JPS6149484B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a load control method for a combined cycle plant including a plurality of gas turbines and a steam turbine driven by utilizing the exhaust heat of these gas turbines.

In a combined cycle plant, the load of the entire plant is usually controlled by distributing the load to multiple gas turbines and steam turbines and controlling each load. By the way, if the load on the gas turbine is changed, the temperature of the steam generated from the exhaust heat boiler connected to it will also change, so there is a method for changing the load that does not consume the life of the steam turbine, which has a small tolerance for the rate of change in steam temperature. is desirable. Therefore, it is possible to change the load of the gas turbines one by one in sequence, and to bypass the steam produced from the waste heat boiler connected to the gas turbine during the load change without guiding it to the steam gas turbine. This means that the load changes while keeping the temperature of the steam that is led to constant. However, the load change rate of the gas turbine is regulated by the conditions on the gas turbine side, and furthermore, it takes a certain amount of time to switch the steam from the waste heat boiler from the steam turbine to the bypass side, so this load change method does not allow rapid load change. Load changes are not possible.

On the other hand, if the steam generated by the gas turbine waste heat boiler is guided to the steam turbine and the load of multiple gas turbines in operation is changed simultaneously, rapid load changes are possible, but the steam temperature changes and the steam This will greatly reduce the lifespan of the turbine.

The present invention was made in view of the shortcomings of the prior art, and its purpose is to provide a load control method that allows load changes to be made as rapidly as possible without excessive wear and tear on the rotor of a steam turbine. Our goal is to provide the following.

The feature of the present invention is that when a load change is requested, initially the operating gas turbines are changed in load all at once to achieve a rapid load change, and when the resulting steam temperature change reaches a certain limit, the output steam is bypassed. The purpose is to sequentially change the load on the gas turbines one by one while the gas turbines are in the same state.

The present invention will be explained below with reference to the drawings.

FIG. 1 shows an example of a combined cycle plant system diagram to which the present invention is applied.

In the figure, 1-1 to 1-4 are gas turbines (hereinafter sometimes abbreviated as GT), 2-1 to 2-4 are gas turbine generators, and 3-1 to 3-4 are exhaust heat recovery boilers (hereinafter abbreviated as HRSG). ), and these 3
The steam turbine 7 (hereinafter referred to as "steam gas turbine")
(sometimes abbreviated as ST). This figure shows a combined cycle plant with four GT/HRSG units, with the second and third units omitted. Note that it is generally composed of 1 to 6 units.

4-1 to 4-4 are boiler outlet valves, 5-1 to 5-
Reference numeral 4 denotes a turbine bypass valve, which is opened when steam is directly guided to the condenser 9 when the GT/HRSG unit is started or stopped. 6 is a main steam header that collects steam from each unit, 7 is a steam turbine, 8 is a steam turbine generator, and 9 is a condenser.

Figure 2 shows the relationship between the gas turbine load and its outlet gas temperature 21, HRSG outlet steam temperature 22, and steam turbine first stage steam temperature 23. The GT outlet gas temperature 11 is approximately 500°C during rated operation. It varies widely depending on the GT load, from about 250℃ at no load. Therefore, HRSG outlet steam temperature 2 and ST
The steam temperature after the first stage also changes in a similar manner.

Figure 3 shows the relationship between the amount of steam temperature change after the first stage of ST and the life wear and tear of the steam turbine rotor (due to low cycle fatigue), and shows the allowable limit temperature (ΔTal... ...This value is a function of the outer diameter of the rotor and the rate of temperature change.) In the following cases, rotor life wear is necessarily small, but Δ
This shows that it increases rapidly when it exceeds Tal.

Note that here, the rate of change in steam temperature after the first stage is determined by the rate of change in GT load, and since the latter is fixed constant depending on the circumstances on the GT side, the former is also constant.

Figure 4 shows the conventional first load change method in a combined cycle plant.
This shows the case where the load is changed all at once on all GTs. GT
Since the individual load change rates are held constant in consideration of the thermal conditions inside the GT, the load change rate can be maximized in this case for the entire plant. Curves 41 to 44 show load changes of the respective GTs, 45 shows load changes of the steam turbine, and 46 shows load changes of the entire plant. In this embodiment, the plant output share is approximately 60% for GT and approximately 40% for ST.

Curves 47 and 48 indicate the HRSG outlet temperature and the steam temperature after the first stage of the ST in this case, but they become large as the GT outlet gas temperature changes.
The temperature change ΔT that the ST rotor undergoes becomes a value larger than the allowable limit temperature ΔTal, resulting in significant wear and tear over its life.

FIG. 5 shows the second conventional load changing method in a combined cycle plant, in which the loads of four GTs are sequentially changed.

To reduce the load on a plant that is currently operating at rated load, first close the #4HRSG outlet valve from time t ₁ to t ₂ to disconnect #4HRSG from linked operation. HRSG generated steam is discharged to the condenser through the turbine bypass valve. Closing the outlet valve reduces the ST load to 75%. Then, from time _t2, the load on #4GT is lowered to 0 load. The reason why the outlet valve is closed first is due to the #4HRSG outlet steam temperature due to the #4GT load drop and other conditions during rated operation.
This is to prevent the main steam header 6 from becoming fatigued due to thermal deformation and thermal stress due to imbalance with the HRSG outlet steam temperature. During the process of #4GT load reduction, the #4GT/HRSG unit is in standalone operation, and heat recovery to the steam turbine is not performed, and operating for a long time in such a state is disadvantageous in terms of plant efficiency. It is necessary to completely stop #4GT at time t, _t3, when 4GT reaches zero load.

In this way, the remaining three GTs will be operating at their rated capacity, the plant load will be 75%, and the plant efficiency will be restored to almost the rated point. Do the same below and further #3GT
If you stop #2 GT, you can achieve a load of 50%, and if you stop #2 GT, you can achieve a load of 25%. During this time shown in 48
Although the steam temperature after the first stage of ST is constant and the rotor does not wear out over its life, this method has the following drawbacks.

(1) The plant load change rate shown in curve 46 in Figure 5 is small. (Less than 1/4 of the first method) (2) Once GT is stopped, it takes at least 20 to 30 minutes to re-align.

In other words, the load change characteristics are poor.

6 and 7 show the load control method of the present invention, which takes the advantages of the two conventional methods described above and eliminates their drawbacks. That is, ST
Until the change in steam temperature after the first stage reaches ΔTal shown in FIG. 3, the GT load is reduced all at once to maintain the rapid change in plant load. If a load increase command is received during this time, the load on all GTs will be increased at the same time as shown in Figure 6, returning the load to the rated load. FIG. 7 shows a case where the load decrease is large and the steam temperature after the first stage of ST reaches the limit value ΔTal.
When the steam temperature change after the first stage of ST reaches ΔTal at time t ₁ , the control system is switched sequentially from that point on, first closing the outlet valve of #4 boiler, and reducing the load on #4 GT from time t ₂ . Stop #4GT after setting it to zero. If further load reduction is required, then close the #3 boiler outlet valve at time t ₃ and close the #3 boiler outlet valve at time t _4.
After setting the load of #3GT to 0, stop #3GT. When a load increase command is received at time t ₅ , first the loads of #1 and #2 GT currently in operation are immediately increased to the rated load, and then the loads of #3 and #4 GT are increased sequentially from time t ₇ , and The outlet valve of the boiler is opened to supply steam to the steam turbine.

Since #3 and #4GT have been temporarily stopped, it will take about 20 minutes to increase their speed again and rejoin them, so when the load increase command is issued, start #3 and #4GT. Instructions shall be given. Therefore, time _t7 in FIG. 7 indicates the point in time when the re-paralleling of #3 GT is completed.

By carrying out the load control as described above, the change in steam temperature after the first stage of ST can be kept within ΔTal as shown in FIG. 748.

After stopping only #4GT or #4, #3, #2GT
Even when a load increase command is issued after the ST has stopped, the operating procedure is the same, and in both cases, the change in steam temperature after the first stage of ST can be maintained within ΔTal.

FIG. 8 shows a combined cycle system diagram to which the method of the present invention is applied, and is equipped with a main steam temperature detector 10 and a temperature switch 11, and an inner wall temperature detector 12 and a temperature switch 13 after the first stage of the ST. .

The temperature switches 11 and 13 operate according to the logic shown in FIG. 9 when the main steam temperature or the inner wall temperature after the first stage changes by more than the limit value ΔTal. i.e. 9
2 is an electrical signal output by the temperature sensor, 93
94 is a delay circuit, and 94 is a deviation calculator 95 which is a monitor switch.If the temperature changes very slowly, the temperature switch is not activated by the function of the delay circuit.

The reason why the first stage steam temperature was not used is that it is difficult to directly detect it due to the structure of the steam turbine.

10 and 11 show a specific control system and logic diagram for implementing the present invention.

In FIG. 10, 101 is a plant load demand, and the deviation from the total plant output added by an adder 103 is obtained by a subtracter 102. This deviation is integrated by the proportional-integral controller 104 and becomes a control signal for each GT.

The control signals to each GT are as shown in 105 to 108.
Relays in parallel GT, b contacts of relays energized when ΔT>ΔTal of steam turbines shown in 109 to 112, and respective gas turbine control devices 117 to 12 via analog memories 113 to 116.
It is input to 0.

As long as ΔT<ΔTal, each GT in parallel receives the same control signal and increases/decreases the load at the same time. However, when the condition of ΔT>ΔTal is satisfied, relay 10
The contacts 9 to 112 are disconnected and each GT output control signal is held at the signal value held in the analog memory.

FIG. 11 shows the logic circuit of this embodiment, and will first be explained using an example during load drop.

At the beginning of the load drop, since ΔT<ΔTal, all GTs are simultaneously controlled as shown in FIG. 10, and the logic in FIG. 11 does not operate. When the amount of load reduction increases and reaches ΔT>ΔTal, the control signal shown in FIG. 10 is disconnected and held at that value. At the same time, in the logic of Fig. 11, ΔT>Δ
The condition of Tal is to close the #4 boiler outlet valve and further lower the load of #4GT (this load drop is
The load signal is lowered at a constant rate to stop each GT, and is given separately from the load signal held in the analog memory shown in FIG. 10), and is disconnected.
As long as the load drop command continues and the condition of ΔT > ΔTal continues, then #3GT and #2GT
is stopped by the same logic. Due to GT stoppage, the entire plant load will fall below the plant load demand, and the load control signal will increase or Δ
When T<ΔTal, the relays 109 to 112 become non-energized again, and a control signal is given to the GTs in parallel.

In the case of load increase, first check the target load and current GT.
The number of additional GTs required to start up to achieve the target load is determined from the number of operating GTs, and a start command is given to that GT.

It takes about 20 to 30 minutes for the additionally started GT to speed up, parallel, and reach full load, but during that time, the currently operating GT becomes ΔT > ΔTal and the increase in the load control signal is not stopped. It will rise as far as it will go to reach full load.

The additionally activated GTs are sequentially brought to full load and supply steam to the steam turbine. This operation will be explained assuming that a load increase is commanded at time _t5 as shown in FIG. At time _t5 , the #3 boiler outlet valve and #4 boiler outlet valve are already closed, and the loads on #3GT and #4GT are also zero. When a command is given to raise the plant load to full load at time _t5 , contacts 105 to 108 in FIG. 10 close, and the loads of #1GT and #2GT that have been operating up to now are immediately raised to full load. On the other hand, in FIG. 11, additional activation of #3GT and #4GT is determined by determining the number of additional activation units. At time t ₇ , when the addition of #3 GT is completed, the load is increased to full load. (The command for this load increase is also given individually to the control device of each GT, not from the analog memory in Figure 10.) When #3GT reaches full load at time _t8 , the #3 boiler outlet valve is opened, and When this is fully opened at t ₉ , the load on #4GT is increased in the same way as above.

As can be understood from the above explanation, in the present invention, when a load change is commanded, the load is first changed on the gas turbines in operation all at once, and only when the resulting steam temperature change to the steam turbine reaches a predetermined value is the boiler output output. Since the gas turbine's unique load changes are made sequentially while the gas turbine is separated from the steam turbine, the life of the steam turbine is not wasted and moreover, rapid plant load changes are possible.

[Brief explanation of the drawing]

Figure 1 is a general combined cycle system diagram.
Figure 2 is a characteristic diagram showing the relationship between GT load, gas temperature, and steam temperature, Figure 3 is a characteristic diagram showing the relationship between temperature change after the first stage of ST and life wear and tear, and Figure 4 is a characteristic diagram showing the relationship between GT load, gas temperature, and steam temperature.
A time chart showing the simultaneous control method, Fig. 5 a time chart showing the conventional GT sequential control method, Figs. 6 and 7 a time chart showing the load control method of the present invention, and Fig. 8 a time chart showing the implementation of the present invention. A system diagram showing an example, FIG. 9 is a block diagram showing details of temperature switches 11 and 13 in FIG. 8, FIG. 10 is a load control system diagram of the embodiment shown in FIG. 8, and FIG. Logic diagram of the embodiment. 1-1 to 1-4... Gas turbine, 2-1 to 2
-4...Gas turbine generator, 3-1 to 3-4...
...Exhaust heat recovery boiler, 4-1 to 4-4...Exhaust heat recovery boiler outlet valve, 5-1 to 5-5...Turbine bypass valve, 7...Steam turbine, 8...Steam turbine generator, 10 ... Steam temperature detector, 12 ... First stage rear inner wall temperature detector, 11, 13 ... Temperature switch, 101 ... Plant load demand, 102
...Subtractor, 103 ...Adder, 113-116
...Analog memory, 117-120...Gas turbine load control device.

Claims (1)

[Claims] 1. In a combined cycle plant consisting of multiple gas turbines and a steam turbine driven by steam generated from the exhaust heat of these gas turbines, when a change in plant load is commanded, the operating gas turbines are first loaded all at once. A combined method characterized in that after the temperature change of the steam supplied to the steam turbine exceeds a predetermined limit value, the generated steam is separated from the steam turbine and a load change unique to the gas turbine is sequentially performed. Load control method for cycle plants.