JP2008002337A - Steam turbine control device and steam turbine control method - Google Patents

Abstract

Provided is a steam turbine control device and a control method for automatically reducing a load and stabilizing a system even when a system frequency fluctuates.
A steam turbine control device includes a rotational speed or frequency deviation detecting means, a speed regulator, a load setter, an output signal of the speed regulator, and an added value of a load set value, and a predetermined value. In a steam turbine control device equipped with a low value priority circuit that selects and outputs any low value of the load limit value, and operates with the steam control valve fully open, a negative value is obtained when the deviation becomes negative. The hysteresis circuit that outputs a positive signal when the deviation becomes almost zero, the load setting value increasing / decreasing means that inputs a negative signal or a positive signal to the load setting device, and monitoring the load setting value, Means for stopping the load set value decreasing operation by the load set value increasing / decreasing means when the load set value is reduced to a value corresponding to the rated output of the steam turbine based on the negative signal output from the hysteresis circuit. .
[Selection] Figure 1

Description

  TECHNICAL FIELD The present invention relates to a steam turbine control device, and in particular, steam in a so-called valve wide open (abbreviation: VWO) operation system in which a steam control valve provided at an inlet of a steam turbine is operated in a fully opened state. The present invention relates to a turbine control device and a steam turbine control method.

  In general, the steam turbine includes a boiler 1, a high-pressure turbine 2, a reheater 3, an intermediate-pressure turbine 4, a low-pressure turbine 5, a condenser 6, and a generator 7, as shown in FIG. The steam is subjected to expansion work by the high-pressure turbine 2, the steam that has lost heat during the expansion work is re-superheated by the reheater 3, and this super-heated steam is re-expanded by the intermediate-pressure turbine 4 and the low-pressure turbine 5. The generator 7 is driven by the power generated at the time, and the turbine exhaust that has finished the expansion work is condensed by the condenser 6 to be condensed water, and after this condensate is purified, it is fed back to the boiler 1 as feed water. Yes.

  The steam turbine is provided with a main steam stop valve 9 and a steam control valve 10 in a main steam pipe 8 that connects the boiler 1 and the high-pressure turbine 2, and a reheat steam pipe 11 that connects the reheater 3 and the intermediate pressure turbine 4. The reheat steam stop valve 12 and the intercept valve 13 are provided. For example, when an accident occurs in the system, the steam control valve 10 and the intercept valve 13 are rapidly closed by a command from the control device 14, and the boiler 1 is switched to the high pressure turbine 2. The supplied steam is cut off to prevent overspeed of the turbines 2, 4, and 5.

  On the other hand, as shown in FIG. 6, the steam control valve 10 slides on the inner peripheral portion of the sleeve 16 in the valve casing 15 having the steam inlet 21 and the steam outlet 22, and advances and retreats by the driving force of the valve rod 17. A body 18, a drive device 19 that applies a driving force to the valve rod 17, and a valve seat 20 that abuts the valve body 18 and closes the steam in a watertight manner. The valve body 18 is opened and closed by the driving force 19 to control the flow rate of the steam.

  As shown in FIG. 7, the steam control valve 10 having such a configuration is a flow characteristic diagram in which the vertical axis represents the steam control valve flow rate and the horizontal axis represents the load setting (steam control valve opening command). While the load setting and the flow rate are maintained in a proportional relationship, the secondary pressure on the downstream side of the valve seat 20 increases and the proportional relationship (linear) does not hold when the valve opening becomes a high region. Since the region appears, the flow rate characteristic is generally a linear increase / decrease range, that is, a flow rate in the range of 90% to 95% of the maximum flow rate is set as the rated load of the steam turbine. In the example of FIG. 7, the 95% position is the steam turbine rated output (100% load), and the load setting is the 100% position.

  Further, when the flow rate is the maximum flow rate (valve fully opened), the load reaches 140%, but the 140% load at this time is handled as the maximum flow rate load.

  As described above, the control device 14 that gives the steam turbine rated load or maximum flow rate load to the steam control valve 10 includes a load setting unit 24 having a load setting increase / decrease circuit 23, and a load limit increase / decrease circuit, as shown in FIG. 25, and a low value priority circuit 27. The adder / subtractor 28 compares the rotation speed setting signal and the actual rotation speed signal, and the deviation generated in the comparison is used as the gain of the speed regulator 29. Multiplying by (1 / speed regulation rate), the valve opening signal is calculated, the load setting signal from the load setting device 24 is added to the calculated valve opening signal by the adder 30, and the low value is given priority to this addition signal. In the circuit 27, the set load signal from the load limiter 26 is matched, and one of the low value signals is given to the steam control valve 10 as a valve opening command signal, and the valve body 18 is opened and closed.

  In a large-scale power generation steam turbine equipped with such a control device 14, the speed regulation rate is generally set to 3% to 5%, and the actual rotational speed of the steam turbine rises by 5% or more compared to the set rotational speed. The valve opening degree of the steam control valve 10 was lowered by 100%. That is, the steam turbine generator load (output) was varied from the rated load to the no load.

  And since the load of the system | strain connected to the generator 7 was interrupted, if the system frequency rises, the control apparatus 14 will reduce the load of a steam turbine along a speed regulation rate, and will make a system | strain load regular (rated). I was trying to recover.

  For example, Patent Literature 1 is disclosed as a control technique for switching between load setting follow-up operation and load limit follow-up operation.

In addition, for example, Patent Document 2 proposes a control technique for preventing a system fault from occurring and preventing overspeed of the steam turbine.
JP-A-5-163903 JP-A-11-153003

  As described above, in the conventional steam turbine, the load when the flow rate of steam flowing through the steam control valve 10 is 90% to 95% of the maximum flow rate is set as the rated load of the steam turbine. Before the maximum flow rate load operation, for example, when an accident occurs in the transmission line system and the actual rotation speed exceeds the set rotation speed, a predetermined speed adjustment rate (gain), for example, 5% speed adjustment rate is obtained. Based on this, the rotational speed control for reducing the actual rotational speed was performed.

  However, even when the rotational speed control is performed under a predetermined speed regulation rate, the steam control valve 10 must narrow the valve opening, and the pressure loss is several percent with respect to the pressure of the inflowing steam. And the power plant efficiency was reduced.

  In recent large-scale steam turbines for power generation, it is required to operate at the maximum flow rate and maximum efficiency of the steam turbine in order to further reduce the pressure loss of the steam flowing through the steam control valve 10 and increase the power plant efficiency. Yes.

  For this reason, the power generation plant adopts a so-called valve wide open (VWO) operation mode, which is called maximum flow load operation in which the steam control valve 10 is kept fully open and the maximum flow rate of steam (equivalent to a load of 140%) flows. Is increasing. That is, the valve opening degree command (load setting) of the steam control valve 10 is operated at 140% or more.

  As described above, when the steam turbine is operated at the maximum efficiency and the maximum flow rate, as already shown in FIG. 7, when the operation is performed at the maximum flow rate load exceeding the rated load of the steam turbine, the load in the system is reduced. However, even if the system frequency rises, the amount of power generation cannot be reduced by an amount corresponding to a predetermined speed regulation rate, and operation is performed with a substantially constant load.

  That is, when viewed from the flow rate characteristic of the steam control valve 10 shown in FIG. 7, when the system frequency originally increases by 0.5%, the turbine load decreases by 10%. If the system frequency does not increase by 2.5%, the load will not decrease by 10%.

  When such rotational speed control is performed, the transmission line system becomes extremely unstable, making it easy to cause disturbance of the system, and inducing chained power outages, so-called blackouts over a wide area in the system. It becomes a factor to generate.

  The present invention has been made based on such circumstances, and in a steam turbine employing a valve wide open (maximum flow rate, maximum efficiency) operation method, even if the system frequency fluctuates for some reason, An object of the present invention is to provide a steam turbine control device and a steam turbine control method that automatically reduce the load (steam turbine generator output) and stabilize the system.

  In order to achieve the above object, a steam turbine control device according to the present invention includes a deviation detecting means for detecting a deviation in rotational speed or frequency, and a speed regulator that multiplies the deviation by a predetermined speed regulator and outputs the result. A steam setting valve that selects a low value of an output signal of the speed regulator and an addition value of the load setting value and a predetermined load limit value; A low-value priority circuit that outputs as an opening degree command of the steam turbine control device that is operated with the steam control valve fully opened, and a negative signal when the deviation becomes a predetermined negative value A hysteresis circuit that outputs a positive signal when the deviation becomes almost zero, and a load setting device by inputting a negative signal or a positive signal output from the hysteresis circuit to the load setting device. Load setting value increasing / decreasing means for performing a small operation or an increasing operation and a load setting value output from the load setting device are monitored, and the load setting value is determined based on a negative signal output from the hysteresis circuit. Means for stopping the load set value decreasing operation by the load set value increasing / decreasing means when the value falls to a value corresponding to the rated output.

  Further, in order to achieve the above object, the steam turbine control method according to the present invention adds a load set value after multiplying the rotation speed or frequency deviation by a predetermined speed regulation rate, and this added value, In a steam turbine control method in which a low value of a predetermined load limit value is selected and output as a steam control valve opening command, and the steam control valve is operated in a fully open state, the deviation is determined in advance. When the specified negative value is reached, a negative signal is output to reduce the load set value from the fully open position of the steam control valve, and the load set value corresponds to the rated output of the steam turbine from the fully open position of the steam control valve When the value is reduced to the value to be stopped, the reduction operation is stopped, and when the deviation returns to almost zero, the load set value is increased from the value corresponding to the rated output of the steam turbine to the fully opened state position of the steam control valve. It is.

  The steam turbine control device and the steam turbine control method according to the present invention automatically and freely outputs a load setting signal from a load setting device when a positive / negative deviation occurs between the rotation speed setting signal and the actual rotation speed signal. It is equipped with a hysteresis circuit that increases / decreases the load and means for increasing / decreasing the load set value, so that even if the system frequency fluctuates during valve wide open operation from the rated load of the steam turbine to the maximum flow rate load, ) The frequency can be recovered to stabilize the system, and the maximum flow rate and maximum efficiency of the steam turbine can be effectively operated.

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments of a steam turbine control device and a steam turbine control method according to the present invention will be described with reference to the drawings and reference numerals attached to the drawings.

(First embodiment)
FIG. 1 is a control block diagram showing a first embodiment of a steam turbine control device and a steam turbine control method according to the present invention.

  The steam turbine control device according to the present embodiment is intended for a steam turbine that is operated with the steam control valve fully opened, and an adder / subtractor 31 that matches the rotational speed setting signal and the actual rotational speed signal, A load setting unit 35 having a load setting increase / decrease circuit 34 connected to the adder / subtractor 31 through a hysteresis circuit 32 and a switching circuit 33, and an output of the load setting unit 35 corresponding to the rated load of the steam turbine (90 to 95%) It is configured to include a comparator 36 that cuts off energization when it is below.

  Furthermore, the hysteresis circuit 32 outputs a negative signal when the deviation signal from the adder / subtractor 31 is, for example, −0.5% or less of a predetermined first set value, and the deviation signal is predetermined. When the obtained second set value becomes, for example, zero or more, the circuit outputs a positive signal.

  On the other hand, when the switching circuit 33 energizes the negative signal or the positive signal from the hysteresis circuit 32 and the load setting signal is 90 to 95% or more, the switching circuit 33 receives the instruction from the comparator 36 and receives the hysteresis circuit 32. It is a circuit that functions to cut the negative signal from.

  In addition, the steam turbine control device according to the present embodiment multiplies the deviation signal from the adder / subtractor 31 by a gain (1 / speed regulation rate) to calculate a valve opening command signal, and a speed regulation rate device (gain device) 37. And an adder 38 for adding the load setting signal from the load setting device 35 to the valve opening command signal from the speed regulator 37, and a load including an addition signal from the adder 38 and a load limit increase / decrease circuit 39. A low value priority circuit 41 is provided that matches the load limit signal from the limiter 40, selects one of the low value signals, and gives it to the steam control valve as a valve opening command signal.

  Next, a steam turbine control method based on the above configuration will be described.

  When the steam turbine is performing the maximum flow rate load operation, the output of the load setting device 35 is, for example, 140% or more, and the steam control valve opening degree command is also outputting 140% or more. And the output of the load limiter 40 is set more than it so that the output of the load setting device 35 may not be disturbed.

  At this time, for example, due to the removal of an accident that occurred in the transmission line system, the system frequency becomes higher than the rated frequency, and the negative deviation signal output from the adder / subtractor 31 is set by the hysteresis circuit 32. When approaching 5%, the load setting signal (steam control valve opening signal) is from the rated load of the steam turbine to the maximum flow load (140% load with the steam control valve fully open) as shown in FIG. Meanwhile, since the curve is a gentle curve flow rate characteristic, the flow rate of the steam flowing through the steam control valve hardly changes. For this reason, the load of the steam turbine hardly changes.

  However, if the operation is below the rated load of the steam turbine, the flow rate characteristic diagram is linear as shown in FIG. 7, and therefore the deviation signal from the adder / subtractor 31 is set by the hysteresis circuit 32. When approaching 0.5%, the load setting signal decreases by (speed regulation rate / 0.5%), that is, 10% when the speed regulation rate is 5%.

  When the system frequency further increases and the deviation signal between the rotational speed setting signal and the actual rotational speed signal exceeds −0.5% by the adder / subtractor 31, the hysteresis circuit 32 outputs a negative signal and passes through the switching circuit 33. Then, the “decrease” signal of the load setting increase / decrease circuit 34 is activated, and the load setter 35 operates in the direction of the load “decrease” in accordance with the activation of this “decrease” signal.

  When the load setting device 35 operates in the direction of “load reduction”, the steam control valve opening command continues to decrease, but when the output signal of the load setting device 35 decreases to a value corresponding to the rated load of the steam turbine, the comparison is made. The device 36 is activated, and the output of the hysteresis circuit 32 is cut off from being input to the load setting increase / decrease circuit 34 via the switching circuit 33.

  As a result, the steam control valve opening command shifts from the maximum flow rate load of 140% to the rated load of the steam turbine, the flow rate of steam flowing through the steam control valve decreases by 10%, and the load of the steam turbine also decreases by 10%.

  In this state, even if the system frequency further increases slightly, the hysteresis circuit 32 maintains a negative value and thus affects the load setting value output from the load setting device 35. Absent.

  Next, when the system frequency is restored to the normal (rated) frequency, when the deviation between the rotational speed setting signal and the actual rotational speed signal becomes a value near zero in the adder / subtractor 31, the output of the hysteresis circuit 32 is positive. And the “increase” signal of the load setting increase / decrease circuit 34 is activated via the switching circuit 33, and the load setter 35 is activated in the direction of the load “increase” in response to the activation of this “increase” signal, and the steam adjustment Fully open the valve and bring the maximum flow load to 140%.

  As described above, in the steam turbine control device and the steam turbine control method according to the present embodiment, the adder / subtracter 31 matches the rotation speed setting signal with the actual rotation speed signal, and a negative or positive (including zero) deviation is obtained. And a hysteresis circuit 32 and a switching circuit 33 for freely outputting either a load “decrease” signal or a load “increase” signal from the load setting device to the steam control valve. For example, when it fluctuates by 0.5% or more, the steam control valve opening command value increases or decreases, and can be automatically lowered to a load determined by a speed regulation rate that should be originally operated as a steam turbine. It can contribute to stabilization of.

  In addition, when the system frequency returns to normal (rated), the opening of the steam control valve can be automatically shifted to the fully open state, and the maximum flow rate and maximum efficiency of the steam turbine can be easily operated. .

  The setting value 0.5% of the hysteresis circuit 32 and the setting value 95% of the comparator 36 quoted in the above description are values determined by the flow characteristics of the steam control valve and the operation of the turbine generator unit. It is not limited to values.

  In the above description, the deviation between the rotation speed setting signal and the actual rotation speed signal is used as the input value of the hysteresis circuit 32. However, a deviation between the actual frequency and the normal (rated) frequency of the power system may be used.

  Further, the fully open position of the steam control valve is set to 140% of the load setting device 35 and the steam control valve opening command. However, the fully open position of the steam control valve may be set to 100%, and each set value may be changed by the ratio. .

(Second Embodiment)
FIG. 2 is a control block diagram showing a second embodiment of the steam turbine control device and the steam turbine control method according to the present invention.

  In addition, the same code | symbol is attached | subjected to the same component as the component of 1st Embodiment, and duplication description is abbreviate | omitted.

  The steam turbine control device and the steam turbine control method according to the present embodiment are connected to the output side of the switching circuit 33, and a lower limiter whose lower limit is 0%, for example, and an upper limiter whose upper limit is 45%, for example. And an adder / subtractor 43 that calculates an output signal from the integrator 42 and a load setting signal from the load setting unit 35 and outputs a load setting increase / decrease signal. .

  In the steam turbine control device having such a configuration, when the system frequency increases and exceeds 0.5% of the normal (rated), for example, when the hysteresis circuit 32 outputs a positive signal, the integrator 42 is preset. A decrease signal is output at a predetermined rate, and the adder / subtractor 43 subtracts the load setting signal from the load setting unit 35 and gradually decreases it. The load setting signal becomes a load setting value corresponding to the rated load of the steam turbine. At this time, the energization of the integrator 42 is cut by the comparator 36, and accordingly, the rated load of the steam turbine is maintained based on the upper limit value from the upper limiter of the integrator 42.

  When the load setting signal is maintained at the rated load of the steam turbine, the steam turbine load is reduced by 10% based on the steam flow characteristic diagram shown in FIG. 7, and the frequency is set to normal (rated).

  Next, when the system frequency is restored to the normal frequency, the hysteresis circuit 32 outputs a positive signal, and inputs the positive signal to the integrator 42 via the switching circuit 33. Then, the integrator 42 gradually lowers toward the value of the upper limiter, and during this time, the steam control valve is fully opened.

  As described above, the steam turbine control device according to the present embodiment is provided with the integrator 42 with the upper and lower limiters on the outlet side of the switching circuit 33 and the adder / subtractor 43 for adding / subtracting the load setting signal from the load setting unit 35. Therefore, the system can be stabilized by slowly increasing or decreasing the load from the maximum flow rate load to the rated load of the steam turbine, and the load fluctuation speed of the steam turbine can be adjusted to a fluctuation speed suitable for plant operation. it can.

(Third embodiment)
FIG. 3 is a control block diagram showing a third embodiment of the steam turbine control device and the steam turbine control method according to the present invention.

  In addition, the same code | symbol is attached | subjected to the same component as the component of 1st Embodiment, and duplication description is abbreviate | omitted.

  The steam turbine control device and the steam turbine control method according to the present embodiment include an integrator 42 with upper and lower limiters on the exit side of the switching circuit 33 and an adder / subtractor 43 that increases or decreases the load setting signal from the load setting device 35. And a function that connects to the inlet side of the integrator 42 and makes the load change rate constant by using the steam control valve opening command signal output from the low value priority circuit 41 (reverses the non-linear part of the steam control valve flow characteristic). A function unit 44 having a function) is provided.

  That is, the steam turbine control device and the steam turbine control method according to the present embodiment are configured so as to adjust the steam during the period from the rated load of the steam turbine to the maximum flow load as shown in the steam control valve flow characteristic diagram shown in FIG. The flow characteristic of the steam flowing through the valve is a gentle convex curve line, and in particular, as the steam turbine approaches the rated load position and the maximum flow load position of the steam turbine, the so-called non-linear flow characteristic becomes a sleeping state. Therefore, the valve opening degree of the steam control valve is operated earlier by the large integration constant of the integrator 42 in the vicinity of the rated load position of the steam turbine and in the vicinity of the maximum flow rate load position, and in the vicinity of the rated load position of the steam turbine. The function unit 44 having a non-linear function characteristic for slowly operating the valve opening degree of the steam control valve with the small integration constant of the integrator 42 is provided.

  As described above, the steam turbine control device and the steam turbine control method according to the present embodiment operate the valve opening degree of the steam control valve earlier with the large integration constant of the integrator 42 in the vicinity of the maximum flow rate load position. A function unit 44 having a nonlinear function characteristic that slowly operates the valve opening degree of the steam control valve with a small integration constant of the integrator 42 in the vicinity of the rated load position is provided, and the flow rate characteristic of the steam flowing through the steam control valve is made linear. The load of the steam turbine can be increased / decreased under a constant load increase / decrease fluctuation rate, and disturbance to the system can be prevented.

(Fourth embodiment)
FIG. 4 is a control block diagram showing a fourth embodiment of the steam turbine control device and the steam turbine control method according to the present invention.

  In addition, the same code | symbol is attached | subjected to the same component as the component of 1st Embodiment, and duplication description is abbreviate | omitted.

  In the steam turbine control device and the steam turbine control method according to the present embodiment, the set value change that can freely change each of the set value incorporated in the hysteresis circuit 32 and the set value incorporated in the comparator 36. A device 45 is provided.

  Thus, since the steam turbine control device and the steam turbine control method according to the present embodiment include the set value changing device 45 that can change the set value in the hysteresis circuit 32 and the set value in the comparator 36, the plant operation is performed. Even in the middle, it is possible to perform an appropriate operation according to the fluctuation of the power system or the output of the steam turbine.

1 is a control block diagram showing a first embodiment of a steam turbine control device and a steam turbine control method according to the present invention. The control block diagram which shows 2nd Embodiment of the steam turbine control apparatus and steam turbine control method which concern on this invention. The control block diagram which shows 3rd Embodiment of the steam turbine control apparatus and steam turbine control method which concern on this invention. The control block diagram which shows 4th Embodiment of the steam turbine control apparatus and steam turbine control method which concern on this invention. The schematic system diagram which shows the conventional steam turbine. The conceptual diagram which shows the conventional steam control valve. The flow rate characteristic diagram which shows the flow volume of the steam which flows through the conventional steam control valve. The control block diagram which shows the conventional steam turbine control apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Boiler 2 High pressure turbine 3 Reheater 4 Medium pressure turbine 5 Low pressure turbine 6 Condenser 7 Generator 8 Main steam pipe 9 Main steam stop valve 10 Steam control valve 11 Reheat steam pipe 12 Reheat steam stop valve 13 Intercept valve 14 Control device 15 Valve casing 16 Sleeve 17 Valve rod 18 Valve body 19 Drive device 20 Valve seat 21 Steam inlet 22 Steam outlet 23 Load setting increase / decrease circuit 24 Load setter 25 Load limit increase / decrease circuit 26 Load controller 27 Low value priority circuit 28 Adder / Subtractor 29 Speed adjustor 30 Adder 31 Adder / Subtractor 32 Hysteresis circuit 33 Switching circuit 34 Load setting increase / decrease circuit 35 Load setter 36 Comparator 37 Speed adjustment adjuster 38 Adder 39 Load limit increase / decrease circuit 40 Load controller 41 Low Value priority circuit 42 integrator 43 adder / subtractor 44 function unit 45 set value changing device

Claims (7)

Deviation detection means for detecting the deviation of the rotational speed or frequency,
A speed regulator that outputs the deviation multiplied by a predetermined speed regulation rate;
A load setting device for outputting the load setting value;
An added value of the output signal of the speed regulator and the load set value, and a low value priority circuit for selecting a low value of a predetermined load limit value and outputting as an opening command of the steam control valve A steam turbine control device that is operated with the steam control valve fully opened,
A hysteresis circuit that outputs a negative signal when the deviation becomes a predetermined negative value, and outputs a positive signal when the deviation becomes substantially zero;
Load setting value increase / decrease means for performing load setting decreasing operation or increasing operation by inputting a negative signal or a positive signal output from the hysteresis circuit to the load setting device,
The load setting value output from the load setting device is monitored, and when the load setting value decreases to a value corresponding to the rated output of the steam turbine based on a negative signal output from the hysteresis circuit, the load setting value is Means for stopping the load set value decrease operation by the value increase / decrease means;
A steam turbine control device comprising: 2. The steam turbine control device according to claim 1, wherein the load set value increase / decrease means comprises an integrator that slows the increase / decrease movement of the load from the rated output of the steam turbine to the maximum flow rate load operation. The steam turbine control device according to claim 2, wherein the integrator includes an upper limiter and an adjustment limiter. 2. The steam turbine control device according to claim 1, wherein the integrator includes a function unit having a function for making a load change rate constant between a rated output of the steam turbine and a maximum flow rate load operation. The steam turbine control device according to claim 1, wherein the hysteresis circuit includes a set value changing device that freely changes the set value. The steam turbine control device according to claim 1, wherein the comparator includes a set value changing device that freely changes the set value. After multiplying the rotation speed or frequency deviation by a predetermined speed adjustment rate, add the load set value, and select the low value of either this added value or a predetermined load limit value to open the steam control valve. In the steam turbine control method, which is operated with the steam control valve fully open,
When the deviation becomes a predetermined negative value, a negative signal is output to reduce the load set value from the fully open state position of the steam control valve,
When the load set value is decreased from the fully open position of the steam control valve to a value corresponding to the rated output of the steam turbine, the decrease operation is stopped,
A steam turbine control method, comprising: increasing the load set value from a value corresponding to a rated output of the steam turbine to a fully open state position of the steam control valve when the deviation returns to substantially zero.

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