DQ is chosen as
controlling, and is thereafter processed to obtain the desired
control signal. Thus, system operation may be controlled on the
basis of heat flow to that turbine region wherein heat flow is
closest to the limit for maintaining maximum strain.
From the above, it is seen that there is disclosed an improved
rotor stress control system based upon heat flow calculations. The
system of operation of this invention may be used independently, or
it may be used in conjunction with further calculations of
operating constraints based upon calculated fatigue, bore thermal
loadings, bore centrifugal loadings, casing wall strain and the
like. Referring again to the Berry patent, a turbine rotor loading
and thermal strain constraint subroutine is shown, in block diagram
form, in FIGS. 9 and 10 of that patent. This subroutine is an
illustration of a manner of calculating further constraints which
may be imposed upon the speed or load control signal. It is to be
noted that such subroutine provides for signals which may be used
as constraints on the change in valve position under either startup
or load control, which constraints are based upon calculated bore
thermal loadings, bore centrifugal loadings, and casing strain. In
addition, the fatigue damage per cycle may be calculated, and a
tally maintained of accumulated fatigue damage, permitting more
accurate rotor plastic strain fatigue supervision and/or control.
The signals produced by such subroutine may be used to modify the
Q.sub.R signal, or may be processed at block 388 to determine
maximum values of steam valve positions. In this manner, while the
system is under the heat flow control of this invention, it may
also be subjected to limit control on the basis of calculated
present loadings and strain at various regions of the turbine.
The application of the invention as described hereinabove produces
generally improved steam turbine operation and, more specifically,
it provides for quicker and more accurate control system
functioning as compared to prior art systems based upon present
rotor stress calculations. The foregoing description has been
presented to illustrate the principles of the invention and it is
to be understood that the means for carrying out the various
functions performed in the practice of this invention are
illustrative of the preferred embodiment. Accordingly, it is
desired that the invention not be limited by the embodiment
described, but, rather that it be afforded a scope consistent with
its broad principles.
It is noted that in the illustration of the preferred embodiment of
this application reference is made to controlling turbine operation
as a function of the thermal condition of a specified portion of
the entire turbine system. As is well known, however, there may be
instances where more than one turbine location may limit operation.
For example, a large steam turbine is normally composed of HP, IP
and LP sections, and thermal stress in any of these sections may be
cause for limiting the speed or loading rate. Thus, the inputs to
difference box 320-A (FIG. 6) may include Q signals from any or all
of the turbine sections, such that low limit select 465 provides
that system operation be controlled by the heat flow representation
of that turbine section having the most limiting thermal
condition.
* * * * *