With the rapid development of modern power electronics technology and microelectronic technology, high-voltage and high-power variable frequency speed control devices have been continuously matured, and the high voltage problem that has been difficult to solve has been solved in recent years by device series or unit series. Then, in order to reduce the occurrence of errors in the measurement process, what principles should be grasped in the selection of high-voltage inverters?
1, voltage level
China's high-voltage motors commonly used voltage levels of 6kV and 10kV, and a small number of high-voltage motors of 3kV class. Most of the high-voltage inverter inputs have an isolation transformer, so the inverter input voltage can usually meet the requirements of different grid voltage levels. The selection of the output voltage level of the inverter usually has different selection principles according to the technical transformation project and the new construction project. For the technical transformation project, the motor is running at the power frequency. Since the motor is usually not replaced, after the inverter is installed, the output voltage of the inverter is required to be the same as the grid voltage, usually 6kV or 10kV.
For new projects, if power frequency bypass is not required, the motor voltage can be optimized with the inverter. For example, the output voltage level of 2.3kV or 3kV can be selected for medium and small power to reduce the cost. If power frequency bypass is required, it is recommended that the inverter output voltage level be consistent with the grid voltage, otherwise the bypass is more complicated.
2, voltage source type and current source type
At present, most of the high-voltage inverters are AC-DC converters, which can be divided into voltage source type and current source type according to the different energy storage components used in the intermediate DC link.
At present, the main current source type inverter is SGCT-PWM current source type inverter. The advantage of the current source type inverter is that the energy can be fed back to the grid and can be operated in four quadrants. The power supply side often uses a three-phase bridge thyristor rectifier circuit, and the input current has a large harmonic. In order to reduce the harmonic components, multiple multiplexes may be used, and sometimes an input filter device must be added. The input power factor is generally low and a power factor compensation device is usually added. Because unlike the voltage source inverter, the large filter capacitor in the DC link can store a large amount of energy. The current source inverter is sensitive to grid voltage fluctuations, and it is easy to stop when the grid voltage fluctuates greatly. This is a relatively large disadvantage.
If the rectifier circuit also uses SGCT for current PWM control, lower input current harmonics and higher input power factor can be obtained, and the input isolation transformer can be omitted. However, PWM rectification will cause the efficiency of the inverter to drop to some extent.
Voltage source inverters currently mainly have three-level high-voltage inverters (using IGBT or IGCT as power devices) and unit series multi-level high-voltage inverters. Because the voltage source type high-voltage inverter adopts the uncontrollable rectification of the diode, the input power factor is relatively high, and it is not easily affected by the grid voltage fluctuation. In the application where the grid condition is relatively poor, it is obviously more stable than the current source type inverter. The maximum voltage of a three-level inverter usually does not exceed 4.16kV, and the output voltage of a unit series multi-level inverter can reach 10kV.
3, V / F control and speed sensorless vector control
The speed sensorless vector control technology can greatly improve the performance of the inverter and expand the application field of the inverter without substantially increasing the hardware cost. That is to say, in the case of a load with relatively low steady-state and dynamic requirements such as a fan pump, the speed sensorless vector control has the functions of torque limiting, fast speed tracking and restarting to effectively prevent the overcurrent trip and deceleration process of the acceleration process. The over-voltage tripping machine and other abnormal shutdown phenomena are very important for ensuring the reliable operation of the inverter, and are more and more recognized by users.
4. Harmonic pollution to the power grid and input power factor
Harmonic pollution to the power grid mainly depends on the structure and characteristics of the rectifier circuit. There are two main ways to reduce harmonic pollution in the power grid: multiple rectification and PWM rectification. The unit series multi-level high-voltage inverter usually has a large number of rectified pulses and less harmonic pollution to the power grid. In order to reduce harmonic pollution to the power grid, current source inverters usually use 18-pulse rectification. The three-level voltage source type inverter needs at least 12 pulses, and when it is high, it can use 24 pulses.
The voltage source type high-voltage inverter has a high power factor in the whole operating range due to the uncontrollable rectification of the diode. The fundamental power factor can generally be maintained above 0.95, and generally no power factor compensation device is required. When the current source type inverter adopts thyristor rectification, the current is delayed due to the thyristor firing angle, the power factor is low, and it will decrease as the rotation speed decreases, and a power factor compensation device is often required.
PWM rectification can effectively reduce harmonics, adjustable power factor, and energy can flow in both directions. The disadvantage is that the cost increases and the efficiency decreases.
5, the output waveform and the applicability to the motor
The influence of output harmonics on the motor mainly includes: causing additional heating of the motor, resulting in additional temperature rise of the motor. The motor is often derated, and harmonics may cause motor torque ripple and increase noise. Dv/dt and common mode voltage can affect motor insulation.
Ordinary two-level and three-level PWM voltage source inverters have large output dv/dt due to large output voltage jump steps and phase voltage jumps reaching half of the DC bus voltage and DC bus voltage, respectively. Larger dv/dt affects the insulation of the motor. Therefore, such a frequency converter generally requires a specially designed motor, and the motor insulation must be strengthened. If you want to use a normal motor, you must add an output dv/dt filter.
The unit series multi-level high-voltage inverter has greatly improved the output waveform due to many output voltage steps and high equivalent switching frequency. Output harmonics, noise, dv/dt, and torque ripple are small. Therefore, the inverter can be used in any ordinary high-voltage motor without a filter, and it is not necessary to derate it.
When there is no input transformer, the common mode voltage will be directly applied to the motor, increasing the insulation stress of the winding to the ground, causing insulation breakdown and affecting the service life of the motor. (There is also a common mode reactor scheme to slow down the common mode voltage to some extent. Impact). If an input transformer is set, the common mode voltage is shared by the input transformer and the motor and is mainly carried by the input transformer.
6, power saving rate
Generally speaking, after a certain equipment, such as a fan or a water pump, the power saving rate of the inverter is not related to the inverter itself, it is mainly determined by the operating conditions of the inverter before the transformation. Such as the original baffle, the opening of the valve, and so on. There may be some differences in the efficiency of different frequency converters, but the impact on the overall energy saving rate is minimal.
7, domestic brands and imported brands
The high-voltage inverters in the early Chinese market mainly used imported brands such as Robin Kang and Rockwell. Now, with the rapid development of the domestic high-voltage inverter industry, the domestic mainstream high-voltage inverter has reached the international level in terms of function and reliability, and it has a great advantage in terms of cost and service.
At present, the domestic high-voltage inverter occupancy rate exceeds 60%. In terms of large capacity (5000 kW or more) and high-performance applications (rolling mills, lifting applications, etc.) and water-cooled inverters, foreign famous brands still have a large advantage. In the conventional applications of small and medium power (fans, pumps), domestic high voltage The frequency converter has been able to fully meet the user's requirements.
1, voltage level
China's high-voltage motors commonly used voltage levels of 6kV and 10kV, and a small number of high-voltage motors of 3kV class. Most of the high-voltage inverter inputs have an isolation transformer, so the inverter input voltage can usually meet the requirements of different grid voltage levels. The selection of the output voltage level of the inverter usually has different selection principles according to the technical transformation project and the new construction project. For the technical transformation project, the motor is running at the power frequency. Since the motor is usually not replaced, after the inverter is installed, the output voltage of the inverter is required to be the same as the grid voltage, usually 6kV or 10kV.
For new projects, if power frequency bypass is not required, the motor voltage can be optimized with the inverter. For example, the output voltage level of 2.3kV or 3kV can be selected for medium and small power to reduce the cost. If power frequency bypass is required, it is recommended that the inverter output voltage level be consistent with the grid voltage, otherwise the bypass is more complicated.
2, voltage source type and current source type
At present, most of the high-voltage inverters are AC-DC converters, which can be divided into voltage source type and current source type according to the different energy storage components used in the intermediate DC link.
At present, the main current source type inverter is SGCT-PWM current source type inverter. The advantage of the current source type inverter is that the energy can be fed back to the grid and can be operated in four quadrants. The power supply side often uses a three-phase bridge thyristor rectifier circuit, and the input current has a large harmonic. In order to reduce the harmonic components, multiple multiplexes may be used, and sometimes an input filter device must be added. The input power factor is generally low and a power factor compensation device is usually added. Because unlike the voltage source inverter, the large filter capacitor in the DC link can store a large amount of energy. The current source inverter is sensitive to grid voltage fluctuations, and it is easy to stop when the grid voltage fluctuates greatly. This is a relatively large disadvantage.
If the rectifier circuit also uses SGCT for current PWM control, lower input current harmonics and higher input power factor can be obtained, and the input isolation transformer can be omitted. However, PWM rectification will cause the efficiency of the inverter to drop to some extent.
Voltage source inverters currently mainly have three-level high-voltage inverters (using IGBT or IGCT as power devices) and unit series multi-level high-voltage inverters. Because the voltage source type high-voltage inverter adopts the uncontrollable rectification of the diode, the input power factor is relatively high, and it is not easily affected by the grid voltage fluctuation. In the application where the grid condition is relatively poor, it is obviously more stable than the current source type inverter. The maximum voltage of a three-level inverter usually does not exceed 4.16kV, and the output voltage of a unit series multi-level inverter can reach 10kV.
3, V / F control and speed sensorless vector control
The speed sensorless vector control technology can greatly improve the performance of the inverter and expand the application field of the inverter without substantially increasing the hardware cost. That is to say, in the case of a load with relatively low steady-state and dynamic requirements such as a fan pump, the speed sensorless vector control has the functions of torque limiting, fast speed tracking and restarting to effectively prevent the overcurrent trip and deceleration process of the acceleration process. The over-voltage tripping machine and other abnormal shutdown phenomena are very important for ensuring the reliable operation of the inverter, and are more and more recognized by users.
4. Harmonic pollution to the power grid and input power factor
Harmonic pollution to the power grid mainly depends on the structure and characteristics of the rectifier circuit. There are two main ways to reduce harmonic pollution in the power grid: multiple rectification and PWM rectification. The unit series multi-level high-voltage inverter usually has a large number of rectified pulses and less harmonic pollution to the power grid. In order to reduce harmonic pollution to the power grid, current source inverters usually use 18-pulse rectification. The three-level voltage source type inverter needs at least 12 pulses, and when it is high, it can use 24 pulses.
The voltage source type high-voltage inverter has a high power factor in the whole operating range due to the uncontrollable rectification of the diode. The fundamental power factor can generally be maintained above 0.95, and generally no power factor compensation device is required. When the current source type inverter adopts thyristor rectification, the current is delayed due to the thyristor firing angle, the power factor is low, and it will decrease as the rotation speed decreases, and a power factor compensation device is often required.
PWM rectification can effectively reduce harmonics, adjustable power factor, and energy can flow in both directions. The disadvantage is that the cost increases and the efficiency decreases.
5, the output waveform and the applicability to the motor
The influence of output harmonics on the motor mainly includes: causing additional heating of the motor, resulting in additional temperature rise of the motor. The motor is often derated, and harmonics may cause motor torque ripple and increase noise. Dv/dt and common mode voltage can affect motor insulation.
Ordinary two-level and three-level PWM voltage source inverters have large output dv/dt due to large output voltage jump steps and phase voltage jumps reaching half of the DC bus voltage and DC bus voltage, respectively. Larger dv/dt affects the insulation of the motor. Therefore, such a frequency converter generally requires a specially designed motor, and the motor insulation must be strengthened. If you want to use a normal motor, you must add an output dv/dt filter.
The unit series multi-level high-voltage inverter has greatly improved the output waveform due to many output voltage steps and high equivalent switching frequency. Output harmonics, noise, dv/dt, and torque ripple are small. Therefore, the inverter can be used in any ordinary high-voltage motor without a filter, and it is not necessary to derate it.
When there is no input transformer, the common mode voltage will be directly applied to the motor, increasing the insulation stress of the winding to the ground, causing insulation breakdown and affecting the service life of the motor. (There is also a common mode reactor scheme to slow down the common mode voltage to some extent. Impact). If an input transformer is set, the common mode voltage is shared by the input transformer and the motor and is mainly carried by the input transformer.
6, power saving rate
Generally speaking, after a certain equipment, such as a fan or a water pump, the power saving rate of the inverter is not related to the inverter itself, it is mainly determined by the operating conditions of the inverter before the transformation. Such as the original baffle, the opening of the valve, and so on. There may be some differences in the efficiency of different frequency converters, but the impact on the overall energy saving rate is minimal.
7, domestic brands and imported brands
The high-voltage inverters in the early Chinese market mainly used imported brands such as Robin Kang and Rockwell. Now, with the rapid development of the domestic high-voltage inverter industry, the domestic mainstream high-voltage inverter has reached the international level in terms of function and reliability, and it has a great advantage in terms of cost and service.
At present, the domestic high-voltage inverter occupancy rate exceeds 60%. In terms of large capacity (5000 kW or more) and high-performance applications (rolling mills, lifting applications, etc.) and water-cooled inverters, foreign famous brands still have a large advantage. In the conventional applications of small and medium power (fans, pumps), domestic high voltage The frequency converter has been able to fully meet the user's requirements.
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