What is the principle of high voltage inverter

High-voltage inverters (called medium-voltage inverters abroad) have been promoted in China since the mid-1990s. After a decade of development, they have been widely accepted by the market today. It is estimated that the market capacity this year will be between 1 billion and 2 billion yuan.

This article analyzes the characteristics of high-voltage inverters from two aspects: product technology and market.

1. Product and technical features of high-voltage inverters From the 1980s to the early 1990s, high-voltage motors were mainly adjusted in three ways:

(1) Hydraulic coupling method. A hydraulic coupling device is connected in series between the motor and the load. The coupling force between the motor and the load is adjusted by the height of the liquid level to achieve load speed regulation.

(2) Cascade speed regulation. Cascade speed regulation must use a wound-rotor asynchronous motor, which sends part of the energy of the rotor winding back to the grid through rectification and inversion. This is equivalent to adjusting the internal resistance of the rotor, thereby changing the slip of the motor. Since the voltage of the rotor and the voltage of the grid are generally not equal, a transformer is required to invert to the grid. In order to save this transformer, the domestic market now generally adopts the form of internally fed motors, that is, a three-phase auxiliary winding is made on the stator to specifically receive the feedback energy of the rotor. The auxiliary winding also participates in the work, so that the energy absorbed by the main winding from the grid will be reduced, achieving the purpose of speed regulation and energy saving.

(3) High-low method. Since the high-voltage frequency conversion technology was not 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 the motor, there are two methods. One method is to use a low-voltage motor; the other method is to continue to use the original high-voltage motor, which requires adding a step-up transformer between the frequency converter and the motor.

The above three methods are all relatively mature technologies. 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 coupling is still a certain distance away from that of variable frequency speed regulation, so the competitiveness of these two technologies is no longer strong. As for the high-low method, it can achieve a relatively good speed regulation effect, but compared with the real high-voltage frequency converter, it has the following disadvantages: low efficiency, large harmonics, strict requirements on the motor, and low reliability when the power is large (above 500KW). The main advantage of the high-low method is its low 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 inverter part uses SGCT to directly connect in series to solve the voltage resistance problem, and the DC part uses a reactor to store energy. The current technical level can achieve an output voltage of 7.2KV, so it adapts to the current situation that most of the voltage in China is 6KV. The power factor on the input side of the current source inverter is relatively low, the heat generated by the reactor is large, and the efficiency is lower than that of the voltage source inverter. Due to the use of current control, the design of the output filter is more complicated, and the common mode voltage, harmonics, and dv/dt problems of the two-level inverter are more prominent, so the requirements for the motor are higher. Although the current source inverter has the advantage of being able to feed back energy, there are not too many loads that need to feed back energy, especially general-purpose inverters, so the market competitiveness of the current source inverter has gradually weakened.

(2) Power unit series multi-level type. As shown in Figure 2. This inverter uses multiple low-voltage power units in series to achieve high voltage. The step-down transformer on the input side adopts a 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 ordinary motors of any voltage. In addition, when a power unit fails, it can automatically exit the system, while the remaining power units can continue to keep the motor running, reducing the loss caused by downtime. The system adopts a modular design, and the faulty module can be quickly replaced. It can be seen that the market competitiveness of the unit series multi-level inverter is very obvious.

(3) Three-level type. As shown in Figure 3. The three-level inverter adopts a clamping circuit to solve the problem of two power devices in series and make the phase voltage output have three levels. The main circuit structure of the three-level inverter has fewer links. Although it is a voltage source structure, it is easy to realize energy feedback. The biggest problem encountered by the three-level inverter in the domestic market is the voltage problem. Its maximum output voltage cannot reach 6KV, so it is often necessary to use alternative methods, either to change the voltage of the motor or to add a step-up transformer on the output side. This weakness limits its application. At present, although some people have proposed other different high-voltage inverter solutions, most of them do not have obvious feasibility, or do not have the potential to replace the above three mainstream inverter structures. With the further reduction of the cost of high-voltage inverters, high and low inverters will withdraw from the competition in the medium-power market and only focus on smaller power occasions. The main disadvantages of the unit series multi-level inverter are the complex conversion link, the large number of power components, and the slightly larger size. However, in the case that other methods cannot meet the needs of domestic applications and the reliability of high-voltage device applications is not too high, its competitive advantage may still be irreplaceable in the near future. Due to the problem of low output voltage, the main application scope of the three-level inverter should be in some special fields, such as rolling mills, ship drives, locomotive traction, hoists, etc. The motors in these fields are specially customized and the voltage may not be standard voltage. At a certain power level, it is a trend of technological development for the three-level inverter to replace the traditional AC-AC inverter. The greater development of the three-level inverter awaits the emergence of power devices with higher withstand voltage and the further improvement of the reliability of existing products. In ultra-high power occasions, that is, power above about 8000KW, the LCI (load commutation inverter) current source inverter composed of thyristors is still the protagonist. Due to the above technical characteristics, the general high-voltage inverter is currently dominated by the unit series multi-level inverter, accounting for more than 70%. At present, there are no less than 20 high-voltage inverter manufacturers in China, represented by Leader Harvest, and basically all of them adopt this circuit structure.

2. Market characteristics of high voltage inverters

(1) Widespread market acceptance. If high-voltage inverters were promoted five years ago, users would generally have to be informed of their principles and why they should be used. However, now, after the joint efforts of many manufacturers and the publicity of market use effects, users have generally accepted high-voltage inverters. It is just a matter of choosing which manufacturer to use.

(2) Performance is very important. High-voltage inverters generally have high power and are used in very critical parts. Therefore, users are most concerned about the reliability of the product. The best way to examine the reliability is to go to the users who have already used it to understand the situation. The more such users there are, the stronger the persuasiveness.

(3) The importance of service cannot be ignored. High-voltage inverters are high-power electronic devices. They will always encounter some problems during use. The working environment of high-voltage inverters is 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 if the price of service and spare parts is high like some foreign manufacturers, 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, the flexibility of product design and whether problems encountered on-site can be solved as soon as possible are very important. Due to the high power consumption and the importance of load, users often do not want the equipment to be tested for a long time. Therefore, if the product design is not rigorous, once a problem is encountered, it will be very difficult to solve. This is the reason why many manufacturers' products have stagnated in recent years.

(5) Further price reduction. Due to fierce competition and the low-price strategy adopted by latecomers in order to gain sales, the price of high-voltage inverters has dropped rapidly. In some projects, the prices quoted by some competing manufacturers are even lower than the cost price. With the advancement of technology, high-voltage inverters will continue to expand their scale in the existing market and further expand their application areas. For many loads, it is necessary to solve the problems of inverter engineering application.