Working Principle of High Voltage Frequency Converter Time: 08-02-28 Source: Soken Heping (Shanghai) Electrical Enter Forum 0 Font Size: Large, Medium, Small

Keywords: Working of High Voltage Frequency Converter Principle inverter

Solcom & Hapn, a well-known trademark in China, has obtained more than 20 national patents, and is a supplier of the Three Gorges and Olympic Games projects. It provides high and low voltage inverters, high, medium and low voltage solid-state starters (soft starter cabinets) , R&D, production and service of switch cabinets, distribution boxes, distribution cabinets, and complete sets of industrial control equipment. Tel: 021-51271111. High-voltage inverters (called medium-voltage inverters abroad) have been promoted in China since the mid-1990s. After ten years of development, they have been generally accepted by the market today. It is estimated that the market capacity this year is between 1 billion and 2 billion. between yuan and renminbi. This article will analyze the characteristics of high-voltage inverters from two aspects: product technology and market. 1. Product and technical characteristics of high-voltage inverters From the 1980s to the early 1990s, three main methods were used to achieve speed regulation of high-voltage motors: (1) Hydraulic coupling method. That is, a hydraulic coupling device is inserted in series between the motor and the load, and the coupling force between the motor and the load is adjusted through the height of the liquid level to achieve speed regulation of the load; (2) Cascade speed regulation. Cascade speed regulation must use a wound asynchronous motor, which rectifies and inverts part of the energy of the rotor winding and then returns it to the grid. This is equivalent to adjusting the internal resistance of the rotor, thus changing the slip of the motor; due to the voltage of the rotor The voltage of the power grid is generally not the same, so inverting to the grid requires a transformer. In order to save this transformer, internally fed motors are now commonly used in domestic market applications, that is, a three-phase auxiliary winding is made on the stator. It specifically accepts the feedback energy from the rotor, and the auxiliary winding also participates in doing work, so that the energy absorbed by the main winding from the power grid will be reduced, achieving the purpose of speed regulation and energy saving. (3) High and low methods. Since high-voltage frequency conversion technology had not yet been solved at that time, a transformer was used to first reduce the grid voltage, and then a low-voltage frequency converter was used to achieve frequency conversion. For motors, there were two methods. One method was to use a low-voltage motor; the other was to use a low-voltage motor. The first method is to continue to use the original high-voltage motor, and a step-up transformer needs to be added between the frequency converter and the motor. The above three methods are relatively mature technologies so far. The speed regulation accuracy of hydraulic coupling and cascade speed regulation is relatively poor, the speed regulation range is small, and the maintenance workload is large. The efficiency of hydraulic coupler is still far behind compared with variable frequency speed regulation, so these two technologies Competitiveness is no longer strong. As for the high-low mode, it can achieve a better speed regulation effect, but compared with the real high-voltage inverter, it has the following shortcomings: low efficiency, large harmonics, strict requirements on the motor, and when the power is large (above 500KW), Reliability is lower. The main advantage of the high-low approach is its lower cost. At present, there are three main types of mainstream high-voltage inverter products: (1) Current source type. As shown in Figure 1. The current source type inverter part uses SGCT to be directly connected in series to solve the voltage withstand problem. The DC part uses reactors to store energy. The current technical level can achieve an output voltage of 7.2KV, so it is suitable for the current situation that most domestic voltages are 6KV. The power factor of the input side of the current source type inverter is relatively low, the reactor generates a large amount of heat, and the efficiency is lower than that of the voltage source type inverter. Due to the use of current control, the design of the output filter is more troublesome, and the two-level inverter has Mode voltage, harmonics, and dv/dt problems are more prominent, so the requirements for the motor are higher. Although current source inverters have the advantage of being able to feed back energy, there are not too many loads that need to feed back energy, especially for general-purpose inverters. Therefore, the market competitiveness of current source inverters has gradually weakened. Figure 1 Current source type high-voltage inverter (2) Power unit series multi-level type. As shown in Figure 2. This inverter uses multiple low-voltage power units connected in series to achieve high voltage. The step-down transformer on the input side adopts phase-shifting method, which can effectively eliminate harmonic pollution to the power grid. The output side adopts multi-level sinusoidal PWM technology, which can be applied to any voltage. Ordinary motors. In addition, when a certain power unit fails, it can automatically exit the system, while the remaining power units can continue to keep the motor running, reducing losses caused by shutdown. The system adopts a modular design and can quickly replace faulty modules. It can be seen that the market competitiveness of unit series multi-level inverters is very obvious.

Figure 2 Power unit series multi-level high-voltage inverter (3) Three-level type. As shown in Figure 3. The three-level inverter uses a clamping circuit to solve the problem of series connection of two power devices and enables the phase voltage output to have three levels. The main circuit structure of the three-level inverter has few links. Although it is a voltage source structure, it is easy to realize energy feedback. The biggest problem encountered by three-level inverters in the domestic market is the voltage problem. Its maximum output voltage cannot reach 6KV, so workarounds are often required, either by changing the voltage of the motor or by adding a step-up transformer on the output side. This weakness limits its application. Figure 3 Three-level high-voltage inverter At present, although some people have proposed other different high-voltage inverter solutions, most of them do not have obvious feasibility, or they do not have the potential to replace the above three mainstream inverter structures. As the cost of high-voltage inverters further decreases, in the medium-power market, high- and low-type inverters will withdraw from competition and only focus on smaller power applications. For unit series multi-level inverters, the main disadvantages are that the converter link is complex, the number of power components is large, and the volume is slightly larger. However, other methods cannot meet the needs of domestic applications, and the reliability of high-voltage device applications is not yet If it is too high, its competitive advantage may still be irreplaceable in the near future. Due to the low output voltage of the three-level inverter, the main application range should be in some special fields, such as steel rolling mills, ship drives, locomotive traction, hoists, etc. The motors in these fields are specially customized. The voltage may not be the standard voltage. At a certain power level, it is a technological development trend for three-level inverters to replace traditional AC-AC inverters. Greater development of three-level inverters awaits the emergence of higher voltage power devices and the further improvement of the reliability of existing products. In ultra-high power situations, that is, power above about 8000KW, LCI (load commutation inverter) current source inverter composed of silicon controlled thyristor is still the protagonist. Due to the above technical characteristics, the majority of general-purpose high-voltage inverters are unit series multi-level inverters, about 70% or more. At present, there are no less than 20 high-voltage inverter manufacturers in China, represented by Li De Huafu, and they basically all use this circuit structure. 2. Market characteristics of high-voltage frequency converters (1) Generally accepted by the market. If high-voltage frequency converters were promoted five years ago, users would generally have to explain their principles and why they should use them. But now, through the joint efforts of many manufacturers and the publicity of the effects of use in the market, users have generally accepted high-voltage inverters. It is just a matter of who to choose among many manufacturers. (2) Performance is important. High-voltage frequency converters generally have high power and are used in very critical parts. Therefore, users are most concerned about product reliability. The best way to test reliability is to go to users who have already used it to understand the situation. The more such users, the stronger the persuasiveness. (3) The importance of service cannot be ignored. High-voltage inverters are high-power electronic equipment. They will always encounter some problems during use. The conditions in which high-voltage inverters work are very critical. Therefore, timely service to users is very important. Service is a very important aspect of maintaining user relationships. If the service is not in place, or like some foreign manufacturers, the price of service and spare parts is higher, it will affect the user's choice. (4) On-site adaptability is very important. It is difficult for general high-voltage inverter development manufacturers to completely imitate the user's on-site conditions in their own laboratories. Therefore, it is very important to consider the flexibility of product design and whether problems encountered on site can be solved as soon as possible. . Due to the high power consumption and the very important load, users often do not want the equipment to be tested for a long time. Therefore, the product design is not rigorous, and once problems are encountered, it is very difficult to solve. In recent years, the products of many manufacturers have stagnated, and this is the reason. (5) Prices fall further. Due to fierce competition and the low-price strategy that latecomers have to adopt in order to gain performance, the price of high-voltage inverters has dropped rapidly. On some projects, the prices quoted by some competing manufacturers are even lower than the cost price. With the advancement of technology, high-voltage frequency converters will not only continue to expand their scale in the existing market, but also further expand their application fields. For many loads, it is also necessary to solve the engineering application problems of the frequency converter. In short, high-voltage frequency converters are ushering in a golden period of development.