CA1071735A - Steam turbine control system and method - Google Patents

Abstract

STEAM TURBINE CONTROL SYSTEM AND METHOD
Abstract of the Disclosure In a steam turbine-generator with provision for determining a load demand signal in accordance with a load reference signal, the valve opening for each admission mode (full-arc and partial-arc) is determined in accordance with the load demand signal. Provision is made for adjusting the ratio between the valve openings under the two admission modes in accordance with a load change, the ratio of steam flow under each admission mode being controlled in accordance with the load change so as to minimize the thermal stresses and reduce the turbine load changing time.

Description

107~735 Background of the Invention:
This invention relates to the rapid loading and - unloading of a steam turbine-generator in accordance with the calculated ratio of steam flow between two types of steam admission no~zles in a manner designed to minimi~e thermal stresses and hence reduce the turbine load changing time.
Start up and loading of a large steam turbine-generator has become more involved in recent years, as the trend toward layer units results in higher thermal stresses for any given temperature transient. Two factors contribute to thermal stresses during start up. Initially, a mis-match exists between the temperature of the admitted steam and that of the metal. The degree of this mis-match depends upon the past operating history, i.e., whether or not the turbine is involved in a cold start or a hot start. This mis-match is essentially corrected during the acceleration phase of the start up.
When the turbine-generator is supplying load and the steam flow is high enough that no substantial mis-match can exist, metal temperatures will follow steam temperatures .
closely. Control of metal temperatures and therefore thermal stresses-is based primarily on an analytical and statistical correltation between stress levels and expected rotor life.
Traditionally, charts and graphs have been provided : to allow the operator to reduce the mis~match at a safe rate during the acceleration phase of the start up and to determine the allowable rate of change of metal temperature during the loading procedure. Various techniques have been employed to speed up the process of loading the turbine, including the use of heat soaking periods on "turning gear" to reduce the initial mis-match. Initial operation in the less efficient "full-arc" steam admission mode has been used to achieve `` 07~735 uniform warming of the high pressure turbine inlet parts.
There have been suggestions in the published prior art of starting up steam turbines using various techniques such as acceleration control, load control, etc., in an effort to minimize start up time without damaging the turbine. These systems are usually predicated on ideal steam generator conditions. Since a turbine start up can take several hours, a system which will reduce this time, as well as allow for fluctuations in steam temperature and pressure from the steam generator, is of great value.
A sophistlcated approach to start up and loading control by continuously calculating rotor surface and bore stresses, using speed and temperature measurements, and then loading at a maximum permissible stress is described in U.S.
Pat. 3,446,224 issued on May 27, 1969 to Everett E. Zwicky, Jr., U.S. Pat. 3,561,216 issued on Feb. 9, 1971 to James H. Moore Jr~, U.S. Pat. 3,588,265 issued on June 28, 1971 to William R. Berry, and U.S. Pat. 3,928,972 issued on Dec. 30, 1975 to Robert L.
- Osborne. Although the proposals in these patents are useful for changing rapid start up and loading, from the standpoint of the delay time involved in the generation of thermal stresses, their teachings are not always satisfactory, because .,'; in effect the turbine is essentially controlled while monitoring the thermal stress produced in the turbine rotor.
Summary of the Invention:
. ~, .~ .
An object of this invention is to provide a steam .;.,~ .
turbine control system which seeks to substantially reduce the generation of thermal stress in the turbine rotor.
~ The invention is based on the fact that the steam ~ 30 temperature with respect to the turbine rotor when steam is admitted into the turbine varies with the steam admission .:
mode, and consequently controls the ratio of steam flow according to the load change under each of the modes, namely `~ the full-arc admission mode and the partial-arc admission mode, thereby premitting load changing without changing the steam temperature with respect to the turbine rotor and hence without generating thermal stress in the turbine rotor.
To this end the invention provides in a steam turbine control system having a turbine and a plurality of valves operable to admit steam to a first stage of the turbine through 10 nozzle arcs, the combination of: (a) means for determining a load demand signal according to a speyd reference signal, a speed feed-back signal, a load reference signal~ a load feed-back signal, and a load change rate signal; (b) means for determining a first valve opening signal under a full-arc admission mode according to said load demand signal; (c) means for determining a second valve opening signal under a partial-arc admission mode according to said the load demand signal;
(d) means for determining first and second ratio control signals between steam flow under the full-arc admission mode . 20 and steam flow under the partial-arc admission mode according to the load reference signal, the load feed-back signal, the load change rate signal~ and a first stage temperature change ~ rate signal; (e) means for adjusting said first valve opening :.-. signal according to said first ratio control signal; (f) means~; for ad~usting said second valve opening signal according to said second ratio control signal; and (g) load control means for positioning said valves to admit a desired total steam flow to said turbine according to the adjusted valve opening signals.
In its method aspect the invention can be defined as providing in a steam turbine control method for a turbine 10717;~5 having a plurality of valves operable to admit steam to a first stage of the turbine through no~zle-arcs, the combination - of: (a) determining a load demand signal according to a speed reference signal, a speed feed-back signal, a load reference signal, a load feed-back signal, and a load change rate signal;
(b) determlning a first valve opening signal under a full-arc admission mode according to said load demand signal; (c) determin-ing a second valve opening signal under a partial-arc admission mode according to said load demand signal; (d) determining first and second ratio control signals between steam flow under the full-acr admission mode and steam flow under the partial-arc admission mode according to the load reference signal, the load feed-back signal; the load change rate signal, and a !

first stage temperature change rate signal; (e) adjusting said first valve opening signal according to said first ratio control signal; (f) ad~usting said second valve opening signal according ~; to said second ratio control signal; and (g) arranging to position said valves to admit a desired total steam flow to said ; turbine according to the adjusted valve opening signals.
",~", ~!~,, 20 Embodiments of the invention are illustrated by way -` of exampl~e only in the accompanying drawings.

~ Brief explanation of the drawings:
,. .
,i Fig. 1 is a simplified schematic diagram of a control system representing an embodiment for carrying out the invention;
Figs. 2a and 2b are simplified schematic diagrams , .c--illustrating admission modes using control valves only;
. .
Fig. 3 is a graph of load vs. temperature under both full-arc and partial-arc conditions;
` Figs. 4a and 4b are graphs of load vs. temperature and load vs. a ratio control signal under full-arc and partial-arc conditions;

Flg. 5 is a simplified schematic diagram of a part of another embodiment of the invention shown in relation to Fig. l;
Fig. 6 is a flow chart showing the principles underlying the process in an important part of the system of Fig. 5;
Fig. 7 is a graph illustrating the variation of the steam temperature of the steam generator and the accompanying variation of the first stage temperature as the turbine load is changed in the course of time;
Fig. 8 is a simplified schematic diagram of a part ; of a further embodiment shown in relation to Fig. ~;
Figs. 9 and 10 are views illustrating the principles underlying the process in an important part of the system of ~ Fig. 8; and ; Fig. 11 is a general flow chart when employing a ~` programmed digital computer for realizing the functions involved in the afore-mentioned embodiments of the invention.
Detailed Description of the Preferred Embodiments Referring to Fig. 1 of the drawing, this schematic diagram shows portions of a reheat steam turbine, its normal speed and load control system, and an automatic ratio-ad~usted, loading system depicted in functional diagrammatic form. It will be understood by those skilled in the art that a large steam turbine-generator control and supervisory sys~em is a very complex affair, and hence only the portions which are material to the present invention are shown here.
Portions of the turbine shown include a high pressure turbine 1, reheat turbine 2~ and one of the double-flow low pressure turbines 3, all arranged in tandem. The number and arrangemènt of additional low pressure turbines, or perhaps -" 107~73~
additional reheat turbines, as well as the number and arrange-ment of generators, are not important to an understanding of the invention. The steam flow is from a steam generator 4 through main stop valves 5 with built in bypass valves 6, and then through control vallves 7, 8, 9, 10, each of the latter ~ -being connected to a different nozzle arc supplying the first stage or high pressure rotor blades. Steam from the turbine 1 id reheated in reheater 11, flows through reheat stop valves (not shown) and intercept valves (not shown) to the reheat turbine 2, and thence through suitable crossover conduits 14 to the low pressure turbines.
The admission of steam is controlled through a number of control valve servo mechanisms shown collectively as 15 and operating the respective valves as indicated by dotted lineæ. The servo mechanisms may be of the electrohydraulic " ` I ~ `
~ type, driving high pressure hydraulic rams in response to ~
f electrical signals, as is well known.

The servo mechanism 15 is under the control of a valve opening control means 16 which provides as its output a suitable valve positioning signal corresponding to a desired $3 ' rate of steam flow.

As is known to those skilled in the art, the control ~, ~
valves 7-10 may be manipulated in such a way as to admit steam ~ uniformly through all of the nozzle arcs dispersed around the ;~ first stage inlet of the turbine, otherwise known as "full-arc"
, , .. -- . .
; admission; or else the control valves 7-10 can be manipulated ; in sequence in a thermo-dynamically more efficient mode, ; admitting steam to one nozzle arc at a time, this being known as "partial-arc" admission.
Reference to Figs. 2a and 2b shows the two extreme positions between full-arc in Fig. 2a and partial-arc in ....
....

~07~73S

Fig. 2b when the control valves are used and the stop valve 5 and its bypass 6 are open. Each of the control valves 7-10 supplies a separate nozzle arc 37-40 respectively. In Fig. 2a, all the control valves 7-10 are partially open admitting steam to all the nozzle arc 37-40. In Fig. 2b, the first control valve 7 is wide open admitting steam to the nozzle arc 37, while the control valve 8 is partially open, admitting a reduced flow of steam to the nozzle arc 38. Valves 9 and 10 are closed so that the nozzle arc 39, 40 are blocked off.
Fig. 3 illuctrates that the first stage temperature difference exists over practically the entire range of rated ~; load, being a maximum at no load, and converging to an identical temperature at full load. At full load, there is no distinction between the full-arc and partial-arc modes. The top line 46 (full-arc) shows a gradually increasing first stage temperature with increase in load. The connected arcuate line segments 47 (partial-arc) show a more pronounced increase in temperature with increase inload, but commencing at a lower temperature.
The discontinuities indicate the points where each of the four control valves commences to open. Theoretical operation with an infinite number of valves is indicated by the broken line 48.
The vertical line 49 in Fig. 3 indicates that, at a point Fa on the fu~l-arc admission line, a high first stage temperature is obtained, while at the same load at point Fb for partial-arc admission, a much lower first stage temperature is obtained. The horizontal line 50 indicates that at a point LL for full-arc admission, a small load is obtained, while at the same first stage temperature at point LH for partial-arc admission, a much larger load is obtained.
When a load change occurs, therefore, the first stage temperature does not change by adequately controlling the ratio between the full-arc admission and the partial-arc admission. In view of this aspect, the invention contemplates that, at the time of a load change, the steam flow is controlled in correspondence with the load change, while the ratio between full-arc admission and partial-arc admission is also controlled so that the first stage temperature is not changed and gradually moves to the~partial admission mode which is more efficient once the load has reached a desired value. Of course, for a load increase after completion of transition to the partial-arc admission mode, the steam flow is increased under this mode at a predetermined rate, since the temperature control of the first stage temperature can no longer be obtained through control of `'i the admission mode ratio. Thus it is possible to realize load control that is essentially free from the generation of thermal stress, without the need for monitoring or supervision of ... .
thermal stress.
In summary, contrary to the teachings of the prior art wherein governing takes place either at full-arc or at partial-arc, the present system comtemplates continuous controll-ing between full- and partial-arc or any intermediate point during transient operation, in order to control the first stage temperature to minimize the thermal stress occurrence.
During constant load operation, control is gradually returned to the more efficient partial-arc admission.
The various functions indicated in Fig. 1 can be carried out by suitable hardware selected to car~y out the indicated functions, or the functions can also be programmed as instructions to a digital computer.
The embodiment employing suitable hardware will be described in conjunction with Fig. 1, and then a description of an example of a flow chart for a ditital computer will be given.

, , .-- . In Fig. 1, designated at 21 is a load demand determining means, to which a speed reference signal NR, a speed feed-back signal NF, a load reference slgnal LR, a load feed-back signal LF and a load change rate signal ~ are coupled to obtain a load demand signal Ld. The load demand signal Ld increases or decreases upon alteration of the load reference signal LR from LRl to LR2 depending upon the magnitude relationship between LRl and LR2, as given by ~ d Rl - yt ~ ~N (NR ~ NF) ::: 1 0 Of course, after LR2 is reached by the load it is d LR2 + ~N (NR _ NF) ..................... (2) where oN is the so-called speed regulation factor, i.e., a factor for converting the speed difference signal (NR - NF) into the corresponding load demand signal. In the instant embodiment, the speed feed-back signal NF and load feed-back :.
signal LF are derlved from the respective outputs of a speed detector and a first stage steam pressure detector, these detectors being schematically indicated at 22 and 23 respectively.
In the means 21, designated at 24, 25 and 26 are adders, at 28, 29 and 30 coefficient multipliers, at 31 a pattern generator, and at 32 a proportional integrated controller. The individual adderR receive their inputs with the illustrated polarities.
Indicated at Kl in the coefficient multiplier 28 is a coefficient for converting the pressure signal into a load signal. The ., pattern generator 31 has an integrating function and responds to changes of the load reference signal, that is it follows the changes of the load reference signal at a specified load change rate y.
Designated at 51 and 52 are respective valve opening determining means. The means 51 determines the openings of the control valves 7 to lO with respect to the load demand signal Ld in the full-arc admission mode, while means 52 -- similarly determines the openings of the control valves 7 to 10 in the partial-arc admission mode. Of course, all the control valves 7 to 10 are positioned at the same opening in the full-arc admission mode while in the partial-arc admission mode they are brought to the fully open position in sequence.
Here, the valve openings are arranged to vary as a linear ;jj function of the load demand signal Ld. This is done by so 10 arranging a servo-mechanism as to make up for non-linear characteristics of the valves, as is shown, for instance, in ` ISA Journal, September 1956, pages 323 through 329 "Control Valve Requirements for Gas Flow System". Designated at 61 ` and 62 are valve opening signal adjusting means which correct -the valve opening signals for the respective admission modes , provided by the respective valve opening determining means ~51 and 52 on the basis of ratio control signals a and ~ to be described hereinafter. Here, ~ and ~ are coefficients related to each other such that a + ~ = 1 (provided 0 < a ~ 1 and 0 < ~ < 1). More particularly, these signals are for causing the ratio between the steam flow into full-arc admission mode and that in the partial-arc admission mode to be a and ~ without changing the steam flow supplied to the turbine. The adjusting valve opening signals obtained from the respective valve opening signal adjusting means 61 and 62 are coupled to a valve opening control means 16, and hence are fed as predetermined positioning signals for each valve to the servo-mechanism 15.
Designated at 71 is a ratio control signal determining means for determining the steam flow ratio between the two admission modes. The load reference signal LR, load feed-back signal LF and load change rate signal ~, as well as a first ;, , :. , ~07~'735 stage temperature change rate signal ~ are coupled to this means 71 to produce the ratio control signals a and ~. The - way of determining the ratio control slgnals a and ~ will now be described with reference to Figs. 4a and 4b, which are characteristic graphs for explaining the translation of a and ;; ~ representing the admission mode ratio, when the load on the turbine is changed from Ll to L2.
In Fig. 4a, when the turbine is in steady operation ~nder load Ll the admission mode is that of partial-arc with the higher efficiency and corresponds to point A in the Figure.
At this ~ime, a and ~ determining the admission mode ratio are found at point A' in Pig. 4b. This means that ~1 = and ~1 = As the load reference signal LR is bhanged from Ll to L2 the steam flow is controlled in such a fashior. that both admission modes coexist, as shown at point B in Fig. 4a, whereby only the load is changed without c~using changes in the first stage temperature. At this time, a and ~ are found at point B' in Fig. 4b and are respectively a2 and ~2. Thereafter, only the admission mode ratio is controlled, without causing load changes to return eventually to the sole partial-arc mode.
As a res~lt,the operation is continued at point C in Fig. 4a and at point C' in Fig. 4b. Here, with the load change between points A and B (Fig. 4a) the admission mode is changed between points A' and B' (Fig. 4b). While in this case the temperature difference in the first state temperature between the two admission modes, as indicated by lines 46 and 48, distributes itself according to the steam flow ratio between the two admission modes, this relation is practically linear; by setting a: ~ = 0.5 : 0.5 the first stage temperature is found mid-way between the lines 46 and 48. Thus the admission mode ratio control signals a and ~ at the time of the load change in Fig. 4a ~071735 are calculated in the following manner.
Since the characteristics 46 and 48 can be regarded practically as straight lines, the first stage ~emperatures TF(LA) and Tp(LA) in the respective full-arc and partial-arc r - modes at a given load LA (%) are given as F(LA) = (TR TF0) loo + TF - - (3) and Tp(LA) = (TR - Tpo) 1OO + Tpo ------- (4) where TR is the first stage temperature under the rated load, TFo is the first stag~ temperature under no load at full-arc admission mode, and Tpo is the first stage temperature under no load at partial-arc admission mode.
Thus, when the turbine is under load Ll (%) and operated at point A, the first stage temperature is obtained as Tp(Ll) from equation (4). Immediately after the change of load from Ll (%) to L2 (%) the first-stage temperature is unchanged, and at this time ~2 and ~2 are as follows.

TP(L2) + a~ { TF(L2) - Tp(L2) } = Tp(Ll) Tp(Ll) - Tp(L2) .......... (5)

2 TF(L2) - T (L ) ~2 ~2 ~ (6) L2 here is obtained from the load reference signal and Ll from the load feed-back signal, so that the first stage temperature in each admission mode under each load is obtained , .. .
from equations (3) and (4) by using TR, Tpo and TFo which are stored as respective constant in the apparatus.
The rate of change of ~ and ~ for correcting the admission mode ratio from ~ = ~1 (= ) to ~ = ~2 in accordance with the load change rate signal ~ is next obtained. The increment A~ of the ratio control signal ~ between the points `~ ~3L07~735 r A and B is -~a = ~2 ~ al ---........................... (7) - The period ~T required for the load change from Ll to L2 i8 ~T = ¦Ll L2¦ ............................... (8) Thus, the rate of change (dt)l of the ratio control signal OL iS
. (dt)l ~T = ¦---Y__~ x (~2-al) .............. (9) ; 10 Consequently, where the control is made by means of special hardware, as is illustrated, the outputs a and ~ of the ratio control signal determining means are l (dt)l ............................. (10) and = .1 - a = l - al ~ (dt)l = ~ (da) t .............................. (11) where ~1 and ~1 are ratio control signals before the commencement of the load change, and t is the period elapsed from the start of the load change. When the control system is realized by a digital computer the control is not continuous but is carried out with a predetermined cycle. In this case, by denoting the control cyclc by T we have a =~ al + (dt)l- ......................... ~10)' these equations (10)' and (11)' corresponding to the respective equations (10) and (11).
It will be apppreciated that in this manner the ratio of steam flow between full-arc and partial-arc admissions is controlled to permit load control without causing changes . - 14 -~()7~735 , in the first stage temperature, thus permitting turbine load control without essentially causing the generation of thermal - stresses. Thus, when the load has to be quickly reduced, this can be effected without being essentially accompanied by thermal stress generation, even with a large load change rate signal.
` After the load has stabilized at L2, the ratio control signals a and ~ are controlled to return to point C' from point B' in Fig. 4b, i.e. point C from point B in Fig. 4a. At this : time it is necessary to detect the completion of the load change, and this is done by determining that the difference between the load reference signal LR and the load feed-back signal LF has been reduced to be within a predetermined range ~L, i.e. stated mathematically R F ¦~ ~L¦ ............................... (12) When this condition is met, the ratio control signals a and ~ are changed to commence transition into the partial-arc admission mode. The ratio control signals a and ~ are so changed that the first stage temperature change rate signal , preset by taking the thermal stress given to the turbine rotor into consideration, is not exceeded, whereby the time ~T' required for trans-ition from point B to ~oint C is give as ~.~T' = Tp(Ll) - Tp(L2) Hence ~T' = Tp(Ll) - Tp(L2) ........................ (13) Thus, the rate of change (ddt)2 of the ratio control signal a ... .
is T . (dt)2 = ~2 da ~2 ~'~2 ............. (14) ~ence (d )2 = ~ ' = ( ) Consequently, like equations (10) and (11~ or equations (10)' and (11)', the ratio control signals a and ~ for bringing ~07~735 .~ , . ..
about transition from point B to point C are ~2 (dt)2 t ........................ (15) ~ a ' 1 - a2 ~ (dt)2 i t .... (16) a = a2 + (dt)2 r ...................... (15)' ~ = 1 - a = 1 - a2 ~ (dt)2 T (16~ ~

When a < O, the ratio control signal ~ may be limited to a = O and ~ = 1, while when >1 it may be limited to a = 1 and ~ = 0. Also, since operation in the partial-arc admisRion mode under low load is liable to result ln local heating of the turbine, it is desirable to exclude this operational mode from the region on the left hand side of the dotted line 51' connecting points D and E in Fig. 4a, that is, to avoid the presence of the ratio control signals a and ~ in the region on the left hand side of the dotted line 52' connecting points D' and E' shown in Fig. 4b. If intrusion into this region is likely the ratio control signal a is desirably limited in the following way. Denoting the loads at the points D and E by LL2 and LLl respectively, the ratio control signal a is limited to aL, that is, = L2 LR = a ........................... (17) L - L L
if LLl ' LR < LL2, while limlting it to a = 1 if LR < LLl.

-~ -In other words, the ratio control signal determining means 71 is arranged such that it also calculates the limit in equation (17) in addition to shose in equations (10) and (11) or equations ; (15) and (16) so that these limited values of the ratio control - signal a may be selectively provided in accordance with the turbine operating conditions.

, ~` ~071735 . . .
The preceding embodiment presents no particular problem insofar as the turbine operation mode can be shifted ~ horizontally, i.e., in the direction parallel to the abscissa `~
in Fig. 4a, such as from point A to point B, when a load change is demanded. However, if it is inevitable to effect transition ;~ along the line 46, 48 or 51' for a load change, for instance when reducing the load from the rated load or reducing the load down to the region on the left hand side of the line 51' or increasing the load from the point C to the point A, generation of some thermal stress is inevitable. In view of this aspect, it is necessary to prepare optimum load change rate signals Yl - y for the individual cases and employ them as in Fig. 1 in accordance with the turbine operating conditions.
Fig. 5 is a schematic diagram similar to Fig. 1 but also showing a load change rate signal determining means 81 which receives the load reference signal LR, the load feed-back signal LF and the ratio control signal from the means 71 to determine the turbine operating condition through its logic circuitry, whereupon it selects and provides the one of prepared load change rate signals ~1 to y4 that corresponds to the operating condition. The load change rate signal Yl is prepared for the locus of the first stage temperature in the direction parallel to the abscissa in Fig. 4a with load change, the signal ~2 for the locus along the line 46, the signal y for the locus along the line 51', and the signal 'f4 for the locus along the line 48. Of course, it is possible to arrange that a separately prepared y value may be selected from the outside by ignoring the ~ value selected through the logic in Fig. 6 whereby to specify the desired ~ at any time.
A further embodiment of the invention, which is developed to include control in co-operation with the steam generator 4, will now be discussed. While the description .~

~o7~735 so far has been based upon the assumption that the steam temperature supplied by the steam generator 4 is constant, the steam temperature actually fluctuates due to various external disturbances. Although various means have been proposed for the control of the steam generator itself, some fluctuations inevitably take place in practice. Fig. 7 shows the characteristics involved in the problem presented in this case and a more sophisticated measure to cope with it by means of a further embodiment of the invention. In this graph, the abscissa ' represents the percent of rated load of the turbine and also the percent of rated steam temperature of the steam generator, taking the ordinate portion below the abscissa for time and that above ;' -~
the abscissa for the first stage temperature. The graph shows that varying the turbine load from 60% to 90% of rated load during a period from instant tl u~til instant t2 causes variation ' of the steam temperature of the steam generator within _ 5%
of the rated temperature TMSo as lndicated by line 92, thus varying the first stage temperature in the manner indicated by line 93. However, the variation shown by the line 93 is not desired, because thermal stress results from the temperature differences.
In one application the rated steam temperature of the steam generator in such case is temporarily reduced by ~ ~-; ~TR, as indicated at T'MSo, to cause variation of the steam temperature in the manner shown by line 92' and of the first stage temperature in the manner shown ~y line 93'. The ratio control signal ~ for the full-arc admission mode is c'orrected' to compensate for the temperature reduction to the values of line 93!, SO that the locus of th'e first stage temperature ~1 30 coincides with the line 48, thus permitting undesired thermal stress to be suppressed. Fig. 8 shows a schematic diagram `. 107~735 showing the essential parts for this purpoæe.
The construction shown in Fig. 8 is similar to that ~ of Fig. 1 except that the performance of the additional load change rate signal determining means 81 is improved so that it can produce a command for correcting the rated steam temperature with respect to the steam generator; also a ratio control signal adjusting means 72 is added. Here, ~TR are provided as changes in rated steam temperature, and this is so because, while in the previous example of a load increase a change of -~TR along the line 48 was required, in the converse case of a load reduction along the line 46 a change of +QTR
is required. Fig. 9 shows the logic construction required for the means 81 in this case. In the ratio control signal ad~ust-ing means 72, the outputs and ~ of the ratio control signal determining means 71 are coupled to respective adders 74 and 75 for adjustment to ' and ~', respectively, in the presence of a correction signal ~' which is calculated from the load demand signal Ld and output TMS of a steam temperature detector (not shown) provided at an output portion of the steam generator, by an equation ~a' = K2 TMSO MS x 100 .................... (18) T - T (100-Ld) Here, TFo and Tpo are those shown in Fig. 4a.
As has been shown, the invention can be carried into practice by the use of suitable hardware. However, since this requires a very complicated system~ it is preferable to employ a programmed digital computer and Figs. 10 and 11 show flow charts for such a program.
While the foregoing embodiments of the invention have ~07~73S
been concerned with a pro~ect power plant system, the invention can also be directly applied to private power generation - equipment connected to an independent load. Further, it is applicable not only to power generation equipment but also to mechanical drive steam turbines such as those for petroleum pipe line pumps and ships. Furthermore, while the above embodi-ments have each used four control valves, it is possible to use as few as two valves for carrying out the invention. Still further, while according to the foregoing procedure the first stage pressure Pl_st is detected as the turbine load and is converted thereto for use, it is also possible to use direct measurement of the generator load, although with some slight sacrifice in precision. As a further alternative, since the time constant of response to the turbine load is comparatively short, typically less than 10 seconds, it is possible to obtain sufficient effect by substituting the output of the pattern generator 31 for the load demand signal Ld for calculation in equation (18). Further, since an insensitivity band ~L is provided with respect to the difference between the turbine load LA and the load reference LR, by controlling the magnitude of this ~L value, a sensitivity adjustment through FA/PA
co-operation control is possible. For example, by setting the ~L to be greater than the governer free width there is no need to respond to turblne load fluctuations due to system frequency fluctuations. Further, the line 51' for limiting the admission mode under low load need not be a straight line between the two output levels LLl and LL2; it is possible to use a curved limiting line by taking the turbine efficiency and the extent of local heating into ~onsideration, while still achieving the effects of the invention and without altering the essential nature thereof. Further, although the first stage steam temperature characteristics are linearly approximated, as by lines 46 and 48, with respect to the turbine load LA, the - actual characteristics are non-linear; thus, if a FA/PA .:
co-operation control of high precision is required, the non-linear characteristics may be used in place of equations (5) and (6). Moreover, as the logic determining function for selectively setting the rate of load change, the sequence in the embodiment of Fig. 6 is not always necessary; it is only necessary to be able to obtain mode determination for the 10 1oCU8 traced by che flrst stage s~ea= tet~perature.

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Claims (8)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a steam turbine control system having a turbine and a plurality of valves operable to admit steam to a first stage of the turbine through nozzle arcs, the combination of:
(a) means for determining a load demand signal according to a speed reference signal, a speed feed-back signal, a load reference signal, a load feed-back signal, and a load change rate signal;
(b) means for determining a first valve opening signal under a full-arc admission mode according to said load demand signal;
(c) means for determining a second valve opening signal under a partial-arc admission mode according to said the load demand signal;
(d) means for determining first and second ratio control signals between steam flow under the full-arc admission mode and steam flow under the partial-arc admission mode according to the load reference signal, the load feed-back signal, the load change rate signal, and a first stage temperature change rate signal;
(e) means for adjusting said first valve opening signal according to said first ratio control signal;
(f) means for adjusting said second valve opening signal according to said second ratio control signal; and (g) load control means for positioning said valves to admit a desired total steam flow to said turbine according to the adjusted valve opening signals.

2. The combination according to claim 1 wherein said second ratio control signal is limited under a predetermined low turbine load.

3. The combination according to claim 1, further comprising means for determining said load change rate signal according to a load change manner of turbine.

4. The combination according to claim 3, further comprising means for adjusting a rated steam temperature of a steam generator furnishing steam to said turbine.

5. In a steam turbine control method for a turbine having a plurality of valves operable to admit steam to a first stage of the turbine through nozzle-arcs, the combination of:
(a) determining a load demand signal according to a speed reference signal, a speed feed-back signal, a load reference signal, a load feed-back signal, and a load change rate signal;
(b) determining a first valve opening signal under a full-arc admission mode according to said load demand signal;
(c) determining a second valve opening signal under a partial-arc admission mode according to said load demand signal;
(d) determining first and second ratio control signals between steam flow under the full-arc admission mode and steam flow under the partial-arc admission mode according to the load reference signal, the load feed-back signal; the load change rate signal, and a first stage temperature change rate signal;
(e) adjusting said first valve opening signal according to said first ratio control signal;
(f) adjusting said second valve opening signal according to said second ratio control signal; and (g) arranging to position said valves to admit a desired total steam flow to said turbine according to the adjusted valve opening signals.

6. The combination according to claim 5, wherein said second ratio control signal is limited under a predetermined low turbine load.

7. The combination according to claim 5, further comprising determining said load change rate signal according to a load change manner of the turbine.

8. The combination according to claim 7, further comprising adjusting a rated steam temperature of a steam generator furnishing steam to said turbine.