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2. Equipment Overview Gas constant pressure supply is one of the important factors for the normal operation of the anode forming heat medium boiler. In practice, due to the instability of the gas production equipment and the randomness of the users, the pressure fluctuations of the gas supply system are relatively large (between 0.6 and 5.1 kpa) and have a great influence on the stable combustion of the boiler: the gas pressure is too low. It may cause problems such as flameout of boiler burners and reduction of calorific value, resulting in increase of heat medium oil temperature or insufficient heat supply, affecting anode quality and production capacity; being too high may cause insufficient boiler combustion and waste of fuel. In order to ensure the normal production of prebaked anode carbon blocks, our gas heating system has been designed with a gas booster. The original gas booster fan is driven by an AC asynchronous motor powered by an industrial frequency power supply, and the baffle opening regulates the flow of the boiler gas to achieve stable boiler combustion and temperature control. The control principle is: When the gas inlet pipe pressure is lower than 4.6kpa, the gas booster fan automatically starts operation; when the inlet gas pressure is higher than 4.8kpa, the pressure fan automatically stops. Although this type of pressurization control method can ensure the gas supply required by the heat medium boiler, the fan is operated at a high speed (2924 r/min) and low load operation state for a long time, and the fan body, the drive shaft support bearing seat and other components are greatly vibrated (such as As shown in the attached table), the bearings are worn out and their life is very short. According to maintenance statistics, four new bearings have to be replaced after 60 to 70 days of continuous use. Fans have high failure rates, frequent maintenance, and high costs. Fans work under low-load (or no-load) conditions for a long time, and they waste a lot of energy. Therefore, in May 2005, China Aluminum Qinghai Branch implemented a converter control reform on the booster fan control system of the heat medium system. The actual proof is that the transformation effect is good. 
Note: 1. Item a is the radial vibration value of the load bearing housing; item b is the vibration value of the fan bearing housing. 2. When the fan bearing seat vibration exceeds 13mm/s, bearing temperature rises above 75°C, it will cause bearing seat cracking, bearing locking and fan impeller cracking and other consequences, so in the daily check, when detected When the vibration reaches 12mm/s, the key monitoring operation is adopted and the maintenance work is arranged in time.
3. Inverter control system design In the system design, through the comparison of performance and control characteristics of various inverters in the domestic market, combined with the working characteristics and actual status of the booster fan, the cas800 series inverter produced by abb was finally selected. Device. In addition to the general control functions of other inverters, this type of inverter also has various control functions such as PID control, multi-signal input, sleep function, set value correction, etc., and it can easily realize the gas constant pressure control system.
3.1 System configuration (1) Fan drag motor parameters: model qu180m2bg; power 22kw; rated turn 2925r/min; rated current 42a; rated voltage 380v.
(2) Frequency converter parameters: cas800-30kva380v.
3.2 Control principle The gas fan drive mechanism is driven by a 22kw AC asynchronous motor and controlled by acs800-30kva inverter. The fan frequency converter compares the set value of the external potentiometer with the real-time measured value of the pressure in the gas inlet pipe as the initial set signal for booster frequency converter speed regulation and corrects it through the pid adjustment function as the actual frequency control given value. Achieve the fan speed adjustment to achieve the purpose of constant pressure control. The system principle is shown in Figure 1. The analog signal wiring of the inverter is shown in Figure 2. 
The PID control function of the inverter can effectively suppress the pressure fluctuation caused by the pressure fluctuation of the gas supply system and the surge of the fan, and suppress the disturbance to the inverter. By setting large integral time constants, the integration of gas pressure disturbances into the system does not work, because the integral adjustment effect is delayed. The larger the lag time of the regulator, the slower the adjustment speed, which is very favorable for the stability of the system, the hysteresis is reduced and the system's anti-interference ability is increased.
3.3 Sleep function parameter setting According to the actual monitoring and statistics, the gas pressure will usually rise at night (up to about 4.5kpa), which will cause the motor to work in an ultra-low speed state. In order to save energy, a sleep function control is set on the frequency converter.
The principle of sleep function setting of the constant pressure control system: when the pressure of the gas supply system reaches more than 4.5kpa (the consumption of other gas users decreases, the pressure in the gas supply network will increase). At this time, in order to meet the pressure requirements for the normal operation of the boiler, the pid process controller will reduce the speed of the motor, the motor will not stop, and maintain a low-speed operation. Due to the natural pressure loss existing in the pipeline and the low efficiency of the centrifugal fan during low speed operation, the pressure value set by the inverter is generally required to be higher than the actual required pressure value. When the sleep function detects this low speed condition, this super-low speed operation will be stopped after the sleep delay. After the control system enters sleep mode, the frequency converter is still monitoring gas pressure. When the pressure is lower than the minimum allowable value, and after the wake-up delay, the fan is restarted. The sleep control logic is shown in Figure 3. 
In Figure 3, the relevant parameter setting requirements and actual setting values ​​are as follows:
(1) 40.21 - sleep settings. If the motor speed is lower than this set value (10hz) and is greater than the sleep delay time within the time difference, the inverter enters sleep mode; in sleep mode, the motor stops running.
(2) 40.22 - sleep delay time setting (20s). When the motor speed is lower than the sleep speed setting, the timer starts timing; when the motor speed is higher than the sleep speed, the timer resets. The setting range is 0.0-3600.0s.
(3) 40.23—Sleep function wake-up setting. If the actual value of the process is lower than the set value and the time is longer than the wake-up delay time, the inverter is awakened. The setting range is 0.0 to 100.0% (expressed as a percentage of the process setpoint).
(4) 40.24 - Sleep function wake-up delay time. When the process actual value is lower than the wake-up set value, the timer starts to work; when the process actual value is higher than the wake-up value, the timer resets. Its setting range is 0.0-3600.0s.
(5) 99.02—Inverter control mode. When "3" is set, it is pid control mode.
The sleep function timing diagram is shown in Figure 4. 
Note: 1, td-sleep delay set time value; tsm-sleep time; tdw-wake-up delay time setting value. 2. Diagnosis: The alarm message “sleepmode†on the front panel of the control panel.
4. Control system debugging After the design and installation of the variable frequency speed control system, the system is commissioned according to the actual production requirements to ensure the actual effect of the system control. The final commissioning work of the new system is extremely important and critical. This also affects the success or failure of the entire renovation project. According to the actual production, systematically correcting parameters and adjusting various control parameters to the optimal values, it is possible to fully exploit advanced system control. Sex and superiority.
The parameter setting of the frequency converter is relatively simple, which is mainly related to the parameter settings that need to be modified after the gas inlet pressure signal acts. The debugging steps are as follows:
(1) Determine the range of the set signal. First disconnect the pressure measurement signal, set the potentiometer to the minimum, start the inverter operation (with load), slowly increase the resistance of the given potentiometer to the fan outlet pressure reaches 5.2kpa, measure the actual resistance value and voltage of the potentiometer Value u (ai3, ai4 side), and record the frequency converter output frequency value.
(2) Connect the pressure signal to the control system and set the given signal to u, set the related parameters of the pid to a certain value, increase the pressure slowly, and measure the degree of influence of the gas inlet pressure on the system (by recording its corresponding data, The linear relationship can be drawn. If the correction to the given value is not ideal, the pid parameter (usually changing the proportional constant) can be reset to obtain the optimal correction parameter value.
(3) Determination of wake-up value. Increase the gas inlet pressure to 4.8kpa, and make the given signal voltage u, start the system work, measure the output frequency of the inverter, and then convert it into the actual given size, you can get the size of the wake-up value.
(4) After the system is put into operation, the degree of influence of the actual interference signal must be verified. If the system oscillates, if it is unstable, the PID differential and integral time constant values ​​can be changed to determine the actual required parameter values.
(5) Actual measurement: The maximum voltage given is 8.4v; when the gas pressure is 0.6kpa, the inverter output frequency is 42hz; the wake-up value is set to 82% (actual operation value).
(6) Matters needing attention in the frequency conversion debugging of the gas constant pressure control system: The acceleration time requirement should be as short as possible (the system lag time can be reduced), because if the setting is too long, the gas pressure suddenly drops sharply, the fan cannot accelerate immediately. Causes the boiler to go out; increasing the deceleration time can reduce the pressure fluctuation of the gas supply and the disturbance of the speed control system. The adjustment of the motor speed is conducive to the stable and reliable supply of the gas, and also prevents the frequency converter from increasing and decelerating. Causes the inverter to overheat and protect the function. Although this setting sacrifices the dynamic response and followability of some systems, it can well meet the safety and stability of the boiler's normal operation.
Practice has proved that this frequency conversion control and regulation device is more suitable for pressure fluctuations, and requires a stable flow, high pressure gas supply users. The results of the transformation prove that the effect is good, the gas pressure control index meets the requirements for the normal operation of the boiler, the temperature of the heat medium oil is adjusted quickly and stably, the combustion is sufficient, the boiler efficiency is increased by 9%, and the daily electricity is 73kw.h or more.
5. Concluding remarks The application of frequency converter to the reformation of the heat medium pressurization fan control system has realized the purpose of gas constant pressure control; it has improved the safety and stability of the boiler operation, and saved significant electric energy. According to statistics, electricity can be saved every month at 2200kw.h The failure rate of the fan was reduced to 4h/month (an average of 23h/month before the transformation); the replacement cycle of the fan bearing bearing was extended to more than 7 months (an average of 2 months before the transformation). And achieved the following benefits:
(1) The reform is difficult to implement. It is suitable for the company's own construction and reconstruction, there is no complicated external control circuit design, and debugging is convenient.
(2) The control parameters are easy to modify. Through the internal setting data modification of the inverter, the adjustment and optimization of various parameters of the control system can be realized.
(3) Low renovation costs. Only need to purchase the inverter; many of the original electrical devices have been fully utilized, such as motors, pressure sensors, control boxes, consoles and so on.
(4) Little maintenance work. Two years after the renovation, the electrical control system only had several cleaning and daily inspections; the system work was stable; the maintenance cost of the fan was reduced by 18,000 yuan per year (purchase of bearing costs).
(5) Save energy. Under the same gas consumption and pressure conditions, the motor current is reduced by an average of 7.2 a, and the motor temperature is reduced by 8 to 11°C.
Application and Realization of Frequency Converter in Gas Constant Pressure Control System
1. Introduction With the development of power electronics technology, frequency converters have been widely used in various industries. Frequency control technology has been applied in equipment drive systems to achieve the purpose of motor soft start, continuous speed regulation and energy saving operation. The low-frequency and low-current start-up of the inverter can significantly reduce the mechanical impact force of the equipment, and also avoid the impact on the power grid when the motor is started; it realizes the smooth speed regulation function of the asynchronous motor; it improves the process performance of the equipment and prolongs the service life. And realize energy-saving operation. This paper mainly describes the application of frequency converter control principle and implementation method for constant pressure supply system of heat medium boiler gas.
The relevant parameters of the gas pressure fan of schedule gas at working frequency
Fig. 1 The principle of frequency converter control
Figure 2 Inverter given signal wiring diagram
Figure 3 Inverter sleep function logic setting
Figure 4 inverter sleep function timing diagram