Abstract: High-voltage, high-capacity variable frequency speed control systems are one of the important research directions in the field of power electronics technology. This paper analyzes some key technical issues in the 6kV/1250kW high-voltage three-level neutral-point clamped frequency converter based on IGCT developed by the author, and presents relevant simulation analysis and experimental research results. 1 Introduction In recent years, China's R&D and production capabilities of variable frequency speed control devices have been continuously improving, and the application level has also made significant progress. Currently, the development is moving towards high performance and high voltage, high capacity. The development of medium and high voltage frequency converters has always been a hot topic, but due to its high technical threshold, large capital investment, and long R&D cycle, it has always been one of the difficulties in this research field. The current high-voltage, high-capacity frequency converter topologies mainly include: ● Capacitor flying-over type; ● Unit cascade type, also known as the Robicon structure; ● Diode neutral-point clamped (NPC) type, represented internationally by products from ABB and Siemens. In contrast, diode-clamped structures are increasingly valued for their advantages, including fewer components, compact structure, simple control algorithm, ease of achieving four-quadrant operation, and suitability for use as high-end frequency converters. However, because this structure directly uses high-voltage switching devices (such as GTOs, IGCTs, or high-voltage IGBTs) as switching units, its high-voltage characteristics are pronounced, reducing device load margins and increasing the requirements for system parameter configuration. In particular, the effects of energy transients and distributed parameters, which were not prominent in small-capacity and low-voltage systems, become more significant, increasing the difficulty and risk of development and making it one of the challenging issues in high-voltage, high-capacity power electronic converters. my country's development of high-voltage (above 3kV) and high-capacity (above 1000kW) three-level NPC variable frequency speed control systems is still in its initial stages. To accelerate the development of my country's own high-end medium- and high-voltage variable frequency speed control devices, in October 2001, the Department of Electrical Engineering of Tsinghua University and China State Grid Nanjing Automation Co., Ltd. jointly established the Tsinghua-Nanjing Automation Power Electronics Application Technology Joint Research Institute, specifically for the development of high-voltage, high-capacity three-level NPC variable frequency speed control systems. In 2004, a prototype of a diode-clamped 6kV/550～1250kW three-level frequency converter based on IGCT was developed. In 2005, it achieved long-term, fault-free, full-load operation in the field. In 2006, it passed all type tests by national-level technical testing departments and ministerial-level technical and product appraisals, and is now fully available on the market. Looking back on the more than five years of research and development, we have gone through an exploration process from theory to practice and then from practice back to theory. In particular, we have gained a deeper understanding of the key issues in high-voltage, high-capacity power electronic conversion devices. These preliminary insights are shared with you. 2. Main Structural Features of the IGCT-Based Three-Level NPC Frequency Converter Due to the high voltage and large current of high-voltage, high-capacity frequency converters, the corresponding voltage and current change rates are also large, and the circuit distributed parameters have a significant impact. This makes the selection of switching devices, the wiring between devices, the matching of absorption circuit component parameters, waveform modulation, and filtering processes significantly different from those of low-voltage, small- and medium-capacity frequency converters. Therefore, the structure is also very different. The structural principle diagram of the frequency converter developed in this paper is shown in Figure 1. Figure 1. A diode-clamped 6kV/550～1250kW three-level frequency converter speed regulation system based on IGCT has the following main structural features: (1) IGCT is used as the main switching device and a press-fit structure is adopted, as shown in Figure 2. Among them, the discrete heat sink is a support device for IGCT heat dissipation and press-fit structure, and also a conductor connecting the two IGCTs. Figure 2. Physical diagram of the structure of the three-level NPC high-voltage frequency converter based on IGCT (2) Two sets of di/dt absorption circuits are shared by the three-phase bridge arms (Ls, Rs and Cs), as shown in Figure 1. (3) The DC bus and the inverter circuit are connected through a stacked flat copper busbar, as shown in Figure 3. Figure 3. Physical diagram of DC bus connection (4) The control system adopts a multi-CPU and fiber optic CAN bus communication system, as shown in Figure 4. Figure 4 Multi-CPU main control board (a) and CAN bus communication board (b) (5) Output adopts integrated filtering and boost structure Output adopts integrated filtering and boost structure, as shown in Figure 5. Figure 5 Integrated boost LC filter system Due to these structural characteristics, some key technical problems that must be solved have emerged. 3 Analysis of several key technical problems High voltage and high capacity frequency converters involve many key technologies. This article describes and analyzes several key issues. 3.1 Effective setting of IGCT safe working area To improve the reliability of high capacity power electronic devices, special attention needs to be paid to the characteristics of semiconductor switching devices. At the same time, the relationship between the application characteristics of the devices and other elements in the power electronic devices should be studied, so as to design and optimize the topology, structure and control strategy of the power electronic devices. Usually, the constraints on various electrical characteristics of devices in the device application manual are often based on specific single tube test circuits. Some key parameters (such as stray inductance) are difficult to guarantee to be completely consistent with the test circuit in actual applications due to structural design problems. Moreover, the actual circuit topology is quite different from the semiconductor test circuit, which often involves the interaction and mutual connection between multiple switching tubes. Meanwhile, differences in operating conditions, load characteristics, and device parameters mean that equipping and operating high-performance devices in specific devices does not necessarily improve device reliability. In other words, the safe operating area (SOA) of a single switching device is not always applicable to the entire device, and may even require significant adjustments in practical applications. In the research on high-voltage, high-capacity variable frequency speed control based on IGCTs, the design and analysis focused on switching devices such as IGCTs and diodes. A definition of the safe operating area of ​​the converter across its entire operating range was proposed. Based on the safe operating area of ​​the IGCT, a quantitative relationship between the safe operation of the IGCT and the safe operation of the converter was given. This relationship was then used to directly optimize the converter's rated operating point, control method, protection measures, structural stray parameter requirements, losses, and efficiency. The optimization diagram is shown in Figure 6. During the optimization process, several patented technologies with independent intellectual property rights were comprehensively used, including three-level inverter midpoint voltage balance and control parameter constraints, switching device loss modeling, and comprehensive analysis of inverter operating conditions. This has achieved good results in practical applications, significantly reducing the device's failure rate. Figure 6. Application of the safe operating range across the entire operating range in optimized design. 3.2 Hybrid modulation high-voltage frequency converters with low output harmonics. Due to the large conversion power, the switching frequency is generally relatively small, resulting in large output harmonics. Conventional sinusoidal PWM (SPWM) and space vector PWM (SVPWM) are difficult to solve the problem of large output harmonics. Specific harmonic elimination PWM (SHEPWM) is an optimized PWM that achieves better harmonic characteristics with fewer switching cycles by optimizing the switching timing. Its main advantages are: high output waveform quality, low torque and current ripple under the same number of switching cycles; reduced requirements for filters, allowing for smaller filter size; low switching cycles and low losses under the same waveform quality, making it particularly suitable for high-voltage, high-power applications with limited switching frequencies, such as those using GTOs and IGCTs; and high DC bus voltage utilization. The disadvantages are: fixed switching angles, requiring offline calculations, difficult to implement online, and insufficient control flexibility, especially at low frequencies where the large number of switching angles necessitates higher storage requirements. This system employs a hybrid PWM method, using asynchronous SVPWM at low frequencies and SHEPWM at high frequencies. This avoids the drawbacks of poor harmonic characteristics in SVPWM at high frequencies and large storage requirements in SHEPWM at low frequencies, fully leveraging the advantages of both. This allows the inverter to effectively suppress low-order harmonics throughout its operating range, resulting in a better output waveform. The challenge lies in the seamless transition between the two methods to ensure the applicability of the hybrid modulation. To address this, a fixed-angle switching method is used. Assuming the operating frequency at the switching moment is 45Hz, for SVPWM, the switching frequency is 600Hz. With a reference vector frequency of 45Hz, the reference vector is sampled 600/45 = 13.33 times in a 360° space within one cycle. One of these samples will inevitably fall within the 0–28° interval. The system switches from SVPWM to SHEPWM only when the reference vector falls within this interval. Similarly, the switch from SHEPWM to SVPWM only occurs when the phase of phase A falls within a fixed angle interval. Since the switching position is fixed, the phenomena and behaviors are repeatable. Based on theoretical analysis, satisfactory results can be obtained by fine-tuning the experiment. The experimental results are shown in Figure 7, where the upper part is the inverter output line voltage and the lower part is the inverter output phase current. Figure 7 Inverter output voltage and current when SVPWM and SHEPWM are switched at 45Hz. 3.3 Design of Integrated Filter Boost System The reliable application of high-voltage, high-capacity variable frequency speed control systems is currently more limited by the manufacturing and assembly processes of semiconductor devices. The IGCT device in the high-voltage three-level NPC inverter used in this system has a withstand voltage rating of 4.5kV. With such a structure, the maximum output line voltage can only be 3.3kV. In addition to the challenge of boosting the output voltage to 6kV, the presence of a large number of harmonic components in the voltage and current waveforms of the system output due to the low switching frequency is also a major problem. These harmonic components will cause serious thermal effects when introduced into the motor; at the same time, the steep rising (falling) edge in the PWM waveform brings a large dv/dt, which will directly threaten the motor insulation and generate shaft current and electromagnetic interference through the coupling capacitor in the line. The higher the voltage level, the more serious the problem. Therefore, a filtering device is needed, such as an RLC filter. To directly apply a 3.3kV/1250kW AC variable frequency speed control system to a 6kV output system, besides effective voltage boosting, it is also necessary to smooth the output voltage waveform and reduce voltage THD. For this purpose, a novel structure using the equivalent leakage inductance of the primary and secondary sides of the step-up transformer for filtering was adopted, and the effectiveness of this design scheme was verified through comparative experiments in large and small capacity prototypes. The principle structure is shown in Figure 8. Figure 8: Simulation filtering effect of the high-voltage, high-capacity filter boosting device is shown in Figure 9. As can be seen from Figure 9, the output harmonics are greatly reduced. Figure 9: Simulation evaluation of the high-voltage, high-capacity LC filtering effect. 3.4 Minimum Pulse Width Design Based on Stray Parameter Calculation The minimum pulse width refers to the minimum pulse width time of the gate control signal set to ensure the complete safe turn-on and turn-off operation of the switching device. The minimum pulse width is limited not only by the highest operating frequency of the switching device itself, but also by the operating state of external circuits such as absorption circuits and voltage equalization circuits. The minimum pulse width mentioned in the literature usually refers only to the minimum pulse width of a single transistor, including the on-state minimum pulse width (tONMIN) and the off-state minimum pulse width (tOFFMIN). In this system, since the three-phase inverter arms share a single absorption circuit, an interphase minimum pulse width (tMIN) must also be added between the commutation processes of two adjacent arms to ensure that the shared absorption circuit is in a stable state before the commutation of each phase arm. Therefore, the setting of the minimum pulse width is one of the keys to the safe and reliable operation of the device and apparatus. Since the transient process of the first-end pulse and the return loop has the greatest impact during the switching process, and these parameters are determined by the loop stray parameters, the design of the minimum pulse width must be based on the calculation of the loop stray parameters. This is one of the challenges. Figure 10 shows the voltage and current test waveforms during the IGCT switching process. Generally, the minimum pulse width setting needs to be determined based on a large number of test waveforms. Figure 10. Measured waveforms of IGCT voltage and load current. 3.4 Multi-CPU Coordinated Control Based on High-Reliability CAN Bus Fiber Optic Communication with Multiple Verification. Due to the large number of peripheral devices, complex input-output relationships, and numerous self-protection and functions in high-voltage variable frequency speed control systems, their control systems are also quite complex. In practice, they are composed of subsystem units with different functions, such as multi-CPU systems consisting of multiple control boards (main control board, input/output board, manual control panel, AD board, etc.). Communication between units is accomplished through the CAN bus, which is centrally distributed near the main control unit. Except for the main control unit, fiber optic cables are used as the communication medium between the CAN controllers and CAN transceivers of the other units. In practical applications, the real-time performance and reliability of communication are critical issues. A fast transmission procedure was developed to enable the interrupt program to initiate transmission in the shortest possible time, ensuring the real-time performance of protection action signal communication. The developed transmission procedure also considers the protection and recovery of the mailbox data field configured in the upper-level program, and retransmits in case of transmission failure, ensuring the rigor of program logic and the reliability of communication. The optimized communication system is stable and reliable during long-term operation, and the protection signal transmission is fast and timely. The waveform on the CAN bus during system operation is shown in Figure 11. Figure 11 CAN bus voltage waveform From Figure 11(a), we can see the data frames containing voltage and current sampling values ​​sent by the data acquisition unit to the main control unit at regular intervals. In the denser areas of the figure, we can also see the data frames sent by the IO unit at irregular intervals and the data frames returned by the main control unit. From Figure 11(b), we can observe that the CAN bus communication is more frequent during protection action. The action of the peripheral relay during protection causes obvious interference to the CAN bus, but the CAN communication can still proceed normally. 4 Problems in field test operation The field operation of high voltage and high capacity frequency converters is very different from laboratory operation. Problems in field operation are one of the key issues for the safe and reliable operation of the system. The problems encountered in the field are roughly as follows: (1) Large temperature variation The field ambient temperature can reach up to +50℃ and down to -30℃. Temperature changes have a great impact on the characteristics of the components in the device. (2) Large vibration and noise Since the device needs to be operated under load for a long time in the field, the vibration caused by the cooling fan, etc., can cause some connecting parts (wires) to loosen or break, which may cause devastating damage. (3) Dust and moisture: The dust is high at the site, especially for integrated devices like IGCT. Dust and moisture on the drive board and main control board can easily cause short circuits and failures. (4) High electromagnetic interference: The site is generally a strong electric field area and is affected by many factors. The power supply voltage fluctuates greatly, which can easily cause device failure. The above problems are important issues affecting the normal operation of high voltage frequency converters at the site and are also problems that must be solved in the development of the device. Previously, these were considered to be some process or protection issues, which are not technical issues of power electronics itself. In fact, these problems include some deep theoretical issues in power electronics technology, such as the characteristics of switching devices under extreme conditions, the power failure of frequency converters, the time delay effect caused by distributed parameters, and the common mode and differential mode problems in the system. These studies are still ongoing, and the accumulation of field operation experience is one of the important links. Since the ultimate goal of the development has always been safe and reliable operation at the site, we have paid more attention to finding problems from reality and then effectively solving them. The first prototype has been running reliably at the site for a year and a half. Figure 12 shows the operation of the high voltage three-level frequency converter at the site. The frequency converter was used to regulate the speed of an ash pump, replacing the original method of regulating the speed through valves and water supply. It adjusts the speed according to the change of water level, realizing automatic control of unattended pumping volume, and the energy saving and consumption reduction effect is obvious. Figure 12 6kV/1250kW three-level frequency converter and its pump load in operation on site 5 Conclusion (1) The foundation of high voltage and high capacity frequency converter is high voltage and high capacity switching device. One generation of device determines one generation of technology. Based on the characteristics of the device and driven by the application of the device, carefully studying the application characteristics of the switching device from principle, simulation, single-piece test and system test is an effective way to develop high voltage and high capacity frequency converter. (2) In high voltage and high capacity or ultra-high capacity power electronic conversion devices, the energy conversion characteristics are more obvious. It is necessary to understand the characteristics of switching transient process, distributed parameters, signal and power delay. (3) Taking the system reliability and adaptability issues as research objectives, and obtaining the main key technologies from the reliability research, is the key to solving the problem of applicable high voltage and high capacity frequency converter.