What people commonly refer to as high-voltage frequency converters and high-voltage motors are actually typically operating at 3-10kV, with 3kV , 6kV , and 10kV being the most common in China . Compared to the mains voltage, this is considered medium voltage, hence some foreign sources also refer to them as medium-voltage frequency converters and medium-voltage electrical equipment.
The development and application of high-voltage frequency converters are inseparable from high-voltage, high-current power electronic devices. Compared with the power switching devices in low-voltage frequency converters, the most important characteristic of high-voltage switching devices is that they can withstand high voltage in the blocking state, while also requiring high current density and low on-state voltage drop in the conducting state; they also need to have sufficiently short on-time and off-time during switching transitions, and be able to withstand high du/dt and di/dt .
Currently, the power electronic devices widely used in high-voltage frequency converters mainly include GTO , IGBT, and IGCT.
I. High-voltage frequency converter control method
1. Constant UIlf control
In industrial drives , variable voltage variable frequency (VVF) systems, i.e. , open-loop control with a constant UIf , are commonly used . The advantages of this method are its simplicity and relatively low cost. It is particularly suitable for large-capacity industrial loads such as fans and pumps. The main problems are poor low-speed performance, inability to maintain a constant magnetic flux over long periods, the need for voltage compensation, and the requirement for forced ventilation and cooling for the asynchronous motor.
2. Vector Control
Vector control can achieve high dynamic and static performance indicators. Since the parameters of the asynchronous motor have a significant impact on it, such systems are often equipped with dedicated motors. For applications such as large rolling mills where high dynamic performance is required, a three-level voltage source high-voltage frequency converter with a vector control dual PWM structure is commonly used.
3. Direct Torque Control
Direct torque control systems offer rapid torque response, limited to within one beat, and have no overshoot. Compared to vector control, they are unaffected by changes in rotor parameters, making them a high-performance AC speed control method with excellent static and dynamic characteristics. They are commonly used in three-level high-voltage frequency converters.
4. Sensorless vector control
This control method is also known as direct vector control. Rockwell Automation's Fowerflex 7000 inverter uses this control method. The key to achieving sensorless control is how to calculate speed-related quantities from readily available stator current and stator voltage.
The core of vector control is to control the magnetic flux of the motor. Therefore, the observation of magnetic flux is also one of the key points of sensorless control. In order to ensure control accuracy, there are reference identification components in sensorless controllers.
II. The impact of high-voltage frequency converters on the power grid and motors
1. Impact on the power grid
Given the generally large capacity of high-voltage frequency converters and their significant proportion in the overall power grid, the harmonic pollution they cause to the grid cannot be ignored. There are two methods to address harmonic pollution: one is to use noise reduction filters to control the harmonics generated by the high-voltage converters to meet the requirements of the power supply department; the other is to use frequency converters with lower harmonic currents, as these converters themselves do not cause harmonic pollution to the grid. This involves using so-called "green" power electronic products, fundamentally solving the harmonic pollution problem.
The 6-pulse thyristor current-source rectifier circuit used in general current-source inverters has a total harmonic current distortion of approximately 30% , far exceeding the 5% current distortion requirement stipulated by the IEEE 519 1992 standard . Therefore, an input harmonic filter must be installed. For a 12 -pulse thyristor rectifier circuit, the total harmonic current distortion is approximately 10% , still requiring a harmonic filter. Most PWM voltage-source inverters use diode rectifier circuits. If the rectifier circuit also uses PWM control, the input current can be essentially sinusoidal, resulting in very low harmonic current. Unit-series multilevel inverters employ a multi-level structure with a very high input pulse number. The total harmonic current distortion can be below 10% , meeting the grid's requirement for harmonic distortion-free operation without any additional filters.
Another comprehensive performance indicator of high-voltage frequency converters is the input power factor. Ordinary current-source frequency converters have a low input power factor, which decreases linearly with decreasing speed, thus requiring a power factor compensation device. Diode rectifier circuits have a high power factor throughout the entire operating range and generally do not require a power factor compensation device. PWW- type rectifier circuits using fully controlled power electronic devices have an adjustable power factor, approaching 1. Multi- watt rectifier circuits with series-connected units offer high power efficiency, with a power factor reaching over 0.95 throughout the entire adjustment range .
Based on the above two indicators, PWM optical PWM ( high-low structure ) frequency converters with fully controlled power electronic devices are both considered "green" power electronic products.
III. Impact on Electric Motors
High-voltage frequency converters can cause harmonic heating ( core ) and torque ripple in motors. Furthermore, output du/dr , common-mode voltage, and noise can negatively impact the motor. Current-source frequency converters, due to their higher output harmonics and common-mode voltage, require motors to be derated and have enhanced insulation, and also suffer from torque ripple issues, thus limiting their application.
Three-level voltage source inverters suffer from output harmonics and du/dt issues, generally requiring an output filter; otherwise, a dedicated motor must be used. For equipment such as fans and pumps that do not typically require four-quadrant operation, unit-series multi-level PWM voltage source inverters offer significant advantages in output harmonics and du/dt , have no special requirements for the motor, and possess broad application prospects.
