Automatic leveling control method for thermal power generating unit parallel operation induced draft fans

Abstract

The invention discloses a thermal power generating unit parallel operation induced draft fan automatic leveling control method which replaces the existing parallel induced draft fan static blade / moving blade opening and frequency converter frequency leveling control logic by changing bilateral balance control, optimizing an adjustment dead zone, optimizing control feedforward, optimizing MFT override control and optimizing the parallel operation induced draft fan operation control logic. The current deviation of the induced draft fans on the two sides is reduced in the parallel operation process of the induced draft fans, it is guaranteed that the phenomena of wind grabbing, surging, vibration increasing and fan stall do not occur in the parallel operation process of the induced draft fans, and the operation safety performance of the induced draft fans is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of operation of thermal power units, and in particular relates to an automatic leveling control method for induced draft fans of thermal power units in parallel operation. Background Art

[0002] As the pollutant emission indicators, energy consumption indicators and peak-shaving capabilities of thermal power units become increasingly stringent, thermal power units must continue to promote the "three-in-one" transformation of energy conservation, heating transformation and flexibility transformation, further reduce the energy consumption of coal-fired power units, and enhance the flexible adjustment capabilities of the units.

[0003] Thermal power units are basically two primary fans, two supply fans and two induced draft fans running in parallel. The "three reforms and linkage" have higher and higher requirements on the equipment quality and automatic control technology of fans. Most of the primary fans of thermal power units are centrifugal fans, which generally use the inlet guide vane opening adjustment; most of the supply fans and induced draft fans are axial flow fans, which generally use the static blade / moving blade adjustment. There is a dead zone in the static blade / moving blade adjustment. The logical control of the fan is the key to the safe and reliable operation of the unit. The logic control system of the fan of the thermal power unit has been tested for many years, and the logic control is basically perfect. However, in the actual operation process, the operating condition leveling of the parallel running fans is achieved by adjusting the inlet guide vanes, static blades or moving blades. After the inlet guide vanes, static blades or moving blades are leveled, the current of the parallel running fans will have a large deviation.

[0004] At present, many thermal power units have carried out frequency conversion energy-saving transformation on fans. The operating condition leveling of parallel fans is achieved through frequency leveling of the frequency converter. In actual operation, the current of parallel fans will also have a large deviation. According to the test standard "Field Performance Test of Power Plant Boiler Fans" DL / T 469-2022, the field performance test measures and calculates the wind speed, wind pressure and air volume of the parallel induced draft fan: (1) When the induced draft fan uses the inlet guide vane / moving blade opening adjustment, the fan air volume is not linearly related to the inlet guide vane and the guide vane / moving blade opening. The fan air volume increases very slowly when the opening is 0-25%, increases rapidly when the opening is 25%-60%, and increases very slowly when the opening is 60%-100%. In some ranges of the guide vane / moving blade opening, the fan air volume remains basically unchanged, and there is a dead zone for adjustment. At the same time, in actual engineering applications, the mechanical design of the fan, the material properties of the blades, and the degree of wear will affect the operating conditions of the fan, resulting in a large deviation in the fan air volume of the parallel fans at the same guide vane / moving blade opening.

[0005] (2) When the induced draft fan is adjusted by the frequency converter, the fan air volume is proportional to the frequency converter frequency. However, in actual engineering applications, the mechanical design of the fan, the material properties of the blades and the degree of wear will affect the operating conditions of the fan. The fan air volume and the frequency converter frequency are in a curve relationship, resulting in a large deviation in the fan air volume when the parallel fans are at the same frequency.

[0006] (3) The induced draft fan air volume and fan current are positively correlated. In actual engineering applications, the fan current can fully represent the fan air volume.

[0007] If the current deviation of the induced draft fans of thermal power units in parallel operation is too large, it is very easy to cause "wind grabbing" and surge of the induced draft fans in parallel operation, resulting in increased fan vibration. In severe cases, the induced draft fans will stall and cause boiler MFT and unit non-stop. Therefore, during the parallel operation of the induced draft fans, it is particularly important to reduce the deviation of the current of the induced draft fans on both sides, improve the safety performance of the induced draft fan operation, and optimize the parallel induced draft fan leveling control logic. Summary of the invention

[0008] In order to solve the above technical problems, the present invention provides an automatic leveling control method for parallel operation of induced draft fans of thermal power units, which adopts the current leveling control logic of parallel operation of induced draft fans to replace the existing parallel induced draft fan static blade / moving blade opening and inverter frequency leveling control logic, so as to reduce the current deviation of the induced draft fans on both sides during the parallel operation of the induced draft fans, ensure that no wind rush, surge, increased vibration and fan stall occur during the parallel operation of the induced draft fans, and improve the safety performance of the induced draft fan operation.

[0009] The technical solution adopted by the present invention is a method for automatically leveling induced draft fans of thermal power units in parallel operation, comprising the following steps: S1. Change the bilateral balance control: Run the current leveling control logic of the induced draft fans in parallel to reduce the current deviation of the induced draft fans on both sides; S2. Optimize the dead zone of the induced draft fan operation control logic: After the furnace negative pressure measurement point is selected as the controlled variable of the control loop, the first-order delay module (LAG) filters the furnace negative pressure measurement point, and sets the dead zone of the induced draft fan control logic PID to 15Pa, that is, when the furnace negative pressure fluctuates within the dead zone of 15Pa, the induced draft fan control logic PID adjustment will not work; S3, optimize control feedforward, use air supply command as feedforward of furnace negative pressure control, so that negative pressure control loop follows air supply adjustment action; use total air volume of boiler after main fuel trip (MFT) as feedforward of furnace negative pressure control; use coal mill trip as feedforward of furnace negative pressure control; S4. Optimize MFT override control: the current furnace pressure minus -1200 multiplied by 0.02 is overridden after limiting. When the furnace negative pressure is lower than -1200Pa, overridden adjustment is performed; S5. Optimize the operation control logic of parallel-operated induced draft fans: optimize the operation control logic of induced draft fans from the perspective of manual cut-off conditions, overspeed increase conditions, overspeed reduction conditions, increase prohibition conditions, and reduction prohibition conditions, so as to automatically lock the unit output and reduce the load when the induced draft fan operates abnormally.

[0010] Preferably, in the above step S1, the automatic leveling logic of the induced draft fans is run in parallel, and the current deviation value is adjusted according to the current deviation of the induced draft fans on both sides. The current deviation is added to the leveling control deviation of each induced draft fan at the same time, with opposite action signs, so that A increases and B decreases or A decreases and B increases. Finally, the currents of the induced draft fans on both sides are balanced, and the outputs of the two fans are adjusted to be balanced, while the total output remains unchanged.

[0011] Preferably, in the above step S4, when MFT or a fan trips occurs, the opening of the induced draft fan is first reduced, maintained for 45 seconds and then restored. The reduction in the opening of the induced draft fan is a function of the total air volume of the boiler when MFT occurs. At the same time, when the coal mill trips, the opening of the induced draft fan is increased by 3% for each tripped mill.

[0012] Preferably, in the above step S5, the manual switching condition of the induced draft fan stator blades / moving blades or the frequency converter is: 1) The furnace negative pressure signal fails or the furnace negative pressure is bad. The furnace negative pressure is bad, including the furnace negative pressure bad point or the deviation between the furnace negative pressure exceeds 400Pa. The delay is 3 seconds and the automatic cut-off is performed. 2) In non-RB state, if the deviation between the furnace negative pressure setting value and the actual value is greater than 300Pa, the system will be automatically cut off after a delay of 3s; 3) In non-RB state, if the deviation between the stationary blade / moving blade opening or the inverter frequency command and the position feedback is greater than 10, the delay is 3s and the automatic cut-off is performed; 4) The circuit breaker of the induced draft fan on this side trips, delays for 3s, and cuts off automatically; 5) If the induced draft fan circuit breaker on this side trips and the induced draft fan circuit breaker on the other side closes, the circuit breaker will be automatically cut off after a delay of 3s; 6) The inlet and outlet guide vanes of the induced draft fan on this side are faulty; 7) The hydraulic oil pumps of the induced draft fans are all stopped and the oil pressure is low.

[0013] Preferably, in the above step S5, the conditions for increasing the opening of the induced draft fan stator blades / moving blades or the frequency overrun of the inverter are: 1) The sequential control system is used to instruct the override to increase the opening of the static / moving blades of the induced draft fan or the frequency of the inverter; 2) Both induced draft fans stop pulsing.

[0014] Preferably, in the above step S5, the conditions for reducing the opening of the induced draft fan stator blades / moving blades or the frequency overrun of the inverter are: 1) The sequential control system is used to instruct the override to reduce the opening of the static / moving blades of the induced draft fan or the frequency of the inverter; 2) The induced draft fan on this side stops and the induced draft fan on the opposite side is running.

[0015] Preferably, in the above step S5, the condition for prohibiting the increase of the opening of the static blades / moving blades of the induced draft fan or the frequency converter frequency is: 1) induced draft fan stall alarm; 2) When the current of the induced draft fan on this side is greater than 95% of the rated current, the induced draft fan static blade / moving blade opening or the current frequency command of the inverter is used as the output upper limit, and the induced draft fan static blade / moving blade opening or the inverter frequency increase is locked; 3) When the furnace pressure is less than -500Pa, the induced draft fan static blade / moving blade opening or the inverter frequency current instruction is used as the output upper limit, and the induced draft fan static blade / moving blade opening or inverter frequency is increased; 4) When the induced draft fan control oil pressure is lower than the set value, the induced draft fan stator / moving blade opening or the current frequency command of the inverter is used as the output upper limit to lock the induced draft fan stator / moving blade or inverter action.

[0016] Preferably, in the above step S5, the condition prohibiting the reduction of the opening of the static blades / moving blades of the induced draft fan or the frequency converter frequency is: 1) When the furnace pressure is >500Pa, the opening of the static / moving blades of the induced draft fan or the current instruction of the frequency converter is used as the output lower limit, and the opening of the static / moving blades of the induced draft fan or the frequency converter is reduced; 2) When the induced draft fan control oil pressure is lower than the set value, the induced draft fan stator / moving blade opening or the current frequency command of the inverter is used as the output lower limit to lock the induced draft fan stator / moving blade or inverter action.

[0017] Compared with the prior art, the present invention has the following beneficial effects: 1) Compared with the existing parallel induced draft fan stator / moving blade opening and inverter frequency leveling control logic technology, the greatest feature of the present invention is that it directly changes the existing parallel induced draft fan output balance adjustment method, adjusts the current of the parallel induced draft fan, so that the current of the two induced draft fans is automatically adjusted to be consistent, avoiding the deterioration of the working conditions caused by the large current deviation of the induced draft fans on both sides at the same stator / moving blade opening and inverter frequency, ensuring that no wind rush, surge, increased vibration and fan stall occur during the parallel operation of the induced draft fans, thereby improving the safety performance of the induced draft fan operation.

[0018] 2) The dead zone, control feedforward and MFT override control of the induced draft fan operation control logic are optimized according to the actual operating conditions. If the induced draft fan current suddenly changes during boiler operation, the control logic can promptly detect the current that deviates from the normal operating range and immediately make corresponding automatic control adjustments.

[0019] 3) The operation control logic of the entire parallel induced draft fan has been optimized to achieve automatic locking of the unit output and load reduction in the event of abnormal operation of the induced draft fan. This truly uses automatic control to prevent the deterioration of the induced draft fan operating condition and ensure the safe operation of the unit. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 A schematic diagram of the automatic leveling control logic of the induced draft fans of the thermal power units in parallel operation provided by the invention; Figure 2 This is a schematic diagram of the control feedforward logic provided by the present invention. DETAILED DESCRIPTION

[0021] The present invention will be further explained below in conjunction with the accompanying drawings to facilitate better understanding by those skilled in the art.

[0022] Example 1 The automatic leveling control method of induced draft fans of thermal power units in parallel operation comprises the following steps: S1. Change bilateral balance control The current leveling control logic of parallel-running induced draft fans reduces the current deviation of the induced draft fans on both sides; when the induced draft fans on both sides are running in parallel, in order to prevent the "wind grabbing" phenomenon caused by the unbalanced output of the induced draft fans on both sides, it is necessary to adjust the output balance of each induced draft fan. The existing adjustment method is to adjust the static blade / moving blade opening and inverter frequency of the parallel-running induced draft fans to make the static blade / moving blade opening and inverter frequency of the two induced draft fans consistent. In the actual operation process, the deviation of the current of the induced draft fans on both sides is large, and the operating personnel need to adjust the deviation value according to the situation to adjust the output balance of the two fans.

[0023] The present invention directly changes the existing output balance adjustment mode, adjusts the current of the parallel-operated induced draft fans, and makes the currents of the two induced draft fans automatically adjusted to be consistent. The automatic leveling logic of the parallel-operated induced draft fans adjusts the current deviation value according to the current deviation of the induced draft fans on both sides. The current deviation is simultaneously added to the leveling control deviation of each induced draft fan, with opposite action signs, so that A increases and B decreases or A decreases and B increases. Finally, the currents of the induced draft fans on both sides are balanced, and the outputs of the two fans are adjusted to be balanced, while the total output remains unchanged.

[0024] When both fans are in automatic mode, the balance control allows operation; when both fans are in manual mode, the balance control does not work. When one induced draft fan is in manual mode and the other is in automatic mode, if the manual induced draft fan current command is increased, the automatic induced draft fan current command will be reduced accordingly; if the manual induced draft fan current command is reduced, the automatic induced draft fan current command will automatically increase.

[0025] S2. Optimize the dead zone of the induced draft fan operation control logic For the automatic control logic of the induced draft fan, the furnace negative pressure measuring point is selected as the controlled variable of the control loop, and the first-order delay module LAG filters the furnace negative pressure measuring point. Since the furnace pressure signal always carries a small-amplitude noise interference signal, directly using such a measurement signal will cause the static blades / moving blades of the induced draft fan and the frequency of the inverter to operate too frequently, which is not conducive to the safe operation of the unit. If the furnace pressure signal is inertially filtered, the reaction time of the furnace pressure measurement value is increased, making the regulation insensitive. Therefore, the dead zone in the regulator is used to improve the regulation performance. Conventional dead zone settings all use empirical values ​​that do not have a scientific basis. The regulation dead zone of the present invention is optimized according to the accuracy of the furnace negative pressure transmitter. At present, the accuracy level of the furnace negative pressure transmitter of the newly built thermal power unit is 0.5%FS, the maximum range is ±3000Pa, and the allowable error is ±15Pa. At the same time, considering that when the thermal power unit is operating normally, the opening of the static / moving blades of the induced draft fan changes by 1%, which affects the furnace negative pressure change by about 150Pa. Therefore, the adjustment dead zone of the induced draft fan control logic PID is optimized to be set to 15Pa. That is, when the furnace negative pressure fluctuation is within the adjustment dead zone of 15Pa, the induced draft fan control logic PID adjustment will not operate, so as to avoid the interference of the furnace pressure signal causing the static / moving blades of the induced draft fan and the frequency of the inverter to operate too frequently, which is beneficial to the safe operation of the unit.

[0026] S3, Optimized control feedforward 1) Using the air supply command as the feedforward of the furnace negative pressure control, the negative pressure control loop follows the air supply adjustment action, reducing the disturbance of the furnace negative pressure during the boiler air supply adjustment; 2) The total air volume of the boiler after the main fuel trip (MFT) is used as the feedforward of the furnace negative pressure control. The fuel entering the furnace is instantly cut off after the MFT trips, and the fuel entering the furnace is instantly reduced after the primary air trips, causing the furnace pressure to drop sharply in a short period of time and then return to normal later. In order to reduce the disturbance of the furnace pressure when the MFT trips, the opening of the induced draft fan is first reduced when MFT or the primary fan trips, and then restored after a period of time (45s). The reduction in the opening of the induced draft fan is a function of the total air volume of the boiler when the MFT occurs; 3) The tripping of the coal mill is used as the feedforward of the furnace negative pressure control. The furnace pressure fluctuation caused by the reduction of fuel after the coal mill trips requires that the opening of the induced draft fan be increased by 3% for each trip.

[0027] S4, optimize MFT override control The current furnace pressure minus -1200 multiplied by 0.02 is over-regulated after limiting. When the furnace negative pressure is lower than -1200Pa, over-regulation is performed. This over-regulation is effective regardless of whether the induced draft fan is in automatic or manual mode.

[0028] When MFT occurs in the boiler, the furnace pressure will drop sharply in a short period of time due to the instantaneous cut-off of the fuel entering the furnace, and then return to normal later. In order to reduce the disturbance of the furnace pressure when MFT trips the furnace, the static blade / moving blade opening or the frequency converter of the induced draft fan is first reduced when MFT occurs, and then restored after a period of time. The whole process lasts for 45 seconds. The amplitude of the static blade / moving blade closing or the frequency converter reduction is a function of the total air volume when MFT occurs. At the same time, when the mill trips, the moving blades of the induced draft fan should also be closed, and the number of fans that trips should be reduced by 3%. This override is effective regardless of whether the induced draft fan is in automatic or manual mode.

[0029] S5. Optimize the operation control logic of parallel induced draft fans The operation control logic of the induced draft fan is optimized based on the manual cut-off conditions, overspeed increase conditions, overspeed reduction conditions, increase prohibition conditions, and reduction prohibition conditions of the induced draft fan, so as to automatically lock the unit output and reduce the load when the induced draft fan operates abnormally.

[0030] Among them, the manual conditions for the induced draft fan static blades / moving blades or inverter cut-off are: 1) The furnace negative pressure signal fails or the furnace negative pressure is bad. The furnace negative pressure is bad, including the furnace negative pressure bad point or the deviation between the furnace negative pressure exceeds 400Pa. The delay is 3 seconds and the automatic cut-off is performed. 2) In non-RB state, if the deviation between the furnace negative pressure setting value and the actual value is greater than 300Pa, the system will be automatically cut off after a delay of 3s; 3) In non-RB state, if the deviation between the stationary blade / moving blade opening or the inverter frequency command and the position feedback is greater than 10, the delay is 3s and the automatic cut-off is performed; 4) The circuit breaker of the induced draft fan on this side trips, delays for 3s, and cuts off automatically; 5) If the induced draft fan circuit breaker on this side trips and the induced draft fan circuit breaker on the other side closes, the circuit breaker will be automatically cut off after a delay of 3s; 6) The inlet and outlet guide vanes of the induced draft fan on this side are faulty; 7) The hydraulic oil pumps of the induced draft fans are all stopped and the oil pressure is low.

[0031] The conditions for increasing the opening of the static blades / moving blades of the induced draft fan or the frequency overrun of the inverter are: 1) The sequential control system is used to instruct the override to increase the opening of the static / moving blades of the induced draft fan or the frequency of the inverter; 2) Both induced draft fans stop pulsing.

[0032] The conditions for reducing the opening of the static blades / moving blades of the induced draft fan or the frequency overrun of the inverter are: 1) The sequential control system is used to instruct the override to reduce the opening of the static / moving blades of the induced draft fan or the frequency of the inverter; 2) The induced draft fan on this side stops and the induced draft fan on the opposite side is running.

[0033] The prohibited conditions for increasing the opening of the induced draft fan static blades / moving blades or the inverter frequency are: 1) induced draft fan stall alarm; 2) When the current of the induced draft fan on this side is greater than 95% of the rated current, the induced draft fan static blade / moving blade opening or the current frequency command of the inverter is used as the output upper limit, and the induced draft fan static blade / moving blade opening or the inverter frequency increase is locked; 3) When the furnace pressure is less than -500Pa, the induced draft fan static blade / moving blade opening or the inverter frequency current instruction is used as the output upper limit, and the induced draft fan static blade / moving blade opening or inverter frequency is increased; 4) When the induced draft fan control oil pressure is lower than the set value, the induced draft fan stator / moving blade opening or the current frequency command of the inverter is used as the output upper limit to lock the induced draft fan stator / moving blade or inverter action.

[0034] The prohibition conditions for reducing the opening of the induced draft fan static blades / moving blades or the inverter frequency are: 1) When the furnace pressure is >500Pa, the opening of the static / moving blades of the induced draft fan or the current instruction of the frequency converter is used as the output lower limit, and the opening of the static / moving blades of the induced draft fan or the frequency converter is reduced; 2) When the induced draft fan control oil pressure is lower than the set value, the induced draft fan stator / moving blade opening or the current frequency command of the inverter is used as the output lower limit to lock the induced draft fan stator / moving blade or inverter action.

[0035] Working principle: like Figure 1-2 As shown, furnace negative pressure signal 1, furnace negative pressure signal 2, and furnace negative pressure signal 3 are selected by a selector 4 among three selections to obtain an actual furnace negative pressure value 5. The deviation 6 between the actual furnace negative pressure value 5 and the given furnace negative pressure value is a furnace negative pressure deviation 7. The furnace negative pressure deviation 7 is judged by an adjustment dead zone 8 to obtain a furnace negative pressure deviation 9 after adjustment dead zone judgment. After judgment, the furnace negative pressure deviation 9, the control feedforward 11, and the locking command increase / decrease 12 are calculated by the induced draft fan control logic PID 10 to obtain an input furnace negative pressure deviation command of the balance control 13. The input furnace negative pressure deviation command is calculated by the balance control 13 to output an induced draft fan current command. Under the regulation of the MFT override control 16, the A induced draft fan operation control logic 14 and the 1B induced draft fan operation control logic 17 are affected by the induced draft fan current command. The A induced draft fan operation control logic 14 outputs an A induced draft fan output current command 15, and the B induced draft fan operation control logic 17 outputs a 1B induced draft fan output current command 18. The current leveling control logic balancing control 13 of the parallel induced draft fans is adopted to replace the existing parallel induced draft fan static blade / moving blade opening and inverter frequency leveling control logic, so as to reduce the current deviation of the induced draft fans on both sides during the parallel operation of the induced draft fans, ensure that there is no wind rush, surge, increased vibration and fan stall during the parallel operation of the induced draft fans, improve the safety performance of the induced draft fan operation, and solve the problems existing in the prior art.

[0036] The logic of the control feedforward 11 is as follows: the A blower position 19, 0 instruction 20, and A blower operation 21 are judged by the A blower switching module 22 to obtain the A blower instruction 27; the B blower position 23, 0 instruction 24, and B blower operation 25 are judged by the 2B blower switching module 26 to obtain the B blower instruction 28. The A blower instruction 27 and the B blower instruction 28 are averaged by the average calculation block 29 to obtain the blower instruction 30, and the blower instruction 30 is calculated by the furnace negative pressure 31 and the blower instruction function to obtain the blower instruction feedforward 32; the boiler MFT34 instruction is calculated by the furnace negative pressure and the boiler total air volume function 35 to obtain the boiler MFT instruction feedforward 36; the blower instruction feedforward 32, the coal mill start-stop feedforward 33, and the boiler MFT instruction feedforward 36 are summed together by the cumulative calculation block 37 to obtain the control feedforward 11.

[0037] According to the actual operating conditions, the adjustment dead zone 8 judgment, control feedforward 11 and MFT override control 16 are optimized. If the induced draft fan current suddenly changes during boiler operation, the automatic leveling control method of the induced draft fans of the thermal power units in parallel operation can promptly detect the current that deviates from the normal operating range and immediately make corresponding automatic control adjustments.

[0038] The entire parallel operation induced draft fan operation control logic A induced draft fan operation control logic 14 and 1B induced draft fan operation control logic 17 have been optimized to achieve automatic locking of unit output and load reduction in the event of abnormal operation of the induced draft fan. This truly uses automatic control to prevent the deterioration of the induced draft fan operating condition and ensure safe unit operation.

[0039] The above-described embodiments are only for describing the preferred implementation of the present invention, and are not intended to limit the scope of the present invention. Without departing from the design spirit and principles of the present invention, various modifications and improvements made by those skilled in the art to the technical solution of the present invention should all fall within the scope of protection determined by the claims of the present invention.

Claims

1. Automatic leveling control method for induced draft fans of thermal power units in parallel operation, characterized in that: The following steps are involved: S1. Change the bilateral balance control: Run the current leveling control logic of the induced draft fans in parallel to reduce the current deviation of the induced draft fans on both sides; S2. Optimize the dead zone of regulation: After the furnace negative pressure measurement point is selected, it is used as the controlled variable of the control loop. The first-order delay module (LAG) filters the furnace negative pressure measurement point, and sets the regulation dead zone of the induced draft fan control logic PID to 15Pa; S3, optimize control feedforward, use air supply command as feedforward of furnace negative pressure control, so that negative pressure control loop follows air supply adjustment action; use total air volume of boiler after main fuel trip (MFT) as feedforward of furnace negative pressure control; use coal mill trip as feedforward of furnace negative pressure control; S4. Optimize MFT override control: the current furnace pressure minus -1200 multiplied by 0.02 is overridden after limiting. When the furnace negative pressure is lower than -1200Pa, overridden adjustment is performed; S5. Optimize the operation control logic of parallel-operated induced draft fans: optimize the operation control logic of induced draft fans from the perspective of manual cut-off conditions, overspeed increase conditions, overspeed reduction conditions, increase prohibition conditions, and reduction prohibition conditions, so as to automatically lock the unit output and reduce the load when the induced draft fan operates abnormally.

2. The automatic leveling control method for parallel operation induced draft fans of thermal power units according to claim 1 is characterized in that: In step S1, the automatic leveling logic of the induced draft fans is run in parallel. According to the current deviation of the induced draft fans on both sides, the current deviation value is adjusted. The current deviation is added to the leveling control deviation of each induced draft fan at the same time, and the action signs are opposite, so that A increases and B decreases or A decreases and B increases. Finally, the current of the induced draft fans on both sides is balanced, and the output of the two fans is adjusted to be balanced, while the total output remains unchanged.

3. The automatic leveling control method for parallel operation induced draft fans of thermal power units according to claim 1 is characterized in that: In step S4, when MFT or a fan trips, the opening of the induced draft fan is first reduced, maintained for 45 seconds, and then restored. The reduction in the opening of the induced draft fan is a function of the total air volume of the boiler when MFT occurs. At the same time, when the coal mill trips, the opening of the induced draft fan is increased by 3% for each tripped mill.

4. The automatic leveling control method for parallel operation induced draft fans of thermal power units according to claim 1 is characterized in that: In step S5, the conditions for switching off the induced draft fan stationary blades / moving blades or the frequency converter are: The furnace negative pressure signal fails or the furnace negative pressure is bad. The furnace negative pressure is bad, including the furnace negative pressure bad point or the deviation between the furnace negative pressures exceeds 400Pa. The delay is 3 seconds and the automatic cut-off is performed. In non-RB state, if the deviation between the furnace negative pressure setting value and the actual value is greater than 300Pa, the system will be automatically cut off after a delay of 3s; In non-RB state, if the opening of the stationary blades / moving blades or the frequency command of the inverter and the position feedback deviation is greater than 10, the delay is 3s and the automatic cut-off is performed; The circuit breaker of the induced draft fan on this side trips, delays for 3s, and cuts off automatically; The circuit breaker of the induced draft fan on this side trips and the circuit breaker of the induced draft fan on the other side closes, delaying 3s, and automatically cutting off; The inlet and outlet guide vanes of the induced draft fan on this side are faulty; The hydraulic oil pumps of the induced draft fans are all stopped and the oil pressure is low.

5. The automatic leveling control method for parallel operation induced draft fans of thermal power units according to claim 1, characterized in that: In step S5, the conditions for increasing the opening of the induced draft fan stator blades / moving blades or the frequency converter frequency overrun are: The Sequential Control System (SCS) is used to instruct the override to increase the opening of the induced draft fan's static / moving blades or the frequency of the inverter; Both induced draft fans stopped pulsing.

6. The automatic leveling control method for parallel operation induced draft fans of thermal power units according to claim 1, characterized in that: In step S5, the conditions for reducing the opening of the induced draft fan stationary blades / moving blades or the frequency converter frequency overrun are: 1) SCS uses an override command to reduce the opening of the induced draft fan's static / moving blades or the frequency of the inverter; 2) The induced draft fan on this side stops and the induced draft fan on the opposite side is running.

7. The automatic leveling control method for parallel operation induced draft fans of thermal power units according to claim 1, characterized in that: In step S5, the conditions for prohibiting the increase of the opening of the static blades / moving blades of the induced draft fan or the frequency converter frequency are: 1) induced draft fan stall alarm; 2) When the current of the induced draft fan on this side is greater than 95% of the rated current, the induced draft fan static blade / moving blade opening or the current frequency command of the inverter is used as the output upper limit, and the induced draft fan static blade / moving blade opening or the inverter frequency increase is locked; 3) When the furnace pressure is less than -500Pa, the induced draft fan static blade / moving blade opening or the inverter frequency current instruction is used as the output upper limit, and the induced draft fan static blade / moving blade opening or inverter frequency is increased; 4) When the induced draft fan control oil pressure is lower than the set value, the induced draft fan stator / moving blade opening or the current frequency command of the inverter is used as the output upper limit to lock the induced draft fan stator / moving blade or inverter action.

8. The automatic leveling control method for parallel operation induced draft fans of thermal power units according to claim 1, characterized in that: In step S5, the conditions for prohibiting the reduction of the opening of the static blades / moving blades of the induced draft fan or the frequency converter frequency are: 1) When the furnace pressure is >500Pa, the opening of the static / moving blades of the induced draft fan or the current instruction of the frequency converter is used as the output lower limit, and the opening of the static / moving blades of the induced draft fan or the frequency converter is reduced; 2) When the induced draft fan control oil pressure is lower than the set value, the induced draft fan stator / moving blade opening or the current frequency command of the inverter is used as the output lower limit to lock the induced draft fan stator / moving blade or inverter action.

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