Currently, research on high-voltage motor variable frequency speed control technology is very active worldwide, and various types of high-voltage frequency converters are emerging. Users naturally hope to choose practical high-voltage frequency converters with good cost performance; therefore, how to choose is a question worth considering. Knowing yourself and your enemy is the key to victory. First, you must determine the technical requirements for the high-voltage frequency converter based on your own operating conditions, and then select a targeted solution, product, and after-sales service. Otherwise, you may encounter unsatisfactory applications and significant investment losses. Different high-voltage frequency converters have different technical levels in their circuit topologies. The technical level determines important indicators such as the stability, reliability, service life, maintenance costs, and cost-effectiveness of the frequency converter and drive system. Just like laptops with essentially the same functions, different technical levels result in quality and price differences ranging from 3,000 yuan to tens of thousands of yuan. Therefore, understanding the inherent technical levels of different types of high-voltage frequency converters and combining the quality of the frequency converter with your operating conditions is crucial to achieving the ideal effect of low investment and high energy-saving returns.
1. The concept of high-voltage frequency converters
According to international conventions and Chinese national standards for voltage level classification, voltage levels ≥10kV are called high voltage, and those between 1kV and 10kV are called medium voltage. We also conventionally refer to motors with a rated voltage of 6kV or 3kV as "high voltage motors." Since frequency converters with rated voltages of 1-10kV share common characteristics, we call frequency converters that drive 1-10kV AC motors high voltage frequency converters. High voltage frequency converters are further divided into two types: current-type and voltage-type, with distinct characteristics:
The main functional characteristic of a frequency converter is its inverter circuit. Based on the type of DC-side filter, inverter circuits can be divided into two categories: voltage-type and current-type. The former has a large capacitor connected in parallel at the DC power input terminal. This capacitor suppresses DC voltage ripple, reduces the internal resistance of the DC power supply, and makes the DC power supply approximately a constant voltage source. It also provides a conduction path for the reactive current from the inverter side. Therefore, it is called a voltage-type inverter circuit.
A large inductor is connected in series on the DC power supply side of the inverter to make the DC power supply approximately a constant current source; this type of circuit is called a current-source inverter circuit. The inductor in series in the circuit can suppress DC current ripple, but the output characteristics are soft. Current-source inverters are an early topology developed before voltage-source inverters.
2. Characteristics and differences between voltage-source inverters and current-source inverters
(1) DC circuit filtering stage. The DC filtering stage of a voltage-source inverter mainly uses a large capacitor, so the power supply impedance is small, which is equivalent to a voltage source. The DC filtering stage of a current-source inverter mainly uses a large inductor, which is equivalent to a constant current source.
(2) Output Waveform. The voltage waveform output by a voltage-source inverter is an SPWM high-frequency rectangular carrier wave, and the output current waveform is approximately a sine wave under inductive loads, containing some high-order harmonic components. A simple input filter can meet the national standards for latent wave content. The current waveform output by a current-source converter is an alternating rectangular wave, and its output voltage waveform is close to a sine wave, containing abundant high-order harmonic components. The motor is prone to overheating, and imported special motors are generally required for use. The input harmonic content is extremely high, requiring a large and bulky filter for operation.
(3) Four-quadrant operation. Because a large inductor is connected in series on the DC power supply side of a current-source inverter, the thyristor rectifier bridge can change the voltage polarity while maintaining the current direction. Therefore, it is easy to make the inverter operate in rectification mode, thereby putting the rectifier bridge in inversion mode and realizing four-quadrant operation. Voltage-source high-voltage frequency converters only use IGBT rectification feedback at two levels and can operate in four quadrants.
(4) Dynamic performance. Current-source inverters have large inductance, making dynamic current response difficult and unable to keep up with the required dynamic torque, resulting in soft characteristics. Voltage-source inverters can be controlled using current feedback loops, offering fast response speeds and adapting to modern control theories: advanced Galerne direct speed control, Fuji vector control, and ABB direct torque control, followed by space voltage vector control and slip optimization F/U control. Under open-loop speed conditions, high-speed and high-precision control of the motor's flux torque can be achieved, allowing the motor characteristics to be either flexible or rigid; dynamic performance is particularly good.
(5) Overcurrent and short-circuit protection are key protection functions of high-voltage frequency converters. Because current-source inverters have a large inductor in series in the circuit, they can suppress the rate of rise of current during faults such as short circuits. Therefore, overcurrent and short-circuit protection of current-source inverters is easy to implement. However, it is more difficult for general voltage-source inverters. Only two-level voltage-source high-voltage frequency converters have a DC inductor, which can suppress the rate of rise of di/dt and easily implement overcurrent and short-circuit protection.
(6) Requirements for switching transistors. Switches in voltage-source inverters require short turn-off times but low withstand voltages; while switches in current-source inverters have no strict requirements for turn-off times but relatively high withstand voltages.
(7) Using a current-source inverter requires two inductors, and the voltage that the switching transistor withstands when it is off is much higher than that of a voltage-source inverter. Currently, only AB has products with this technical solution.
The above differences indicate that voltage-type high-voltage frequency converters have a more promising application prospect than current-type high-voltage frequency converters.
3. Characteristics of the topologies of four voltage-source high-voltage frequency converters
3.1 Current topologies for achieving high voltage in voltage-source high-voltage frequency converters
In recent years, the development needs of power electronics technology have spurred the rapid development of power electronic devices. Conversely, once a new generation of devices or a new technology overcomes some of the shortcomings of older devices, it will drive revolutionary changes in power electronic application devices, including frequency converters.
IGBTs developed rapidly in the 1990s. Their insulation, modularity, and operating frequency up to 20kHz ushered in a "silent" era for frequency converters. They were free from the problem of secondary breakdown, and their effectiveness in frequency conversion speed regulation of 380V and 660V asynchronous motors was widely accepted, leading to a boom in the development of low-voltage frequency converters.
In high-voltage motor frequency conversion speed regulation with voltages ranging from 1140V to 3-10kV, the operating voltage of IGBT modules is far from meeting the requirements. While IGBTs are currently manufactured at 3.3kV and IGCTs at 4.5kV , this still doesn't meet the voltage levels required for direct use. Furthermore, their performance is poor and their prices are high, making manufacturing expensive. The series connection of IGBTs presents several world-class technical challenges, including high peak values of dynamic dv/dt in multiple stages at high switching frequencies, line inductance, lead inductance, motherboard technology, series synchronous control, and dynamic voltage equalization. These challenges create critical difficulties and have led domestic and international R&D experts to consider them off-limits. Therefore, the choice of components for high-voltage frequency converters has become a hot topic in electrical design research worldwide. Consequently, different technologies and products have emerged in high-voltage frequency converters throughout history.
Class A: High-voltage frequency converters for fans and pumps. Driven by: High-voltage AC asynchronous motors, specifically for fans and pumps (for applications requiring low square torque and low dynamic control requirements).
(1) High-low-high mode, using step-down transformer low-voltage frequency converter special step-up transformer motor;
(2) 12-pulse transformer rectifier IGBT three-level two-potential overlap indirect high voltage method;
(3) Indirect high voltage method with series connection of multi-pulse transformer rectifier IGBT unit and multi-potential overlap;
Note: Indirect – refers to the process in the frequency converter's power conversion stage where a transformer is used for voltage transformation.
Class B: General-purpose high-voltage frequency converter. Driven by: High-voltage AC asynchronous motors; High-voltage AC synchronous motors. Suitable for a wide range of loads: (Applicable to fans, water pumps, and various mechanical transmission controls requiring full-range high-torque control and four-quadrant operation.)
(4) Direct high voltage method with IGBT elements connected in series for direct rectification;
3.2 High-Low-High Pattern
Voltage conversion method: step-down transformer (R1) low-voltage frequency converter (R2) step-up transformer (R3) motor (R4).
The system's equivalent impedance R = R1 + R2 + R3 + R4
Output transformers require special manufacturing, resulting in high costs, low power factor, low efficiency, high self-loss, and bulkiness. The system performance is poor; it can be used for general process speed regulation but is unsuitable for speed regulation and energy-saving applications.
3. 3IGBT three-level two-potential overlapping indirect high-voltage method (abbreviated as: three-level high-voltage frequency converter)
Voltage conversion method: power supply step-down transformer (R1) IGBT three-level inverter (R2) motor (R3).
The system's equivalent impedance R = R1 + R2 + R3 (with the step-up transformer impedance R4 added during voltage boosting).
Three-level high-voltage frequency converters, also known as "neutral-point clamped" (or NPC (Netural Point Clamped) high-voltage frequency converters), are a type of high-voltage frequency converter developed and introduced in recent years. High-voltage variable frequency speed control systems utilize neutral-point clamped three-level technology. The frequency converter mainly consists of an input 12-pulse transformer, rectifier, neutral-point clamping circuit, three-level mode inverter, output filter, and control unit.
The rectifier circuit typically uses diodes, with high-voltage fast recovery diodes used for clamping. The inverter section uses GTOs, IGBTs, or IGCTs as power devices. The output voltage level is 4.16kV .
Initially, because the output voltage and the motor's operating voltage are not directly matched, for 6KV high-voltage motors, it is necessary to adjust the voltage settings.
The “Y” connection was changed to a “” connection. When the frequency converter malfunctioned, it was changed back to the mains frequency operation.
Currently, an autotransformer can be added to the output end, allowing direct use with 6KV and 10KV high-voltage motors, similar to a high-low-high configuration. This is currently a product solution from ABB and Siemens.
