Preface 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 studying. Knowing yourself and your enemy is the key to victory. First, you must formulate the technical requirements for the high-voltage frequency converter according to your own operating conditions, and then select the appropriate high-voltage frequency converter 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 basically 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 technical levels of different types of high-voltage frequency converters and combining the quality of the frequency converter with the operating conditions is crucial to achieving the ideal effect of low investment and high energy-saving returns. 1. Concept of High-Voltage Frequency Converters According to international conventions and Chinese national standards, voltage levels are classified as follows: voltage ≥ 10kV is called high voltage, and 1kV to 10kV is 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. Their main functional characteristic is the inverter circuit. Based on the DC-side filter type, inverter circuits can be divided into voltage-type and current-type. The former has a large capacitor connected in parallel at the DC power input terminal. On the one hand, this suppresses DC voltage pulsation and reduces the internal resistance of the DC power supply, making the DC power supply approximately a constant voltage source; on the other hand, it provides a conduction path for 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 connected in series in the circuit can suppress the pulsation of the DC current, but the output characteristics are soft. The current-source inverter is an early topology developed before the voltage-source inverter. 2. Characteristics and differences between voltage-source inverters and current-source inverters (1) DC circuit filtering stage. The DC filtering stage of the 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 the current-source inverter mainly uses a large inductor, which is equivalent to a constant current source. (2) Output waveform. The voltage waveform output by the voltage-source inverter is an SPWM high-frequency rectangular carrier wave, and the output current waveform is approximately a sine wave under inductive load. It contains some high-order harmonic components. The input can meet the national standard for latent wave content by using simple filtering. The current waveform output by the current-type inverter is an alternating rectangular wave, and its output voltage waveform is close to a sine wave, containing rich high-order harmonic components. The motor is prone to high heat, and imported special motors are generally required for use. The input harmonic content is extremely high, and a huge and bulky filter is required for its use. (3) Four-quadrant operation. Because the current-type inverter has a large inductor connected in series on its DC power supply side, the thyristor rectifier bridge can change the voltage polarity while maintaining the current direction unchanged. Therefore, it is easy to make the inverter operate in the rectification state, so that the rectifier bridge is in the inversion state, realizing four-quadrant operation. The voltage-type high-voltage frequency converter only uses IGBT rectification feedback at two levels and can operate in four quadrants. (4) Dynamic performance. Current-type inverters have large inductance, making dynamic current response difficult and requiring dynamic torque to keep up, resulting in soft characteristics; voltage-type inverters can be controlled by current feedback loops, with fast response speed and adaptability to modern control theories: advanced Galering direct speed control, Fuji vector control, ABB direct torque control, and secondary space voltage vector control and slip optimization F/U control. Under open-loop speed conditions, high-speed and high-precision control of motor flux torque can be achieved, making the motor characteristics flexible or rigid; dynamic performance is particularly good. (5) Overcurrent and short-circuit protection are key protection functions of high-voltage inverters. Because current-type inverters have large inductance in series in the circuit, they can suppress the rate of current rise during faults such as short circuits, so overcurrent and short-circuit protection of current-type inverters is easy to achieve, while it is more difficult for general voltage-type inverters. Only two-level voltage-type high-voltage inverters have DC inductors, which can suppress the rate of rise of di/dt, making it easy to achieve overcurrent and short-circuit protection. (6) Requirements for switching transistors. The switching transistors in voltage-type inverters require short turn-off times but have low withstand voltages; while the switching transistors in current-type inverters have no strict requirements for turn-off times but have relatively high withstand voltage requirements. (7) Current-type inverters require two inductors, and the voltage that the switching transistors withstand when they are off is much higher than that of voltage-type inverters. Currently, only AB has products with this technical solution. The above differences show that voltage-type high-voltage inverters have a better application prospect than current-type high-voltage inverters. 3 Characteristics of the topology of four types of voltage-type high-voltage inverters 3.1 Current topology of voltage-type high-voltage inverters to achieve high voltage In recent years, with the development needs of power electronics technology applications, power electronic devices have developed rapidly; conversely, once a new generation of devices or a new technology overcomes some of the shortcomings of old devices, it will drive revolutionary changes in power electronic application devices, including inverters. IGBTs developed rapidly in the 1990s, and their insulation, modularity and operating frequency of up to 20kHz enabled inverters to enter the "silent" era. Without the problem of secondary breakdown, its effectiveness in frequency conversion speed regulation of 380V and 660V asynchronous motors has been widely accepted, leading to a boom in the development of low-voltage frequency converters. However, 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 made at 3.3kV and IGCTs at 4.5kV, this still doesn't meet the voltage levels for direct use. Furthermore, their poor performance and high price make 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 to them being considered off-limits by R&D experts both domestically and internationally. Therefore, determining the appropriate components for high-voltage frequency converters has become a hot topic of research and innovation in electrical design worldwide. Therefore, high-voltage frequency converters have different technologies and products in different historical periods: Class A: High-voltage frequency converters for fans and pumps. Driven objects: High-voltage AC asynchronous motors for fans and pumps (for applications with low requirements for square torque and dynamic control) (1) High-low-high mode, using step-down transformers, low-voltage frequency converters, and special step-up transformers for motors; (2) 12-pulse transformer rectifier IGBT three-level two-potential overlap indirect high-voltage mode; (3) Indirect high-voltage mode with multiple pulse transformer rectifier IGBT units in series and multiple potential overlap; Note: Indirect - refers to the process of voltage transformation using transformers in the frequency converter's conversion stage. Class B: General-purpose high-voltage frequency converters. Driven objects: High-voltage AC asynchronous motors; High-voltage AC synchronous motors. General load type: (Applicable to fans, water pumps, and various mechanical transmission controls for full-process high-torque control and four-quadrant operation.) (4) Direct rectification IGBT element series direct high voltage method; 3.2 High-low-high method Voltage conversion method: step-down transformer (R1) Low voltage frequency converter (R2) Step-up transformer (R3) Motor (R4). System equivalent impedance R=R1+R2+R3+R4 The output transformer needs to be specially manufactured, which is costly, has a low power factor, low efficiency, large self-loss, and is bulky. The system performance is poor and can be used for general process speed regulation, but is not suitable for speed regulation and energy saving applications. 3.3 IGBT 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 equivalent impedance R = R1 + R2 + R3 (with the step-up transformer impedance R4 added during voltage boosting). The three-level high-voltage frequency converter, also known as a "neutral point clamped" (NPC) high-voltage frequency converter, is a type of high-voltage frequency converter developed and introduced in recent years. The high-voltage variable frequency speed control system adopts 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 section. The rectifier circuit generally uses diodes, the clamping uses high-voltage fast recovery diodes, and the power devices in the inverter section use GTOs, IGBTs, or IGCTs. The output voltage level is 4.16kV. Initially, because the output voltage and the motor operating voltage did not directly match, the "Y" connection of the high-voltage motor had to be changed to a "+" connection for 6kV. When the frequency converter failed, it was changed back to mains frequency operation. Currently, an autotransformer can be added to the output terminal, allowing direct use with 6KV and 10KV high-voltage motors, similar to a high-low-high configuration. This is currently a product based on ABB and Siemens technologies. 3.3.1 Technical Features This series of frequency converters adopts a structure similar to traditional voltage-type frequency converters. The key technology lies in handling the midpoint drift. Drift is small under no-load and light-load conditions. However, as the load increases or changes dynamically, the capacitors struggle to support the midpoint, especially due to unequal capacitive reactances. This can cause the clamped midpoint to become unstable, resulting in voltage fluctuations. The magnitude of this midpoint fluctuation will cause output voltage asymmetry, output harmonics, waveform distortion, and increased common-mode voltage. This manifests as severe vibration and overheating of the motor if a high-voltage motor is directly connected to the output without a reactor. This is a phenomenon that no other type of frequency converter would produce. Therefore, regardless of the distance to the motor, a three-level high-voltage frequency converter must be equipped with an output reactor to address the problems of high motor vibration and noise. However, the potential for common-mode voltage leads to motor insulation aging. Due to the multiple dead zones in the three-level inverter switching mode, a long dead time is required to ensure smooth switching, resulting in high common-mode voltage. This defect is caused by circuit characteristics and hardware; simply optimizing the control software can only achieve a minor effect. Similar to the Jialing JCS type, adding an output common-mode suppressor is necessary for effective operation. When the output voltage of a three-level inverter is low, it is actually equivalent to a two-level voltage waveform, with high levels of 11th, 13th, and 17th harmonics and large harmonic currents. Without a filter, only a dedicated motor from the supplier can be used, and its output voltage can only reach 4200V, which is actually achieved by adding a step-up transformer. 3.3.2 Product Features 1) Extremely Low Efficiency The three-level inverter has a simple structure, but the increased number of diodes and circuits, coupled with the inconsistent drive waveforms of each IGBT, will inevitably lead to inconsistent clamping and switching performance. The turn-on and turn-off of power components are guaranteed by clamping diodes. Clamping diodes have high withstand voltage requirements, good fast recovery performance, and a large number of main components, resulting in a relatively complex system structure and limited expansion capabilities. 2) Inverter Capacity Requires a 20% Increase, High Investment Inconsistent Conductive Load of Switching Devices. The conductive load of switches near the bus and those near the output is unbalanced, which will lead to different current ratings for the switching devices. In a circuit, if the current rating of components is designed based on the most severe conduction load, then 2*(m-2) outer components per phase will have excessively high current ratings, resulting in waste. The inverter output line voltage is 4.16kV, and the motor's delta connection is 3.3kV, so the inverter output voltage must be stepped down to 3.3kV. The inverter will generate useless power: 4.16kV - 3.3kV = 0.86kV. When selecting a inverter, the capacity must be increased by at least 20% to match the required capacity, thus increasing investment. 3) Because a star/delta converter is required to achieve power frequency/variable frequency switching for a 6kV high-voltage motor , a three-level inverter uses a Y/Δ reconnection method to convert a Y-connected 6kV motor to a Δ connection. However, after the Y/Δ connection was changed, the motor voltage was inconsistent with the grid voltage, making bypass function impossible. When the frequency converter malfunctions, to ensure normal production, the motor must first be changed back to Y connection and then connected to the 6kV grid. Therefore, the motor connection change must be achieved by installing a Y/Δ switching cabinet to enable bypass function. 4) High output harmonic content requires a dedicated variable frequency motor. Due to the inherent high harmonic components in the output waveform of the three-level frequency converter, the output performance is poor. The output current and voltage waveforms are shown in Figure 2. The waveform of the frequency converter in the low-speed range is extremely poor and basically cannot meet the requirements of the working conditions. Therefore, an LC filter must be configured on the output side of the frequency converter to be used for ordinary squirrel-cage motors. Similarly, due to harmonics, the power factor, efficiency, and even lifespan of the motor will be affected to a certain extent. It can only reach the optimal working state at the rated operating point, but as the speed decreases, the power factor and efficiency will decrease accordingly. Output voltage harmonics are high at 5th and 7th harmonics, and the 11th and 13th harmonics reach over 20%, which can cause harmonic reactive power heating and torque pulsation in the motor, which are fatal to both cables and motors. Therefore, foreign companies generally recommend using dedicated motors. 3.4 Unit Series Multiplexing Frequency Converter Voltage Conversion Method: Power Transformer (R1) → Unit Series Frequency Converter (R2) → Motor (R3) System Equivalent Impedance R = R1 + R2 + R3 3.4.1 Main Circuit The unit series multiplexing high-voltage frequency converter utilizes a phase-shifting main transformer to step down the voltage, then connects multiple low-voltage single-phase frequency converters in series with a controller. Each power unit is powered by a phase-shifting main transformer with multiple windings. The transformer is a crucial component in the circuit structure of the unit series high-voltage frequency converter. There are 12 power units at 3kV, with 4 power units connected in series to form one phase. The 6kV series has 15 power units, with 5 power units connected in series to form one phase. The 10kV series has 21 power units, with 7 power units connected in series to form one phase. In the phase-shifting transformer, the 6kV inverter requires 3×5 windings and 48 main terminals (the 10kV inverter requires 3×7 windings and 66 main terminals). The transformer input uses an internal delta connection, and the output uses an external star connection with an extended delta configuration. The so-called multiplexing technology involves each phase consisting of several low-voltage PWM power units connected in series. Each power unit is powered by a multi-winding isolation transformer with multi-stage phase-shifting rectification, controlled by the CPU and driven by fiber optic isolation. The output side supplies power to the motor by connecting the U and V output terminals of each unit in series to form a star connection. By recombining and multiplexing the PWM waveform of each unit, a lower harmonic content (higher harmonic content at the output) can be achieved at the input (when the inverter outputs at 50Hz in the high-frequency range) conditions. This is the main circuit topology diagram of a 6kV frequency converter. Each group consists of 5 power units with a rated voltage of 690V connected in series, therefore the phase voltage is 690V × 5 = 3450V, and the corresponding line voltage is 6000V. Each power unit is powered by 15 secondary windings of the input isolation transformer. These 15 secondary windings are divided into 5 groups, with a 12° phase difference between each group. Taking the middle delta connection as a reference (0°), there are two sets of 4 windings above and below, leading (+12°, +24°) and lagging (-12°, -24°) ​​respectively. The required phase difference angle can be achieved by different connection groups of the transformer. Each power unit is a low-voltage PWM voltage-type inverter with three-phase input and single-phase output, composed of low-voltage (IGBT) circuitry. Each power unit outputs PWM voltages with different phase differences according to a pre-programmed timing sequence, resulting in three state levels: 1, 0, and -1. Five units per phase are superimposed in a stepped manner, generating 11 different gradient level waveforms. Figure 4 shows the output positive wave envelope voltage waveform synthesized for one phase. This voltage waveform has no special requirements for series-connected power units and can be used for common squirrel-cage motors. This multi-level technology-based high-voltage frequency converter, also known as a unit-series voltage-type frequency converter, uses power units in series with double "Y" circuits. It employs multi-winding transformers to group and equalize voltage across rectifier units, superimposing unit levels to output a high-voltage sinusoidal envelope stepped voltage waveform. It is suitable for variable frequency speed control drives of ordinary squirrel-cage motors. The term "perfect harmonic-free" used in multi-level technology is a marketing term from a foreign company. This misleading advertising, based on the Chinese public's lack of understanding of frequency conversion technology, uses the full-load harmonic content at the input end. This is a conceptual confusion and a deceptive tactic. In fact, the harmonics generated by the frequency converter should be strictly divided into two parts: 1. Input harmonic content index, which refers to the interference effect of the frequency converter on the power grid. 2. Output harmonic content index refers to the high-frequency radiation of the frequency converter and its side effects on the motor, such as operational pulsation, temperature rise, insulation aging, and bearing fatigue. It is well known that transformers, while performing isolation functions, generate new harmonic sources. Even a perfectly sinusoidal power frequency transformer has excitation harmonics; how can a transformer with nonlinear rectification and superposition be "perfectly harmonic-free"? Harmonics are still present. It can be said that the input harmonic content is low and meets standards. In fact, many high-voltage frequency converters meet the input harmonic content indexes in GB/T14549-93, Power Quality, Harmonics in Public Power Grids, and GB/T12668.4, High-Voltage Frequency Converter standards. Unit series multiplexing is based on a 120° square wave at the output. The frequency converter has a better waveform and lower harmonic content at rated frequency and rated heavy load. At low frequencies or light loads, waveform distortion is large, the asymmetry of the three-phase output voltage increases, the motor flux linkage pulsation increases, a potential difference appears between the motor neutral point and the frequency converter neutral point, and harmonics increase dramatically. Because of the presence of a transformer in this type of inverter, if the neutral point of the motor is not grounded, the motor will have a common-mode voltage. When the neutral point of the motor is grounded, the common-mode voltage still exists and does not disappear, but is transferred to the transformer through the grounding point. The transformer bears the impact of the common-mode voltage on the insulation and the harmonic heat energy. This is one of the main reasons why the transformer of this unit series high-voltage inverter is prone to failure. Inverters are often used for energy-saving operation below the power frequency, which is extremely detrimental to the life of the motor. In the early application of a foreign company's high-voltage inverter in China, it was necessary to replace the motor with a special motor produced by them. This also indirectly indicates the severity of the output harmonics of the unit series multi-stage inverter. 3.4.2 Technical characteristics of unit series multi-stage inverter (1) It is an effective method of single transformer high-low-high. The power unit series voltage summing circuit is adopted, and the transformer multi-winding group voltage division rectifier unit is used to equalize the voltage. The unit level is superimposed, and the inverter outputs a high voltage stepped voltage waveform. After filtering by the motor stator inductor, the phase voltage is a sine wave (in fact, even if the output waveform of any inverter is very poor, as long as it is filtered by the motor stator inductor, the phase voltage will be a sine wave). (2) Mature technology is easy to combine different voltage output requirements. Since the power unit is series, the mature technology of low-voltage inverter is adopted. The inverter unit is composed of low-voltage IGBTs. The number of series units can be used to adapt to different output voltage requirements. (3) The power unit is modular, standardized, and interchangeable. Since the multiple power units have the same structure and parameters, it is easy to have interchangeability between units and realize redundant design. Even if individual units fail, the unit can be short-circuited through the unit bypass function, and the system can still operate at a reduced rate. (4) The operation of switching between power frequency and inverter is simple. If the power frequency operation after the inverter failure is considered, a simple switching device can be added. It can be easily switched to power frequency operation through the switching switch. (5) Complex and expensive phase-shifting transformers are required. Due to the presence of necessary phase-shifting transformers in the system, it is not easy to further improve the system efficiency. In the phase-shifting transformer, the 6kV three-phase 6-winding × 3 (10kV requires 12-winding × 3) delta connection will generate internal circulating current when the three-phase voltage is unbalanced (in fact, the three-phase voltage cannot be absolutely balanced). This will inevitably lead to an increase in internal resistance and current loss, which will also cause an increase in the copper loss of the transformer. At this time, coupled with the inherent loss of the transformer core (the excitation power is a constant factor), the efficiency of the transformer will decrease. This will affect the efficiency of the entire system, and the efficiency will decrease further as the load rate decreases. The average efficiency of the frequency converter system is low. If the transformer is damaged, the repair is extremely complicated and expensive. The total cost is at least about 45% of the purchase price. (6) Low input harmonic content under heavy load. Due to the use of necessary phase-shifting transformers to achieve multiple rectifications, the low harmonic content index at the input end is indirectly obtained. (7) Too many power units and power devices are used. The 6kV system requires 150 power devices (90 diodes and 60 IGBTs); there are too many phase-shifting main transformer contacts, the wiring is complicated, and the internal resistance and loss of the system increase. There are many drive components and wiring. Correspondingly, there will inevitably be more faults during long-term use, and the maintenance is complicated and the workload is large. (8) The output voltage waveform is good at rated load, but the distortion is prominent below 25Hz, and the harmonic content increases significantly. The motor vibrates greatly when starting from 0Hz, the motor temperature is high, and it cannot accelerate quickly. (9) It can only be used for variable frequency speed regulation of fans and water pumps. (10) The dynamic characteristics are soft, the response speed is slow, and the acceleration and deceleration time is long. (11) It is not easy to use for mechanical rotation with braking conditions; it is not easy to achieve four-quadrant operation with energy feedback, and it cannot achieve braking. (12) The device is too big, heavy, and has a large installation area. 3.5 Direct Series Connection of IGBT Elements in High-Voltage Frequency Converter (General High-Voltage Frequency Converter) Direct rectification of IGBT elements in series in a direct high-voltage method (without an internal input transformer). In the medium- and high-voltage field, the focus of the problem is the withstand voltage of self-turn-off power devices such as IGBTs. The withstand voltage of IGBTs operating at 3kV, 6kV, 10kV, or higher cannot be solved in the short term, and the series connection problem of high-speed power switching devices is a globally recognized unsolved cutting-edge problem. Voltage conversion method: Power supply IGBT elements in series with the high-voltage converter (R1) → Motor (R2). System equivalent impedance R = R1 + R2 3.5.1 Main Circuit The advantages of multilevel and multiplexing have been discussed above. With the development of modern PWM control technology, the generated voltage waveform can basically eliminate low-order harmonics. Two-level is better than three-level overall performance, and is not much different from multiplexing. In the low-frequency range, the waveform is superior to multilevel and multiplexing. At the same time, the problems brought by multilevel and multiplexing are considerable compared to direct series connection. 3.5.2 Static and Dynamic Performance Direct series two-level inverters can be equipped with DC braking circuits or energy feedback, similar to low-voltage inverters, and their dynamic performance can be as excellent as low-voltage inverters, while maintaining a simple circuit. This is not easy for multi-level inverters, especially multi-level inverters, limiting their application to some applications with low speed control requirements. Therefore, IGBT element direct series high-voltage inverters (general-purpose high-voltage inverters) utilize Jialing's core DSC technology. Direct speed control (DSC) is an optimal motor control method for AC drives, allowing direct control of all core variables of AC motors. It provides precise speed and torque control without the need for a pulse encoder on the motor shaft to provide rotor position feedback. Crucially, the control is unaffected by changes in stator and rotor temperatures that affect motor parameters (vector control deteriorates with stator temperature, and direct torque control deteriorates with rotor temperature). DSC has developed unprecedented capabilities in AC drives and provides excellent service for all applications. Direct Speed ​​Control (DSC) represents a revolution in motor control for AC drives. It achieves precise speed and torque control from zero speed without using pulse encoder feedback on the motor shaft. It can generate full-load torque even at zero speed. In DSC, stator flux, rotor magnetic field, and speed are the primary control variables. Slip is used as the error, torque as the adjustment variable, and robust control design ensures stability and reliability. A high-speed digital signal processor combined with advanced motor software models updates the motor state 40,000 times per second. Because the motor state and the comparison between actual and setpoint values ​​are constantly updated, each switching state of the inverter is determined individually. This means the inverter can generate optimal switching combinations and respond quickly to dynamic changes such as load disturbances, momentary power outages, and grid voltage fluctuations. DSC eliminates the need for separate PWM modulators for voltage and frequency control. Open-loop dynamic speed control accuracy reaches that of closed-loop flux vector control. DSC static speed control accuracy is 0.1% to 0.4% of the nominal speed (50Hz to 2Hz), meeting the requirements of most industrial applications. For more precise speed regulation, a pulse encoder can be added as an option. The open-loop torque step rise time of DSC is less than 5 milliseconds, while the open-loop torque step rise time of a flux vector control inverter without a speed sensor is more than 100 milliseconds, equivalent to direct torque control, with a torque ripple of 0.3%, which is better than direct torque control. The JL5000 inverter's excellent robustness, i.e., reliability and stability, is unparalleled. 3.5.3 Complexity In comparison, three-level inverters require 6 more fast diodes, and five-level inverters require even more. In multi-level inverters, each switch must be controlled independently; in multi-level inverters, each of the 4 switching devices in each unit must be controlled independently, and all require bulky, complex, costly, and self-damaging input transformers. IGBT components directly connected in series without an input transformer require only one switching quantity control for the same component. Its efficiency and reliability are, in principle, much higher. 3.5.4 Energy Saving Effect Multi-level inverters use a delta-stretcher method for transformers to obtain several different independent voltages, making it difficult to obtain balanced three-phase phase-shifted voltages. This inevitably creates circulating current, increasing copper and iron losses. Even with minimal load changes, the internal and external connections of hundreds of transformers will also increase losses and reduce reliability. Input transformers reduce efficiency. The primary purpose of using frequency converters is to achieve energy savings and generate economic benefits. IGBT-based direct-series high-voltage frequency converters can save more than 5% more energy under the same operating conditions. The benefits generated after several years of using more efficient energy-saving equipment are also considerable. Taking a 2000kW high-voltage frequency converter as an example, the transformer's self-loss alone amounts to 360 days × 24 hours × 100kW × 0.5 yuan/kW·h = 360,000 yuan per year. 3.5.5 Input and output harmonic content complies with national standards. IGBT-based direct-series high-voltage frequency converters employ passive correction technology at the input end. This technology can compensate for the phase shift of the fundamental frequency or suppress certain specified harmonics. Specifically, passive components are added at the input end to compensate for the input current of the filter capacitor. An inductor is inserted in series in the input circuit to limit the rise rate of the input current and extend the conduction time of the rectifier diode, thereby improving the power factor to over 0.9. Harmonics are shifted to the vicinity of the modulation frequency, ensuring that the input harmonic content (THD) fully complies with national standards. A voltage sine wave shaper is used at the output to shape the PWM voltage waveform output by the high-voltage inverter into a standard sinusoidal voltage waveform identical to the grid voltage. The waveform remains unchanged regardless of whether the inverter operates at high or low frequencies or under heavy or light load conditions. Furthermore, a common-mode voltage regulator with a world-patented "anti-common-mode technology" is incorporated at the output, making it the only high-voltage inverter to solve the high-voltage EMC problem. Its output harmonic content fully complies with international standards. 3.5.6 The world's only IGBT universal high-voltage inverter suitable for any motor load type The high-efficiency JCS series high-voltage inverter, due to its absence of input and output main transformers and its incorporation of internationally advanced technology, is an unparalleled high-efficiency, high-quality, and cost-effective product among current high-voltage inverters. Its versatility includes: use in variable-condition speed regulation and energy-saving applications for fans and pumps; use in potential load applications, such as cranes, hoists, elevators, and belt conveyors; use for precise control of angle and displacement, such as rolling mills; and use in general mechanical transmission systems with constant torque. 4. Conclusion In summary, it can be said that in the field of high-voltage motor variable frequency speed regulation, scientific and technological researchers have designed various high-voltage frequency converters to promote the development of human society and the leap in science and technology. Their active promotion and application during a certain period have made significant historical and scientific contributions. The updating or replacement of new science and technology is an inevitable law of social development. Any new technology has an inevitable process of development from understanding, acceptance, and reinvention. From the analysis of the above common high-voltage frequency converter electrical schemes, current-type, high-low-high, and three-level technology schemes of high-voltage frequency converters deserve careful consideration when selecting them. Unit series superimposed multi-level technology high-voltage frequency converters may have application value within a certain period. The IGBT-based direct-series high-voltage frequency converter (general-purpose high-voltage frequency converter) features no input or output main transformer and incorporates advanced international technology, making it an unparalleled product in terms of quality and cost-effectiveness among current high-voltage frequency converters. In particular, its advanced control technology achieves versatility and represents a power electronic device with purely national intellectual property rights, showcasing national wisdom and strength. The JCS high-voltage frequency converter is a purely domestically produced high-tech product that has received support, assistance, and recognition from renowned domestic experts and is worthy of widespread promotion and application. JCS high-voltage frequency converters should continue to incorporate new technologies and improve quality to achieve sustainable and long-term development.