Abstract : This article introduces an application example of HIVERT high-voltage frequency converters in the high-temperature blower at the kiln tail of a 2500t/d clinker production line. It summarizes the development and selection of high-voltage frequency converters, problems encountered and their handling during installation and commissioning, precautions for daily work, and post-use benefit analysis.
Keywords : high voltage frequency converter, harmonics, energy saving, economic benefits
I. Introduction
Lunan Zhonglian Cement Co., Ltd. originally had two 2300t/d clinker production lines, which were key national "Seventh Five-Year Plan" projects. After more than 20 years of operation, the equipment had become aging, leading to increasing maintenance workload and costs. Furthermore, high consumption of raw materials, fuels, and electricity resulted in a year-on-year increase in production costs. In response to the national call for energy conservation and emission reduction, our company upgraded the No. 2 2300t/d clinker production line at the end of 2009. The main equipment is now all high-efficiency equipment, and many fans utilize energy-saving and consumption-reducing electrical products such as high-voltage frequency converters, low-voltage frequency converters, and soft starters, laying a solid foundation for the high efficiency and low consumption of the 2500t/d clinker production line. The original kiln tail high-temperature fan speed control of the 2300t/d clinker production line used a hydraulic coupling, which had high energy consumption. To improve production efficiency and reduce consumption, after comprehensive comparison, we selected HIVERT high-voltage frequency converters, and the raw material system circulating fans also adopted HIVERT high-voltage frequency converters.
II. Introduction to Lunan Zhonglian Cement Co., Ltd.
Lunan Zhonglian Cement Co., Ltd., formerly known as Lunan Cement Plant, was established in 1987 as a key national "Seventh Five-Year Plan" project. It was the first cement company in China to pass three international certifications: ISO9001 quality management system, ISO14001 environmental management system, and ISO10012 metrology system. In 1999, it became a core enterprise under China United Cement Co., Ltd., the cement business platform of China National Building Materials Group. In March 2006, the company was listed overseas. To meet the further development needs of the group and to unify the brand and management model, it was renamed Lunan Zhonglian Cement Co., Ltd. on January 1, 2007. It is also the location of China United Cement's Shandong operations management area.
While prioritizing environmental protection, the company makes rational use of natural resources and adheres to the development philosophy of "making good use of resources to serve construction." It was the first to propose the concept of "green cement," fully utilizes technological means, and continuously strengthens scientific research on resource utilization. The utilization rate of waste residue replacing clay resources is 100%, and the comprehensive utilization rate of limestone is over 98%. The company continuously promotes the improvement of the local social environment and achieves good economic and social benefits.
III. Significance of Variable Frequency Drive Applications
Cement production enterprises are major energy consumers in the national economy, facing the arduous task of energy conservation and consumption reduction. Therefore, they must reduce energy consumption through various means to achieve optimal economic benefits and the highest labor productivity. In a cement plant with a daily output of 5,000 tons of cement clinker, the installed capacity of the plant's blowers accounts for approximately 34.8% to 37.7% of the total installed power, and their electricity consumption accounts for approximately 30% to 40% of the plant's total electricity consumption. Therefore, it is extremely important to do a good job in reducing energy consumption and increasing efficiency of blower motors.
As is well known, fans are designed for full-load operation. However, in actual production, the required air volume varies due to changes in operating conditions and output. Most fans achieve this by adjusting the opening of the inlet and outlet valves, but this method increases air resistance and sacrifices fan efficiency. Variable frequency drive (VFD) technology, by changing the equipment's operating speed to regulate air volume, can both meet production requirements and save energy.
IV. Development and Selection of High Voltage Frequency Converters
1. Development of high-voltage frequency converters
With the development of power electronics, computer technology, and automatic control technology, modern AC speed control technology, represented by variable frequency speed control, has developed rapidly. AC variable frequency speed control overcomes the shortcomings of DC motors, leverages the inherent advantages of AC motors (simple structure, robustness, durability, economy, reliability, good dynamic response, etc.), and effectively solves the inherent limitations of AC motor speed control performance. AC motor variable frequency speed control is a major means of saving energy, improving production processes, enhancing product quality, and improving the operating environment. Due to its high efficiency, high power factor, and excellent speed control and starting/braking performance, variable frequency speed control is widely recognized both domestically and internationally as the most promising speed control method.
Currently, there are numerous low-voltage frequency converter manufacturers in China, most of which produce small to medium-power AC 380V products. However, only a few companies with research capabilities or financial strength (such as Hekang Yisheng) are capable of researching, manufacturing, and providing services for high-voltage frequency converters. With the rapid development of the domestic high-voltage frequency converter industry, mainstream domestically produced high-voltage frequency converters have reached international standards in terms of functionality and reliability, while holding a significant advantage in cost and service. Currently, domestically produced high-voltage frequency converters account for over 60% of the market share, greatly improving the transmission level of my country's industry and promoting frequency conversion energy-saving retrofits in various sectors.
2. Selection of frequency converter
(1) Voltage level
In my country, the commonly used voltage levels for high-voltage motors are 6kV and 10kV, with a small number of 3kV high-voltage motors. Most high-voltage frequency converters have an isolation transformer at the input, so the input voltage of the converter can usually meet the requirements of different power grid voltage levels. For technical upgrade projects, where the motor is originally running at mains frequency and the motor is usually not replaced, the output voltage of the frequency converter after installation must match the grid voltage.
(2) Voltage source type and current source type
Currently, most high-voltage frequency converters are AC-DC-AC type. Based on the different energy storage elements used in the intermediate DC link, they can be divided into voltage source type and current source type.
The advantages of current source inverters are that energy can be fed back to the grid and they can achieve four-quadrant operation. The power supply side often uses a three-phase bridge thyristor rectifier circuit, which results in relatively high harmonics in the input current, generally low input power factor, and a tendency to shut down when the grid voltage fluctuates significantly.
Voltage source type high voltage frequency converters have a relatively high input power factor due to the use of diode uncontrolled rectification, and are not easily affected by grid voltage fluctuations, making them suitable for applications with poor grid conditions.
(3) Harmonic pollution of power grid and input power factor
Harmonic pollution to the power grid primarily depends on the structure and characteristics of the rectifier circuit. There are two main ways to reduce grid harmonic pollution: multiplexing rectification and PWM rectification. Unit-series multilevel high-voltage frequency converters typically have a higher number of rectifier pulses, resulting in less grid harmonic pollution. To reduce grid harmonic pollution, current-source frequency converters usually use 18-pulse rectification. Three-level voltage-source frequency converters require at least 12 pulses, and 24 pulses can be used for higher requirements.
(4) Output waveform and applicability to motors
The main effects of output harmonics on motors are: causing additional heating, leading to extra temperature rise in the motor, which often requires derated operation; harmonics can also cause torque pulsation and increased noise. Therefore, the waveform to be detected should be stable and have few harmonics.
(5) Energy saving rate
For devices like fans or water pumps, the energy-saving rate after applying a frequency converter is not significantly related to the frequency converter itself, but is mainly determined by the original operating conditions of the frequency converter, such as the opening degree of the original baffles and valves.
(6) Bypass cabinet or switching cabinet (optional)
The bypass cabinet or switching cabinet switch can be selected from isolating knife switches, vacuum contactors, or a combination of both, depending on the user's operating conditions.
The function of the bypass cabinet is to connect the motor to the power grid after the frequency converter is taken out of operation, so as to ensure the continuity of production.
The function of the switching cabinet is to switch the inverter output to different motors.
V. Problems and solutions encountered during commissioning of HIVERT high-voltage frequency converters
1. Overcurrent fault alarm (train trip) occurs.
Cause Analysis: The kiln tail exhaust gas supplies air to the raw material system through a high-temperature fan. Due to the long and steeply angled outlet duct of the high-temperature fan, dust in the kiln tail exhaust gas adheres to the duct wall. When the dust accumulates to a certain extent, material collapse occurs, and the dust instantly enters the high-temperature fan, increasing its load and even causing it to reverse. In severe cases, this can lead to motor blockage, causing the high-voltage frequency converter to trigger an overcurrent fault alarm (trip). See Figure 1a for a schematic diagram.
Solution: Drill a hole in the middle of the high-temperature fan's outlet duct, and weld a tapered pipe underneath. When dust adhering to the duct wall collapses, it will pass through the pipe into a small storage bin on the ground for processing. See Figure 1b for a schematic diagram.
2. A drive unit failure occurred.
High-temperature fan high-voltage frequency converter data: Power unit model: HPU690/250D1P, rated current: 250A, no bypass system. After approximately one day of operation following power-on, three power units failed, located at A4, B4, and C4 on the frequency converter. The IGBT inside unit C4 exploded. The other two power units showed no component damage and functioned normally after power-on testing.
Cause analysis:
The screws on the absorber plate of the C4 unit IGBT were loose, which increased the contact resistance between the absorber plate and the positive and negative terminals of the IGBT. As a result, the absorber plate failed to absorb the damage, causing the C4 unit IGBT to break down and explode. It is suspected that the screws loosened during long-distance transportation.
The other two power units, A4 and B4, experienced drive failures not due to device damage, but rather due to the IGBT's own protection mechanism. Previously, power unit C4 had malfunctioned, causing an imbalance in the three-phase output, which in turn caused current fluctuations. The IGBT has a built-in current anomaly protection function. When the current is very large and the cycle is very short (the current fluctuation exceeds the inverter's own protection response time, and the inverter cannot protect itself in time), if it exceeds the IGBT's maximum limit, an alarm signal will be issued and the output will be automatically blocked to prevent IGBT damage. This causes the inverter to shut down, but the inverter's fault type is still classified as a drive fault; in reality, the IGBT is not damaged.
Solution: Replace with a power unit that has been tested and certified by the manufacturer.
3. No fault symptoms or fault records, but the motor trips.
Cause analysis: During equipment operation, when external interference signals, especially pulse signals, interfere with the high-voltage frequency converter, the high-voltage frequency converter may malfunction and stop, i.e., the equipment stops unexpectedly.
Solution: Ensure proper grounding of the high-voltage frequency converter signal lines, and also ensure proper grounding of the input and output signal lines of the DCS system to reduce faults caused by signal interference.
VI. Several issues to pay attention to in daily work
1. Environmental and heat dissipation issues in frequency converter applications
Due to the high dust levels and relatively harsh environment at cement production sites, high-voltage frequency converters, as power electronic devices, generate approximately 3% to 4% heat through losses, requiring heat dissipation (generally achieved through air cooling). Therefore, these converters should undergo enhanced dust prevention measures and routine maintenance, including regular dust removal.
2. Harmonic interference problem
Excessive harmonics can cause additional harmonic losses in the power grid components, leading to malfunctions in relay protection and automatic devices, and affecting the normal operation of various electrical equipment. To avoid control interference, the control lines of high-voltage frequency converters should use shielded cables with single-end grounding, and should be installed separately from power cables in different cable trays during wiring.
3. Problem of tripping due to sudden increase in load caused by changes in fan operating conditions
Comprehensive and targeted testing is conducted on the irregular peak load operation of the fan to eliminate potential faults caused by sudden increases in load due to changes in fan operating conditions, ensuring stable operation of the equipment and reducing the failure rate of motors and fan equipment.
VII. Post-renovation benefit analysis
1. Direct benefits: i.e., energy saving analysis after the renovation.
(1) Energy-saving principle of the fan (as shown in Figure 2):
In the figure, curve L1 is the characteristic curve of the fan when it operates at speed N1, curve L2 is the characteristic curve of the fan when it operates at speed N2, and curves L3 and L4 are the resistance curves of the pipeline network.
When the fan operates at speed N1, it is at point A with an airflow of Q1 and a pressure of H1. The fan power is PA = H1 * Q1. If the production process requires the airflow to be reduced from Q1 to Q2 (using a baffle), the fan will still operate at speed N1, but the air pressure will rise to H3, and the fan's operating point will shift to point B. The fan power will then be PB = H3 * Q2. Due to the baffle's flow-blocking effect, the pipeline resistance curve changes from L3 to L4. Although Q2 < Q1, H3 > H1, so the fan power change between points A and B is minimal.
If the fan airflow is not adjusted using speed settings, the pipe network resistance curve remains constant at L3. By changing the fan speed to reduce the airflow, the fan operates at speed N2, with the operating characteristic curve at L2. The fan operates at point C, with the airflow still at Q2 and the pressure at H2. Compared to points B and C, the reduction in shaft power is: ΔP = PB - PC = (H3 - H2) × Q2
Under the condition that the duct resistance characteristics remain unchanged, the air volume Q, pressure H, shaft power P, and speed N of a centrifugal fan satisfy the following relationships (similarity theorem): Q∝N, H∝N² , P∝N³ .
Therefore: Q1/Q2 = N1/N2, H1/H2 = (Q1/Q2) ^2 , PA/PC = (Q1/Q2) ^3
The above equations show that by changing the fan air volume through speed regulation, when the fan air volume drops to half, the fan shaft power will decrease by 87.5% (PA/PC=0.5 3 =0.125, 1-0.125=0.875) without considering efficiency. Therefore, the energy-saving effect of using frequency converters in fans is very significant.
(2) Energy saving calculation
Exhaust fan motor parameters: rated power 1600KW, rated voltage 6000V, rated current 192.6A, rated speed 1485r/min, power factor 0.88, and the current of the fan motor under normal power frequency operation is 160A.
The power of the fan at industrial frequency: P <sub>valve</sub> = 1.732UI <sub>valve</sub> COS<sub>φ</sub> = 1.732 × 6 × 160 × 0.88 = 1463.2 KW
The power of the fan in variable frequency speed control mode: From the relationship between speed and frequency f/fe=n/ne, and the relationship between speed and power P/Pe=(n/ne) ³ , we can get: P= Pe×（f/fe） ³ . The inverter outputs a frequency of 40Hz during normal operation, and the power frequency is 50Hz. Therefore: P <sub>inverter</sub> =1600*（40/50） ³ =819.2 KW
Therefore: P <sub>section </sub> = P <sub>valve </sub> - P<sub> inverter</sub> = 644KW
Daily savings: 644 × 24 × 0.5 × 95% = 7341.6 yuan
Note: The calculation is based on a 95% operating rate of the high-temperature fan, and the electricity cost is calculated at 0.5 yuan per kilowatt-hour.
2. Indirect benefits:
After adopting high-pressure variable frequency speed control for high-temperature fans, in addition to achieving energy-saving and consumption-reducing economic benefits, other benefits are also significant:
(1) It extends the service life of motors and other equipment. After the frequency conversion, the high-voltage motor achieves soft start, the starting current is greatly reduced, and the impact of the starting current on the power grid and motor is avoided. At the same time, it realizes automatic control of air volume in production, and improves the degree of automation control of equipment and the safety and reliability of equipment operation;
(2) Improved process conditions, guaranteed speed regulation accuracy, and smooth and stable adjustment of air volume and air pressure, which improved production efficiency, especially guaranteed the utilization rate of production line equipment, and further increased output;
(3) Equipment operation and maintenance costs have decreased, with maintenance costs reduced by nearly 70% after the adoption of frequency conversion regulation.
(4) The harmonic content of the input current of the high voltage frequency converter is less than the limit required by the national standard, and the power factor can be guaranteed to be above 0.95. There is no need to invest in power compensation equipment, and the related equipment investment and maintenance costs can be reduced.
VIII. Daily Inspection and Maintenance
1. Operational data logging and fault logging
Daily records of inverter and motor operating data are essential, including inverter output frequency, output current, and output voltage. These data should be compared with established parameters to facilitate early detection of potential faults. If the inverter trips due to a fault, the fault and the inverter's operating conditions at the time of tripping must be recorded for detailed analysis of the cause of the fault.
2. Routine inspection of frequency converters
Perform this check every two weeks, recording the ambient temperature and radiator temperature; observe the inverter for any abnormal vibrations or noises, and check if the fan is operating normally.
3. Inverter maintenance
The frequency converter should be cleaned and maintained once a quarter. Maintenance involves removing accumulated dust and dirt from inside the frequency converter and in the air duct, wiping the surface of the frequency converter clean, and carefully inspecting the inside of the frequency converter for any overheating or discoloration.
(1) When the temperature is high in summer, ventilation at the inverter installation site should be strengthened. Ensure that there are no excessive dust, acids, salts, corrosive and explosive gases in the surrounding air;
(2) Summer is the rainy season. Prevent rainwater from entering the inverter room and ensure that the inverter is installed in a clean and dry environment.
4. After each inverter maintenance, carefully check for any missing screws and wires to prevent short circuits caused by small metal objects. Especially after significant modifications to the electrical circuit, ensure that all electrical connections are correct and reliable.
IX. On-site services provided by Hekang Yisheng Company
On-site service is the terminal for communication between service enterprises and customers. A large number of service initiatives of enterprises are realized through on-site employees providing services to customers. The quality of on-site service is directly related to the enterprise's service terminal - on-site service, and has a direct impact on the implementation of enterprise management system and the enhancement of enterprise competitiveness.
As a leading enterprise in the research and development and production of high-voltage frequency converter technology in China, Hekang Yisheng Company attaches great importance to the quality of its products and on-site service. Unexpected events can occur at any time during on-site service, and many problems must be resolved immediately. The emphasis here is not on a rapid response to customer complaints, but rather on the responsibility of on-site commissioning personnel to make "real-time decisions" and quickly handle emergencies when service conditions change. The on-site commissioning engineers worked alongside the electrical operation personnel, disregarding rest and fatigue, and used their superb skills to promptly resolve numerous problems, demonstrating the rapid response capabilities and dedication of your company's technical staff.
Meanwhile, your company attached great importance to the problems encountered in the application of high-voltage frequency converters in high-temperature fans, and specially dispatched your company's technical supervisor to conduct technical training and exchanges with our electrical personnel. Our electrical personnel not only learned a great deal theoretically and practically, but also benefited greatly from the "decisive, rapid, and efficient" professionalism of your company's technicians, which received unanimous praise from our company's leaders and employees. Hekang Yisheng Company is not only making products, but also building a brand, which is the best explanation for why Hekang Yisheng Company maintains its leading position in the industry.
X. Conclusion
Based on the actual operation of the HIVERT high-voltage frequency converter in our company for more than three months, the equipment is relatively stable, the speed adjustment is simple, and the maintenance is convenient, which brings great convenience to operators and maintenance personnel. It has played a significant role in high and stable production, energy consumption reduction, safety and environmental protection, achieving the expected design results and creating good economic benefits. It is worth promoting and using in new dry process cement production lines.
