阅读说明：本技术 一种亚临界火电机组主蒸汽温度自动控制系统及方法 (Automatic control system and method for main steam temperature of subcritical thermal power generating unit ) 是由 宋国鹏 郭红日 唐峻 高德鹏 崔景辉 刘俊峰 梁健 吉海喆 张渊杰 金国强 王辰 于 2021-11-08 设计创作，主要内容包括：本发明公开了一种亚临界火电机组主蒸汽温度自动控制系统及方法,该系统包括低温过热器来的蒸汽,低温过热器来的蒸汽通过屏式过热器换热升温,再通过减温器对屏式过热器出口的蒸汽温度进行温度控制后,最后通过末级过热器进行换热升温,末级过热器出口的温度即为主蒸汽温度,减温器内部的减温水喷水量由减温水调节阀来控制,减温水调节阀相连的PID控制器和第二加法运算器,本发明控制方法由两部分组成,一部分为PID控制,主要对主蒸汽温度与主蒸汽温度设定值之间的偏差进行修正调整,另一部分为预测控制回路；本发明可提高机组的自动化水平,大大减小运行人员的操作负担,并使机组具有更良好的动态响应品质,获得理想的调节特性。(The invention discloses a system and a method for automatically controlling the temperature of main steam of a subcritical thermal power generating unit, wherein the system comprises steam from a low-temperature superheater, the steam from the low-temperature superheater is subjected to heat exchange and temperature rise through a screen superheater, then the temperature of the steam at the outlet of the screen superheater is subjected to temperature control through a desuperheater, and finally the steam is subjected to heat exchange and temperature rise through a final superheater, wherein the temperature at the outlet of the final superheater is the temperature of the main steam, the water spray quantity of desuperheater in the desuperheater is controlled by a desuperheater regulating valve, and a PID (proportion integration differentiation) controller and a second addition arithmetic unit which are connected with the desuperheater regulating valve are connected with each other; the invention can improve the automation level of the unit, greatly reduce the operation burden of operators, ensure that the unit has better dynamic response quality and obtain ideal regulation characteristics.)

1. The utility model provides a subcritical thermal power unit owner steam temperature automatic control system which characterized in that: the steam temperature monitoring device comprises steam (1) from a low-temperature superheater, wherein the steam (1) from the low-temperature superheater is subjected to heat exchange temperature rise through a screen superheater (2), then the temperature of the steam at the outlet of the screen superheater (2) is controlled through a desuperheater (3), and finally the steam is subjected to heat exchange temperature rise through a final superheater (4), the temperature at the outlet of the final superheater (4) is the main steam temperature, the water spray quantity of desuperheater in the desuperheater (3) is controlled through a desuperheater regulating valve (5), a PID (proportion integration differentiation) controller (9) and a second addition arithmetic operator (17) which are connected with the desuperheater regulating valve (5) are provided with a plurality of desuperheater rear temperature signal measuring points (6) at the outlet of the desuperheater (3), and a plurality of main steam temperature signal measuring points (7) at the outlet of the final superheater (4);

the input signal of the PID controller (9) comprises two paths, the first path is a main steam temperature set value (8), and the set value is directly set by an operator; the second path is the main steam temperature which needs to be controlled and adjusted, is directly measured by a plurality of main steam temperature signal measuring points (7), and is averaged to obtain a main steam temperature calculated value; the PID controller (9) has control functions of proportion P and integral I, the PID controller (9) performs small-amplitude adjustment on the deviation of the main steam temperature, and the large-amplitude adjustment is completed by a predictive control loop; the prediction control loop comprises three paths, the first path is a coal quantity prediction control loop, and the control thought is as follows: introducing a total coal quantity (10) signal, carrying out time delay processing on the total coal quantity (10) signal through a pure lag arithmetic unit (11), and then calculating through a first differential arithmetic unit (12) to obtain an action instruction of the temperature-reducing water regulating valve (5) corresponding to the variation of the total coal quantity (10) signal; the second path is a temperature prediction control loop behind the desuperheater, and the control idea is as follows: averaging a plurality of post-desuperheater temperature signal measuring points (6) obtained by direct measurement to obtain post-desuperheater temperature calculated values, filtering the post-desuperheater temperature calculated values through a filtering block (13), and calculating through a second differential operator (14) to obtain action instructions of a desuperheater water regulating valve (5) corresponding to the variation of the post-desuperheater temperature calculated values; the third path is a main steam temperature protection control loop, and the control idea is as follows: calculating the calculated value of the main steam temperature after the averaging treatment by a function generator (15) to obtain an action instruction of the desuperheating water regulating valve (5) corresponding to the calculated value of the main steam temperature; the three signals are added by a first addition arithmetic unit (16) to obtain a total action command of the temperature-reducing water regulating valve (5) corresponding to the prediction control loop, and then the total action command and the output of the PID controller (9) are calculated by a second addition arithmetic unit (17) to obtain a final action command of the temperature-reducing water regulating valve (5).

2. The automatic control system for the main steam temperature of the subcritical thermal power generating unit according to claim 1, characterized in that: the number of the desuperheater rear temperature signal measuring points (6) and the number of the main steam temperature signal measuring points (7) are two.

3. The control method of the automatic main steam temperature control system for the subcritical thermal power generating unit according to claim 1 or 2, characterized by comprising the following steps: the control method comprises two parts, wherein one part is PID control and mainly corrects and adjusts the deviation between the main steam temperature and the main steam temperature set value, and the other part is a prediction control loop and mainly extracts parameters mainly influencing the steam temperature change according to the reaction characteristics in the boiler so as to carry out intervention control in advance and overcome the characteristic of large steam temperature control inertia; when the temperature of the main steam rises, a positive deviation occurs between the calculated value of the main steam temperature and the signal of the set value of the main steam temperature (8), so that the proportional P action and the integral I action of the PID controller (9) start to act, and an action instruction for increasing the output of the PID controller (9) is sent out; similarly, when the temperature of the main steam is reduced, a negative deviation occurs between the calculated value of the main steam temperature and the signal of the set value (8) of the main steam temperature, so that the proportional P action and the integral I action of the PID controller (9) start to act, and an action instruction for reducing the output of the PID controller (9) is sent; three paths of prediction control loops are designed; the first path of prediction control loop: the main factor influencing the main steam temperature of the subcritical thermal power generating unit is the heat generated by coal combustion, and the main steam temperature is inevitably increased along with the increase of combustion energy when the coal amount is increased under the normal operation working condition of the unit; similarly, when the coal amount is reduced, the temperature of the main steam is inevitably reduced along with the reduction of the fuel energy, meanwhile, the change of the coal amount is considered to generate the influence on the temperature of the main steam after a certain reaction time, the change of the temperature of the main steam is intelligently predicted according to the change of the coal amount, the temperature advance of the main steam is controlled, the specific realization mode is that the reaction time influencing the temperature of the main steam is simulated when the coal amount is changed, a total coal amount (10) signal is processed by a pure lag arithmetic unit (11), the working principle of the pure lag arithmetic unit (11) is to delay and output an input signal, the delay time is delay time arranged in the pure lag arithmetic unit (11), the reaction time of the coal amount is approximately simulated by the pure lag arithmetic unit (11), and then the corresponding action command of the temperature reduction water regulating valve (5) is obtained by the calculation of a first differential arithmetic unit (12), when the coal quantity value output by the pure lag arithmetic unit (11) is increased compared with the previous moment, the first differential arithmetic unit (12) starts to act and sends out a working instruction for increasing the output; similarly, when the coal quantity value output by the pure lag arithmetic unit (11) is reduced compared with the previous time, the first differential arithmetic unit (12) starts to act and sends out a work instruction of reducing the output; the second predictive control loop: under the normal operation condition of a unit, the change trend of the post-desuperheater temperature is consistent with the change trend of the main steam temperature and can change before the main steam temperature, so that the temperature signal is selected to carry out intelligent prediction control, the post-desuperheater temperature signal measuring point (6) is subjected to averaging processing to obtain a post-desuperheater temperature calculated value, the post-desuperheater temperature calculated value is subjected to filtering processing by a filtering block (13), the filtering time is set for 3-5 seconds according to the fluctuation condition of the post-desuperheater temperature signal measuring point (6), the filtering block (13) is used for carrying out smoothing processing on the post-desuperheater temperature signal measuring point (6) to eliminate unnecessary disturbance of the measuring point due to measurement errors of electronic components and fluctuation in the line transmission process, then, a corresponding action instruction of a desuperheating water regulating valve (5) is obtained through calculation of a second differential arithmetic unit (14), and when the post-filtered desuperheater temperature calculated value is increased compared with the last moment, the second differential operator (14) starts to operate and sends out a work instruction for increasing the output; similarly, when the post-desuperheater temperature calculation value after filtering is reduced compared with the last time, the second differential operator (14) starts to act and sends out a work instruction for reducing output; a third control loop: under some abnormal operation conditions of the unit, the method cannot well control the temperature of the main steam, so a protection control loop is set to prevent the temperature of the main steam from being too high or too low, the main steam temperature signal measuring point (7) is subjected to averaging processing to obtain a main steam temperature calculated value by combining a linear relation between the unit desuperheating water regulating valve (5) and the main steam temperature signal measuring point (7), and then a corresponding desuperheating water regulating valve (5) action instruction is obtained by calculating through a function generator (15), wherein the function generator (15) specifically sets parameters as shown in table 1:

table 1: opening function table of main steam temperature corresponding desuperheating water regulating valve

Temperature of main steam (. degree.C.) 520 530 535 542 545 550 Opening of desuperheating Water regulating valve (%) -100 -50 0 0 20 100

The function generator (15) is set in the sense that the main steam temperature of the subcritical thermal power unit is generally set to be 540 ℃ for operation, so that the temperature is considered to be in a normal operation range from 535 ℃ to 542 ℃, and the protection control loop does not intervene; when the temperature of the main steam exceeds 542 ℃, the protection control loop starts to open the desuperheating water regulating valve (5) to quickly lower the main steam temperature signal measuring point (7) so as to prevent the main steam temperature from being over-heated; when the temperature of the main steam is lower than 535 ℃, the protection control loop starts to close the temperature reduction water regulating valve (5) so that the temperature signal measuring point (7) of the main steam is quickly raised to prevent the temperature of the main steam from being too low; the three prediction control loops add through a first addition arithmetic unit (16) to obtain a final action instruction of the desuperheating water regulating valve (5) of the prediction control loop, and then add with the output of the PID controller (9) through a second addition arithmetic unit (17) to obtain a final action instruction of the desuperheating water regulating valve (5), so that accurate control of the main steam temperature signal measuring point (7) is realized.

Technical Field

The invention relates to the technical field of automatic control of thermal power stations, in particular to a system and a method for automatically controlling the temperature of main steam of a subcritical thermal power unit.

Background

The subcritical thermal power generating unit is a generating unit with steam pressure of 15.8-18.5 MPa and temperature of 540 ℃, and is mainly characterized in that a steam drum is arranged in a boiler, the steam drum boiler has strong heat storage capacity due to large water volume, and when the temperature of main steam and the pressure of main steam fluctuate, the temperature and the pressure of the main steam are controlled to be slow by changing the quantity of coal and the like under the influence of inertia of the steam drum, so that the timely adjustment of parameters is not facilitated.

For many years, the control of the main steam temperature is always a difficult point and a key point in the control of the subcritical thermal power generating unit, because the main steam temperature is an object with serious delay phenomenon, the main steam temperature is easily influenced by various factors, the process flow of a controlled object is complex, the main steam temperature is one of essential important parameters for improving the economic benefit of a power plant and ensuring the safe operation of the unit

For a subcritical thermal power generating unit, the temperature control of main steam is to maintain the temperature of steam at the outlet of a final superheater within an allowable range, protect the final superheater so that the temperature of the pipe wall does not exceed the allowable working temperature, and is one of important indexes for verifying the operation quality of a boiler. The main steam temperature is high, the heat efficiency of the unit is high, and in order to improve the comprehensive efficiency of the unit as much as possible, the designed operating values of the main steam temperature of the current unit are all close to the allowable limit temperature of steel. However, the superheater is located in a high-temperature and high-pressure area of the boiler, and the steel is irreversibly damaged due to the excessively high temperature, so that the operation safety of the whole unit is threatened, and therefore, the control of the temperature of the main steam within a reasonable range is very important.

The current main steam temperature control strategy widely applied is a cascade control strategy, and viewed from the dynamic characteristics of a controlled object, the dynamic characteristics of the main steam temperature have certain time delay and larger inertia, and a main steam temperature control system designed only by the outlet steam temperature of a final superheater is difficult to meet the production operation requirements. The cascade control strategy is characterized in that the PID of the main loop controller controls the temperature of main steam, the output of the PID is a set value of the PID of the auxiliary loop controller to adjust the temperature behind the desuperheater, and finally the control of the temperature of the main steam is realized by controlling the water spraying amount of the desuperheater temperature adjusting valve. In the strategy, as long as the temperature after the desuperheater changes, the PID of the secondary loop controller changes the opening degree of the desuperheating water regulating valve, changes the desuperheating water amount, primarily maintains the steam temperature after the desuperheater, and has a coarse adjustment effect on the main steam temperature at the outlet of the final superheater. The temperature of the main steam at the outlet of the final superheater is controlled by a main loop controller PID. As long as the temperature of the main steam at the outlet of the final superheater does not reach a set value, the output of the PID of the main loop controller is continuously changed, so that the PID of the secondary loop controller continuously changes the desuperheating water amount until the temperature of the main steam is restored to the set value of the temperature of the main steam. Although the cascade control strategy is widely applied, the characteristic that the adjusting effect is inconsistent with the expectation under certain working conditions exists, and misunderstanding of operators is easily caused.

Disclosure of Invention

In order to solve the problems in the control of the main steam temperature, the invention aims to provide an automatic control system and method for the main steam temperature of a subcritical thermal power generating unit, which can meet the requirement of more reasonable and effective control of the main steam temperature of the subcritical thermal power generating unit and provide further guarantee for the safe, stable and economic operation of the unit.

In order to achieve the above purpose, the invention is implemented by the following technical scheme:

a subcritical thermal power unit main steam temperature automatic control system comprises steam 1 coming from a low-temperature superheater, wherein the steam 1 coming from the low-temperature superheater is heated through heat exchange of a screen superheater 2, then the temperature of the steam at the outlet of the screen superheater 2 is controlled through a desuperheater 3, and finally the heat exchange and heating are carried out through a final superheater 4, the temperature at the outlet of the final superheater 4 is the main steam temperature, the water spray quantity of desuperheater water in the desuperheater 3 is controlled through a desuperheater water regulating valve 5, a PID controller 9 and a second addition arithmetic unit 17 are connected with the desuperheater water regulating valve 5, a plurality of desuperheater rear temperature signal measuring points 6 are arranged at the outlet of the desuperheater 3, and a plurality of main steam temperature signal measuring points 7 are arranged at the outlet of the final superheater 4;

the input signal of the PID controller 9 comprises two paths, wherein the first path is a main steam temperature set value 8 which is directly set by an operator; the second path is the main steam temperature which needs to be controlled and adjusted, is directly measured by a plurality of main steam temperature signal measuring points 7, and is averaged to obtain a main steam temperature calculated value; the PID controller 9 has control functions of proportion P and integral I, the PID controller 9 performs small-amplitude adjustment on the deviation of the main steam temperature, and the large-amplitude adjustment is completed by a predictive control loop; the prediction control loop comprises three paths, the first path is a coal quantity prediction control loop, and the control thought is as follows: introducing a total coal quantity 10 signal, carrying out time delay processing on the total coal quantity 10 signal through a pure lag arithmetic unit 11, and then calculating through a first differential arithmetic unit 12 to obtain an action instruction of the temperature reduction water regulating valve 5 corresponding to the variation of the total coal quantity 10 signal; the second path is a temperature prediction control loop behind the desuperheater, and the control idea is as follows: averaging a plurality of post-desuperheater temperature signal measuring points 6 obtained by direct measurement to obtain post-desuperheater temperature calculated values, filtering the post-desuperheater temperature calculated values by a filtering block 13, and calculating by a second differential operator 14 to obtain action instructions of a desuperheater water regulating valve 5 corresponding to the variation of the post-desuperheater temperature calculated values; the third path is a main steam temperature protection control loop, and the control idea is as follows: calculating the calculated value of the main steam temperature after the averaging treatment by a function generator 15 to obtain an action instruction of the desuperheating water regulating valve 5 corresponding to the calculated value of the main steam temperature; the three signals are added by a first addition arithmetic unit 16 to obtain a total action command of the temperature-reducing water regulating valve 5 corresponding to the prediction control loop, and then the total action command and the output of the PID controller 9 are calculated by a second addition arithmetic unit 17 to obtain a final action command of the temperature-reducing water regulating valve 5.

The number of the desuperheater rear temperature signal measuring points 6 and the number of the main steam temperature signal measuring points 7 are two.

The control method of the automatic main steam temperature control system of the subcritical thermal power generating unit comprises two parts, wherein one part is PID control and mainly corrects and adjusts the deviation between the main steam temperature and a main steam temperature set value, the other part is a prediction control loop and mainly extracts parameters mainly influencing steam temperature change according to the reaction characteristics in a boiler, intervention control is carried out in advance, and the characteristic of large steam temperature control inertia is overcome; when the temperature of the main steam rises, a positive deviation occurs between the calculated value of the main steam temperature and the set value of the main steam temperature 8, so that the proportional P action and the integral I action of the PID controller 9 start to act, and an action instruction for increasing the output of the PID controller 9 is sent out; similarly, when the temperature of the main steam is reduced, a negative deviation occurs between the calculated value of the main steam temperature and the signal of the set value 8 of the main steam temperature, so that the proportional P action and the integral I action of the PID controller 9 start to act, and an action instruction for reducing the output of the PID controller 9 is sent; three paths of prediction control loops are designed; the first path of prediction control loop: the main factor influencing the main steam temperature of the subcritical thermal power generating unit is the heat generated by coal combustion, and the main steam temperature is inevitably increased along with the increase of combustion energy when the coal amount is increased under the normal operation working condition of the unit; similarly, when the coal amount is reduced, the temperature of the main steam is inevitably reduced along with the reduction of the fuel energy, meanwhile, the change of the coal amount is considered to generate the influence on the temperature of the main steam after a certain reaction time, the change of the temperature of the main steam is intelligently predicted according to the change of the coal amount, the temperature advance of the main steam is controlled, the specific realization mode is that the reaction time of the coal amount changing to the temperature of the main steam is simulated, a total coal amount 10 signal is processed by a pure lag arithmetic unit 11, the working principle of the pure lag arithmetic unit 11 is that an input signal is output in a delay mode, the delay time is delay time arranged in the pure lag arithmetic unit 11, the reaction time of the coal amount is approximately simulated by the pure lag arithmetic unit 11, then, a corresponding action command of the temperature reduction water regulating valve 5 is obtained through the calculation of a first differential arithmetic unit 12, when the coal amount output by the pure lag arithmetic unit 11 is increased at a moment, the first differential operator 12 starts to operate and issues a work instruction to increase the output; similarly, when the coal amount output by the pure lag operator 11 is decreased from the previous time, the first differential operator 12 starts to operate to issue a work instruction for decreasing the output; the second predictive control loop: under the normal operation condition of the unit, the variation trend of the post-desuperheater temperature is consistent with the variation trend of the main steam temperature and can change before the main steam temperature, so that the temperature signal is selected for intelligent prediction control, the post-desuperheater temperature signal measuring point 6 is subjected to averaging processing to obtain a post-desuperheater temperature calculated value, the post-desuperheater temperature calculated value is subjected to filtering processing of a filtering block 13, the filtering time is set to be 3-5 seconds according to the fluctuation condition of the post-desuperheater temperature signal measuring point 6, the filtering block 13 is used for smoothing the post-desuperheater temperature signal measuring point 6 to eliminate unnecessary disturbance of the measuring point caused by measurement errors of electronic components and fluctuation in the line transmission process, then, the corresponding action instruction of the desuperheating water regulating valve 5 is obtained through calculation of a second differential arithmetic unit 14, when the post-filtered post-desuperheater temperature calculated value is increased at a moment, the second differential operator 14 starts to operate and issues a work instruction to increase the output; similarly, when the post-desuperheater temperature calculation value after filtering is reduced compared with the last time, the second differential operator 14 starts to act to send out a work instruction for reducing output; a third control loop: under some abnormal operation conditions of the unit, the method can not well control the temperature of the main steam, so a protection control loop is set to prevent the temperature of the main steam from being too high or too low, the main steam temperature signal measuring point 7 is subjected to averaging processing to obtain a main steam temperature calculated value by combining the linear relation between the unit desuperheating water regulating valve 5 and the main steam temperature signal measuring point 7, then a corresponding action instruction of the desuperheating water regulating valve 5 is obtained by calculating through a function generator 15, and the function generator 15 specifically sets parameters as shown in table 1:

table 1: opening function table of main steam temperature corresponding desuperheating water regulating valve

Temperature of main steam	520	530	535	542	545	550
							Opening degree of the temperature-reducing water regulating valve%	-100	-50	0	0	20	100

The function generator 15 is set in the sense that the main steam temperature of the subcritical thermal power unit is generally set to be 540 ℃ for operation, so that the range from 535 ℃ to 542 ℃ is regarded as a normal operation range, and the protection control loop does not intervene; when the temperature of the main steam exceeds 542 ℃, the protection control loop starts to open the desuperheating water regulating valve 5 to quickly lower the main steam temperature signal measuring point 7 so as to prevent the main steam temperature from being over-heated; when the temperature of the main steam is lower than 535 ℃, the protection control loop starts to close the temperature reduction water regulating valve 5, so that the temperature signal measuring point 7 of the main steam is quickly raised, and the temperature of the main steam is prevented from being too low; the three prediction control loops add through a first addition arithmetic unit 16 to obtain a final action instruction of the desuperheating water regulating valve 5 of the prediction control loop, and then add with the output of the PID controller 9 through a second addition arithmetic unit 17 to obtain a final action instruction of the desuperheating water regulating valve 5, so that the accurate control of the main steam temperature signal measuring point 7 is realized.

Compared with the prior art, the invention has the following advantages:

1) aiming at the characteristic of large inertia of main steam temperature control of a subcritical thermal power generating unit, the change of coal quantity is creatively introduced, a model of reaction time of the change of the coal quantity is established, the main steam temperature is controlled from a combustion angle, and the problem is fundamentally solved.

2) The post-desuperheater temperature is introduced into a control strategy, and the signals are subjected to filtering smoothing, so that the characteristics of large delay and large inertia of main steam temperature control are further overcome, and the accurate control of the main steam temperature is realized.

3) The occurrence of abnormal working conditions is considered, a protection control loop is additionally set, the dangerous conditions that the temperature of main steam is too high or too low and the like are prevented, and further guarantee is provided for the safe operation of a power plant.

4) The main steam temperature under the control of the strategy can increase the input time of automatic control of the desuperheating water regulating valve, greatly reduce the operation burden of operators and provide further guarantee for the intelligent operation of a power plant.

Drawings

FIG. 1 is a schematic diagram of a control system according to the present invention.

Description of reference numerals:

1-steam from a low temperature superheater; 2-platen superheater;

3-desuperheater; 4-final superheater; 5-temperature reducing water regulating valve;

6-measuring point of temperature signal after desuperheater; 7-main steam temperature signal measuring point;

8-main steam temperature set point; 9-PID controller;

10-total coal amount; 11-pure hysteresis operator;

12-first differential operator; 13-a filtering block;

14-a second differential operator; 15-function generator;

16-first addition operator; 17-second addition operator;

Detailed Description

The present invention will be described in further detail with reference to the accompanying drawings and specific embodiments.

As shown in fig. 1, an automatic control system for main steam temperature of a subcritical thermal power generating unit comprises steam 1 from a low-temperature superheater, wherein the steam 1 from the low-temperature superheater is heated through heat exchange of a platen superheater 2, the temperature of the steam at the outlet of the platen superheater 2 is controlled through a desuperheater 3, and finally the steam is heated through heat exchange of a final superheater 4, the temperature at the outlet of the final superheater 4 is the main steam temperature, the water spray amount of the desuperheater in the desuperheater 3 is controlled through a desuperheater regulating valve 5, a PID controller 9 and a second addition arithmetic unit 17 which are connected with the desuperheater regulating valve 5 are provided with a desuperheater rear temperature signal measuring point 6 (two measuring points are usually arranged) at the outlet of the desuperheater 3, and a main steam temperature signal measuring point 7 (two measuring points are usually arranged) at the outlet of the final superheater 4.

The input signal of the PID controller 9 comprises two paths, wherein the first path is a main steam temperature set value 8 which is directly set by an operator; the second path is the main steam temperature which needs to be controlled and adjusted, and is directly measured by main steam temperature signal measuring points 7, 2 measuring points are usually arranged on site, and the calculated value of the main steam temperature is obtained after the two-way averaging processing is carried out on the measuring points; the PID controller 9 has control functions of proportion P and integral I, the PID controller 9 performs small-amplitude adjustment on the deviation of the main steam temperature, and the large-amplitude adjustment is completed by a predictive control loop; the prediction control loop comprises three paths, the first path is a coal quantity prediction control loop, and the control thought is as follows: introducing a total coal quantity 10 signal, carrying out time delay processing on the total coal quantity 10 signal through a pure lag arithmetic unit 11, and then calculating through a first differential arithmetic unit 12 to obtain an action instruction of the temperature reduction water regulating valve 5 corresponding to the variation of the total coal quantity 10 signal; the second path is a temperature prediction control loop behind the desuperheater, and the control idea is as follows: averaging a plurality of post-desuperheater temperature signal measuring points 6 obtained by direct measurement to obtain post-desuperheater temperature calculated values, filtering the post-desuperheater temperature calculated values by a filtering block 13, and calculating by a second differential operator 14 to obtain action instructions of a desuperheater water regulating valve 5 corresponding to the variation of the post-desuperheater temperature calculated values; the third path is a main steam temperature protection control loop, and the control idea is as follows: calculating the calculated value of the main steam temperature after the averaging treatment by a function generator 15 to obtain an action instruction of the desuperheating water regulating valve 5 corresponding to the calculated value of the main steam temperature; the three signals are added by a first addition arithmetic unit 16 to obtain a total action command of the temperature-reducing water regulating valve 5 corresponding to the prediction control loop, and then the total action command and the output of the PID controller 9 are calculated by a second addition arithmetic unit 17 to obtain a final action command of the temperature-reducing water regulating valve 5.

The control method of the invention comprises two parts, one part is PID control which mainly corrects and adjusts the deviation between the main steam temperature and the main steam temperature set value, the other part is a prediction control loop which mainly extracts the parameters which mainly affect the steam temperature change according to the reaction characteristics in the boiler, carries out intervention control in advance and overcomes the characteristic of larger steam temperature control inertia; when the temperature of the main steam rises, a positive deviation occurs between the calculated value of the main steam temperature and the signal of the set value of the main steam temperature 8, so that the action of the proportional P action and the integral I action of the PID controller 9 starts to act, and an action instruction for increasing the output of the PID controller 9 is sent out; similarly, when the temperature of the main steam is reduced, a negative deviation occurs between the calculated value of the main steam temperature and the signal of the set value 8 of the main steam temperature, so that the proportional P action and the integral I action of the PID controller 9 start to act, and an action instruction for reducing the output of the PID controller 9 is sent; considering that a subcritical thermal power unit has fewer steam temperature control means, automatic adjustment is performed only by main steam temperature deviation too late, and the effect is not ideal, some control ideas are performed in advance from the combustion angle of a boiler, and three prediction loops are designed in the invention; the first path of prediction control loop: the main factor influencing the main steam temperature of the subcritical thermal power generating unit is the heat generated by coal combustion, and the main steam temperature is inevitably increased along with the increase of combustion energy when the coal amount is increased under the normal operation working condition of the unit; similarly, when the coal amount is reduced, the temperature of the main steam is inevitably reduced along with the reduction of the fuel energy, meanwhile, the change of the coal amount is considered to generate the influence on the temperature of the main steam after a certain reaction time, the change of the temperature of the main steam is intelligently predicted according to the change of the coal amount, and the temperature advance of the main steam is controlled, and the specific implementation mode is that the reaction time from the change of the coal amount to the influence on the temperature of the main steam is simulated, a total coal amount 10 signal is processed by a pure lag arithmetic unit 11, the working principle of the pure lag arithmetic unit 11 is that an input signal is output in a delayed mode, the delay time is delay time arranged in the pure lag arithmetic unit 11, the reaction time of the coal amount can be approximately simulated through the module, a certain 300MW subcritical thermal power unit is taken as an example, the boiler reaction characteristic is combined, the delay time is set to be 300 seconds, namely the input value of the total coal amount 10 at the current moment, the coal quantity value output by the pure lag arithmetic unit 11 is output after 300 seconds, and then a corresponding action command of the temperature-reducing water regulating valve 5 is obtained through calculation of the first differential arithmetic unit 12, when the coal quantity value output by the pure lag arithmetic unit 11 is increased compared with the last moment, the first differential arithmetic unit 12 starts to act and sends out a working command of increasing the output; similarly, when the coal amount output from the pure lag operator 11 decreases from the previous time, the first differential operator 12 starts to operate to issue an operation command for decreasing the output, and the second prediction control loop: under the normal operation condition of the unit, the variation trend of the post-desuperheater temperature is consistent with the variation trend of the main steam temperature and can change before the main steam temperature, so that the temperature signal is selected for intelligent prediction control, the post-desuperheater temperature signal measuring point 6 is subjected to secondary averaging to obtain a post-desuperheater temperature calculated value, the post-desuperheater temperature calculated value is subjected to filtering processing of a filtering block 13, the filtering time is usually set to be 3-5 seconds according to the fluctuation condition of the post-desuperheater temperature signal measuring point 6, the filtering block 13 is used for carrying out smoothing processing on the post-desuperheater temperature signal measuring point 6 to eliminate unnecessary disturbance of the measuring point due to measurement errors of electronic components and fluctuation in the line transmission process, then, a corresponding action instruction of the desuperheating water regulating valve 5 is obtained through calculation of a second differential arithmetic unit 14, when the post-filtered desuperheater temperature calculated value is increased at a moment, the second differential operator 14 starts to operate and issues a work instruction to increase the output; similarly, when the post-desuperheater temperature calculation value after filtering is reduced compared with the last time, the second differential operator 14 starts to act to send out a work instruction for reducing output; a third control loop: under some abnormal operation conditions of the unit, the method can not well control the temperature of the main steam, so a protection control loop is set to prevent the temperature of the main steam from being too high or too low, the linear relation between the unit desuperheating water regulating valve 5 and the main steam temperature signal measuring point 7 is combined, the main steam temperature signal measuring point 7 is subjected to two-time averaging processing to obtain a main steam temperature calculated value, then the calculated value is calculated by a function generator 15 to obtain a corresponding action instruction of the desuperheating water regulating valve 5, and the function generator 15 specifically sets parameters as shown in table 1:

table 1: opening function table of main steam temperature corresponding desuperheating water regulating valve

Temperature of main steam (. degree.C.)	520	530	535	542	545	550
							Opening of desuperheating Water regulating valve (%)	-100	-50	0	0	20	100

The function generator 15 is set in the sense that the main steam temperature of the subcritical thermal power unit is generally set to be 540 ℃ for operation, so that the range from 535 ℃ to 542 ℃ is regarded as a normal operation range, and the protection control loop does not intervene; when the temperature of the main steam exceeds 542 ℃, the protection control loop starts to open the desuperheating water regulating valve 5 to quickly lower the main steam temperature signal measuring point 7 so as to prevent the main steam temperature from being over-heated; when the temperature of the main steam is lower than 535 ℃, the protection control loop starts to close the temperature reduction water regulating valve 5, so that the temperature signal measuring point 7 of the main steam is quickly raised, and the temperature of the main steam is prevented from being too low; the three prediction control loops add through a first addition arithmetic unit 16 to obtain a final action instruction of the desuperheating water regulating valve 5 of the prediction control loop, and then add with the output of the PID controller 9 through a second addition arithmetic unit 17 to obtain a final action instruction of the desuperheating water regulating valve 5, so that the accurate control of the main steam temperature signal measuring point 7 is realized.