1. Introduction Shandong Fengguang Electronics Co., Ltd., based on years of research and development of medium and low voltage frequency converters, integrated the advantages and disadvantages of various domestic and international high voltage high power frequency converter schemes, and successfully developed the optimal scheme. In December 2002, it passed the provincial-level scientific and technological achievement and product appraisal, becoming one of the few domestic enterprises producing high voltage high power frequency converters. 2. Schemes and Advantages/Disadvantages of Domestically Produced High Voltage High Power Frequency Converters Currently, two schemes dominate domestically produced high voltage high power frequency converters: one is the multi-level technology of power units connected in series to form high voltage; the other is the three-level structure using high-voltage modules. Other schemes using high-low-high voltage are basically obsolete due to the high technical difficulty, high cost, and large footprint of the output step-up transformer. Therefore, the high-high voltage scheme is the main development direction for high voltage high power frequency converters. The high-high voltage scheme is further divided into multi-level technology (CSML) and three-level (NPC) schemes. Currently, some manufacturers produce high voltage high power frequency converters using the three-level scheme, while most manufacturers use multi-level technology with low-voltage modules and multiple units connected in series. Comparing these two schemes, each has its advantages and disadvantages, mainly in the following aspects: (1) The CSML scheme uses a large number of components, but they are all low-voltage components, which are not only inexpensive but also easy to purchase and replace. The technology of low-voltage components is also relatively mature. In contrast, the NPC scheme uses fewer components, but the cost is high, and it is difficult to purchase and maintain. (2) Voltage equalization problem (including static voltage equalization and dynamic voltage equalization) Voltage equalization is an important factor affecting high-voltage frequency converters. When using the NPC scheme, when the output voltage is high (such as 6kV), a single component cannot meet the withstand voltage requirements, and components must be directly connected in series. This will inevitably lead to voltage equalization problems, lose the advantages of the three-level structure in voltage equalization, and the reliability of the system will also be affected. However, the CSML scheme does not have voltage equalization problems. The only exception is when the inverter is in a fast braking state, the motor is in a generator braking state, which causes the DC bus voltage in the unit to rise. The degree of rise of the DC bus voltage in each unit may be different. By detecting the DC bus voltage of the power unit, when the DC bus voltage of any unit exceeds a certain threshold, the deceleration time is automatically extended to prevent the DC bus voltage from rising, which is the so-called overvoltage stall prevention function. This technology is widely used in low-voltage inverters and is very successful. (3) Harmonic pollution and power factor of the power grid Since the pulse number of the input rectifier circuit of the CSML method exceeds that of the NPC method, the former has a clear advantage in terms of input harmonics, and therefore also has a certain advantage in terms of comprehensive power factor. (4) Output waveform The output phase voltage of the NPC method is three levels and the line voltage is five levels. The output phase voltage of the CSML method is 11 levels and the line voltage is 21 levels (for five units in series). Moreover, the equivalent switching frequency of the latter is much higher than that of the former, so the quality of the output waveform of the latter is also higher than that of the former. (5) The output voltage jump step of the NPC mode is half of the high voltage DC bus voltage, which is about 4kV for a 6kV output inverter. The output voltage jump step of the CSML mode is the DC bus voltage of the unit, which will not exceed 1kV. Therefore, the difference between the former and the latter is also very obvious. (6) In terms of transformer and inverter circuit, the efficiency of the NPC mode and the CSML mode is very close. However, due to the difference in output waveform quality, if a common motor is used, the former must set an output filter, while the latter does not. The presence of the filter will affect the efficiency by about 0.5%. (7) Four-quadrant operation When the input adopts a symmetrical PWM rectifier circuit, the NPC mode can achieve four-quadrant operation and can be used for rolling mills, hoists and other equipment; while the CSML mode cannot achieve four-quadrant operation. It can only be used for fan and pump loads. (8) Redundancy design Redundancy design of the NPC mode is difficult to achieve, while the CSML mode can easily adopt power unit bypass technology and redundant power unit design scheme, which greatly helps to improve the reliability of the system. (9) Maintainability Besides reliability, maintainability is also an important factor in evaluating the quality of high-voltage, high-power frequency converters. The CSML method adopts a modular design; when replacing a power unit, only three AC input terminals, two AC output terminals, and one fiber optic connector need to be removed to extract the entire unit, which is very convenient. The NPC method is not so convenient. In summary, the three-level voltage-type frequency converter has a simple structure and can be made into a four-quadrant frequency converter, with a wide range of applications. When the voltage level is high, direct series connection of components can lead to voltage equalization problems, as well as output harmonics and dv/dt issues. Generally, an output filter needs to be set, and when the power grid has high requirements for harmonic distortion, an input filter also needs to be set. The multi-level PWM voltage-type frequency converter does not have voltage equalization problems and has significant advantages in terms of input harmonics and dv/dt. For ordinary fans and pumps that do not require four-quadrant operation, the CSML frequency converter has a broad application prospect. This type of frequency converter is also called a perfect harmonic-free frequency converter by designers at home and abroad. After extensive discussions and considering the advantages and disadvantages of various solutions, our company's designers finally selected the CSML solution, a perfect harmonic-free frequency converter, as our best choice. This is the JD-BP37 and JD-BP38 series of high-voltage, high-power frequency converters that we are launching to the market. 3 Performance characteristics of the frequency converter (1) The frequency converter adopts a multi-power unit series scheme, with low output waveform distortion, and can be connected to ordinary AC motors without the need for an output filter. (2) The input side adopts multi-phase shift rectification technology, resulting in low current harmonics and a high power factor. (3) Communication between the controller and the power unit is achieved through multi-path parallel optical fiber, which improves anti-interference and reliability. (4) The controller adopts a power supply system independent of the high-voltage source, which is beneficial for the overall machine debugging and operator training. (5) It adopts a full Chinese Windows color LCD touch interface. (6) The main circuit is modularly designed, making installation, debugging, and maintenance convenient. (7) Complete fault monitoring and alarm protection functions. (8) On-site control and remote control are optional. (9) Built-in PID regulator, can operate in open loop or closed loop. (10) Can print out operation reports as needed. 4 Working Principle 4.1 Basic Principle This frequency converter is an AC-DC-AC type unit series multi-level voltage source frequency converter, and the principle block diagram is shown in Figure 1. The number of units depends on the voltage level. Here, we take 8 units per phase, for a total of 24 units as an example. Each power unit bears the entire motor current, 1/8 of the phase voltage, and 1/24 of the output power. Each of the 24 units has an independent three-phase input winding on the transformer. The power units and the secondary windings of the transformer are mutually insulated. The secondary winding adopts the extended delta connection method to achieve multiplexing and reduce the harmonic components of the input current. The 24 secondary windings are divided into three phase groups, with a 20° phase difference. Using phase B as the reference, the eight secondary windings corresponding to the eight units in phase A lead phase B by 20°, and the eight secondary windings corresponding to the eight units in phase C lag phase B by 20°, forming an 18-pulse rectifier circuit structure. The overall schematic diagram is shown in Figure 2. 4.2 Power Unit Circuit: All units have six diodes to achieve three-phase full-wave rectification, and four IGBTs form a single-phase inverter circuit. The main circuit of the power unit is shown in Figure 3. The four IGBTs are represented by T1, T2, T3, and T4, and their gate voltages are UG1, UG2, UG3, and UG4, respectively. The output of each power unit is the same PWM wave. The output waveform of the power unit is shown in Figure 4. The inverter uses multi-level phase-shift PWM technology. Power units in the same phase output the exact same reference voltage (same amplitude, same frequency, same phase). The output waveform of multiple units superimposed is shown in Figure 5. 4.3 System Structure and Control (1) System Structure The entire system consists of an isolation transformer, 3 frequency converter cabinets and 1 control cabinet, see Figure 6. [align=center] [/align] a) The primary side of the isolation transformer is star-connected, and the secondary side has 24 independent three-phase windings. In order to adapt to the power grid conditions on site, taps are left on the primary side of the transformer. b) The three phases A, B and C of the frequency converter cabinet are installed in 3 cabinets, which can be called cabinet A, cabinet B and cabinet C respectively. c) The control cabinet contains the control system, and the front panel of the cabinet is equipped with control panel, control terminal block, etc. Due to the different voltage levels and capacities, the number of units in different models is different, and the panel layout will also be different. 4.4 System Control The whole machine control system has a 16-bit microcontroller as the main controller, and each of the 24 power units has its own auxiliary CPU, which is handled by an 8-bit microcontroller. In addition, there is another CPU, which is also an 8-bit microcontroller, responsible for managing the keyboard and display screen. (1) The third harmonic compensation technology improves the power supply voltage utilization rate. (2) The controller has a power supply system independent of the high-voltage power supply. When the high voltage is not applied, the waveform at each point of the equipment is the same as when the high voltage is applied, which greatly facilitates the reliability and debugging of the whole machine. (3) The system adopts advanced carrier phase shift technology. Its feature is that the fundamental wave of the unit output is superimposed and the harmonics cancel each other out. Therefore, the distortion of the total output waveform after series connection is particularly small. 5 Field Application In August and October 2002 and March and April 2003, our company applied two JD-BP37-630F high-voltage high-power frequency converters, one JD-BP38-355, and one JD-BP37-550F to the ironmaking plant of Shandong Laiwu Iron and Steel Co., Ltd., the Jinzhou Oil Production Plant of Liaohe Oilfield, and Zhejiang Yongsheng Chemical Fiber Co., Ltd., respectively. From the perspective of operation: (1) The inverter has a compact structure and is easy to install. Since all parts of the inverter are installed in the cabinet, there is no need for other reactors, filters, compensation capacitors, starting equipment and other devices. Therefore, it is small in size, compact in structure, easy to install, has less on-site wiring, and is convenient to debug. (2) The motor and unit run smoothly and all indicators meet the process requirements. The motors driven by the inverter are all three-phase ordinary asynchronous motors. The motors always run smoothly and the temperature rise is normal throughout the entire operating range. The noise and starting current of the fan are very small, and there is no abnormal vibration or noise. Within the speed range, the maximum temperature rise of the bearing is within the allowable range. (3) The three-phase output waveform of the inverter is perfect and very close to the sine wave. According to the on-site test, the three-phase output voltage waveform and current waveform of the inverter are very standard, indicating that the inverter can completely control the operation of ordinary motors and has no special requirements for motors. (4) The inverter operates stably and performs well. Since the equipment was put into operation, the inverter has been running very stably. During equipment operation, our company's technicians regularly inspected the temperature rise of the inverter's input transformer and the power unit, and found them to be completely normal. The output voltage and current waveforms have excellent sinusoidal properties, very low harmonic content, and efficiency exceeding 97%, which is superior to similar imported equipment. (5) Improved operating conditions reduce the labor intensity of workers. The inverter can automatically adjust the motor speed according to production needs to achieve the best effect, greatly reducing the labor intensity of workers. (6) The inverter is simple to operate, easy to master and maintain. The inverter's start-up, shutdown, and frequency change are simple to operate, and operators can master them fully after half an hour of training. In addition, the inverter has a full range of functions, which improves the reliability of the equipment and has a significant energy-saving effect. Taking the JD-BP37-630F frequency converter used by Shandong Laiwu Steel Co., Ltd. as an example, the system's production cycle is approximately 1 hour, with a 20-minute iron tapping time and an interval of about 40 minutes. The rated current of the motor configured in the system is 80A. Based on the operating conditions and the actual operating conditions of other production lines, the expected operating current of this motor should be 60A. At the frequency converter's upper limit of 45Hz, the current is 45A; at an interval operating frequency of 20Hz, the current is 20A. According to the formula, the energy-saving effect reaches 42.7%. 6. Conclusion: Based on the operation of these units over the past few months, the high-voltage, high-power frequency converter independently developed and manufactured by our company operates stably and reliably, with significant energy-saving effects, improving the working environment of staff and reducing the labor intensity of on-duty personnel. The frequency converter has comprehensive motor protection functions, reducing maintenance costs, extending the service life of motors and fans, bringing significant economic benefits to users, and has won high praise from users. According to experts, there are more than 30,000 high-voltage high-power motors above 6kV in our country, which is equivalent to about 6.5 million kW. Therefore, the market for high-voltage high-power frequency converters is extremely broad. Reference [1] Xu Furong. Technical and economic analysis of speed regulation and energy-saving operation of high-power fans and water pumps [J]. Frequency Converter World, 2001, (8).