Application of variable frequency negative pressur

2022-08-21
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Application of variable frequency negative pressure control of induced draft fan of 300MW unit

Abstract: This paper expounds the modification of the original induced draft logic and the addition of a new variable frequency control scheme design in the Siemens distributed control system telepermme, briefly describes the application level of this transformation in sunshine power plant, and puts forward the advantages and disadvantages of this control design and transformation suggestions

key words: furnace pressure variable frequency technology decentralized control system

variable frequency speed regulation device can optimize the operation state of the motor, greatly improve its operation efficiency and achieve the purpose of energy saving. In the past, limited by factors such as price, reliability and capacity, it has not been widely used in China's fan market. In recent years, with the rapid development of electronic devices and control technology, the price of high-voltage inverter continues to decline, the reliability continues to increase, and the modular design makes its capacity almost unlimited. Accordingly, high-voltage high-capacity inverter is also gradually widely used

Shanxi sunshine Power Generation Co., Ltd. installed a set of 2000kw/6kv high-voltage frequency conversion device produced by Beijing Lide Huafu Electric Technology Co., Ltd. on two induced draft fan motors for the technical transformation of 1 . The controller is composed of high-speed MCU, industrial PC and PLC. Single chip microcomputer realizes PWM control. The industrial control PC provides a friendly all Chinese windows monitoring and operation interface, and can realize remote monitoring and networking control at the same time. The built-in PLC is used for the logic processing of switch signals in the cabinet, which can flexibly interface with the user on site to meet the special needs of users. The high-voltage inverter uses s in Siemens' PLC and has good interface ability with DCS system. According to the operation characteristic requirements of induced draft fan and the specific requirements of high-voltage frequency converter control, the following interface and control scheme between DCS system and variable frequency speed regulation system are determined

1. Design of interface scheme between DCS system and high-voltage frequency converter

there are 22 signals between DCS system and high-voltage frequency converter, including 18 switching signals and 4 analog signals. The switching value signals of high-voltage frequency converter of each induced draft fan include: ① standby state; ② Operation status; ③ Stop status; ④ Light fault alarm; ⑤ Serious fault alarm; ⑥ High voltage closing is allowed; ⑦ Unit bypass status; ⑧ Start command; ⑨ Stop command. The analog signal of high-voltage frequency converter of each induced draft fan includes: speed control command and speed feedback signal

the frequency conversion control scheme is realized by defining the logical configuration of the above signals in the DCS system

2. The following contents are added to the DCS control.

in order to realize the start and stop control and speed regulation of the variable frequency induced draft fan, the following contents are added to the DCS display and control:

(1) the start and stop operation of the high-voltage converter is realized through the DCS system for the remote start and stop of the high-voltage converter

(2) DCS controls the speed control of high-voltage frequency converter to realize the manual automatic control of induced draft frequency conversion

(3) add light fault alarm block, heavy fault alarm block and power frequency bypass status of high-voltage converter to the display alarm of DCS system

3. Description of operation mode and control logic

3.1 operation mode of induced draft fan high voltage frequency converter

under normal circumstances, the induced draft fan starts in frequency conversion mode. Considering the possible failure of high voltage frequency converter, it also has 1 frequency conversion mode, 1 power frequency operation mode and 2 power frequency operation modes

the operation mode of high-voltage inverter is divided into local and remote control. In the local control state, the speed command signal output by DCs tracks the speed feedback of the high-voltage converter. At this time, the remote operation of the high-voltage converter is invalid

the high-voltage frequency converter is controlled by DCS in two ways: automatic and manual. In the manual state, the operator controls the speed of the high-voltage converter by changing the picture speed control block to realize the adjustment of negative pressure

3.2 allowable conditions for starting the high-voltage frequency converter of induced draft fan

the following three conditions must be met for starting: ① the high-voltage part of induced draft fan a and B has been started; ② The start ready switch sent by the high-voltage frequency converter of induced draft fan a and B locally from its PLC is put into operation. ③ The output of the speed setting value of the high-voltage frequency converter of induced draft fan a and B is less than 30%

since the premise of starting the high-voltage frequency converter is that the high-voltage switch of the induced draft fan motor must be closed and the start feedback is 1, the original conditions for starting the induced draft fan continue to function in the whole logic, that is, the original conditions for starting the fan remain as the allowable conditions for starting the high-voltage frequency converter of the induced draft fan. In addition, considering the actual local conditions of the high-voltage converter, the ready signal sent by the high-voltage converter and the frequency conversion ready of a/b induced draft fan are added as another condition for starting

in the debugging process of remote start-up of high-voltage converter, it was found that since the command in the speed setting block of high-voltage converter may be at a higher speed, at this time, starting the high-voltage converter will inevitably have a large disturbance to the furnace negative pressure, and it is easy to cause misoperation, so the limit that the command must be less than 30% was added in the start-up

3.3 automatic adjustment of the speed adjustment of the high-voltage frequency converter of the induced draft fan

(1) the switching value part of the automatic speed of the A and B high-voltage frequency converters

when the stationary blades of the induced draft fan are put into automatic operation, the speed of the A and B high-voltage frequency converters will be locked for automatic operation. In addition, when the value formed in the deviation circuit exceeds a certain value (tentatively 50%), the high-voltage converter will be automatically cut off. When the furnace negative pressure signal fails, the sensor fault signal will be sent, and the high-voltage converter will exit automatic. When the furnace negative pressure is triggered at a low value, the oxygen index of the locking speed is convenient to increase the fire resistance of the tested material after a delay of 3S. When the furnace negative pressure is triggered at a high value, the locking speed decreases after a delay of 3S

(2) analog quantity of automatic speed of a and B high-voltage frequency converters

since the variable-frequency regulation object is the same as the regulation object of the stationary blade of the induced draft fan, a lifelong infringement investigation system is established, so the original deviation is directly led out as the deviation of the existing variable-frequency regulation, which acts on the existing induced draft variable-frequency control. The balance circuit combined with the speed is added according to the characteristics of frequency conversion to balance the output on both sides. At the same time, the gain circuit of its single and double fan frequency conversion mode is also added independently. Because the original deviation forming circuit includes the feedforward part of the total air volume, it will not be added in the new frequency conversion speed circuit. Considering that once a single induced draft fan frequency conversion trip occurs, and the frequency conversion mode operation cannot be restored, the current balance circuit in the original baffle control circuit is changed to the position feedback balance circuit, At the same time, after the frequency conversion of the other induced draft is gradually increased to the maximum, the automatic induced draft is put into operation

3.4 relevant tripping protection involved in induced draft fan frequency conversion

(1) the frequency conversion tripping of the fan on one side trips the forced draft fan on the corresponding side, and the logic of closing the corresponding baffle and stationary blade in parallel remains unchanged

(2) after the frequency conversion tripping of the fans on both sides, the MFT tripping circuit in the original boiler main protection PLC controller remains unchanged due to the high-pressure switching of the corresponding fans a and B

(3) in the original induced draft fan tripping circuit, the function of high-voltage frequency converter tripping induced draft fan in case of heavy fault is added, so as to ensure that the induced draft fan is tripped in case of frequency conversion mode, and this condition is locked in case of power frequency mode

the flow of variable frequency control of induced draft fan is shown in Figure 1

Figure 1: induced draft variable frequency control flow chart

4. Induced draft variable frequency automatic parameter setting test and related commissioning

(1) start a and B induced draft fans and high-voltage frequency converters, adjust the static blades of the original two induced draft fan baffles to 100%, and set the furnace negative pressure to -50pa

(2) after starting a and B forced draft fans, open their moving blades (forced draft fan baffle) to 10%, set the frequency conversion of a and B induced draft fans at the lowest speed of 225r/min, and put the induced draft frequency conversion into automatic mode at the same time, then conduct the constant value disturbance test, change the set value of furnace negative pressure by 20%, and record the automatic change of frequency conversion

(3) according to the characteristics of pressure regulation, first put the integration time to a larger 4min and the proportional coefficient to 0.3, and then gradually change the proportional coefficient, using the critical proportional band method to set the parameters. After the regulated constant amplitude oscillation occurs, according to the algorithm of the critical proportional band, the preliminary design is carried out first, and there is a basic parameter. P=0.025,Ti=100s;

(4) test the opening of the moving blades of a and B supply fans on the up stroke of every 10% of the opening, observe the change of the negative pressure in the furnace, record the size of the deviation and the time to eliminate the deviation, carry out the down stroke test after completion, and carry out the disturbance test with the moving blades of a/b supply fans

(5) change the opening of one of them to 30%, observe the speed change of induced draft variable frequency and the response time of negative pressure, and then carry out the moving blade disturbance test of forced draft fan. Observe the change of furnace negative pressure every 10% opening, record the size and elimination time of deviation, as well as the command output of high-voltage frequency converter and the actual value of speed. After completion, carry out the down stroke test to verify the proportional gain of single and double fan operation

(6) simulate the action conditions of MFT. When the opening of moving blades a and B of air supply is 50%, observe the change of furnace negative pressure and the action of induced draft override link after fire extinguishing. After the automatic test, when the induced draft frequency conversion is put into automatic, conduct an actual action test on the interlock of induced draft frequency conversion

(7) during the test, it is also necessary to observe whether the negative pressure can be controlled to a satisfactory range automatically by frequency conversion after the forced draft fan is pulled off on one side and the forced draft fan on one side is tripped in the simulation operation

(8) the safe operation of the boiler is the fundamental guarantee of the power of the whole plant. Although the variable frequency speed regulation device is reliable, once there is a problem, it must ensure the safe operation of the boiler, so the switching between power frequency and variable frequency operation must be realized. If one induced draft inverter fails and cannot be recovered in a short time, the automatic control of induced draft shall be adjusted by the original stationary blade. For this reason, it is necessary to test and stop one induced draft frequency converter, turn up the other induced draft frequency converter, and put the original induced draft automatic (static electromechanical reversing blade) into operation for corresponding disturbance. Through the test, some of the parameters are adjusted and modified

according to the above commissioning, the PID parameters of induced draft frequency conversion are gradually optimized. Under the frequency conversion mode, the negative pressure adjustment is stable and reliable, and the adjustment quality has been significantly improved. At the same time, the original static blade baffle is adjusted under the condition of one power frequency and one frequency conversion, and the original static blade adjustment PID parameters have also been modified accordingly. When one frequency conversion fault is switched back to power frequency, The original static blade baffle can still automatically avoid the smoke generated when the temperature is high to the indoor control negative pressure, which provides a spare space for improving the safety of operation

5. Achieve the effect of induced draft variable frequency speed regulation

(1) after the fan variable frequency transformation, the motor realizes soft start, the peak current and peak time are greatly reduced, the impact on the power and load is eliminated, the motor insulation is avoided from being damaged by operating overvoltage, and the service life of the motor and fan is prolonged

(2) variable frequency regulation is adopted to realize the full opening of the baffle, reduce the throttling loss of the baffle, and the speed can be adjusted evenly to meet the needs of peak shaving, which can save a lot of electric energy

(3) the speed decreases under low load, which reduces the wear and vibration of the mechanical part, prolongs the overhaul cycle of the fan, and thus saves a lot of maintenance costs

(4) it has the characteristics of high control accuracy, strong anti-interference ability, low harmonic content, and perfect protection function, which can realize the stable start of zero speed, which is conducive to the safe operation of motors and fans

6. Conclusion

(1) the comparison test of two different operation modes of variable frequency regulation and static blade damper regulation of induced draft fan on site shows that:

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