Application Research of High Voltage Frequency Converter in High Power Fan Pump

As we all know, high-voltage motor is widely used, it is the main power of industrial and mining enterprises. In metallurgy, steel, petroleum, chemical, water treatment and other industries in large and medium-sized factories and mines, widely used in drag fans, pumps, compressors and various other large machinery. Its energy consumption accounts for more than 70% of the total motor energy consumption, and most of them have speed regulation requirements, but the current speed and start-up methods are still backward, wasting a lot of energy and reduce the mechanical life. With the electric drive technology, especially the development of frequency control technology, as the high-voltage drive technology of high-voltage frequency conversion has also been widely used. Incidentally, the current habit of high-voltage inverter, in fact the voltage is generally 2.3-10kV, mainly domestic 3kV, 6kV and 10kV, compared with the grid voltage, can only be counted as medium voltage, it often become MediumVoltageDrive abroad.

Jigang high-pressure fan pump speed control system

China's high-voltage motors are mostly 6kV and 10kV, the old factory in Jinan Iron and Steel into the line power is 6kV, high-voltage motor speed control are mostly direct start and hydraulic coupling speed; new plant into the line power supply voltage of 10kV, high-pressure fan speed System, the use of hydraulic coupler speed control mode. Direct start or step-down start not only starting current, causing the grid voltage decreases, affecting the normal operation of other electrical equipment; and the mechanical impact of the spindle, easy to cause fatigue fracture, affecting the mechanical life. When the grid capacity is not big enough, it may even fail to start. The hydraulic coupler adds an impeller between the motor shaft and the load shaft to adjust the pressure of the liquid (usually oil) between the impellers so as to adjust the rotational speed of the load. This speed control method is essentially a power-poor type of slipper, energy-saving effect is not very good, and as the speed is getting lower and lower efficiency, the need to disconnect the motor and load installation, maintenance workload, after a period Time need to seal, bearings and other components to be replaced, the scene is generally dirty, it appears that the low grade equipment, is a phase-out technology.

In general, the use of high-pressure (medium voltage) frequency control system for fans, pump load has two important characteristics: First, due to the elimination of the valve (or baffle) energy loss and the fan, pump close to the operating point The peak efficiency of the line, the total efficiency than the hydraulic coupler increased by 25% to 50%; Second, high-pressure (medium voltage) frequency control start performance, the use of high-voltage inverter, you can achieve "soft" start. The characteristics of the inverter device ensure that there is enough torque when starting and accelerating, and eliminating the impact of starting on the motor, ensuring the stability of the power grid and improving the service life of the motor and the machine.

Now the third steelmaking in Jinan Iron and Steel Company, for example, to analyze the high-pressure (medium voltage) inverter in the actual production of energy-saving effect. In the third steel mill in Jinan Iron and Steel used a total of 10 sets of high-voltage dust motor, installed capacity of 23.1MW, accounting for 40% of the total installed capacity of the three steel. However, the actual working current monitored from the site has a higher proportion, the current value is shown in Table 1, and the fan-like load accounts for 60% of the total capacity. The high-pressure frequency converter than the hydraulic coupler efficiency can be increased by 25% to 50%, according to the monthly 20% energy saving fan, the total monthly electricity consumption can be reduced by 8%, three steel 10 million yuan per month electricity, so that each year can be reduced Cost nearly 800000 yuan, from the above rough calculation point of view, high pressure (medium pressure) frequency control in Jigang high pressure fan, pump applications, the prospects are wide, energy-saving effect is huge.

Table 1 Jinan Iron and Steel No. 3 Steel Mill fan current ratio

Application Status of High Voltage Inverter

Although the industrialization of high-voltage inverters started to take shape in the mid-1980s due to the high voltage, high power and complex technology, with the rapid development of high-power power electronic devices and the driving force of huge market, the high-voltage inverters nearly ten Years of rapid development, the use of devices has been developed from SCR, GTO, GTR to IGBT, IGCT, IGET and SGCT, the power range from hundreds of kilowatts to tens of megawatts. Technically mature, reliability is guaranteed, the use of more and more widely. High-voltage inverter can be used with standard medium and high-power AC asynchronous motor or synchronous motor to form AC variable frequency speed control system to drive fans, pumps, compressors and various mechanical transmission devices to save energy, improve efficiency and improve product quality the goal of.

In recent years, a variety of high-voltage inverter constantly appear, high-voltage inverter so far has not been as close as a unified low-voltage inverter topology. According to the high-voltage components can be divided into direct high-pressure type and high - low - high, depending on whether the intermediate DC link points, can be divided into AC - AC inverter and AC - DC - AC inverter, AC - DC - Inverter, according to the middle part of the DC filter can be divided into voltage source and current source. The following will be more widely used in several current high-voltage inverter analysis, pointing out their strengths and weaknesses.

1 high - low - high frequency converter

The inverter is a low-voltage inverter, which adopts the input step-down transformer and the output step-up transformer to realize the interface with the high-voltage power grid and the motor. This is a transitional technology when the high-voltage frequency conversion technology is immature. Due to the low voltage drive voltage is low, the current can not be unlimited rise, limiting the capacity of this drive. Due to the existence of the output transformer, the efficiency of the system is reduced and the area of ​​the output transformer is increased. In addition, the magnetic coupling ability of the output transformer is weakened at low frequencies, so that the load capacity of the inverter is weakened at startup. The harmonics of the power grid, if the use of 12-pulse rectifier can reduce the harmonics, but can not meet the stringent requirements of harmonics; output transformer while boosting, the same dv / dt converter also amplified, you must install the filter Can be applied to ordinary motor, otherwise it will produce corona discharge, insulation damage. Siemens early production of such a structure of the inverter, has now stopped production, only to provide spare parts.

2 current source high-voltage inverter

The input side is rectified by silicon controlled rectifier (SCR), using inductive energy storage. The inverter side uses SGCT as the switching element, which is a traditional two-level structure. Due to the limited voltage level of the device, multiple devices must be connected in series. Device series is a very complex engineering application technology, in theory, low reliability, but some companies can do the point of product. Since there are only two levels on the output side, the motor has a large dv / dt and an output filter must be used. Grid-side multi-pulse rectifier is optional, the user needs to address their own factory conditions. The main advantage of this drive is that the inertial energy of the load can be fed back to the grid without the need for additional circuitry. The main disadvantage of the current source inverter is the low power factor of the grid side, large harmonics, and changes as the conditions change, not good compensation. Current source high-voltage inverter represents the manufacturer is AB.

3 voltage source three-level inverter

Inverter adopts diode rectification, capacitor storage, IGBT or IGCT inverter. Three-level inverter mode, the use of diode clamp method to solve the problem of two devices in series, the technology is simple and direct than the two devices in series easy, at the same time, an increase of an output level, the output waveform than the two power Good level. The main problem with this drive is that due to the use of high-voltage components, the du / dt at the output side is still relatively severe and an output filter is required. Due to the device voltage level limit, the maximum voltage can only do 4160V, to adapt to the needs of 6kV and 10kV power grid, replacing the motor is a practice, but when the fault caused by the bypass to the grid more trouble. For a 6kV motor has a workaround, is to change the motor from the star connection to the angle connection, so that the voltage of the motor becomes 3kV; this approach makes the motor circulation losses rise, there have been cases of burning motor, It may be related to this. Three-level inverter generally uses 12 pulse rectifier. Voltage source three-level inverter on behalf of manufacturers ABB, Siemens and so on.

4 power module series multi-level inverter

Inverter low-voltage inverter series way to achieve high-voltage output, is the voltage source converter. Its input side using phase-shifting step-down transformer to achieve more than 18 pulses rectifier to meet the international power grid harmonic the most stringent requirements. With a load, the grid-side power factor can reach more than 95%. In the output side of the multi-level PWM technology, dv / dt small, less harmonic, to meet the needs of ordinary induction motor. According to the needs of the load design of the inverter output voltage is to solve the 6kV, 10kV motor speed better way. Standard modular design of the power circuit, easy to replace, the device used in the domestic procurement is relatively easy. This inverter uses low-voltage IGBT as the inverter component, compared with the three-level inverter using high-voltage IGBT, the number of power components more, but the technology is more mature. Compared with the three-level inverter with high-voltage IGCT, the number of power components is larger, but the total number of components is smaller, because IGCT requires a very complex auxiliary shutdown circuit. As the rectifier transformer and the power module connection more, so the transformer can not be placed separately from the inverter, space is limited in the occasion is not very flexible. The power module series multi-level inverter on behalf of manufacturers Siemens Robincon, 利德 华福 companies.

5 high-voltage inverter application overview

Current source inverter technology is mature and can be four quadrant operation, but due to high voltage devices in series voltage equalization problems, input harmonics on the power grid and output harmonics on the motor and other issues, its application is limited. Moreover, the performance of the inverter is related to the parameters of the motor. The commonality is poor, the harmonic components of the current are large, the pollution and loss are large, and the common-mode voltage is high, affecting the insulation of the motor. AB company PowerFlex7000 series with SGCC voltage 6.5kV, the maximum voltage is only 6.6kV.

Voltage source inverter due to the use of high-voltage devices, the output side of the dv / dt more serious, the need to use the output filter. Due to the device voltage level limit, the maximum voltage can only do 4160V.

Unit series multi-level PWM voltage source inverter with harmonic pollution on the grid is small, high input power factor, do not have to use the input harmonic filter and power factor compensation device. Output waveform is good, there is no harmonic heating caused by additional heating and torque ripple, noise, output dv / dt, common mode voltage and other issues, you can use the ordinary asynchronous motor. Unit series multi-level inverter output voltage can reach 10kV, or even higher.

Comparison of the above three types of high-voltage inverter, the unit series multi-level inverter input and output waveform is good, the harmonic pollution of the power grid is small, the output for ordinary motor, rapid development in recent years, has become a high-voltage variable speed The mainstream program. China's high-voltage motors are mostly 6kV and 10kV level, the current three-level inverter by the device withstand voltage limits, it is still difficult to achieve this level of direct high-voltage output, and the unit series multi-level inverter output voltage can reach 10kV or even more High, so it has been widely used in our country, especially in energy-saving areas such as fan pumps, almost a monopoly situation. The high-voltage motors used in Jinan Iron and Steel are the ordinary cage-type induction motors with voltage levels of 10kV and 6kV. The unit series multi-level voltage source inverter is the most suitable choice.

Unit series multi-level inverter principle, technical advantages and manufacturers technical characteristics

1 unit series multi-level inverter principle

(1) Unit series multi-level inverter adopts several independent low-voltage power units in series to realize high-voltage output. Its principle is shown in Fig.1.

Figure 1 voltage superimposition principle

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(2) 6kV output voltage level, for example, the inverter main circuit topology shown in Figure 2.

Figure 2 main circuit topology

(3) After the grid voltage is stepped down by the secondary side multiplex transformer, the power unit is supplied with power. The power unit is an AC-DC-AC PWM voltage source inverter with three-phase input and single-phase output.

Figure 3 power cell structure

Principle summary, the output of adjacent power units connected in series to form a Y-junction structure, to achieve high-voltage direct conversion of variable frequency, high-voltage motors. Each power unit is respectively fed by a set of secondary windings of the input transformer, and between the power units and the secondary windings of the transformer are insulated from each other. For inverters with rated output voltage of 6kV, each phase consists of 5 series power units with 690V nominal voltage. The output phase voltage can reach up to 3450V and the line voltage up to 6kV. Each power unit withstands full output current but only provides 1/5 of the phase voltage and 1/15 of output power. Therefore, the unit's voltage level and the number of series connections determine the inverter output voltage. The unit's rated current determines the inverter output Current. Due to the use of the entire power unit in series, there is no equalization problem caused by the series connection of the devices.

2 unit series multi-level inverter technical advantages

Since Siemens Robincon introduced the first inverter in 1994, after more than a decade of continuous development, the unit serial multi-level inverter gradually formed the following complete technologies.

(1) input transformer multiple design

Input transformer implementation of multiple design, to reduce the harmonic current purposes. Input high power factor, do not have to use the input harmonic filter and power factor compensation device. Taking 6kV inverter as an example, 15 secondary windings of the transformer are connected by Yanbian triangle and divided into 5 different phase groups. Mutual difference of 12 °, the formation of 30 pulse diode rectifier circuit structure, so theoretically 29 times the following harmonics can be eliminated, the input current waveform close to the sine wave. The total harmonic current distortion can be less than 1%.

(2) inverter output multi-level phase shift PWM technology

In the PWM modulation, the phase shift PWM is adopted, that is, the modulation signals of each unit in the same phase are the same, and the carrier signals differ from each other by an electrical angle and are paired up and down. The output of each cell is thus the same pattern of PWM waves, but at a different angle from each other. The equivalent switching frequency of the output voltage increases greatly after superimposing. Change the amplitude and frequency of the reference wave, you can achieve high-voltage output variable frequency. In fact, in order to improve the power utilization, the reference wave is not a strict sine wave, but a certain third harmonic is injected to form a "saddle-shaped" waveform.

(3) power unit bypass technology

A bidirectional thyristor (or two SCRs in anti-parallel) is connected in parallel at the output terminals T1 and T2 of each power unit. When the power unit fails, block the unit, and then turn on the SCR to create a bypass. Bypass, the circuit can continue to work, but the output voltage decreased slightly. If the load is very important, redundant design can be carried out to install spare power unit. Power cell bypass technology greatly improves the reliability of the unit series multi-level inverter, to a large extent, make up for the number of components more than the reliability of the problem.

3 units in series multi-level inverter manufacturers technical performance characteristics

Unit series multi-level voltage source inverter invented by the United States Robin Kang and apply for a patent, named perfect harmonic drive. Since Robin Motion's patent application is only in the United States, the technology in China is a well-known technology and the high-voltage inverter produced by Beijing Lide Huafu also adopts such a structure. In addition to the above two, at home and abroad have TMEIC (Toshiba, Mitsubishi), Fuji, Oriental Hitachi and many other manufacturers of this type of converter.

The following on the Robin Hood, Ridley Huafu, TMEIC three high-voltage inverter performance indicators to compare, select the current domestic grid and equipment for the actual situation of the product, the performance of three inverters shown in Table 2.

Table 2 inverter performance comparison

Comparison of three inverter performance, high-voltage inverter has the following characteristics:

(1) The basic control mode can be done without encoder vector control mode;

(2) Unit bypass function is continuously improved, especially Robin Kang's center point offset scheme is more secure;

(3) The price of high-voltage frequency converter is declining continuously, which is not an elusive thing for domestic enterprises.

If you consider the cost and other factors, the domestic company's inverter is the first choice, and currently supporting the main high-voltage inverter industry, such as multi-pulse rectifier transformer, electrolytic capacitors, heat sinks, high voltage soft cable and other basic mature maintenance costs will be very low. If you do not consider the cost factor, since the first unit series multi-level inverter was introduced in the field of technology Robinhood has been in a leading position, and sales of 500 units per year, also shows the company's products excellent Technical performance.

Conclusion

High-voltage motor using high-voltage inverter can realize stepless speed regulation to meet the production process of the motor speed control requirements to improve product yield and quality, but also substantial savings in energy and reduce production costs. Although one-time high investment, but the investment is generally 2 to 3 years to recover. High-pressure (medium voltage) inverter in Jigang high-pressure fans, pump motor system, once applied not only energy-saving effect will be very significant, but also for the development of circular economy in Jinan Iron and Steel Co., Ltd. far-reaching significance.