Plant control apparatus and plant control method

Abstract

According to one embodiment, a plant control device controls a plant comprising: a gas turbine driven by gas from a burner that ignites fuel; a heat recovery boiler that uses heat from the exhaust gas of the gas turbine to generate steam; a steam turbine driven by steam; and a clutch that engages the first and second shafts when the rotational speed of a first shaft connected to the gas turbine catches up with the rotational speed of a second shaft connected to the steam turbine. The aforementioned device, when shutting down the plant, stops the steam turbine, causing the rotational speed of the second shaft to decrease. Then, combustion continues, causing the rotational speed of the first shaft to decrease. When the rotational speed of the first shaft decreases to a first rotational speed, fuel is cut off to stop the gas turbine. At this time, the clutch engages so that the rotational speed of the first shaft catches up with the rotational speed of the second shaft at a second rotational speed lower than the first rotational speed, thus stopping the gas turbine.

Description

[Technical Field]

[0001] The embodiments disclosed herein relate to plant control devices and plant control methods. [Previous Technology]

[0002] Combined cycle (C / C) power plants, consisting of a combined burner, a gas turbine, a waste heat recovery boiler, and a steam turbine, are well-known. The burner burns fuel and exhausts combustion gases, while the gas turbine is driven by these combustion gases. The waste heat recovery boiler recovers heat from the exhaust gases of the gas turbine to generate steam, and the steam turbine is driven by the steam generated by the waste heat recovery boiler. [Summary of the Invention]

[0003] In a rigidly coupled C / C power plant, the gas turbine and the steam turbine are connected to the same rotating shaft. On the other hand, in a clutch-connected C / C power plant, the gas turbine is connected to a first rotating shaft, and the steam turbine is connected to a second rotating shaft. The first rotating shaft is engaged or disengaged from the second rotating shaft using a clutch. Specifically, the first and second rotating shafts are engaged via clutch engagement, and disengaged via clutch disengagement. Furthermore, generally, the generator and the gas turbine are connected together to the first rotating shaft.

[0004] The shutdown method for a clutch-engaged C / C power plant differs from that for a rigidly engaged C / C power plant. Generally, when shutting down a clutch-engaged C / C power plant, the steam turbine begins to rotate downwards after the clutch disengages, followed by the gas turbine. In this case, because the gas turbine rotates downwards faster than the steam turbine, the gas turbine speed quickly catches up with the steam turbine speed, and the clutch re-engages.

[0005] When the clutch is engaged, the gas turbine and steam turbine systems, already in a engaged state, experience a decrease in rotational speed. This decrease in speed is hindered, resulting in a slower, more gradual decrease. Consequently, the time it takes for the gas turbine speed to decrease to the point where the combustor's fuel is cut off becomes longer, delaying the timing of fuel cut-off. As a result, the combustor system maintains flame for an extended period before fuel is cut off, consuming unnecessary fuel.

[0006] According to one embodiment, a plant control device controls a power plant, the power plant comprising: a burner for burning fuel; a gas turbine driven by combustion gases from the burner; a heat recovery boiler for generating steam using heat from exhaust gases from the gas turbine; a steam turbine driven by the steam from the heat recovery boiler; a first shaft connected to the gas turbine; a second shaft connected to the steam turbine; and a clutch for engaging the first shaft and the second shaft when the rotational speed of the first shaft catches up with the rotational speed of the second shaft. The device comprises: a first shutdown control unit that, when shutting down the power plant, shuts down the steam turbine and causes the rotational speed of the second shaft to decrease from its rated speed, and after the steam turbine stops, continues combustion in the burner while causing the rotational speed of the first shaft to decrease from its rated speed. The aforementioned device further includes: a second shutdown control unit, which shuts off the fuel in the aforementioned burner and stops the aforementioned gas turbine when the speed of the aforementioned first shaft drops to the first speed. The second shutdown control unit is configured to engage the aforementioned clutch to stop the aforementioned gas turbine so that the speed of the aforementioned first shaft catches up with the speed of the aforementioned second shaft at a second speed below the aforementioned first speed. [Simplified Explanation of the Diagram]

[0041] [Figure 1] is a schematic diagram showing the structure of the power plant according to the first embodiment.

[0042] [Figure 2] is a flowchart showing the plant shutdown method of the first embodiment.

[0043] [Figure 3] is a trend graph used to illustrate the plant shutdown method of the first embodiment.

[0044] [Figure 4] is a trend graph used to illustrate the plant shutdown method of the second embodiment.

[0045] [Figure 5] is a trend graph used to illustrate the plant shutdown method of the third embodiment.

[0046] [Figure 6] is a schematic diagram showing the structure of a power plant for a comparative example.

[0047] [Figure 7] is a flowchart showing the plant shutdown method of the comparative example.

[0048] [Figure 8] is a trend graph used to illustrate the plant shutdown method of the comparative example.

Implementation Method

[0049] The embodiments will now be described with reference to the accompanying drawings. In Figures 1 to 8, the same element symbols are given to the same or similar configurations, and repeated descriptions are omitted. (Comparative Example)

[0050] Hereinafter, the plant configuration or plant shutdown method of the comparative example will be described. The plant configuration of this comparative example is equivalent to a clutch-engaged type, and the plant shutdown method of this comparative example is equivalent to applying a rigid-engaged plant shutdown method to a clutch-engaged type. In the rigid-engaged type, the gas turbine and the steam turbine are connected to the same rotating shaft without a clutch. (1) Factory structure of the comparative example

[0051] Figure 6 is a schematic diagram showing the structure of a power plant 100 in the comparative example.

[0052] The power plant 100 in Figure 6 is equipped with a plant control device 101 for controlling the operation of the power plant 100, and further includes: a gas turbine (GT) 102, a steam turbine (ST) 103, a heat recovery boiler 104, an MCV valve (increase / decrease valve) 105, a fuel regulating valve 106, a compressor 107, a burner 108, an evaporator 109, a furnace drum 110, a superheater 111, a turbine bypass regulating valve 112, a condenser 113, a circulating water pump 114, a seawater intake and discharge section 115, a fuel supply section 116, a generator 117, a first rotating shaft 118, a second rotating shaft 119, a clutch 120, a first detection gear 121, a second detection gear 122, a steam pipe 123, a main pipe 124, a bypass pipe 125, a turbine exhaust pipe 126, and a generator circuit breaker 127. The generator circuit breaker 127 is connected to the system grid 129 via transmission line 128. The power plant 100 in Figure 6 goes a step further, equipped with: a flame detector FD1, a GT speed detector SP1, an ST speed detector SP2, and a MW transducer MW-Tr. This power plant 100 is a "clutch-engaged single-shaft compound cycle power plant" in which the gas turbine 102 and the steam turbine 103 are coupled via the engagement of a clutch 120.

[0053] Initially, the clutch 120 will be described. The actual construction of the clutch 120 is complex, and Figure 6 illustrates it in a stylized manner. This clutch 120 is generally referred to as an SSS clutch (Synchro-Self-Shifting), which can engage or disengage the first rotating shaft 118 connected to the gas turbine 102 and the generator 117, and the second rotating shaft 119 connected to the steam turbine 103. In the former case, the clutch 120 is in an engaged state; in the latter case, the clutch 120 is in a disengaged state. These engagements or disengagements are not caused by the plant control device 101. As the SSS clutch is named, if the rotational speed of the second rotating shaft 119 reaches the rotational speed of the first rotating shaft 118 (synchronous), the clutch 120 will automatically engage due to centrifugal force. If the rotational speed of the second rotating shaft 119 drops to a speed slower than that of the first rotating shaft 118, the clutch 120 will automatically disengage.

[0054] When the power plant 100 is started up or shut down, the clutch 120 disengages, resulting in an operating condition where the rotational speed of the first rotating shaft 118 differs from that of the second rotating shaft 119. Here, a first detection gear 121 is provided on the first rotating shaft 118, and a GT speed detector SP1 is installed near the first detection gear 121. The GT speed detector SP1 detects the rotational speed of the first rotating shaft 118 (the rotational speed of the gas turbine 102 and the generator 117) and outputs the detection result to the power plant control device 101. Furthermore, a second detection gear 122 is provided on the second rotating shaft 119, and an ST speed detector SP2 is installed near the second detection gear 122. The ST speed detector SP2 detects the rotational speed of the second rotating shaft 119 (the rotational speed of the steam turbine 103) and outputs the detection result to the power plant control device 101.

[0055] After the power plant 100 completes its startup and enters normal operating condition, the clutch 120 engages, and the GT speed detector SP1 and ST speed detector SP2 detect the same speed. In this situation, because the gas turbine 102 and the steam turbine 103 are essentially connected to a single rotating shaft, the generator 117 is driven to generate electricity via both the gas turbine 102 and the steam turbine 103.

[0056] The fuel regulating valve 106 is located on the fuel piping. When the fuel regulating valve 106 is opened, fuel 116 is supplied from the fuel piping to the burner 108. The compressor 107 introduces air through its inlet and supplies compressed air to the burner 108. The burner 108 causes the fuel 116 to burn together with the oxygen in the compressed air, producing high-temperature, high-pressure combustion gases. The flame detector FD1 detects the flame inside the burner 108 and outputs the flame detection result to the plant control device 101.

[0057] The gas turbine 102 is driven by combustion gases from the combustor 108, thereby rotating the first rotating shaft 118. A generator 117 is connected to the first rotating shaft 118 and generates electricity using the rotation of the first rotating shaft 118. The electricity generated by the generator 117 is sent to a transmission line 128 equipped with a generator circuit breaker 127 or a megawatt sensor MW-Tr, and then transmitted to the system grid 129 via the transmission line 128. At this time, the megawatt sensor MW-Tr measures the power output of the generator 117 and sends the measured power to the plant control unit 101.

[0058] The gas turbine exhaust gas A1 discharged from the gas turbine 102 is sent to the heat recovery boiler 104. The heat recovery boiler 104 uses the heat of the gas turbine exhaust gas A1 to generate main steam A2. The evaporator 109, furnace 110, and superheater 111 are installed inside the heat recovery boiler 104 and constitute part of the heat recovery boiler 104. Water in the furnace 110 is sent to the evaporator 109, where it is heated by the gas turbine exhaust gas A1 to become saturated steam. The saturated steam is sent to the superheater 111, where it is superheated by the gas turbine exhaust gas A1 to become superheated steam. The superheated steam generated by the heat recovery boiler 104 is discharged as main steam A2 to the steam piping 123.

[0059] Steam piping 123 branches off at the main piping 124 and the bypass piping 125. The main piping 124 is connected to the steam turbine 103, and the bypass piping 125 is connected to the condenser 113. The MCV valve 105 is located on the main piping 124. The turbine bypass regulating valve 112 is located on the bypass piping 125.

[0060] When the MCV valve 105 is opened, the main steam A2 from the main piping 124 is supplied to the steam turbine 103. The steam turbine 103 is driven by the main steam A2, thereby rotating the second rotating shaft 119. With the clutch 120 engaged, the generator 117 is driven by both the gas turbine 102 and the steam turbine 103 because the steam turbine 103 rotates together with the gas turbine 102 on one rotating shaft. The main steam A2 driving the steam turbine 103 is discharged from the steam turbine 103 as exhaust steam and sent to the condenser 113 through the turbine exhaust pipe 126.

[0061] On the other hand, if the turbine bypass regulating valve 112 is opened, the main steam A2 from the bypass pipe 125 is sent to the condenser 113 via the steam turbine 103. The main steam A2 sent to the condenser 113 or the aforementioned exhaust steam is cooled and condensed by seawater 115 supplied by the circulating water pump 114.

[0062] The plant control device 101 controls the start-up and shutdown of the power plant 100, and various other operations of the power plant 100. For example, the plant control device 101 controls the fuel regulating valve 106, the MCV valve 105, and the turbine bypass regulating valve 112 to operate the gas turbine 102, the steam turbine 103, or the heat recovery boiler 104. The shutdown method of the power plant 100 in this comparative example will be described below. (2) Factory shutdown method in the comparative example

[0063] Figure 7 is a flowchart showing the plant shutdown method of the comparative example.

[0064] This process is implemented by software housed within the plant control device 101. Furthermore, the specific values ​​used in the following description are merely examples for ease of explanation and understanding. Also, please refer to the trend graph in Figure 8, which will be described later, for easier understanding.

[0065] Before the shutdown of power plant 100 (plant shutdown) is initiated, power plant 100 is in normal operation and clutch 120 is engaged. In normal operation, gas turbine 102 operates at a rated output of 200MW, and steam turbine 103 also operates at a rated output of 100MW, with the two turbines operating at a total output of 300MW (hereinafter referred to as combined output).

[0066] When the plant shutdown begins (step S100), the opening degree of the fuel regulating valve 106 is reduced at a constant rate to decrease the output of the gas turbine (GT) (step S101), and the combined output is gradually reduced from 300MW. During plant shutdown, the focus is on minimizing the load on the gas turbine 102 and steam turbine 103. As in step S101, by reducing the opening degree of the fuel regulating valve 106 at a constant rate, the amount of fuel 116 is gradually reduced. As a result, for example, the temperature of the gas turbine exhaust gas A1 also gradually decreases, and the thermal stress generated when shutting down the gas turbine 102 is also reduced. Because the heat energy retained in the gas turbine exhaust gas A1 decreases, the heat (temperature, pressure, flow rate) of the main steam A2 also decreases, so the output of the steam turbine (ST) is reduced (step S102), and its power generation is gradually reduced from 100MW.

[0067] The decrease in gas turbine output is interrupted when the combined output detected by the megawatt sensor MW-Tr drops to 60MW (step S104 "Yes") (step S105). Furthermore, the reduction of the opening degree of the fuel regulating valve 106 is also interrupted, and the gas turbine 102 maintains a constant output state. At this time, the details of the 60MW combined output are as follows: That is, the gas turbine 102 maintains 25MW, and the steam turbine 103 maintains 35MW, generating a total of 60MW of electricity.

[0068] However, this is a general characteristic of a combined cycle power plant. It is observed that, under low output conditions, the steam turbine output is greater than the gas turbine output in terms of thermal equilibrium. This is because, even at low output, the gas turbine 102 supplies a considerable amount of fuel 116 for power generation and also for driving the compressor 107. Therefore, the output of the steam turbine 103 increases because the gas turbine exhaust gas A1 generates high-heat main steam A2.

[0069] While maintaining the gas turbine 102 at a constant output of 25MW, the steam turbine 103 is shut down (step S106), resulting in a decrease in the power generation of the steam turbine 103 from 35MW. During this shutdown operation, the opening degree of the MCV valve 105 is gradually reduced at a constant rate, causing the output of the steam turbine 103 to decrease until the MCV valve 105 is fully closed. When the MCV valve 105 is fully closed, the steam turbine 103 is shut down, and because its power generation is zero, the combined output is only 25MW, the output of the gas turbine. At this time, because the MCV valve 105 is closed, the remaining main steam A2 is allowed to escape through the turbine bypass valve 112 into the condenser 113, and the pressure in the furnace 110 is maintained at a constant level.

[0070] In step S105, prior to step S106, the gas turbine output is maintained at 25MW. However, maintaining the output at 25MW is necessary from the following perspectives: i) If the gas turbine output drops below 25MW, the temperature of the gas turbine exhaust gas A1 becomes too low. As a result, cold main steam A2 flows into the steam turbine 103, generating thermal stress and hindering the operation of the steam turbine 103. ii) Conversely, if the gas turbine output is maintained above 25MW and the steam turbine 103 is shut down, the main steam A2 retains high heat. The main steam A2 flowing into the condenser 113 through the turbine bypass valve 112 puts a burden on the condenser 113. Specifically, due to this high heat, the temperature difference between the inlet and outlet seawater of the condenser 113 exceeds the environmentally permissible temperature (generally 7°C). Maintaining the output at 25MW is necessary by choosing between i) and ii).

[0071] The steam turbine 103 is a turbine corresponding to 60 Hz, and its rated speed is 3600 RPM (Revolutions Per Minute). When the steam turbine 103 stops (MCV valve 105 is fully closed), the torque driving the steam turbine 103 disappears, and the speed of the steam turbine 103 (hereinafter also referred to as ST speed) begins to decrease from 3600 RPM (step S107). Then, precisely at the moment when the speed of the steam turbine 103 decreases to slightly lower than the speed of the gas turbine 102 (hereinafter also referred to as GT speed), the clutch 120 automatically disengages (step S108). Afterward, the ST speed decreases further, but due to the large inertia of the steam turbine 103, the ST speed decreases at a gentle and slow rate.

[0072] After the steam turbine 103 is shut down, the opening degree of the fuel regulating valve 106 is reduced at a constant rate, causing the gas turbine output to decrease before restarting (step S109), and its power generation gradually decreases from 25MW. Until the gas turbine output detected by the megawatt sensor MW-Tr drops to 5MW (step S110 "Yes"), the generator circuit breaker 127 is disconnected to disconnect the turbine (step S111). When disconnected, the power generation (MW) is zero, and the combined output is also zero. Furthermore, because the plant shutdown method in this comparative example is based on the shutdown method of a rigidly coupled power plant, the gas turbine output is reduced and restarted shortly after the steam turbine 103 is shut down. This is the point of criticism as a step different from the first embodiment described later. Before disconnection, the gas turbine 102's speed is a rated 3600RPM; however, after disconnection, it begins to decrease to below 3600RPM. However, at this time, a considerable amount of fuel 116 is still being supplied to the gas turbine 102 to drive the compressor 107, making it impossible to suddenly stop the gas turbine 102 while it is supplied with a large amount of fuel 116. Therefore, the opening degree of the fuel regulating valve 106 is further reduced (step S112), causing the speed of the gas turbine 102 to decrease (step S113). Because the gas turbine 102, while rotating at GT, also drives the compressor 107, the compressor effectively acts as a brake, causing the GT speed to decrease rapidly at a high rate.

[0073] Suddenly, the GT speed, which decreases rapidly at a high speed, catches up with the ST speed, which decreases slowly. Specifically, when the GT speed drops to 3000 RPM (step S114 "Yes"), the speeds of the two turbines are the same (step S115 "Yes"), at which point the clutch 120 engages again (step S116). Then, the gas turbine 102 and the steam turbine 103, in the engaged state, decrease together from 3000 PM (step S117). The speeds in this engaged state are referred to below as "GT-ST speeds". The GT-ST speed is the midpoint of the rate of decrease of the speeds of the two turbines up to that point, that is, a rate of decrease slower than the GT speed, but decreasing at a rate of decrease faster than the ST speed.

[0074] Until the GT‧ST speed detected by the GT speed detector SP1 drops to 1200 RPM (step S118 "Yes"), the fuel regulating valve 106 is fully open, and the fuel 116 becomes very low. At this point, the gas turbine 102 is shut down (step S119), and the fuel regulating valve 106 is closed. Thus, the fuel 116 is cut off and becomes zero. The GT‧ST speed drops further and finally enters rotating operation at a very low speed (step S120), and the plant shutdown operation is completed (step S121).

[0075] Figure 8 is a trend chart used to illustrate the plant shutdown method of the comparative example. It is prepared to visualize the plant shutdown method of this comparative example more directly.

[0076] To reiterate the above, the combined output was operating at 300MW before the plant shutdown began. Upon starting the plant shutdown, fuel 116 was reduced at a predetermined rate, and the combined output gradually decreased from 300MW.

[0077] When the combined output drops to 60MW (gas turbine output is 25MW), the reduction of fuel 116 is temporarily interrupted. Then, the steam turbine (ST) 103 is shut down. Once the steam turbine 103 has shut down, clutch 120 disengages, and the ST speed begins to decrease from 3600 RPM. Simultaneously, the gas turbine output reduction is restarted, gradually decreasing from 25MW, and is disconnected when it reaches 5MW. Component symbol T3 indicates the time it takes for the combined output to decrease from 25MW to 5MW.

[0078] After being decoupled, the GT speed drops from 3600 RPM at a high speed, and catches up with the ST speed when it drops to 3000 RPM. The speeds of the two turbines are the same, and at this time the clutch 120 engages again.

[0079] After the clutch 120 is engaged, the GT speed (GT‧ST speed) decreases at a slower rate than the GT speed at that time. As a result, it takes a long time (T1) for the GT speed to decrease from 3600 RPM to 1200 RPM. When the GT speed decreases to 1200 RPM, the fuel 116 is cut off, stopping the gas turbine 102. Afterwards, with the clutch 120 engaged, both turbines gradually decrease in rotation and enter rotational operation. (3) Problem 1 of the Comparative Example

[0080] As clearly shown in Figure 8, the comparative example's plant shutdown method involves re-engaging the clutch while the GT speed is still at a high speed of 3000 RPM after decoupling, and re-engaging the clutch while fuel is still being supplied. As a result, the GT speed after clutch engagement is affected by the large inertia of the steam turbine and can only decrease at a slow rate, requiring a long time (T1) to reach 1200 RPM. This results in a long and continuous period of unnecessary fuel consumption (T1) that does not contribute to power generation. (4) Problem 2 of the Comparative Example

[0081] When the clutch is engaged, the first rotating shaft 118 and the second rotating shaft 119 are physically "shaking" together, which inevitably causes a shock to the gas turbine or generator. Specifically, increased bearing vibration of the gas turbine or generator is observed, especially significant vibration of the generator bearing near the clutch. This vibration occurs not only during the power plant shutdown process but also during the power plant startup process. However, while the plant startup is always performed at the rated speed (3600 RPM) when the clutch is engaged, there is a possibility that the plant shutdown will be performed at a dangerous speed as described below, and the problem becomes more serious in such cases. The relationship between dangerous speed and clutch engagement will be explained below.

[0082] Generally, rotating shafts have a dangerous speed (dangerous rotational speed) resulting from large vibrations caused by resonance, and the rotating shafts of turbines or generators in power plants are no exception. The dangerous speed is the speed (rotational speed) generated by resonance caused by the inherent vibration of the rotating shaft, and it depends on the natural frequency (resonance frequency) of the rotating shaft. Therefore, the value of the dangerous speed varies from one rotating shaft to another in a power plant, but it is mostly in the range of 70% to 80% of the rated speed. The reason is that when designing rotating shafts, the rated speed is usually set at about 1.2 to 1.3 times the dangerous speed.

[0083] Hereinafter, an example will be described where the rotating shaft (the combination of the first rotating shaft 118 and the second rotating shaft 119) of the comparative example has a dangerous speed of 80% of its rated speed. This 80% speed is 2880 RPM (3600 × 0.8), and the engagement speed of the clutch 120, which is 3000 RPM, falls well near 2880 RPM. That is, the engagement speed of the clutch 120 in the comparative example is within the dangerous speed range. In other words, under the plant shutdown method of the comparative example, the timing of the dangerous speed caused by large vibrations overlaps with the timing of clutch engagement, which exacerbates bearing vibration and could potentially damage the equipment in the worst case. To prevent such equipment damage, protective measures are considered to activate an emergency gas turbine shutdown if bearing vibration is detected to exceed a specified threshold. However, this is not a stable plant shutdown method. [First Implementation] (1) Plant structure of the first embodiment

[0084] Figure 1 is a schematic diagram showing the structure of the power plant 100a according to the first embodiment.

[0085] The power plant 100a in Figure 1 is equipped with a plant control device 101a for controlling the operation of the power plant 100a. The power plant 100a in Figure 1 further includes: a gas turbine (GT) 102, a steam turbine (ST) 103, a heat recovery boiler 104, an MCV valve (increase / decrease valve) 105, a fuel regulating valve 106, a compressor 107, a burner 108, an evaporator 109, a furnace 110, a superheater 111, a turbine bypass regulating valve 112, a condenser 113, a circulating water pump 114, a seawater intake and discharge section 115, a fuel supply section 116, a generator 117, a first rotating shaft 118, a second rotating shaft 119, a clutch 120, a first detection gear 121, a second detection gear 122, steam piping 123, main piping 124, bypass piping 125, a turbine exhaust pipe 126, and a generator circuit breaker 127, all having the same functions as the power plant 100 in Figure 6. The generator circuit breaker 127 in Figure 1 is the same as that in Figure 6, and is connected to the system grid 129 via transmission line 128. The power plant 100a in Figure 1 further includes: a flame detector FD1, a GT speed detector SP1, a ST speed detector SP2, and a megawatt sensor MW-Tr, all having the same functions as the power plant 100 in Figure 6. This power plant 100a is a "clutch-engaged single-shaft type compound cycle power plant" in which the gas turbine 102 and the steam turbine 103 are coupled via the engagement of a clutch 120.

[0086] The plant control device 101a of this embodiment has the same functions as the plant control device 101 of the comparative example, but it also has functions that are different from those of the plant control device 101 of the comparative example. On the other hand, the functions of other constituent elements of the power plant 100a shown in FIG1 are the same as the functions of the corresponding constituent elements of the power plant 100 shown in FIG6. (2) Plant shutdown method of the first embodiment

[0087] Figure 2 is a flowchart illustrating the plant shutdown method of the first embodiment.

[0088] This process is implemented by software housed within the plant control device 101a. Furthermore, the specific values ​​used in the following description are merely examples for ease of explanation and understanding. Also, please refer to the trend graph in Figure 3, which will be described later, for easier understanding.

[0089] Before the shutdown of power plant 100a (plant shutdown), power plant 100a is in normal operation and clutch 120 is engaged. In normal operation, gas turbine 102 is operating at its rated output of 200MW, and steam turbine 103 is also operating at its rated output of 100MW, with the two turbines operating at a total output of 300MW (combined output).

[0090] When the plant shutdown begins (step S200), the opening degree of the fuel regulating valve 106 is reduced at a constant rate to decrease the output of the gas turbine (GT) (step S201), and the combined output is gradually reduced from 300MW. During plant shutdown, the focus is on minimizing the load on the gas turbine 102 and steam turbine 103. As in step S201, by reducing the opening degree of the fuel regulating valve 106 at a constant rate, the amount of fuel 116 is gradually reduced. As a result, for example, the temperature of the gas turbine exhaust gas A1 also gradually decreases, and the thermal stress generated when the gas turbine 102 is shut down is also reduced. Because the heat energy retained in the gas turbine exhaust gas A1 decreases, the heat (temperature, pressure, flow rate) of the main steam A2 also decreases, so the output of the steam turbine (ST) is reduced (step S202), and its power generation is gradually reduced from 100MW.

[0091] The decrease in gas turbine output is interrupted (step S205) when the combined output detected by the megawatt sensor MW-Tr drops to 60MW (step S204 "Yes"). Furthermore, the reduction of the opening degree of the fuel regulating valve 106 is also interrupted, and the gas turbine 102 maintains a constant output state. At this time, the details of the 60MW combined output are as follows: That is, the gas turbine 102 maintains 25MW, and the steam turbine 103 maintains 35MW, generating a total of 60MW of electricity.

[0092] While maintaining the gas turbine 102 at a constant output of 25MW, the steam turbine 103 is shut down (step S206), resulting in a decrease in the power generation of the steam turbine 103 from 35MW. During this shutdown operation, the opening degree of the MCV valve 105 is gradually reduced at a constant rate, causing the output of the steam turbine 103 to decrease until the MCV valve 105 is fully closed. When the MCV valve 105 is fully closed, the steam turbine 103 is shut down, and because its power generation is zero, the combined output is only 25MW, the output of the gas turbine. At this time, because the MCV valve 105 is closed, the remaining main steam A2 is allowed to escape through the turbine bypass valve 112 into the condenser 113, and the pressure in the furnace 110 is maintained at a constant level.

[0093] Furthermore, the reason for maintaining the gas turbine output at 25MW in step S205 before step S206 is the same as that in step S105 of the comparative example, and the explanation is omitted.

[0094] The steam turbine 103 is a turbine corresponding to 60 Hz, and its rated speed is 3600 RPM. When the steam turbine 103 is completely shut down (MCV valve 105 is fully closed), the torque driving the steam turbine 103 disappears, and the speed of the steam turbine 103 (ST speed) begins to decrease from 3600 RPM (step S207). Then, precisely at the instant the speed of the steam turbine 103 decreases to slightly below the speed of the gas turbine 102 (GT speed), the clutch 120 automatically disengages (step S208). Afterwards, the ST speed decreases further, but due to the large inertia of the steam turbine 103, the ST speed decreases at a gentle and slow rate.

[0095] The above factory shutdown procedure is the same as that in the comparative example. The factory shutdown procedure in this embodiment differs from that in the comparative example in the following aspects.

[0096] During the period when the ST speed begins to decrease from 3600 RPM, the output of the gas turbine 102 remains at 25 MW. Then, until the ST speed detected by the ST speed detector SP2 decreases to 1200 RPM (step S209 "Yes"), the gas turbine output decrease is restarted (step S210). When the gas turbine output decreases again, the power generation of the gas turbine gradually decreases from 25 MW. Until the gas turbine output detected by the megawatt sensor MW-Tr decreases to 5 MW (step S211 "Yes"), the generator circuit breaker 127 is disconnected to disconnect the generator (step S212). When disconnected, the power generation (MW) is zero, and the combined output is also zero.

[0097] Before disconnection, the GT speed is rated at 3600 RPM. However, after disconnection, the GT speed only begins to decrease below 3600 RPM. At this time, a considerable amount of fuel 116 is still supplied to the gas turbine 102 to drive the compressor 107 or burner 108, further reducing the opening degree of the fuel regulating valve 106 (step S213), causing the GT speed to decrease further (step S214). Because the gas turbine 102, which is rotating downwards, also drives the compressor 107, the compressor 107 can be said to have a braking effect, causing the GT speed to rapidly decrease to 1200 RPM at a relatively high rate. At this time, because the clutch 120 is disengaged, it is not affected by the large inertia of the steam turbine 103, and the gas turbine 102 rotates rapidly and independently to 1200 RPM. This differs from the comparative example. The plant control device 101a performs the processing steps S206 to S214, which is one example of the first shutdown control unit. Furthermore, the speed of "1200 RPM" in step S209 is one example of the third speed.

[0098] Until the GT speed detected by the GT speed detector SP1 drops to 1200 RPM (step S215 "Yes"), the fuel regulating valve 106 is fully open, and the fuel 116 becomes very small, at which point the gas turbine 102 is shut down (step S216). At this time, the fuel regulating valve 106 is closed, and the fuel 116 is cut off to zero. The reason for shutting down the gas turbine 102 only when the GT speed drops to 1200 RPM is, as mentioned above, to allow the amount of fuel 116 to be sufficiently reduced while the exhaust gas temperature of the gas turbine 102 is low, so that there will be no great burden on the gas turbine 102 in terms of thermal stress, etc. In this embodiment, combustion of the combustor 108 continues until step S216, and the fuel 116 of the combustor 108 is cut off in step S216. The speed "1200 RPM" in step S215 is one example of the first speed.

[0099] After the gas turbine 102 is shut down, the GT speed decreases at a high rate. Then, until the GT speed drops to 900 RPM (step S217 "Yes"), the GT speed catches up with the decreasing ST speed at a slow rate, and the speeds of the two turbines are the same (step S218 "Yes"). At this time, the clutch 120 engages again (step S219). The plant control device 101a performs the processing of steps S215 to S219, which is one example of the second shutdown control unit. Moreover, the speed "900 RPM" in step S217 is one example of the second speed.

[0100] Next, the gas turbine 102 and steam turbine 103, in the combined state, decrease in rotation speed together from 900 RPM (step S220). The rotational speed in this combined state is referred to as "GT-ST speed" as described above. The rate of decrease of GT-ST speed is the rate between the rate of decrease of GT speed and the rate of decrease of ST speed at that time. GT-ST speed decreases further and finally enters rotating operation at a very low speed (step S221), and the plant shutdown operation is completed (step S222). GT-ST speed decreases slowly from 900 RPM, but since the gas turbine 102 is in a shut-down state, even if the rotational decrease is delayed in this operating range, it will not cause a major problem.

[0101] Figure 3 is a trend chart for illustrating the plant shutdown method of the first embodiment. It is prepared to visualize the plant shutdown method of this embodiment more directly.

[0102] To reiterate the above, the combined output was operating at 300MW before the plant shutdown began. Upon starting the plant shutdown, fuel 116 was reduced at a predetermined rate, and the combined output gradually decreased from 300MW.

[0103] When the combined output drops to 60MW (gas turbine output is 25MW), the reduction of fuel 116 is temporarily interrupted. Then, the shutdown of steam turbine (ST) 103 begins. Once the shutdown of steam turbine 103 is complete, clutch 120 disengages, and the ST speed begins to decrease from 3600RPM. At this time, the ST speed decreases at a gentle and slow rate, taking time (T4) for the ST speed to decrease from 3600RPM to 1200RPM.

[0104] The gas turbine output will only begin to decrease when the ST speed drops to 1200 RPM (the 3rd speed). When the gas turbine output begins to decrease again, it will gradually decrease from 25 MW to 5 MW before being disconnected. The time required for the combined gas turbine output (output) to decrease from 25 MW to 5 MW is T3.

[0105] After decoupling, the GT speed decreases rapidly from 3600 RPM. When the GT speed drops to 1200 RPM, fuel 116 is cut off, causing the gas turbine 102 to shut down. At this point, the GT speed decreases from 3600 RPM to 1200 RPM (the first speed) in a relatively short time (T2). This differs from the comparative example.

[0106] After the gas turbine 102 is shut down, the GT speed continues to decrease. Then, when the GT speed drops to 900 RPM (the second speed), the GT speed catches up with the ST speed, and the speeds of the two turbines are the same. At this time, the clutch 120 engages again. Afterwards, with the clutch 120 engaged, the two turbines slowly rotate down and enter rotating operation. (3) Effects of the first embodiment 1

[0107] Comparing Figures 3 and 8, it can be seen that the time (T2) required for the turbine speed to drop from 3600 RPM to 1200 RPM in this embodiment is extremely short compared to (T1) in the comparative example. As a result, the "problem of unnecessary fuel supply for a long time" that accompanied the plant shutdown method in the comparative example is eliminated or greatly mitigated in this embodiment. To achieve this effect, in this embodiment, the ST speed is waited for to drop to 1200 RPM before the gas turbine output is further reduced from 25 MW. Therefore, when the GT speed drops to 1200 RPM in this embodiment, the ST speed must drop to below 1200 RPM. That is, in this embodiment, the speeds of the two turbines are the same and the clutch is engaged, which is always below 1200 RPM (the speed at which fuel is cut off).

[0108] Furthermore, in this embodiment, the clutch is engaged when the fuel supply is cut off. When the clutch is engaged, the two rotating shafts "shake" against each other, resulting in increased vibration of the bearings of the gas turbine or generator, as has already been explained. In the comparative example, this shaking occurs during combustion in the gas turbine; in contrast, in this embodiment, it occurs after the fuel supply is cut off (the gas turbine has already stopped). Therefore, according to this embodiment, a safer and less risky plant shutdown method can be achieved compared to the comparative example.

[0109] Furthermore, in this embodiment, during the period when the GT speed decreases from 3600 RPM to 1200 RPM, the clutch is always disengaged, and the gas turbine (and generator) rotates and decreases independently. In this way, it is not affected by the large inertia of the steam turbine, and the gas turbine speed can decrease independently and quickly from 3600 RPM to 1200 RPM, allowing for rapid fuel cutoff. If such a plant shutdown method can be achieved, the gas turbine's rotational decrease will only begin after the ST speed has decreased to the fuel cutoff speed of the gas turbine, which is 1200 RPM. (4) Effects of the first embodiment 2

[0110] In the first embodiment, because the clutch 120 is engaged at 900 RPM, which is significantly away from the dangerous speed range of 2880 RPM, the problem of increased bearing vibration in the gas turbine or generator due to prolonged clutch engagement, as in the comparative example, can be eliminated. That is, according to this embodiment, a plant shutdown method that avoids the dangerous speed range by engaging the clutch is realized.

[0111] As described above, the danger speed is the speed (rotational speed) generated by resonance due to the inherent vibration of the rotating shaft, and is dependent on the natural frequency (resonance frequency) of the rotating shaft. Therefore, the value of the danger speed differs from that of each rotating shaft in the power plant, but it is mostly in the rotational speed range of approximately 70% to 80% of the rated speed. Furthermore, the danger speed also varies between the comparative example (the danger speed of the combination of the first rotating shaft 118 and the second rotating shaft 119) and the first embodiment (the danger speed of the first rotating shaft 118 alone). However, taking into account the variation in the value of the danger speed, the plant shutdown method of the first embodiment has the effect of engaging the clutch while avoiding the danger speed range. This is because the rotational speed at which fuel is cut off is significantly lower than that of the danger speed. In other words, even if the danger speed varies between the comparative example and the first embodiment, the danger speed of the first embodiment is close to 2880 RPM (e.g., 2800 RPM), which is significantly different from 900 RPM. Furthermore, in the following description, for ease of understanding, the dangerous speed of the first embodiment or other embodiments will be described as 2880 RPM.

[0112] This specification describes an example where the fuel cutoff speed is set at 1200 RPM (33% of rated speed). However, this cutoff speed varies from power plant to power plant (and from gas turbine to gas turbine model). Gas turbines that cut off fuel at relatively high speeds of 70% to 80% of rated speed are very rare. For reasonable commercial gas turbine designs, fuel cutoff at speeds as low as 30% to 40% of rated speed is generally preferred. This is because cutting off fuel at 1200 RPM (33% of rated speed) significantly reduces the amount of fuel and results in lower exhaust gas temperatures, thus avoiding significant stress on the gas turbine. Cutting off fuel at high speeds of 70% to 80% means that a sudden cutoff leaves a large amount of fuel remaining, necessitating a harsh plant shutdown due to the gas turbine's inherent characteristics. In other words, the speed at which fuel is cut off is generally a sufficiently low speed compared to the high speed range of 70% to 80% where the dangerous speed range exists. Therefore, by adopting the plant shutdown method of this embodiment, which engages the clutch after fuel is cut off, the overlap between the dangerous speed range and the clutch engagement speed can be avoided.

[0113] Furthermore, in the above description, it was explained that in the first embodiment, the clutch 120 was engaged at 900 RPM, which is significantly away from the danger speed range of 2880 RPM. In this embodiment, the engagement speed of the clutch 120 is preferably at least 200 RPM away from the danger speed, and more preferably at least 300 RPM away from the danger speed. Therefore, when the danger speed is 2880 RPM, the engagement speed of the clutch 120 is preferably at least 2680 RPM, and more preferably at least 2580 RPM. In this way, the generation of resonance as described above can be suppressed. (5) Variations of the first embodiment

[0114] However, according to power plants, there are cases where the plant shutdown method caused by the modification of the first embodiment described herein is necessary. The reason is that the power plant (first rotating shaft 118) in this modification has dangerous speed ranges in addition to the dangerous speed range of about 80% of the high rotational speed range (around 2880 RPM). That is, the first rotating shaft 118 in this modification has a dangerous speed of 30% of the rated speed, which is 1080 RPM (3600 × 0.3).

[0115] In the shutdown method of the first embodiment, the 900 RPM at which the clutch engages is sufficiently close to the critical speed range, resulting in increased bearing vibration due to resonance. Here, this modified example employs a plant shutdown method that engages the clutch at 700 RPM, significantly away from 1080 RPM. Therefore, in the first embodiment, when the ST speed drops to 1200 RPM (step S209 "Yes"), the gas turbine output reduction begins (step S210); in contrast, in this modified example, the gas turbine output reduction begins when the ST speed drops to 1000 RPM. Corresponding to this 200 RPM reduction in ST speed, the clutch engagement speed also decreases by 200 RPM, engaging at 700 RPM. This avoids the problem of the critical speed range overlapping with the clutch engagement speed. In this variation, 1000 RPM is one example of the third speed, 1200 RPM is one example of the first speed, and 700 RPM is one example of the second speed.

[0116] Furthermore, it is important to note that the difference between the dangerous speed of 1080 RPM and the speed at which the clutch 120 engages is 380 RPM. Therefore, the speed at which the clutch 120 engages in this situation is more than 200 RPM away from the dangerous speed, and even more than 300 RPM away from the dangerous speed. (6) Research on the first implementation method

[0117] In the first embodiment, the time (T2) required to decrease from 3600 RPM to 1200 RPM is shorter; however, the waiting time (T4) to maintain the gas turbine output at 25 MW is longer. However, since 25 MW of power generation is carried out during the waiting time of T4, the fuel supplied during T4 (such as the fuel in T1 in the comparative example) will not be wasted. However, on the other hand, there is a need to advance the plant shutdown time by transitioning to rotating operation (step S221) in the shortest possible time. Typically, there are situations where it is desirable to start plant equipment inspection, repair, maintenance, etc., as early as possible. From this point of view, the waiting time (T4) at 25 MW has the aspect of extending the time required for plant shutdown. The second embodiment, which will be described next, is a plant shutdown method for improving this point. [Second Implementation]

[0118] The following describes the second embodiment. The power plant configuration in the second embodiment is the same as that of the power plant 100a in the first embodiment, except for the software installed in the plant control device 101a. The difference between the software in the first and second embodiments lies in the timing of the gas turbine output reduction starting from 25MW. That is, in the first embodiment, the gas turbine output reduction begins when the ST speed drops to 1200 RPM (step S210), while in the second embodiment, the gas turbine output reduction begins when the ST speed drops to R1 [RPM]. In this case, there is a relationship of R1 > 1200 between R1 [RPM] and 1200 RPM. By starting the gas turbine output reduction earlier in the second embodiment, the time required for plant shutdown can be shortened. The method for calculating the value of R1 will be detailed below.

[0119] Furthermore, please refer to FIG1 for the plant structure of this embodiment. Also, please refer to FIG2 for the flow of the plant shutdown method of this embodiment. Here, for example, "1200RPM" in step S209 is referred to as "R1[RPM]" as described above.

[0120] Figure 4 is a trend graph used to illustrate the plant shutdown method of the second embodiment.

[0121] According to this explanation, the combined output was operating at 300MW before the plant shutdown began. Upon starting the plant shutdown, fuel 116 was reduced at a prescribed rate, and the combined output gradually decreased from 300MW.

[0122] When the combined output drops to 60MW (gas turbine output is 25MW), the reduction of fuel 116 is temporarily interrupted. Then, the shutdown of the steam turbine (ST) 103 begins. Once the steam turbine 103 has been shut down, the clutch 120 disengages, and the ST speed begins to decrease from 3600 RPM. The above plant shutdown procedure is the same as in the first embodiment.

[0123] The plant shutdown procedure in the second embodiment differs from that in the first embodiment in the following aspects. The ST speed decreases at a gentle and slow rate, and the gas turbine output decreases only when the ST speed drops to R1 [RPM]. The value of R1 will be described later. In this embodiment, R1 [RPM] is one example of the third speed.

[0124] If the gas turbine output starts to decrease again, the gas turbine output will gradually decrease from 25MW, and disconnect when it reaches 5MW. The time (T3) required for the gas turbine output (combined output) to decrease from 25MW to 5MW is the same as in the first embodiment.

[0125] After decoupling, the GT speed decreases rapidly from 3600 RPM. When the GT speed drops to 1200 RPM, it catches up with the ST speed, and the speeds of the two turbines become identical. At this point, clutch 120 re-engages. Simultaneously, fuel 116 is cut off, causing gas turbine 102 to stop. The time (T2) required for the GT speed to decrease from 3600 RPM to 1200 RPM is the same as in the first embodiment. Furthermore, the time (T4) required for the ST speed to decrease from 3600 RPM to 1200 RPM is also the same as in the first embodiment. Afterwards, with clutch 120 engaged, the two turbines gradually decrease in rotation and enter rotational operation. The 1200 RPM in this embodiment is one example of the first speed and one example of the second speed.

[0126] Thus, in the second embodiment, when the GT speed and ST speed drop to 1200 RPM, the fuel 116 is simultaneously cut off and the clutch 120 is engaged. The R1 [RPM] required to achieve this can be calculated as follows.

[0127] In Figure 4, the ST speed drops from 3600 RPM to 1200 RPM for T4. The rate of decrease of ST speed is expressed by the following formula (1).

[0128] The rate of decrease in ST speed = (3600-1200) / T4 = 2400 / T4…(1)

[0129] Here, the time of T4 (and T2 and T3) is all in the unit of "minute".

[0130] Therefore, the time required for the ST speed to drop from R1[RPM] to 1200RPM is expressed by formula (1) as follows (2).

[0131] Time from R1 to 1200 RPM = (R1-1200) ÷ ST Rate of decrease in RPM = (R1-1200)T4 / 2400…(2)

[0132] On the other hand, the time required for the gas turbine output to drop and then start again, and for the GT speed to drop from 25MW to 1200RPM after decoupling, is expressed by the following equation (3).

[0133] The time from 25MW to 1200RPM = T3 + T2…(3)

[0134] In the plant shutdown method of the second embodiment, the timing of ST speed reaching 1200 RPM is consistent with the timing of GT speed reaching 1200 RPM. For this purpose, the relationship between equation (2) and equation (3) should be satisfied. That is, R1 that satisfies the following equation (4) is obtained.

[0135] (R1-1200)T4 / 2400=T3+T2…(4)

[0136] Solve equation (4), and use R1 as equation (5) and find it.

[0137] R1=2400(T3+T2) / T4+1200…(5)

[0138] First, the relationship between R1 and 1200 is recorded. However, R1 is the speed that is only 1200 RPM higher than the speed difference expressed in equation (6) below.

[0139] Speed ​​difference = 2400(T3+T2) / T4…(6)

[0140] The second embodiment is shorter because the gas turbine output reduction is started earlier than that of the first embodiment, which only uses the speed difference of formula (6).

[0141] In calculating the shortening time, equation (6) can be divided by the rate of decrease of ST speed (equation (1)).

[0142] Shortening time = 2400(T3+T2) / T4÷(2400 / T4) = T3+T2…(7) (1) Effects and research of the second implementation method

[0143] In the second embodiment, the shutdown of the 1200 RPM gas turbine and the re-engagement of the clutch are performed simultaneously, which eliminates the problems in the comparative example and shortens the plant downtime compared to the first embodiment. The shortened time is T3 + T2 given in formula (7). To explain the reason briefly, in the first embodiment (Fig. 3), the waiting time for maintaining the gas turbine output at 25 MW is T4; in contrast, in the second embodiment (Fig. 4), the waiting time for maintaining 25 MW is shortened to T4 - (T3 + T2). This can be easily seen from the comparison between Fig. 3 and Fig. 4.

[0144] Regarding the above, when comparing the second embodiment (Fig. 4) and the first embodiment (Fig. 3), it is helpful to keep in mind that the timing is based on the ST speed. Specifically, in Fig. 4 and Fig. 3, the "time when the ST speed starts to decrease from 3600 RPM" and the "time when the ST speed reaches 1200 RPM" are the same (in this case, the time is calculated from 0, which is the starting point when the plant shutdown begins). Therefore, relative to the time when the ST speed reaches 1200 RPM, it is possible to compare how much earlier or how much later the fuel cutoff is. Based on this view, it is determined that the fuel cutoff in Fig. 4 is earlier (T3+T2) than in Fig. 3.

[0145] In addition, in the comparative example (Fig. 8), the time for the ST speed to start decreasing from 3600 RPM is the same as these, but the time for the ST speed to reach 1200 RPM is different from these (because the rate of decrease is different from that in (1) when the clutch is engaged), so the ST speed reference cannot be applied.

[0146] Hereinafter, a brief mention will be made of the plant shutdown method under the condition of earlier fuel cut-off (earlier than T3+T2). In this case, depending on the degree of acceleration, if the gas turbine output is reduced and then restarted at a speed higher than R1 calculated by equation (5), the fuel cut-off can be accelerated. However, in this case, the ST speed and GT speed are the same speed of 1200 RPM or higher, that is, the clutch engages before the fuel is cut off, which is similar to the plant shutdown method of the comparative example. This is the meaning of T3+T2 as the limit of acceleration. (2) Variations of the second embodiment

[0147] In calculating R1 in the second embodiment, actual machine data (the rate of decrease of ST speed (1)) during plant shutdown is necessary. Therefore, when actually implementing the second embodiment, it is necessary to initially try the plant shutdown method caused by the first embodiment, and then transfer to the second embodiment step by step after obtaining actual machine data. In this case, the ST speed decrease graph in Figure 4 (and Figure 3) is represented by a straight line in the lower right corner. However, in reality, the ST speed decreases in a slightly concave curve, so the R1 calculated by temporarily defining it as a "straight line" has some error. Taking this into consideration, it is practical to give an appropriate margin to the actual applicable R1, and to engage the clutch at a speed slightly lower than 1200 RPM, which is a bit lower than the R1 calculated by the above (5) formula. According to the shutdown method described in this variation, after fuel cutoff (gas turbine shutdown) at 1200 RPM, the clutch engages after a wait of several seconds. Therefore, it can be expected that the mechanical impact on the turbine equipment (gas turbine shutdown and clutch engagement) will be mitigated due to the time difference, thus achieving a more satisfactory shutdown. The reduction in plant downtime compared to equation (7) is sufficiently justified. [Third Implementation]

[0148] Figure 5 is a trend graph used to illustrate the plant shutdown method of the third embodiment.

[0149] The third embodiment is similar to the second embodiment and is a plant shutdown method aimed at saving fuel. Therefore, during the waiting period for the ST speed to decrease, the gas turbine output is maintained at 5MW in the third embodiment, compared to maintaining the gas turbine output at 25MW in the second embodiment, in order to save fuel.

[0150] Furthermore, please refer to FIG1 for the plant structure of this embodiment. Also, please refer to FIG2 for the flow of the plant shutdown method of this embodiment. Here, for example, "1200RPM" in step S209 is referred to as "R2[RPM]" as described later.

[0151] The following explains Figure 5. Before the plant shutdown begins, the combined output is operating at 300MW. When the plant shutdown begins, fuel 116 is reduced at a prescribed rate, and the combined output gradually decreases from 300MW.

[0152] When the combined output drops to 60MW (gas turbine output is 25MW), the reduction of fuel 116 is temporarily interrupted. Then, the shutdown of the steam turbine (ST) 103 begins. Once the steam turbine 103 has been shut down, the clutch 120 disengages, and the ST speed begins to decrease from 3600 RPM. The above plant shutdown procedure is the same as in the second embodiment.

[0153] The plant shutdown procedure in the third embodiment differs from that in the second embodiment in the following aspects. After the steam turbine 103 is shut down, the gas turbine output is immediately reduced and then restarted. When the gas turbine output is reduced and restarted, the gas turbine output gradually decreases from 25MW, and is maintained at 5MW when it reaches 5MW. The time (T3) required for the output to decrease from 25MW to 5MW is the same as in the second embodiment. During this period, the ST speed decreases at a gentle and slow rate, and the turbine is de-energized when the ST speed reaches R2 [RPM]. The value of R2 will be described later. In this embodiment, R2 [RPM] is one example of the third speed.

[0154] After decoupling, the GT speed decreases rapidly from 3600 RPM. When the GT speed drops to 1200 RPM, it catches up with the ST speed, and the speeds of the two turbines become identical. At this point, clutch 120 re-engages. Simultaneously, fuel 116 is cut off, causing gas turbine 102 to stop. The time (T2) required for the GT speed to decrease from 3600 RPM to 1200 RPM is the same as in the second embodiment. Furthermore, the time (T4) required for the ST speed to decrease from 3600 RPM to 1200 RPM is also the same as in the second embodiment. Afterwards, with clutch 120 engaged, the two turbines gradually decrease in rotation and enter rotational operation. The 1200 RPM in this embodiment is one example of the first speed and one example of the second speed.

[0155] Thus, in the third embodiment, similar to the second embodiment, when the GT speed drops to 1200 RPM, the gas turbine is simultaneously stopped and the clutch is engaged. The R2 [RPM] for achieving this is calculated in the same way as when R1 was calculated, as follows (the calculation process is omitted).

[0156] R2=2400(T2 / T4)+1200…(8) (1) Effects and Research of the Third Implementation

[0157] The third embodiment maintains the gas turbine output at 5MW, which is more economical than the second embodiment, which maintains the gas turbine output at 25MW. This reduces fuel consumption and saves fuel, thus enabling an economical plant shutdown method.

[0158] However, on the other hand, since 5MW is 2.5% of the rated output of the gas turbine of 200MW (generally referred to as the extremely low load operating range), depending on the situation, the third embodiment may cause obstacles to the operation of the stable gas turbine. For example, in the case of so-called system disturbance in the system grid 129, the system frequency changes drastically, and the droop control system of the gas turbine control causes a rapid reduction in fuel, which in the worst case leads to de-energization due to reverse power (reverse electricity).

[0159] In comparison, in the second implementation at 25MW, even if the same droop control causes a sharp reduction in fuel, the higher 25MW will not result in reverse power, and the gas turbine’s power generation operation is more stable.

[0160] The values ​​of 5MW or 25MW used in this specification (the gas turbine output maintained during the waiting period for the ST speed to decrease) are merely convenient examples for illustrative purposes. However, even when any of the first to third embodiments are adopted, these gas turbine output values ​​equivalent to 5MW or 25MW are carefully selected from the viewpoint of balancing the stability and economy of gas turbine operation.

[0161] Hereinafter, the plant shutdown time is compared between the third embodiment (Fig. 5) and the second embodiment (Fig. 4). The ST speed mentioned earlier is used as the time reference, which also applies to this case. The time when the ST speed reaches 1200 RPM is the same as in Fig. 5 and Fig. 4 (when T4 has elapsed since the ST shutdown was completed). That is, in both embodiments, the fuel cut-off time is the same, and the plant shutdown time is the same. At first glance, because the process of reducing the gas turbine output from 25MW to 5MW is performed first (equivalent to time T3), the third embodiment can achieve plant shutdown earlier. However, because R2 given in equation (8) is a lower speed than R1 given in equation (7) (the time is delayed), the plant shutdown time is the same in both embodiments. If the R2 (earlier time) is set higher than that in formula (8) for disengagement, the ST speed and GT speed are the same as the speed of 1200 RPM or higher. This is the factory shutdown method of engaging the clutch before the fuel is cut off, which is not the desired shutdown method. (2) Variations of the third embodiment

[0162] The R2 calculated in this embodiment has a slight error, similar to R1 in the second embodiment. Here, in the variation of the third embodiment, an appropriate margin is given to the R2 applied to reality, which is a speed slightly lower than 1200 RPM than the R2 calculated by the above equation (8). This variation is more realistic, and therefore the more desirable shutdown method is the same as that in the variation of the second embodiment.

[0163] Although certain embodiments have been described, these embodiments are given by way of example only and are not intended to limit the scope of the invention. In fact, the novel apparatus, methods, and devices described herein can be embodied in many other forms; furthermore, various omissions, substitutions, and changes can be made to the forms of the apparatus, methods, and devices described herein without departing from the spirit of the invention. The appended claims and their equivalents are intended to cover these forms or modifications that fall within the scope and spirit of the invention.

Claims

1. A plant control device for controlling a power plant, the power plant comprising: a burner for igniting fuel; a gas turbine driven by combustion gases from the burner; a heat recovery boiler for generating steam using heat from exhaust gases from the gas turbine; a steam turbine driven by steam from the heat recovery boiler; a first shaft connected to the gas turbine; a second shaft connected to the steam turbine; and a clutch for engaging the first shaft and the second shaft when the rotational speed of the first shaft catches up with the rotational speed of the second shaft; characterized in that the plant control device comprises: a first shutdown control unit that, when shutting down the power plant, disengages the first shaft from the second shaft via the clutch. The second shaft is configured to stop the aforementioned steam turbine and cause the speed of the second shaft to decrease from the rated speed. After the aforementioned steam turbine stops, the speed of the aforementioned first shaft is caused to decrease from the aforementioned rated speed while the aforementioned burner continues to burn, causing the speed of the aforementioned first shaft to decrease from the aforementioned rated speed to a first speed. The second shutdown control unit is configured to shut down the aforementioned gas turbine by cutting off the fuel in the aforementioned burner when the speed of the aforementioned first shaft decreases to the aforementioned first speed. During the period when the fuel in the aforementioned burner is cut off, the gas turbine is shut down in order to allow the speed of the aforementioned first shaft to catch up with the speed of the aforementioned second shaft at a second speed lower than the aforementioned first speed. Then, the aforementioned first shaft and the aforementioned second shaft are engaged via the aforementioned clutch.

2. As in request item 1, the plant control device, wherein, The aforementioned first shutdown control unit causes the speed of the aforementioned first shaft to decrease from the aforementioned rated speed when the speed of the aforementioned second shaft decreases to the third speed.

3. As in request item 2, the plant control device, wherein, The aforementioned third rotational speed is below the aforementioned first rotational speed.

4. The plant control device as requested in item 2, wherein, The aforementioned third rotational speed is higher than the aforementioned first rotational speed.

5. The plant control device as requested in item 1, wherein, The aforementioned first shaft has a dangerous speed that is related to the resonant frequency of the aforementioned first shaft; the aforementioned second rotational speed is even lower than the aforementioned dangerous speed.

6. The plant control device as requested in item 5, wherein, The difference between the aforementioned dangerous speed and the aforementioned second rotational speed is more than 200 RPM.

7. A method for controlling a power plant, the power plant comprising: a burner for burning fuel; a gas turbine driven by combustion gases from the burner; a heat recovery boiler for generating steam using heat from exhaust gases from the gas turbine; a steam turbine driven by the steam from the heat recovery boiler; a first shaft connected to the gas turbine; a second shaft connected to the steam turbine; and a clutch for engaging the first shaft and the second shaft when the rotational speed of the first shaft catches up with the rotational speed of the second shaft; characterized in that the method comprises: disengaging the first shaft and the second shaft via the clutch when the power plant is shut down. The process includes: stopping the steam turbine and causing the speed of the second shaft to decrease from the rated speed; continuing combustion in the burner after the steam turbine stops and causing the speed of the first shaft to decrease from the rated speed to a first speed; and cutting off the fuel supply to the burner to stop the gas turbine when the speed of the first shaft decreases to the first speed. During the period when the fuel supply to the burner is cut off, the gas turbine is stopped so that the speed of the first shaft catches up with the speed of the second shaft at a second speed lower than the first speed. Then, the first shaft and the second shaft are engaged via the clutch.

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