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Some technical analysis of high-voltage inverters in the Chinese market

Oct 12, 2020

Some technical analysis of high-voltage inverters in the Chinese market

Foreword: High-voltage inverters have been popularized in China since the mid-1990s. After ten years of development, they are generally accepted by the market today.

1. Product and technical characteristics of high voltage inverter

From the 1980s to the early 1990s, there were three main ways to achieve speed regulation for high-voltage motors:

(1) Hydraulic coupling method. That is, a hydraulic coupling device is connected in series between the motor and the load, and the coupling force between the motor and the load is adjusted by the height of the liquid level to realize the speed adjustment of the load;

(2) Cascade speed regulation. The cascade speed regulation must use a wound asynchronous motor, and part of the energy of the rotor winding is sent back to the grid through rectification and inverter. This is equivalent to adjusting the internal resistance of the rotor, thereby changing the slip of the motor; due to the voltage of the rotor The voltage of the grid is generally not equal to the grid voltage, so a transformer is needed to invert the grid. In order to save this transformer, the form of internal-fed motor is generally used in domestic market applications, that is, a three-phase auxiliary winding is made on the stator. Specially accept the feedback energy of the rotor, and 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 and low mode. Because the high-voltage frequency conversion technology did not solve at that time, a transformer was used to reduce the grid voltage first, and then a low-voltage frequency converter was used to achieve frequency conversion; for the motor, there are two methods, one is to use a low-voltage motor; the other One way is to continue to use the original high-voltage motor, and a step-up transformer needs to be added between the inverter and the motor.

The above three methods are relatively mature technologies. The speed regulation accuracy of fluid coupling and cascade speed regulation is relatively poor, the speed regulation range is small, the maintenance workload is large, and the efficiency of fluid coupling is still a certain gap compared with frequency conversion speed regulation, so these two technologies The 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, stricter requirements on the motor, and high power (above 500KW). The reliability is low. The main advantage of the high-low method is its lower cost.

At present, there are three main types of mainstream high-voltage inverter products:

(1) Current source type.

The current source inverter part uses SGCT to directly solve the withstand voltage problem. 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 domestic voltage is 6KV. The power factor on the input side of the current source inverter is relatively low, the reactor has a larger heat, and the efficiency is lower than that of the voltage source inverter. Due to the current control, the design of the output filter is more troublesome. Common mode voltage and harmonics, dv/dt problems are more prominent, so the requirements for the motor are higher. Although current source inverters have the advantage of repaying energy, there are not too many loads that need to return energy, especially general-purpose inverters. Therefore, the market competitiveness of current source inverters has gradually weakened.

(2) Multi-level power unit series connection.

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 shift method, which can effectively eliminate harmonic pollution to the grid. The output side adopts multi-level sinusoidal PWM technology, which can be applied to any voltage. 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 the loss caused by shutdown. The system adopts modular design, which can quickly replace faulty modules. It can be seen that the market competitiveness of multi-level inverters connected in series with units is very obvious.

(3) Three-level type.

The three-level inverter adopts a clamping circuit to solve the problem of connecting two power devices in series and make the phase voltage output have three levels. The main loop 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 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 by changing the voltage of the motor or adding 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, in the medium-power market, high-low-type inverters will withdraw from the competition and focus only on smaller power occasions. For multi-level inverters connected in series, the main disadvantage is that the converter links are complicated, 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 If it is too high, its competitive advantage may still be irreplaceable in the recent period. Due to the low output voltage of the three-level inverter, the main application range should be in some special fields, such as rolling mills, ship drives, locomotive traction, hoists, etc. The motors in these fields are all specially customized. The voltage may not be a standard voltage. At a certain power level, it is the trend of technological development that three-level inverters replace traditional AC-AC inverters. The greater development of three-level frequency converters is awaited by the emergence of higher withstand voltage power devices and the further improvement of the reliability of existing products. In super-high power occasions, that is, the power of about 8000KW or more, the LCI (load commutating inverter) current source inverter composed of thyristor is still the protagonist. Due to the above-mentioned technical characteristics, the general-purpose high-voltage frequency converter is currently the unit series multi-level frequency converter accounting for the majority, about 70% or more.