I. Overview

For a long time, the Chinese government has attached great importance to energy conservation. China's "Five-Year Plan" for energy conservation and comprehensive utilization of resources has proposed high-voltage, high-power frequency conversion as one of the key energy-saving technologies, and it is required to vigorously promote the demonstration of high-voltage and high-power frequency conversion. engineering.

At present, the competition in the cement industry is very fierce, but the key is the competition of manufacturing costs, and the motor power consumption accounts for nearly 30% of the cost, and the high-voltage motor used to drive the fan occupies a large proportion in the motor, so do the motor down It is extremely important that consumption efficiency work be done. At present, many cement plants have serious phenomena of large horse-drawn carts. If the use of variable-frequency speed control technology to change the operating speed of equipment to adjust the size of the air volume, it can not only meet the production requirements, but also to save energy, while reducing the caused by adjusting the baffle Economic losses caused by wear and frequent shutdowns of baffles and pipes. Therefore, in the cement plant fan frequency control technology, can save a lot of energy, improve production efficiency, bring greater benefits for the cement plant. According to the specific situation, after the fan adopts variable frequency speed regulation, the energy saving rate is in the range of 30%-50%, and the total investment such as the equipment and other installation costs of the inverter can usually be recovered within one and a half to two years.

Second, the traditional baffle regulation problems

The traditional adjustment method of the fan is to adjust the opening of the inlet baffle to adjust the air volume, which is a backward method with poor economic efficiency, large energy consumption, serious equipment damage, difficult maintenance, and high operating costs. The main problems are as follows:

1. When using baffle adjustment, a large amount of energy is lost during the closure of the baffle. For wind turbines, the most effective energy-saving measure is the use of speed regulation to regulate the flow. Since most of the fans are square torque loads, the shaft power is roughly related to the speed, so when the fan water pump speed drops, the power consumed is greatly reduced. Fig. 1 shows the curve of the power consumption vs. air volume when the wind turbine adopts various adjustment methods. Curve 1 is the power consumed by the motor when the output damper is controlled, 2 is the power consumed by the motor when the input damper is controlled, and 3 is the power consumed by the motor when the slip speed control (slip motor, fluid coupling) is used. 4 is the power consumed by the motor when the frequency conversion speed control is controlled. The bottom curve is the actual required shaft power (that is, the output power of the motor shaft) during the speed control. It can be seen that among the many adjustment methods, the best energy-saving effect is frequency control.

2. The medium has a large impact on the baffle valves and pipes, and the equipment is seriously damaged.

3. The baffle is slow to move, manual operation is not easy for personnel, and improper operation may cause fan vibration. Baffle actuators are generally high-torque electric actuators. They have many failures, they cannot adapt to long-term frequent adjustments, and the adjustment of linearity is poor, making closed-loop automatic control more difficult, and the dynamic performance is not ideal.

4. The starting current of the asynchronous motor when directly starting is generally 6-8 times of the rated current of the motor, which will cause a large impact on the power grid and will also cause the motor to generate heat. The strong impact torque has many adverse effects on the mechanical life of the motor and the fan. . There are winding motor using water resistance method to start, there are disadvantages such as complex equipment and low reliability. In the past, cement plants also used hydraulic couplers for speed control. The disadvantages of hydraulic coupling devices are large volume, large noise, narrow speed range, low efficiency, and complicated oil system maintenance.

Third, the advantages of using frequency control

1. Frequency control can save a lot of energy that was originally lost in the baffle closure process, greatly improving the economic benefits. 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.

It can be seen from the formula n=60f(1-s)/p 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.

For centrifugal fans, hydrodynamics has the following principles: output air flow Q is proportional to speed n; output pressure H23

It is proportional to the speed n; the output shaft power P is proportional to the speed n; that is:
Q1/Q2=n1/n2
H1/H2=(n1/n2)2
P1/P2=(n1/n2)3

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 damper and pipeline system resistance in vain. If the use of variable frequency speed regulation air flow, shaft power can be reduced significantly with the flow. In frequency control, when the fan is lower than the rated speed, the theoretical power saving is

E=[1-(n'/n)]×P×T (kWh)

Where: n-rated speed

n'—— actual speed

P——motor power at rated speed

T - working hours

The above formula provides sufficient theoretical basis for frequency conversion energy conservation.

2. After adopting variable frequency speed regulation, soft start can be realized, and the impact on the power grid and the impact of mechanical load are greatly reduced, and the life of the motor and the fan are prolonged. At the same time, after adopting frequency conversion speed regulation, the reactive power of the motor is instantaneously compensated by the filter capacitor in the DC link of the inverter, and the input power factor of the inverter can reach more than 0.95. Relative to the direct power frequency motor operation, the power factor is greatly improved, especially for low speed motor. After the realization of variable frequency speed regulation, the fans are often operated below the rated speed, the wear of the fan and the baffle of the fan, the wear of the bearings, and the damage of the seal are all greatly reduced and the maintenance workload is reduced. The vibration and noise of the motor operation are also significantly reduced.

3. After adopting variable frequency speed regulation, it can conveniently form closed-loop control and automatically adjust. The 420mA signal output from the regulator is sent to the inverter. Through the inverter to adjust the motor speed, the air volume can be adjusted smoothly, and the linearity is better. Dynamic response is quick, making the equipment safe and stable in a more economical state.

Third, the principle and characteristics of Epa high voltage inverter

Shanghai Inavert Power Electronics Co., Ltd. Innovert series high-voltage inverter (unit series multi-level PWM voltage source inverter) is a direct high-voltage output voltage source inverter. It uses several low-voltage PWM inverter power units in series to achieve direct high-voltage output. The frequency converter has small harmonic pollution to the power grid and high input power factor. It is not necessary to use input harmonic filters and power factor compensation devices. The output waveform is good, there is no problem of harmonics caused by the motor additional heating and torque ripple, noise, output du/dt, common-mode voltage and other issues, do not have to set the output filter, you can use the ordinary asynchronous motor.

Innovert series uses speed sensorless vector control technology, all digital control, with a large starting torque, high speed accuracy, anti-power grid fluctuations and strong load disturbance characteristics.

The grid voltage (eg 6KV) is stepped down by the secondary side isolation transformer to supply power to the power unit, the power unit is a three-phase input, the single-phase output AC/DC PWM source inverter structure, the output of the adjacent power unit The terminals are connected in series, the center points are connected to form a Y-connected structure, and the other three terminals realize the high-voltage output of the variable-frequency conversion and are supplied to the motor. 3KV output voltage level The inverter consists of three power units with rated voltage of 690V in series. By changing the number of series-connected power units per phase, high-voltage output at different voltage levels can be achieved. Each phase of 6KV frequency converter is made up of 5 power units in series, and each phase of 10KV frequency converter is made up of 8 power units in series.

Each power unit is powered by a set of secondary windings of the input transformer, and the power units and the transformer secondary windings are 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 6KV voltage class inverters, the 15 secondary windings that supply 15 power units are grouped into 3 groups and divided into 5 different phase groups. The difference between them is 12o electrical angle to form a 30-pulse rectifier circuit structure. The input current waveform is close to a sine wave, and the total harmonic current distortion is about 1%. Since the input current harmonic distortion is very low, and the diode rectification method is used, the integrated power factor of the inverter input can reach more than 0.95. Figure 3 shows the inverter's input voltage and current waveforms.

The inverter output adopts multi-electrical phase shift PWM technology. The power unit of the same phase outputs the fundamental wave voltage of the same amplitude and phase, but the carriers of the series units are staggered by a certain electrical angle to realize multi-level PWM output. The voltage is very close to a sine wave. The level of the output voltage at each level is only the unit DC bus voltage, so du/dt is small. The power unit uses a relatively low switching frequency to reduce switching losses and increase efficiency. The rated efficiency of the inverter can be 98.5%. The overall efficiency after input transformers is still above 97%. Due to the use of phase-shift PWM, the equivalent switching frequency of the motor voltage is greatly increased, and the number of output levels is increased. Take the 6KV output inverter as an example. The output phase voltage is 11 levels, the line voltage is 21 levels, and the output is equivalent. The switching frequency is 6KHZ. The increase of the level number and the equivalent switching frequency will help to improve the output waveform and reduce the output harmonics. The harmonic heating causes the motor to generate heat, and the noise and torque ripple are greatly reduced. There is no special requirement that can be directly used for ordinary asynchronous motors.

In comparison with conventional current source inverters and three-level voltage source inverters that are directly connected in series with high voltage devices, since the power cells are connected in series, the maximum voltage that the device can withstand is the voltage of the DC bus in the cell. The devices do not have to be connected in series and do not exist. The problem of voltage equalization caused by the series connection of the devices. The power unit adopts the conventional IGBT power module, the driving circuit is simple, and the technology is mature and reliable.

The power unit adopts a modular structure. All power units in the same frequency converter can be interchanged, and maintenance is also very convenient. Due to the power unit series structure, the power unit bypass option can be adopted. When the power unit is faulty, the control system can automatically bypass the faulty unit, and the inverter can still derate and continue to run, greatly improving the reliability of the system.

Fourth, application examples

In May 2006, a cement plant in the south used the Innovert series of high-voltage variable-frequency speed control devices produced by Shanghai Aipa Power Electronics Co., Ltd. on the exhaust gas treatment fan at the kiln tail. So far it has been operating well and it has achieved significant energy savings.

The kiln exhaust gas treatment fan is a 10kV high-voltage motor. The motor parameters are as follows:

Winding Type Asynchronous Motor, Model:YRKK-630-10 Rated Power:800KW Frequency:50HZ Rated Voltage:
10000V Wiring: Y/Y Rated current: 62.21A Open-circuit rotor voltage: 1591V Rated rotor current: 307A
Rotation speed: 594rpm Power factor: 0.781 Insulation class: F Protection class: IP54 Lanzhou Motor Factory was manufactured in 2004.

Inverter model: Innovert 10/10-70, rated voltage 10KV, rated current 70A, capacity 1050KVA.

In the original production process, according to how much material is added in the kiln and the rotational speed of the kiln body, the air volume of the exhaust fan is adjusted by adjusting the opening of the damper damper. The design margin is relatively large. During the normal production process, the flap damper opening is small, and the wind pressure difference between the damper flaps is large, causing a large throttling loss. The operating mode is full-open damper, and the speed of the motor is adjusted by frequency conversion to achieve the purpose of adjusting the air volume. The inverter controls access to the original DCS system and the DCS system completes normal operation. The normal operating frequency of the inverter is around 40Hz.

Before and after the frequency conversion transformation, statistics were made on the corresponding operational data. Now part of the data is analyzed as follows.

Before the transformation, the average power consumption of the motor was 680 kW, and the average power consumption after the transformation was 410 kW. After the transformation, the power consumption decreased by 270 kW. Calculated according to 330 days of annual operation,
The annual electricity is: 270*24*330=2138,400 degrees
At 0.5 yuan per kilowatt hour, the annual energy saving benefit is 2138,400 * 0.5 = 1.106 million yuan
Electricity saving rate: 270/680=39.7%

The performance comparison list of the fans before and after the transformation is as follows:

Average power consumption after transformation (kW) 680 410
10kV Switching Side Current (A) 52.7 27.8
Starting method Rotor string water resistance start Frequency conversion soft start fan noise
Bearing temperature rises high and low

The frequency converter is also designed with an industrial frequency bypass circuit (Figure 6). In the event of a failure of the frequency converter, the motor can be automatically cut to the original power frequency power supply circuit, and the original water resistance is used to start the motor to ensure that the motor Normal operation without affecting production. When the power frequency is running, the fan speed will increase and the wind pressure will change greatly. Therefore, the damper of the fan should be timely adjusted on the DCS in time to reduce the output air volume of the fan to the required value of the working condition.

Fifth, the problem that should pay attention to when adopting frequency conversion velocity modulation

1. Reliability considerations. The continuous production nature of the cement industry determines that high-voltage frequency converters used in cement plants require high reliability and ensure safe production. The Innovert series adopts mainstream power unit series technology solutions, instead of directly connecting the power devices in series, which avoids the problem of voltage equalization caused by the direct series connection of the devices and essentially guarantees the reliability of the system. At the same time, the product's unique speed sensorless vector control technology improves the starting torque and speed accuracy, while improving the ability to resist power grid fluctuations and load disturbances, greatly improving the reliability.

2. The influence of input harmonics of the inverter on the power grid. If the inverter input current harmonics are large, the following hazards will occur:
Misoperation of relay protection devices in the power supply system may lead to large-scale power outages. Measurement instrumentation errors increase, affecting the measurement accuracy and control performance. Affects the normal operation of other power electronic devices, electronic computer systems and communications equipment. Harmonics increase the loss of electrical equipment such as motors, transformers, and capacitors. In severe cases, they can overheat or burn. Innovert series high-voltage frequency inverters have very low input current harmonic distortion and do not generate harmonic pollution to the power grid. The IEEE meets IEEE 5191992 and GB/T14549-93 standards. Large and medium-sized cement plants have a high level of automation. Most of them use automated instrumentation and computer control systems. They have high harmonic requirements for power systems, and Innovert's inverters have great advantages.

3. The effect of inverter output waveform on the motor. Since the application of frequency conversion in cement plants is a large part of the transformation of old equipment, the original ordinary motor is designed to operate directly in the power grid, and the voltage waveform of the power grid is basically a sine wave. If the inverter output waveform quality is not good, it will have a bad influence on the motor. Inverter output harmonics can cause additional heating and torque ripple of the motor, noise increases, output dv/dt and common-mode voltage can affect the motor insulation. Innovert series high-voltage inverter can use the original ordinary asynchronous motor because of the good quality of output waveform and no need to set the output filter.

Sixth, the conclusion

At present, many blowers and large horse-drawn carts in cement plants are serious. The air volume adjustment method of the fan is basically adjusted by the baffle plate, which consumes large energy, has poor economic efficiency, and has serious equipment damage. It is urgent to adopt advanced high-voltage frequency conversion speed control for technological transformation to reduce the power consumption of the cement plant and improve the economic efficiency of the enterprise.

The cement industry adopts high-voltage variable frequency speed control technology for wind turbines. It has been proved that the application of high-voltage frequency converters by Shanghai Electric Power Electronics Co., Ltd. is necessary and possible, and the economic benefits are significant.

Innovert series high-voltage frequency inverter has high reliability and good input and output waveform quality. It is suitable for frequency conversion and speed regulation of cement plant fans, can improve the reliability of equipment operation, save a lot of energy, and bring greater economic and social benefits for cement plants. , with high promotion value

references

1 Yan Wei, Chen Boshi, High-voltage Frequency Conversion Technology, Electrotechnical Journal, 1999.3
2 Yan Wei, Chen Boshi, Zhou Heliang, Zhao Xiangbin The origin, current situation and prospect of multi-level high-voltage inverters in series units. Electric Drive 2006.6

Shanghai Aipa Power Electronics Co., Ltd.