Overview
Cement production enterprises are large energy consumers in the national economy, and the cement industry has been listed as one of the country's key areas for resource conservation. Under the policy of the State Council to speed up the construction of a conservation-minded society, it is of great significance to improve the energy-saving manufacturing and application of the cement industry and establish a conservation-oriented cement industry system. Given the current contradiction between supply and demand of energy at home and abroad, cement manufacturers must reduce energy consumption through various means to obtain the best economic benefits and the highest labor productivity. In the production of cement, the motor load power consumption accounts for nearly 30% of the cost, while the high-voltage motor used to drive the fan occupies a large proportion in the motor. For a cement production line, 25% to 30% of the power is used for Drag all kinds of fans, so it is very important to do a good job of reducing the efficiency of the fan motor.
First, the problem of the traditional baffle and hydraulic coupler adjustment
At present, many cement plants have serious phenomena of large horse-drawn carts. At the same time, due to changes in operating conditions and output, the amount of air flow required by the system also changes. Most fans adopt the traditional method of adjusting the opening of the inlet and outlet valves. Realization, and this method is to increase the wind resistance, sacrificing the efficiency of the fan to meet the requirements, serious losses, maintenance is difficult and high operating costs.
Comparison of power consumption characteristics of different adjustment methods of centrifugal fans,
Frequency converter and hydraulic coupling comparison:
According to the theory of fluid mechanics, in the ideal condition of the fan, the shaft power is proportional to the third power of the speed. When the speed of the fan is reduced, the shaft power decreases with the third power of the speed, and the power required to drive the motor also decreases accordingly. Obvious power-saving effect.
Because the hydraulic coupling is controlled by hydraulic technology, according to the principle of energy conservation, the mechanical energy transferred by the hydraulic oil is converted into heat energy when the output speed of the fluid coupling is low, and the rotational speed of the motor is constant. Therefore, energy is saved. The effect is relatively much worse, that is, the use of frequency converter is the best solution, and the size of energy-saving rate varies with the speed control method, generally up to 30% ~ 70%.
In addition, because the hydraulic coupler is placed between the motor and the blower pump, if the hydraulic coupler fails, the blower and the pump will stop, which will affect the operation of the complete set. The use of high-voltage inverter, motor and fan pump is directly connected, if the high-voltage inverter failure, through a simple switch, the motor can run at frequency, the entire unit can continue to run.
Second, the principle of frequency conversion speed saving
Asynchronous motor variable frequency speed regulation is to change the synchronous speed by changing the stator power supply frequency f to achieve speed control, in the speed control from high speed to low speed can maintain a small slip, and therefore consumes a small slip power, efficiency High is the most reasonable speed control method for asynchronous motors.
By the formula n=60f/p(1-s)
It can be seen that if the power supply frequency f is uniformly changed, the synchronous rotation speed of the motor can be smoothly changed. Asynchronous motor variable frequency speed regulation has the advantages of wide speed range, high smoothness and hard mechanical characteristics. At present, frequency conversion speed regulation has become the most important speed regulation mode for asynchronous motors and has been widely used in many fields.
Centrifugal fan, fluid mechanics has the following principles: output air volume Q is proportional to speed n; output pressure H is proportional to speed n2; output shaft power P is proportional to speed n3; ie:
When the air volume of the fan needs to be changed, such as adjusting the opening degree of the damper, a large amount of electric energy will be consumed in the resistance of the valve and the pipeline system in vain. If the use of variable frequency speed regulation air flow, shaft power can be reduced significantly with the flow. The transformation of the fan through frequency conversion not only saves energy but also greatly improves the performance of the equipment. The above formula provides sufficient theoretical basis for frequency conversion energy conservation.
Third, three-ring high-voltage frequency converter principle and characteristics
Three-ring high-voltage frequency conversion adopts several low-voltage PWM frequency conversion power units in series to achieve direct high-voltage output. The frequency converter has the characteristics of extremely low harmonic pollution, high input power factor, good output waveform quality, no additional harmonics, motor heating, torque ripple, noise, dv/dt and common mode voltage. Without adding output filters, ordinary asynchronous motors can be used, including domestic motors.
The main circuit diagram of the high voltage frequency converter is as follows:
High-voltage inverter system diagram
The 6KV grid voltage is stepped down by the secondary side isolation transformer to supply power to the power unit. The power unit is a three-phase input, single-phase output AC-DC PWM voltage source inverter structure, which realizes the high-voltage direct output of variable-voltage and frequency conversion. Supply high voltage motor. Taking 6KV output voltage level as an example, each phase consists of 6 power units rated at 690V. The output phase voltage reaches 3780V and the line voltage reaches 6.6KV. Each power unit is powered by a set of secondary sides of the input transformer. , between the power units and transformer secondary windings insulated from each other. The secondary winding adopts a delta-edge delta connection method to realize multiplexing in order to reduce the input harmonic current. For the 6KV voltage level conversion, it is a 36-pulse rectification circuit structure, and the input current waveform is close to a sine wave. Because input current harmonic distortion is very low, the integrated factor of the input of the frequency converter can reach above 0.95. The following figure shows the input voltage and current of the inverter.
Frequency input voltage and current
The inverter output adopts multi-phase-shifting PWM technology, 6KV output is equivalent to 13 levels, the output voltage is very close to a sine wave, dv/dt is very small.
The increase in the number of levels is conducive to improving the output waveform. The heating of the motor caused by harmonics, noise and torque ripple are greatly reduced, so this inverter has no special requirements on the motor, can be directly used for ordinary asynchronous motors, no output filter. The following figure shows the inverter output voltage and current waveforms.
Machine input line voltage and current
Compared with inverters directly connected in series with high-voltage devices, instead of using traditional devices in series to achieve high-voltage output, instead of using the entire power unit in series, the maximum voltage the device can withstand is the voltage of the DC bus in the unit and can be used directly. In low-voltage power devices, devices do not have to be connected in series, and there is no voltage equalization problem caused by the series connection of devices. The low-voltage IGBT power module used in the power unit has a simple drive circuit and is mature and reliable.
The characteristics of the three-ring high-voltage frequency converter compared with other frequency converters:
Unlike other inverters, SH-HVF series high-voltage inverters do not produce undesired side effects from users during the conversion process:
(1) It will not cause significant harmonic distortion in the plant's power distribution system; no power filter is required; no interference with sensitive equipment; no resonance problem with power factor compensation capacitors.
(2) The power factor is very high, 95% or more in the entire adjustment speed range, without power factor compensation.
(3) It is not necessary to reduce any rated value of the motor due to output harmonics. The motor does not generate extra heat compared to the direct use of the grid voltage.
(4) Torque ripple that causes mechanical resonance will not occur.
(5) The motor noise will not be significantly increased compared to the direct use of the grid power supply.
(6) Compared with the direct use of grid power, it will not have a significant effect on the motor insulation.
(7) The motor's rated torque may not be limited within the entire motor speed range, but only by the motor's own overheat limit value.
(8) Fan noise is less than 75 decibels, so normal conversation can be performed even near inverters operating at full load.
(9) The unit adopts a modular structure, so if it is needed, the damaged module can be replaced in a few minutes. The microprocessor-based advanced diagnostic program can accurately find any fault location.
Fourth, three-ring high-voltage frequency converter in the Wannian cement plant dust fan application
1, site overview
Wannian Cement Plant is located in Wannian County, Jiangxi Province, and the Zhe-Ji Line in the east. It is adjacent to Poyang Lake and Handan Railway Station. The 206 State Road is extended by the factory and the transportation is very convenient. Wannian Cement Plant is a key cement producer in China and the largest cement manufacturer in Jiangxi Province. It has wet and dry cement production lines and produces 1.2 million tons of ordinary cement annually.
Milling mill raw material mill exhaust fan, high temperature fan for ordinary asynchronous motor drag, the original mode of operation for the raw material mill motor running at full speed, by adjusting the opening of the outlet windshield to adjust the amount of air volume; high temperature fan used Fluid coupling speed control. This operation has the following disadvantages:
(1). Regulatory response lags, slow adjustment, and low adjustment accuracy.
Reliance on damper adjustment actuators to adjust the damper opening itself is a last resort, because the damper adjustment mechanism contains a considerable part of the mechanical mechanism, limited by the mechanical part to adjust the speed is limited, the adjustment accuracy is also affected, often on the scene of the air volume control is not It is in place and can not even meet the requirements of on-site craftsmanship.
(2). The throttle adjustment wastes electrical energy, which is unscientific and uneconomical.
Although the damper adjustment is simple in structure and investment is small, the adoption of damper adjustment artificially changes the resistance curve of the air duct, and a large amount of energy is wasted on the damper.
(3). The motor and fan have been tested at full speed and the maintenance cycle is short.
Due to the full-speed operation of the motor, mechanical parts such as motor bearings are heavily worn and the fan dampers are worn out.
(4) There is a large amount of fluid coupling maintenance and there is oil leakage.
In summary, the motor frequency conversion transformation is imperative. To completely change the existing technology, the speed of the fan must be adjusted by changing the motor speed, so as to achieve the purpose of adjusting the air volume, so as to meet the requirements of on-site process and extend the equipment and equipment use. Lifespan, to achieve the purpose of energy saving.
2, frequency conversion reform program
In 2009, Sanhuan changed the frequency of kiln fans #3, #4, and #5 in Wannian Cement Plant. According to Wannian cement plant's motor parameters and load conditions, the high-voltage frequency converters produced by Sanhuan are deployed and operated in a one-to-one manner; this product has the advantages of large design margin, strong overload capacity, convenient operation, and reliable quality.
In order to fully ensure the reliability of the system, the frequency converter is equipped with an industrial frequency bypass device. When the frequency converter is abnormal, the frequency converter can not operate normally, and the motor can be automatically switched to the power frequency running state to ensure the production needs; the schematic diagram is as follows:
In the figure, k1, k2 and k3 are vacuum contactors in the same cabinet, k2 and k3 are electrically interlocked, and the inverter and its automatic incoming line cabinet are provided by Party B. When the frequency converter fails to operate, it is automatically divided into k2, k1, and k3. The frequency conversion time from frequency conversion to power frequency is about 5 seconds. The motor and user side high voltage circuit breaker QF retains Party A's original equipment.
Control method: The raw material grinding exhaust fan automatically controls the rotation speed of the motor by collecting the pressure difference between the inlet and outlet in the dust collector; the temperature control of the original system by the high temperature fan is changed from the original hydraulic coupler adjustment to the inverter adjustment, and the original fluid is removed. The force coupler connects the hydraulic coupler with a connecting shaft between the motor and the hydraulic coupler. The frequency converter regulates the motor itself and finally adjusts the pressure at the kiln preheater (high temperature fan inlet). Condition requirements.
3, energy-saving analysis
Now after the frequency conversion of #3 raw material grinding and exhaust fan, it has achieved remarkable energy saving effect. Before the transformation, the fan runs at about 740r/min (the motor runs at full speed). After the transformation, the fan speed is often running at about 450r/min (that is, running At around 40 Hz, the power consumption when adjusting the baffle is greatly reduced, and the power saving effect and economic efficiency are remarkable. Before and after the frequency conversion transformation, the operating data of the motor is shown in the following table
In the above table, the calculation method of the average motor power before reconstruction is: 40×1.732×6×0.80=332kW
After the transformation, the average motor power is calculated as: 21×1.732×6×0.96=216 kW
Note: When the inverter is running, the power factor of the inverter input is 0.96; when the motor is running, the power factor is 0.8.
Based on the above actual data, it can be concluded that the energy saving rate of the fan after reconstruction is: (332-216)/332=35%
The equipment is operated for 300 days a year. The actual electricity charge is 0.45 yuan/kWh, and the electricity cost saved in a year is: (332-216) x 300 x 24 x 0.45 = 380,000 yuan.
Wannian Cement Plant's frequency conversion reform project was completed and put into production in July 2009. Under the same conditions of cement production, the energy consumption of the fan under the frequency conversion conditions and power frequency conditions is compared with the energy-saving test. The highest energy saving is calculated: The rate reached 37.93%, and the average electricity saving rate was 24.68%. After the actual operation, due to the fluctuation of cement production, the electricity saving rate was far greater than the above value. After six months of operation, it is estimated that the annual electricity consumption will be about 9.34 million kwh, and the annual electricity savings will be 4.67 million yuan. Has significant economic benefits.
V. Conclusion
Zhou Shaoliang, deputy director of Wannian Cement Plant, explained to us the actual effect of the plant's transformation: Since the 6 high-voltage frequency conversion transformation of the wind turbines, the power consumption of the plant has been significantly reduced, and the soft start function of the high-voltage frequency converter has greatly reduced the impact of start-up. The impact of the current on the motor and the power grid effectively reduces the failure of the motor, thereby prolonging the maintenance period and service life of the motor, and also effectively avoiding the adverse impact of the impact load on the power grid. After the frequency conversion transformation, the fan speed is about 80% of the original, which greatly extends the service life of the fan, motor bearings and fan impeller. The original damper is fully open, greatly reducing its wear, prolonging the service life of the damper, reducing maintenance and maintenance costs, and further reducing the Windage resistance; high temperature fan to eliminate the original hydraulic coupler, reduce the stoppage of the kiln due to hydraulic coupling failure, improve the operating rate of the large kiln, and save the high maintenance cost of the hydraulic coupling; high-voltage inverter unique smooth regulation The mechanical wear of the fan and the motor is reduced, the temperature of the bearing and the bearing bush are reduced, the maintenance cost is effectively reduced, and the service life of the equipment is prolonged.