[ Abstract ] This paper reviews the performance characteristics and system principle of high-voltage frequency converters in conjunction with the production process of vertical kiln Roots blowers in cement plants. It also details the overall transformation scheme and introduces the power-saving effect of the frequency converter.
[ Keywords ] High-voltage frequency converter, energy saving and consumption reduction, cement plant, vertical kiln, Roots blower.
I. The Necessity of High-Voltage Variable Frequency Retrofitting of Vertical Kiln Roots Blower
The Roots blower in the vertical kiln of Guizhou Jiulong Cement is a wound-rotor asynchronous motor. The original operating mode was for the motor to run at full speed at the industrial frequency, with the required air volume adjusted by the outlet vent valve. Due to the limited capacity of the company's power grid, the motor was not allowed to start at full voltage. Therefore, the motor was started using a rotor series resistor, and after starting, the starting resistor was automatically short-circuited, allowing the motor to run at full speed. However, after commissioning, the following drawbacks were found in the adjustment method via the outlet vent valve:
1. The outlet vent valve has a delayed response, slow adjustment speed, and low adjustment accuracy, which often fails to control the required air volume on site, affecting the on-site production process requirements.
2. The outlet vent valve adjustment is unscientific, uneconomical, and wasteful of resources. Although the vent valve adjustment is simple in structure and requires little investment, it is clearly unsuitable in today's increasingly energy-conscious environment. In the early stages of the cement plant's construction, considering the possibility of future expansion and operational safety, the motors and fans were selected with a large margin, resulting in a serious problem of oversized motors pulling small loads. In fact, the rated current of the motor is 40A, while the actual operating current is only about 28A on average. Due to the use of outlet vent valve adjustment, the air pressure and air volume in the duct are artificially released, and a large amount of energy is wasted by the vent valve. In today's increasingly energy-scarce environment, this is clearly outdated, and energy-saving renovation is imperative.
3. When the motor is running at full speed at the power frequency, the mechanical parts such as the motor bearings are severely worn. In addition, since it is a wound-rotor asynchronous motor, the carbon brushes on its slip rings are also severely worn when the rotor is running at high speed, resulting in a short replacement cycle and a large amount of equipment maintenance.
4. The starting process is complex. The rotor series resistance starting method involves many secondary circuits, which requires a lot of maintenance and has low efficiency, but it is indispensable. The starting method is an outdated technology.
In conclusion, to fundamentally change the existing process, we must improve it from the source, that is, by changing the motor speed to adjust the fan speed, thereby adjusting the air volume to meet the on-site process requirements.
II. Selection of Speed Regulation Method
Currently, the main speed control methods for high-power high-voltage asynchronous motors in China include the following: cascade speed control, internal feedback cascade speed control, hydraulic coupling speed control, and frequency conversion speed control.
1. Cascade speed regulation: The advantage is that it can recover slip power, so the speed regulation efficiency is relatively high. However, it also has many problems. It is not suitable for existing wound-rotor asynchronous motors. The starting process is complicated, the starting current is large, the speed regulation range is limited, the response is slow, the power factor and efficiency are low, it is difficult to achieve coordination with control systems such as PLC and DCS, and the harmonic pollution is large, which has a significant impact on the power grid.
2. Internal Feedback Speed Regulation: Internal feedback speed regulation is developed based on cascade speed regulation. Its advantages include all the advantages of cascade speed regulation and small size. Its disadvantages are: it requires a special motor, improper slip ring handling can easily lead to accidents, although a frequency-sensitive resistor is used for starting, the starting current is still very large, which has a great impact on the motor and the power grid, the speed regulation range is limited, the input power factor and efficiency are low, a large number of high-order harmonics are generated on the motor side due to the inverter of the thyristor, which causes aging of the motor insulation and easily causes torque pulsation of the motor, as well as additional heat generation and noise pollution. The cable needs to be thickened, and the motor surge phenomenon cannot be eliminated.
3. Speed regulation of hydraulic coupling: The speed regulation range is limited. The slip loss at high speed is about 3%-5%, and the slip power loss at low speed is even greater (1 - speed ratio), which can reach up to 15% of the rated transmission power. Because the efficiency is proportional to the speed, the efficiency is extremely low at low speed, the accuracy is low, the operation cannot be switched, the maintenance is complicated, and the cost is high.
4. High - voltage frequency converter speed regulation: Due to the application of advanced power electronics technology, computer control technology, modern communication technology, high-voltage electrical engineering, motor drive and other comprehensive scientific and technological fields, it has unparalleled advantages over other speed regulation methods.
III . Technical Advantages of the Kelun CL2700 Series High Voltage Variable Frequency Speed Control System
1. High efficiency, pollution-free, and high power factor
The CL2700 series high-voltage variable frequency speed control system adopts a high-high scheme with power units connected in series. It uses a multi-winding high-voltage phase-shifting transformer. When the current flowing through the secondary winding is superimposed on the primary side of the transformer, it forms a current waveform that closely approximates a sine wave. Actual testing shows that at 50Hz operation, the grid-side current harmonics are <2%, the motor-side output voltage harmonics are <1.5% (even at 40Hz, they are still <2%), the efficiency of the complete system is >97%, and the power factor is >0.96. This fully meets the requirements of IEEE 519-1992 for voltage and current harmonic content.
Furthermore, by employing a self-developed dedicated PWM control method, output voltage harmonics can be further reduced by 1-2% compared to other similar methods.
2. Advanced fault unit bypass operation (professional core technology)
To improve system reliability, a certain output voltage margin is considered in the entire variable frequency speed control system, and bypass circuits are added to each power unit. When a power unit fails, the fault can be automatically detected and the bypass circuit can be activated, preventing that unit from operating. Simultaneously, the program automatically performs calculations and adjusts the algorithm to ensure that the three-phase line voltages remain perfectly symmetrical, and the motor operation is unaffected. This control method is at an internationally advanced and domestically leading level, significantly improving system reliability and operational stability.
3. High - performance control technology
The CL2700 series high-voltage variable frequency speed control system pioneered the implementation of simplified vector control technology, enabling rapid dynamic response to constant torque. It also features adaptive acceleration and deceleration functions, automatically adjusting acceleration and deceleration times based on actual changes in operating parameters to quickly reach the set speed without exceeding the maximum allowable current. Simultaneously, the system can automatically identify the motor speed, allowing users to start, accelerate, decelerate, or stop the motor without considering its current operating status or stopping it.
The CL2700 series high-voltage variable frequency speed control system can also realize the automatic feedback energy limiting function.
4. High reliability
The control power supply can automatically switch between external 220V power supply and high-voltage auxiliary power supply. It is also equipped with a UPS, so even if both power supplies fail, the control system can still work for a long enough time to safely shut down the entire system, issue an alarm, and record all status parameters at the time of the fault.
High-speed fiber optic communication can effectively avoid electromagnetic interference.
When the number of unit failures exceeds the set value, the system can automatically switch to mains frequency operation.
The rectifier transformer has a complete temperature monitoring function.
The uniquely designed power cabinet air duct places the main heat-generating components close to or within the air duct, resulting in excellent heat dissipation and ensuring the system's overload capacity.
It has strong resistance to grid voltage fluctuations. When the grid voltage fluctuates within the range of -15% to +15%, the system can operate normally. For power units, it can operate normally when the voltage varies within the range of -25% to +20%.
5. Other features
It has strong self-diagnostic capabilities and monitors all major parameters and interface signals in the system.
The 10.4-inch LCD touchscreen human-machine interface is simple, user-friendly, and displays a wealth of information.
With a rich set of interfaces, it can interface with a variety of automation equipment and systems to meet different needs in various fields. Special interfaces can be customized.
It has both local and remote operation capabilities, and can be remotely monitored via DCS;
Maintenance is simple, all units are interchangeable, and spare parts are scarce.
IV. Detailed Implementation Plan and Process of the Renovation Project
Based on the actual site conditions, the bypass cabinet adopts a one-to-one manual configuration. This structure is a typical manual bypass solution, consisting of three high-voltage isolating switches QS1 , QS2 , and QS3. Among them, QF1 is the high-voltage isolating main switch provided to the customer.
Its working principle is as follows: When operating at power frequency, QS2 is closed, and QS1 and QS3 are open; when operating at variable frequency, QS1 and QS3 are closed, and QS2 is open. QS2 and QS3 are double high-voltage isolating switches and will not close simultaneously, thus achieving mechanical interlocking.
Advantages: In the event of a high-voltage frequency converter failure, there is a clear point of power loss, ensuring personal safety. Simultaneously, the load can be manually connected to the mains power grid.
Disadvantage: When the high-voltage frequency converter fails, it cannot automatically switch from frequency conversion to mains frequency.
The existing resistor starting device on site will be retained. When the high-voltage frequency converter fails, the high-voltage input cable of the frequency converter can be directly switched to the resistor starting device. The operation is simple and quick.
V. Improvement Results
1. Motor parameters
Rated power | Rated voltage | Rated current | Operating current | Power factor | Rated speed |
560kW | 10kV | 40A | 32A | 0.83 | 594 |
2. Inverter parameters
Inverter Model | Input voltage | Rated power | Power factor | Rated current |
CL2700-10-0700-9QY | 10kV | 700kVA | 0.96 | 46A |
3. Energy consumption comparison before and after frequency converter retrofit
Operating mode | Adjustment method | Input voltage | Input current | Operating frequency | 24-hour meter reading energy consumption | Energy saving rate |
Power frequency operation | Vent valve | 10.5kV | 28A | 50.00Hz | 8310kW.h | |
Variable frequency operation | Variable frequency speed control | 10.5kV | 11A | 30.27Hz | 4609kW.h | 44.53% |
4. Comparison of on-site operating conditions before and after frequency converter retrofit
Startup method | Starting current | Bearing temperature rise | Adjusting the reaction rate | Repair costs |
Series resistor start-up | big | high | slow | high |
Variable frequency soft start | Small | Low | quick | Low |
Based on the above table, it can be seen that:
The indirect effects of frequency conversion retrofitting the vertical kiln Roots blower are also very significant. Because the frequency conversion speed control system often operates at around 30 Hz, the rotational speed of the motor and blower is reduced, the shaft temperature of the motor and blower is reduced, the noise is reduced, and the overall maintenance cycle is greatly shortened. Operators can easily adjust the operating frequency of the motor through the monitoring interface on the remote control box. The frequency resolution of the high-voltage frequency converter is accurate to 0.01 Hz, and the adjustment is timely and highly accurate.
VI. General Knot
Energy conservation is a fundamental national policy in my country. The country has formulated a "Medium- and Long-Term Special Plan for Energy Conservation." To implement the goals of this plan, the National Development and Reform Commission recently launched ten key energy-saving projects. The cement industry, a major energy consumer, is a key focus of national macro-control and energy conservation efforts. Through various measures, if the electricity consumption per ton of cement produced can be controlled within 100 kWh, based on my country's annual cement production of 1.2 billion tons, achieving the energy-saving target would save millions of tons of coal and tens of billions of kilowatt-hours of electricity annually. It would also reduce emissions, benefiting the environment and lowering enterprise costs. The frequency conversion retrofit of the high-pressure Roots blower in the vertical kiln at Guizhou Jiulong Cement Plant has achieved both energy savings and process improvement, proving a complete success. Currently, compared to other industries such as metallurgy and power, the progress of high-voltage frequency converter retrofitting in my country's cement industry is relatively slow. If most cement manufacturers in China can rationally and deeply tap their own potential, there is reason to believe that China's skies will be bluer, its waters clearer, and the bright prospect of an energy-saving and harmonious society will surely unfold before us!
