JPH0230405B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a boiler steam temperature control device that uses a feedforward control method to suppress fluctuations in steam temperature at a boiler outlet when load changes.

[Technical background of the invention and its problems]

Overview In general, boiler steam temperature control equipment:
A so-called feedback control method that detects the boiler outlet steam temperature (control amount) and operates the spray valve, fuel valve, water heat ratio, and recirculation gas damper using a PID controller to maintain the boiler outlet steam temperature at a predetermined value. Control is being carried out by

However, there is a long dead time or delay in the response of the controlled variable to the manipulated variable. Therefore, the feedback control method alone cannot sufficiently suppress fluctuations in the boiler outlet steam temperature during load changes. For this reason, recently, a feedback control method and a feedback control method are used together.

The following is an example of a boiler steam temperature control system that uses a feed forward control system. That is, focusing on the steam temperature in the upstream region near the steam generation section, the steady-state value of this steam temperature is determined in advance as a function of the load amount or a signal corresponding to the load amount such as the steam flow rate. The device controls the boiler outlet steam temperature in advance (feedforward) by controlling the steam temperature in the upstream portion so as to match the determined steady-state value.
The feature of this control device is that when the load changes, the steam temperature in the upstream section is controlled to a new steady value according to the load amount, thereby proactively controlling the steam temperature at the outlet of the boiler located downstream. The present invention is aimed at improving controllability by reducing the burden on a feedback control element (for example, element 6 in FIG. 1, which will be described later) for the boiler outlet steam temperature.

However, according to the above conventional control device,
The boiler outlet temperature fluctuates when the load changes due to a delay in the change in heat transfer characteristics of the steam reactor in response to a load change, and a difference between this delay time and the transmission delay time of the steam temperature change in the upstream section to the downstream steam temperature change. Not only will it not be possible to suppress this, but in severe cases, it may even aggravate load fluctuations.

Details The phenomenon outlined above will be explained in detail below using the boiler steam temperature superheating process shown in FIG. 1 as an example.

First, regarding the process configuration, the radial superheater 1 (characteristics: Figure 2 a) decreases the amount of heat transfer as the load increases, and the contact type superheater 2 (characteristics: Figure 2 b) where the amount of heat transfer increases as the load increases. are continuously connected (composite characteristic: see Figure 2c). A desuperheater 3 is provided upstream of the radial superheater 1, and is configured to adjust the outlet steam temperature of the radial superheater 1 by spraying spray water W _S regulated by a spray valve 4. . Note that W _B indicates steam.

Next, regarding the characteristics of this process, in a steady state where the outlet temperature of the contact type superheater 2 is controlled to a constant value regardless of the load, the steady value of the radiant type superheater outlet steam changes depending on the load as shown in Figure 3b. Furthermore, the steady value of the steam temperature at the outlet of the attemperator takes on the characteristics shown in Figure 3c depending on the load.

Next, control of the above process will be explained. The steam temperature target value _C1 is given by the function generating element 7 (FIG. 1) which receives the signal L indicating the current load amount. This function generating element 7 has the characteristics shown in FIG. 3c. Therefore, a steady value of the desuperheater outlet steam temperature C ₂ corresponding to the load amount at that time is given to this process as a target value.

At the same time, the outlet steam temperature _C2 of the attemperator 3 is detected, and the spray valve 4 is operated by the attemperator outlet steam temperature control element 5 so that the temperature _C2 matches the target value _C1 . Outlet steam temperature of superheater 2 a ₂
control in advance (feed forward control).
In addition, the contact type superheater outlet steam temperature _a2 is detected and a signal is outputted to correct the target value _a1 of the desuperheater outlet steam temperature _C2 using the contact type superheater outlet steam temperature control element 6. Some have.

The state when the load increases in a process controlled by the control system described above will be described.
Note that when the load decreases, the changes in the process gain and delay time are different from when the load increases, so strictly speaking, they are not symmetrical to when the load increases. However, qualitatively, the description will be made by replacing the terms "temperature rise" and "temperature fall", so the explanation for the load drop will be omitted.

When the load increases, the contact superheater outlet steam temperature a ₂ increases first due to an increase in the amount of heat transfer from the contact superheater 2, which affects the contact superheater 2 outlet steam temperature a ₂ the fastest.

Next, if the attemperator outlet steam temperature C ₂ is constant, the radial superheater outlet steam temperature b decreases due to a decrease in the amount of heat transferred in the radial superheater 1. After a certain period of time has elapsed, this decrease in temperature b affects the contact type superheater outlet steam temperature a ₂ , suppressing the increase in the contact type superheater outlet steam temperature a ₂ and even lowering it. However, if the attemperator outlet steam temperature C ₂ is preliminarily controlled by the function generating element 7 as described above so as to have the characteristics shown in FIG. ₂ )
increases, and the decrease in the radial superheater outlet steam temperature b is suppressed to the point where the decrease in the amount of heat transfer in the radial superheater 1 cancels out this effect. As a result, the effect of suppressing the increase in the contact type superheater outlet steam temperature a ₂ is weakened, and the problem is that the peak value of increase in the contact type superheater outlet steam temperature a ₂ that occurs at the beginning of a load change is increased. It was hot.

In order to solve this problem, conventionally, a first-order delay element 8, shown as a broken line block in _FIG . This reduces the effect of suppressing the decrease in the radial superheater outlet steam temperature b, thereby reducing the contact superheater outlet steam temperature.
There is an example of suppressing the increase in the peak value of _a2 .

However, in this type of system, although the contact superheater outlet steam temperature _a2 has increased,
The purpose is to increase the desuperheater outlet steam temperature C ₂ , and it is not intended to actively suppress the increase in the contact type superheater outlet steam temperature a ₂ . Therefore, in the case of such a method, the peak value of rise in the contact type superheater outlet steam temperature _a2 will still be large. This phenomenon is particularly noticeable when the delay in the change in heat transfer amount with respect to a change in the load of the radial superheater 1 is large or the rate of change is small.

[Purpose of the invention]

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to solve the above-mentioned conventional problems and provide a boiler steam temperature control device that is capable of controlling boiler steam temperature, and in particular, can significantly suppress temperature fluctuation peak values at the time of load changes. do.

[Summary of the invention]

The boiler steam temperature control device of the present invention uses feed-forward control to control the steam temperature in a boiler in which steam from the boiler is sprayed with a desuperheater and then passed through a radial superheater and a contact superheater in series. A boiler steam temperature control device that controls a boiler steam temperature by feedback control, wherein the feedforward control is a boiler steam temperature control device that controls the temperature of steam at the outlet side of the attemperator and the steam temperature obtained as a function of a load equivalent signal corresponding to a load amount. A feedback control signal obtained based on the temperature target value is applied to the attemperator control means for controlling the attemperator, and the feedback control is performed at the outlet of the contact superheater. In the boiler steam temperature control device, which is achieved by applying a feedback control signal obtained based on the side steam temperature and the target value to the desuperheater control means, the steam temperature target value is inputted, and the steam temperature target value is inputted. When the steam temperature target value changes in response to a change in the load equivalent signal, the modified steam temperature target value having a characteristic that follows the steam temperature target value after once changing in the opposite direction to the change direction is set to the desuperheater. It is configured to include an inverse characteristic conversion element added to the control means.

By setting the target value with such an inverse characteristic, for example, when the load increases, the upstream steam temperature is temporarily lowered, and as a result, an increase in the boiler outlet steam temperature is suppressed. Next, in order to follow (that is, increase) the steam temperature in the upstream part of the boiler, the upstream part temperature is set in advance of the steady value corresponding to the changed load in order to compensate for the temperature drop due to the decrease in heat transfer in the radial superheater. can be raised to As a result, the boiler outlet steam temperature can be prevented from falling too low and can be suppressed to small fluctuations. When the load decreases, the above explanation may be reversed.

〔Effect of the invention〕

Therefore, according to the present invention, fluctuations in boiler outlet steam temperature can be significantly reduced. That is,
According to the present invention, the steam temperature in the boiler in which the steam from the boiler is sprayed at a reduced temperature and then passed through a radiant superheater and a contact superheater in series is controlled by feedback control and feedback control. In the boiler steam temperature control device, when the load increases or decreases, the inverse characteristic conversion element
Feedback control is performed as if the load was increased or decreased once, and then increased or decreased, so as the load increases or decreases, the feedback in the opposite direction is first affected by the contact type superheater that appears on the outlet side. As a result, the outlet steam temperature can be properly controlled and the vertical fluctuation range of the temperature can be kept small. As a result, it is possible to eliminate obstacles to increasing the load change rate of a thermal power generation plant, and improve the ability to respond to sudden load changes. In addition, conventionally, the set value of the boiler outlet steam temperature was set with a margin below the upper limit value by the amount of fluctuation, but it can be set higher by the amount that fluctuation is reduced, so thermal efficiency increases. Furthermore, thermal stress on the turbine and superheater metal parts is reduced, which can extend their lifespan.

[Embodiments of the invention]

Next, the present invention will be described in detail based on illustrative embodiments. FIG. 4 shows the configuration of a target value setting circuit for the steam temperature in the upstream part of the boiler according to the present invention.

In FIG. 4, the function generating element 10 receives a signal 20 corresponding to the load amount (hereinafter referred to as load equivalent signal) 20 and generates a steady value signal 21 of the upstream steam temperature corresponding to this signal 20, In terms of content, it corresponds to element 7 in FIG. 1, and in terms of characteristics, it corresponds to FIG. 3c. This steady-state value signal 21 is input to the inverse response characteristic element 11.

As shown in FIG. 5a, the inverse response characteristic element 11 changes the output (solid line) signal in response to a change in the input (dashed line) signal in the opposite direction of the input signal, and then changes the output signal in the direction of the input change. A tracking output signal 22 is output. A typical transfer function of this inverse response characteristic element 11 is expressed by the following equation.

1-T ₁ S/1+T ₂ S output signal 22 is input to the next subtraction element 12.

In the subtraction element 12, the inverse response characteristic element 11
A separately detected upstream steam temperature detection value 23 is subtracted from the output signal 22 from the output signal 22, and a deviation value signal 24 thereof is output. This deviation value signal 24 is output to the upstream steam temperature control element 13.

The upstream steam temperature control element 13 performs control calculations so that the value of the input deviation value signal 24 becomes zero, determines the corresponding operation amount, and drives the operation end of the plant with the operation signal 25. .

In this way, the upstream steam temperature is preliminarily controlled in a change pattern in which the boiler outlet steam temperature does not fluctuate when the load changes. Note that the target value 22 of the upstream steam temperature may be corrected by using the output signal of the boiler outlet steam temperature control element as the target value correction signal 26. This is shown in broken lines in FIG.

Next, the operation when the control device of the present invention is applied to the Plato shown in FIG. 1 will be explained. For example, when the load increases, a steady-state value signal 21 corresponding to the increase in the load equivalent signal 20 is output from the function generation element 10 and input to the inverse response characteristic conversion element 11. The inverse response characteristic conversion element 11 outputs an output signal 22 which decreases once according to its characteristic (FIG. 5a). As a result, the boiler outlet steam temperature is once lowered, and therefore the rise in the boiler outlet steam temperature is controlled.

Next, the output signal 22 rises, but the rise characteristic is such that it precedes the steady value corresponding to the load after changing the upstream steam temperature so as to compensate for the temperature drop due to the decrease in the amount of heat transfer in the radial superheater 1. shall be raised to As a result, it is possible to prevent the boiler outlet steam temperature from falling too low and to suppress fluctuations to a very small magnitude.

On the other hand, when the load decreases, contrary to the above, the upstream steam temperature is raised once and then lowered in advance to the steady value corresponding to the changed load, so the boiler outlet steam temperature fluctuates in the same way as when the load increases. can be suppressed to a very small size.

Here, FIG. 6 shows an example of a control response when the control device according to the present invention is applied to an actual plant. In FIG. 6, the contact superheater outlet steam temperature a, which is the boiler outlet steam temperature, fluctuated by 20.4°C when the conventional control device was used (dotted line), whereas in the case of the present invention ( In the solid line), the fluctuation range decreased to 3.2℃. b is the steam temperature characteristic at the outlet of the desuperheater, c
indicates the load change characteristics. In addition, as shown in Figure 5b, the desuperheater outlet steam temperature, which is the upstream steam temperature, does not increase monotonically to a new steady value once descending, but has a trough-like characteristic, which is due to contact superheating. This is the effect of correction when the target value correction signal 26 from the outlet steam temperature control element 6 (FIG. 1) is added.

Thus, according to the present invention, it is possible to significantly reduce fluctuations in boiler outlet steam temperature.

[Brief explanation of drawings]

Figure 1 is a block diagram showing the conventional boiler steam temperature control process, Figure 2 is a characteristic diagram showing the heat transfer characteristics with respect to the load in the superheater, and Figure 3 is a characteristic diagram showing the steady value characteristics of the steam temperature of each part with respect to the load. ,
FIG. 4 is a block diagram showing the target value setting circuit portion of the boiler steam temperature control device according to the present invention, and FIG. 5a
is a characteristic diagram of the reverse response characteristic element, b is a characteristic diagram when the target value is corrected, and FIG. 6 is a characteristic diagram showing an example of control response when the boiler steam temperature control device according to the present invention is applied to an actual plant. . 1... Radial superheater, 2... Contact superheater, 3
... desuperheater, 4 ... spray valve, 5 ... desuperheater outlet temperature control element, 6 ... contact type superheater outlet steam temperature control element, 10 ... function element, 11 ... reverse response characteristic element, 12... Subtraction element, 13... Upstream steam temperature control element, 20... Load equivalent signal, 21...
Steady value signal, 22... Reverse response characteristic output signal, 23
... Upstream steam temperature detection signal, 25... Upstream steam temperature control element.

Claims (1)

[Claims] 1. After spraying the steam from the boiler with a desuperheater,
A boiler steam temperature control device for controlling the steam temperature in a boiler in which a radial superheater and a contact superheater are passed in series through feedforward control and feedback control, the feedforward control being is a feedforward control signal obtained based on the steam temperature on the outlet side of the attemperator and a steam temperature target value obtained as a function of a load equivalent signal corresponding to the load amount,
The feedback control is performed by adding a feedback control signal to a desuperheater control means for controlling the attemperator, and the feedback control is performed by adding a feedback control signal obtained based on the outlet side steam temperature of the contact superheater and a target value. In the boiler steam temperature control device which is implemented by adding the steam temperature control means to the desuperheater control means, when the steam temperature target value is input and the steam temperature target value changes in accordance with a change in the load equivalent signal, , a boiler steam temperature characterized by having an inverse characteristic conversion element that applies a corrected steam temperature target value of a characteristic that follows the steam temperature target value to the desuperheater control means after once changing in the opposite direction to the change direction. Control device.