Abstract: This paper introduces the technical characteristics and working principle of a two-level high-voltage frequency converter based on IGCT devices. The experimental waveforms of the two-level high-voltage frequency converter, its operation in power plants, and its energy-saving effects are analyzed. The application prospects of a two-level high-voltage frequency converter for power sources are discussed. Keywords : IGCT device; two-level; frequency converter Key words : IGCT devices; two-level; frequency converter 0 Introduction In the face of the severe situation of international energy shortage and global warming, reducing energy consumption and reducing greenhouse gas emissions have become urgent tasks. Practice has proven that variable frequency speed regulation of industrial motors under certain operating conditions is an effective way to save energy. According to statistics, there are more than 40 million wind turbines in China that need to be speed-regulated for energy saving, with an annual power consumption of about 150 billion kWh, and an energy saving potential of 45 billion kWh/year, which is huge [1]. The reason why the promotion of high voltage frequency converters with significant energy-saving effects was slow in the early stage was, on the one hand, because the initial investment was large and the reliability of some products was not high, and on the other hand, because the application of frequency converters would bring some new problems, such as power grid harmonic pollution, motor harmonic loss heating and motor insulation aging. With the increasing emphasis on energy conservation and emission reduction by the state and the development of inverter technology, high-performance, cost-effective high-voltage inverters are being used more and more. Ordinary inverters often have high di/dt and du/dt electrical stresses, resulting in high harmonic emissions in the circuit output. However, by adopting a reasonable circuit topology, di/dt and du/dt can be effectively reduced, and by employing a reasonable control algorithm, harmonic output can be reduced, thereby greatly improving the reliability of the inverter itself while reducing harmonic pollution to the power grid. Two-level high-voltage inverters based on IGGT devices have an output harmonic content of less than 2% and an efficiency of up to 98%. 1. Introduction to Inverter Technology The main circuit topology and internal circuit of a typical cascaded high-voltage inverter are shown in Figures 1 and 2, respectively. The main circuit topology of a two-level inverter based on IGCT devices is shown in Figure 3. As shown in Figure 3, the rated voltage that a single power transistor in a two-level high-voltage (6 kV) frequency converter connected in series must withstand is 6000 × √2 ÷ ≈ 32800 (V), and the maximum operating voltage reaches 2800 × 1.15 × 1.08 ≈ 3500 (V) (considering an overvoltage of 15% and a transformer impedance of 8%). This places high demands on the single-transistor absorption circuit and its control. As shown in Figures 1 and 2, the rated voltage that each power transistor in a cascaded product needs to withstand is 6000 ÷ √3 × √2 ÷ 5 × 1.15 × 1.08 ≈ 1200 (V). Therefore, the implementation of a two-level topology high-voltage frequency converter is much more difficult than that of a cascaded frequency converter. The two-level high-voltage (6 kV) frequency converter has the following characteristics: (1) Cascaded frequency converters require 150 power transistors, while two-level frequency converters only require 36 power transistors. Therefore, the failure probability of two-level frequency converters can be greatly reduced, and the reliability can be greatly improved; (2) Cascaded frequency converters have 45 transformer outputs, while two-level frequency converters have only 9 outputs, which significantly reduces the failure probability; (3) Cascaded frequency converters do not allow the motor to be placed too far away from the frequency converter to prevent abnormal protection of the frequency converter or even damage to the frequency converter and motor due to reasons such as flyback at the machine end. However, two-level frequency converters have an LC filter, and there is no large du/dt in the output cable. There is no flyback phenomenon at the machine end, so there is no such restriction; (4) In terms of energy flow, two-level frequency converters can achieve 4-quadrant operation by slightly modifying the rectifier circuit, which is something that current cascaded frequency converter technology cannot do. (5) High-voltage metal film capacitors are used in two-level frequency converters instead of electrolytic capacitors, so that the service life of the whole machine can reach more than 10 years, which is difficult to achieve in cascaded frequency converters. 2 Introduction to two-level technology based on IGCT device Figure 3 is the main circuit topology of two-level high-voltage frequency converter. The inverter section uses high-voltage, high-current power switching device IGCT, which integrates anti-parallel freewheeling diodes. In order to reduce the impact on the power grid, the rectifier circuit adopts an 18-pulse diode rectifier structure. Its core technologies are specific harmonic elimination technology, LC filtering technology and multi-IGCT series voltage equalization technology. In the design, specific harmonic elimination technology and LC filtering technology are perfectly combined to achieve a relatively ideal effect. The output voltage and current waveforms of the two-level high-voltage frequency converter measured by the experiment are shown in Figure 4. There are almost no harmonics and large du/dt, which does not damage the motor insulation. At the same time, grounding the midpoint of the filter capacitor can eliminate the common-mode voltage (i.e., eliminate the motor axial voltage) so that the service life of the motor is not affected. The design difficulty of two-level high-voltage frequency converter based on IGCT device lies in the dynamic voltage equalization of multiple series power transistors. After rigorous calculations, precise simulations, and repeated experimental improvements, the measured waveforms of power transistor turn-off voltage equalization under rated current conditions were obtained, as shown in Figure 5. The second peak voltage in Figure 5 are 3525V, 3360V, and 3240V, respectively. Here, the deviations of the voltages of the three power transistors from the average voltage are all less than 5%, achieving excellent voltage equalization. The dynamic voltage equalization circuit is shown in Figure 6. While this circuit provides good voltage equalization, it causes excessive di/dt at the moment of IGCT turn-on, which is unacceptable for IGCT devices. From the perspective of suppressing di/dt, the resistance value needs to be increased, but a large resistance will inevitably reduce the effect of dynamic voltage equalization. Therefore, the voltage equalization shown in Figure 5 is not the optimal effect, but rather a comprehensive result considering di/dt. Regarding key processes, due to the adoption of lateral power transistor press-fit technology, the power density of the power circuit is greatly improved. Even with a filter circuit, the volume and weight of the two-level high-voltage inverter are significantly smaller than other products. 3. Operational Results Before the frequency converter upgrade of the condensate pump (1120kW) of the 330MW unit at Yangzhou Power Generation Co., Ltd., the motor operating current was 126A when the unit was running at full load, with only a slight decrease in current when the load decreased. After the frequency converter upgrade, the inverter input current was 92A at full load and 41A at 210MW load, resulting in a comprehensive energy saving of over 35%. Before the upgrade, the pump consumed approximately 8 × 1,000,000 kWh annually; after the upgrade, it consumed approximately 5 × 1,000,000 kWh annually. Based on a grid electricity price of RMB 0.39/kWh, this translates to at least RMB 1.1 million in electricity savings annually, demonstrating considerable economic benefits. Simultaneously, the automation level of the power plant's motor control was significantly improved. 4. Conclusion The above analysis shows that the two-level high-voltage frequency converter exhibits excellent performance, offering significant advantages in reducing output harmonic content, improving converter efficiency, extending motor lifespan, and enhancing system stability. In applications such as power systems, coal mines, and urban water supply where equipment reliability and safety requirements are very high, using a two-level high-voltage frequency converter with low output harmonic content and minimal damage to the load is a good choice. References : [1] Yang Zhiyong, Zhao Zhengming, Application of three-level frequency converters in energy saving [c]//Proceedings of the Technical Forum of the Power and Energy Session of the World Congress of Engineers 2004, Beijing: China Electrotechnical Society, 2004: 167-169,