As we all know, the application of high-voltage motors is extremely wide, and it is the main driving force in industrial and mining enterprises. It is widely used to drive fans, pumps, compressors and various other large-scale machinery in large and medium-sized factories and mines in various industries such as metallurgy, steel, petroleum, chemical industry, and water treatment. Its energy consumption accounts for more than 70% of the total energy consumption of the motor, and most of them have speed regulation requirements, but the current speed regulation and starting methods are still very backward, wasting a lot of energy and reducing the life of the machinery. With the development of electric drive technology, especially frequency conversion speed regulation technology, high-voltage frequency conversion speed regulation technology as a large-capacity drive has also been widely used. By the way, the current habitually called high-voltage inverter actually has a voltage of 2.3-10kV, mainly 3kV, 6kV and 10kV in China. Compared with the grid voltage, it can only be counted as medium voltage, so it is often called MediumVoltageDrive abroad.
Jinan Iron and Steel’s high-pressure fan and water pump speed control system.
Most of China’s high-voltage motors are 6kV and 10kV. The incoming power supply is 6kV in the old factory of Jinan Iron and Steel. It is 10kV. In the high-pressure fan speed control system, the hydraulic coupling speed control method is adopted. Direct start or reduced voltage start not only has a large starting current, which causes the voltage of the power grid to decrease, which affects the normal operation of other electrical equipment; and the large mechanical shock of the main shaft is likely to cause fatigue fracture and affect the life of the machine. When the grid capacity is not large 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 to achieve the purpose of adjusting the load speed. This method of speed regulation is essentially a slip power consumption method, and the energy saving effect is not very good. Moreover, as the speed drops, the efficiency becomes lower and lower. It is necessary to disconnect the motor and the load for installation and maintenance. It takes time to replace parts such as shaft seals and bearings. The site is generally dirty and the equipment appears to be of low grade, which is an obsolete technology.
Generally speaking, the use of high pressure (medium pressure) frequency conversion speed regulation system has two important characteristics for fans and pumps: First, because the energy loss of the valve (or baffle) is eliminated and the working point of the fan and pump is close The total efficiency of the peak efficiency line is 25%-50% higher than that of the hydraulic coupler; second, the high-voltage (medium voltage) frequency conversion speed regulation performance is good, and the high-voltage frequency converter can be used to achieve “soft” starting. The characteristics of the frequency conversion device ensure sufficient torque when starting and accelerating, and eliminate the impact of starting on the motor, ensure the stability of the power grid, and increase the service life of the motor and machinery.
Now take Jigang No. 3 Steelmaking Plant as an example to analyze the energy-saving effect of high-voltage (medium-voltage) frequency converters in actual production. A total of 10 high-pressure dust removal motors were used in the No. 3 Steelmaking Plant of Jinan Iron and Steel, with a total installed capacity of 23.1MW, accounting for 40% of the total installed capacity of the No. 3 Steelmaking Plant. The actual working current monitored from the scene has a higher proportion. The current value is shown in Table 1. The fan load accounts for 60% of the total capacity. The efficiency of the high-voltage inverter can be increased by 25%-50% than that of the hydraulic coupler. The total monthly electricity consumption can be reduced by 8% based on the monthly energy saving of the fan by 20%. The cost is nearly 800,000 yuan. From the above rough calculation, the application of high-pressure (medium-voltage) frequency conversion speed regulation in Jinan Iron and Steel’s high-pressure fans and pumps has broad prospects and great energy-saving effects.
Table 1 Fan current ratio of the No. 3 Steelmaking Plant of Jinan Iron and Steel Co., Ltd.
The application status of high-voltage inverters
Although the industrialization of high-voltage inverters only began to take shape in the mid-1980s due to factors such as high voltage, high power, and complex technology. However, with the rapid development of high-power power electronic devices and the driving force of the huge market, High-voltage inverters have developed rapidly in the past ten years. The devices used have evolved from SCR, GTO, and GTR to IGBT, IGCT, IGET, and SGCT, with power ranging from hundreds of kilowatts to tens of megawatts. The technology has matured, the reliability is guaranteed, and the application range is getting wider and wider. The high-voltage frequency converter can be matched with standard medium and high-power AC asynchronous motors or synchronous motors to form an AC variable frequency speed regulation system to drive fans, water pumps, compressors and various mechanical transmission devices to achieve energy saving, high efficiency and improve product quality the goal of.
In recent years, various high-voltage inverters have appeared continuously, and high-voltage inverters have not had a nearly unified topology like low-voltage inverters so far. According to the high-voltage composition mode, it can be divided into direct high-voltage type and high-low-high type. According to whether there is an intermediate DC link, it can be divided into AC-AC inverter and AC-DC-AC inverter. In the frequency converter, according to the difference of the intermediate DC filter link, it can be divided into voltage source type and current source type. The following will analyze several high-voltage inverters that are currently widely used, and point out their respective advantages and disadvantages.
1 High-low-high frequency
converter The frequency converter is a low-voltage frequency converter, which uses an input step-down transformer and an output step-up transformer to realize the interface with the high-voltage power grid and the motor. This was a transition technology when the high-voltage frequency conversion technology was immature at that time. Due to the low voltage of the low-voltage inverter, the current cannot rise without limit, which limits the capacity of this inverter. Due to the existence of the output transformer, the efficiency of the system is reduced and the floor space is increased; in addition, the magnetic coupling capacity of the output transformer is weakened at low frequency, which weakens the load capacity of the inverter when it is started. The harmonics of the power grid are large. If 12-pulse rectification is used, the harmonics can be reduced, but the strict requirements for the harmonics cannot be met; while the output transformer is boosting, the dv/dt generated by the frequency converter is also amplified, and filtering must be installed It can be suitable for ordinary motors, otherwise it will cause corona discharge and insulation damage. Siemens produced the inverter with this structure in the early stage, but it has stopped production and only provides spare parts.
2 Current source type high voltage inverter
The input side uses thyristor for rectification, inductance energy storage, and the inverter side uses SGCT as the switching element, which is a traditional two-level structure. Due to the limited withstand voltage level of the device, multiple devices must be connected in series. Device series connection is a very complicated engineering application technology. In theory, the reliability is very low, but some companies can achieve productization. Since there are only two levels on the output side, the dv/dt that the motor bears is relatively large, and an output filter must be used. The multi-pulse rectifier on the grid side is an optional part, and users need to make requirements for their own factory conditions. The main advantage of this type of inverter is that it can feed back the inertial energy of the load to the grid without an external circuit. The main disadvantages of current source inverters are low power factor on the grid side, large harmonics, and changes with changes in working conditions, so it is not easy to compensate. The representative manufacturer of the current source type high voltage inverter is AB.
3 Voltage source type three-level frequency
converter The frequency converter adopts diode rectification, capacitor energy storage, IGBT or IGCT inverter. The three-level inverter form adopts the diode clamp method to solve the problem of connecting two devices in series. It is technically simpler and easier to connect in series than two devices. At the same time, an output level is added to make the output waveform larger than that of two devices. Okay. The main problem of this kind of inverter is: due to the use of high-voltage devices, the du/dt on the output side is still serious, and an output filter is required. Due to the limitation of the device’s withstand voltage level, the maximum voltage can only be 4160V. To adapt to the needs of 6kV and 10kV power grids, replacing the motor is a way to do it, but it is troublesome to bypass the power grid when a fault occurs. There is a workaround for a 6kV motor, which is to change the motor from star connection to angle connection, so that the voltage of the motor becomes 3kV; this method increases the circulating current loss of the motor, and there have been cases of burning the motor in China. It may be related to this. Three-level inverters generally use 12-pulse rectification. Voltage source type three-level inverters represent manufacturers ABB, Siemens, etc.
4 power module series multi-level inverter
The inverter uses low-voltage inverters in series to achieve high-voltage output, and is a voltage source inverter. Its input side adopts a phase-shifting step-down transformer to achieve a rectification mode of more than 18 pulses, which meets the most stringent international requirements for harmonics of the power grid. When loaded, the power factor on the grid side can reach more than 95%. Multi-level PWM technology is adopted on the output side, with low dv/dt and low harmonics, which meets the needs of ordinary asynchronous motors. The output voltage of the inverter can be designed according to the needs of the load, which is a better way to solve the speed regulation of 6kV and 10kV motors. The power circuit adopts a standard modular design, which is easy to replace, and it is relatively easy to purchase the components used in China. This type of inverter uses low-voltage IGBTs as inverter components. Compared with the three-level inverters that use high-voltage IGBTs, it has a larger number of power components, but the technology is more mature. Compared with the three-level inverter using high-voltage IGCT, the number of power components is more, but the total number of components is less, because IGCT requires a very complicated auxiliary shutdown circuit. Since there are many connections between the rectifier transformer and the power module, the transformer cannot be placed separately from the inverter, and it is not very flexible in occasions with limited space. The power module series multi-level frequency converter represents manufacturers Siemens Robincon Company, Lead Huafu Company and so on.
5 Overview of the application of high-voltage inverter
Current source inverter technology is mature and can operate in four quadrants. However, due to the problem of voltage equalization in series connection of devices at high voltage, the impact of input harmonics on the power grid and the impact of output harmonics on the motor, etc. Its application is restricted. Moreover, the performance of the frequency converter is related to the parameters of the motor, with poor versatility, large current harmonic components, large pollution and loss, and high common-mode voltage, which has an impact on the insulation of the motor. The PowerFlex7000 series of AB company uses SGCT tubes with a withstand voltage of 6.5kV, and the highest voltage is only 6.6kV.
Because voltage source type frequency converter adopts high-voltage components, the dv/dt on the output side is more serious, and an output filter is required. Due to the limitation of the device withstand voltage level, the maximum voltage can only be 4160V.
The unit series multi-level PWM voltage source inverter has low harmonic pollution to the power grid, high input power factor, and no need to use input harmonic filters and power factor compensation devices. The output waveform is good, and there is no additional heating, torque ripple, noise, output dv/dt, common mode voltage and other problems of the motor caused by harmonics, and ordinary asynchronous motors can be used. The output voltage of the multi-level inverter connected in series with the unit can reach 10kV or even higher.
Comparing the above three types of high-voltage inverters, due to the good input and output waveforms of the unit series multi-level inverter, the harmonic pollution to the power grid is small, and the output is suitable for ordinary motors. In recent years, it has developed rapidly and has gradually become a high-voltage variable frequency speed control. Mainstream program. Most of the high-voltage motors in our country are 6kV and 10kV grades. At present, three-level inverters are limited by the withstand voltage of the device, and it is difficult to achieve this level of direct high-voltage output. The output voltage of the unit series multi-level inverter can reach 10kV or even more. High, so it is widely used in our country, especially in energy-saving fields such as fans and pumps, almost forming a monopoly. The high-voltage motors used in Jinan Iron and Steel are all common cage asynchronous motors with voltage levels of 10kV and 6kV, and the unit series multi-level voltage source inverter is the most suitable choice.
The principle, technical advantages and technical characteristics of the
unit series multi-level inverter 1 unit series multi-level inverter principle
(1) The unit series multi-level inverter uses several independent low-voltage power units in series to achieve high-voltage output
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