As we all know, the application of high-voltage motors is extremely extensive, and it is the main driving force in industrial and mining enterprises. It is widely used in large and medium-sized factories and mines in metallurgy, steel, petroleum, chemical, water treatment and
Other industries to drive fans, pumps, compressors and various other large machinery. The energy consumed accounts for more than 70% of the total energy consumption of the motor, and most of them have the requirements of speed regulation, but the current speed regulation and starting methods are still backward, a large amount of energy is wasted and the mechanical life is reduced. With the development of electric drive technology, especially the frequency conversion speed control technology, the high-voltage variable frequency speed control technology as a large-capacity transmission has also been widely used. By the way, the high-voltage inverters that are commonly used to be called, in fact, the voltage is generally 2.3-10kV, and the domestic main is 3kV, 6kV and 10kV. Compared with the grid voltage, it can only be regarded as medium voltage, so foreign countries often become Medium Voltage Drive. .
Jinan Steel High Pressure Fan Pump Speed ​​Control System
China's high-voltage motors are mostly 6kV and 10kV. In Jinan Steel's old factory, the incoming power supply is 6kV. The high-voltage motor speed regulation is mostly direct start and hydraulic coupling speed regulation; the new factory entrance line power supply voltage is 10kV, and the high-pressure fan speed regulation In the system, the hydraulic coupler speed control mode is adopted. Direct start or step-down start is not only a large starting current, which causes the grid voltage to drop, affecting the normal operation of other electrical equipment; and the mechanical impact of the main shaft is large, which is easy to cause fatigue fracture and affect the mechanical life. 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 (generally oil) between the impellers to achieve the purpose of adjusting the load speed. This kind of speed regulation method is essentially the practice of slip power consumption type, the energy saving effect is not very good, and the efficiency is lower and lower as the speed of rotation decreases, and the installation and maintenance work of the motor and the load needs to be disconnected. Time needs to replace the shaft seal, bearings and other components, the site is generally dirty, it seems that the equipment grade is low, is a phase-out technology.
Generally speaking, the use of high-voltage (medium-voltage) variable frequency speed control system has two important characteristics for the load of fan and water pump: First, because the energy loss of the valve (or baffle) is eliminated and the working point of the fan and water pump is close. Its peak efficiency line, its total efficiency is 25% to 50% higher than that of the hydraulic coupler. Second, the high-voltage (medium voltage) variable frequency speed control has good starting performance, and the high-voltage inverter can be used to achieve "soft" starting. The characteristics of the frequency conversion device ensure sufficient torque during starting and acceleration, and eliminate the impact of the starting on the motor, ensure the stability of the power grid, and improve the service life of the motor and the machine.
Taking Jinan Steel's three steelmaking as an example, the energy saving effect of high voltage (medium voltage) frequency converter in actual production is analyzed. A total of 10 high-pressure dust-removing motors were used in the Jinan Iron and Steel Co., Ltd., with a total installed capacity of 23.1 MW, accounting for 40% of the total installed capacity of the three steels. The working current actually monitored from the site has a higher proportion, the current value is shown in Table 1, and the fan type load accounts for 60% of the total capacity. The high-voltage inverter can increase the efficiency of the hydraulic coupler by 25% to 50%. According to the monthly energy saving of 20%, the total monthly electricity can be reduced by 8%. The annual electricity cost of the three steelmaking is 10 million yuan, which can be reduced every year. The cost is nearly 800,000 yuan. From the above rough calculations, the application of high-voltage (medium-voltage) variable frequency speed regulation in Jinan high-pressure fan and water pump has broad prospects and huge energy-saving effect.
High-voltage inverter application status
Although the industrialization of high-voltage inverters began to take shape in the mid-1980s due to high voltage, high power, and complicated technology, with the rapid development of high-power power electronic devices and the driving force of huge market, high-voltage inverters are nearly ten. Years of development have been very rapid, and the use of devices has evolved from SCR, GTO, GTR to IGBT, IGCT, IGET and SGCT, with power ranging from a few hundred kilowatts to tens of megawatts. The technology is mature, the reliability is guaranteed, and the use surface is more and more extensive. The high-voltage inverter can be matched with standard medium and high-power AC asynchronous motor or synchronous motor to form an AC variable frequency speed control system, which is used to drive fans, 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 emerged, and high-voltage inverters have not yet had a nearly uniform topology like low-voltage inverters. According to the high-pressure composition method, it can be divided into direct high-voltage type and high-low-high type. According to the presence or absence of intermediate DC link, it can be divided into AC-AC inverter and AC-DC-AC inverter. In the inverter, according to the difference of the intermediate DC filter, the voltage source type and the current source type can be divided. 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 inverter
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 is a transitional technology when the high-voltage frequency conversion technology is not mature. Due to the low voltage of the low-voltage inverter, the current cannot rise unrestricted, which limits the capacity of the 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 capability of the output transformer is weakened at low frequencies, so that the load capacity of the frequency converter is weakened at startup. The harmonics of the power grid are large. If 12-pulse rectification is used, the harmonics can be reduced, but the strict requirements for harmonics cannot be met. When the output transformer is boosted, the dv/dt generated by the inverter is equally amplified, and filtering must be added. The device can be applied to ordinary motors, otherwise corona discharge and insulation damage will occur. The early production of inverters of this type by Siemens has stopped production and only spare parts are available.
2 current source type high voltage inverter
The input side is rectified by a thyristor, and the inductor is used for energy storage. The inverter side uses SGCT as a switching element, which is a traditional two-level structure. Since the device's withstand voltage level is limited, multiple devices must be connected in series. Device series is a very complicated engineering application technology. In theory, reliability is very low, but some companies can achieve productization. Since the output side has only two levels and the motor is subjected to a large dv/dt, an output filter must be used. Multi-pulse rectifiers on the grid side are optional and users need to make requests for their own plant conditions. The main advantage of this type of frequency converter is that the inertial energy of the load can be fed back to the grid without the need for an external circuit. The main disadvantage of the current source type inverter is that the power factor on the grid side is low, the harmonics are large, and it changes with the change of working conditions, which is not easy to compensate. The representative of the current source type high voltage inverter is AB.
3 voltage source type three-level inverter
The frequency converter uses diode rectification, capacitor energy storage, IGBT or IGCT inverter. The three-level inverter form adopts the diode clamp method to solve the problem that the two devices are connected in series. It is technically easier than direct connection between the two devices. At the same time, an output level is increased, so that the output waveform is better than two. Good. The main problem with this type of frequency converter is that due to the use of high voltage devices, the du/dt on the output side is still severe, requiring an output filter. Due to the limitation of the withstand voltage level of the device, the maximum voltage can only be 4160V. To adapt to the needs of 6kV and 10kV power grids, it is a practice to replace the motor, but it is more troublesome to bypass the grid when the fault occurs. There is a workaround for the 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 are already cases of burning the motor in China. It is possible to 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 a low-voltage inverter in series to achieve high-voltage output, which 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 requirements on the international harmonics of the power grid. When the load is loaded, the grid side power factor can reach more than 95%. Multi-stage PWM technology is adopted on the output side, which has small dv/dt and less 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 6kV, 10kV motor speed regulation. The power circuit adopts standard modular design, and the replacement is simple. The used devices are also easy to purchase in China. This kind of frequency converter uses low voltage IGBT as the inverter component. Compared with the three-level inverter using high voltage IGBT, the number of power components is large, but it is technically mature. Compared to a three-level inverter using a high-voltage IGCT, the number of power components is larger, but the total number of components is smaller 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 places where space is limited. The power module serial multi-level inverter represents the manufacturers Siemens Robin Kang, Li De Hua Fu and so on.
5 Overview of high voltage frequency converter applications
The current source type inverter is mature in technology and can operate in four quadrants. However, due to the voltage equalization problem of the series connected in high voltage, the influence of input harmonics on the power grid and the influence of output harmonics on the motor, its application is limited. Moreover, the performance of the inverter is related to the parameters of the motor, the versatility is poor, the harmonic components of the current are large, the pollution and loss are large, and the common mode voltage is high, which has an influence on the insulation of the motor. The AB Power Flex 7000 series uses SGCT tubes with a withstand voltage of 6.5kV and the maximum voltage is only 6.6kV.
Due to the high voltage device used in the voltage source type inverter, the dv/dt on the output side is relatively serious, and an output filter is required. Due to the device's withstand voltage level, the maximum voltage can only be 4160V.
The unit series multi-level PWM voltage source type inverter has small harmonic pollution to the power grid, high input power factor, and does not need to adopt an input harmonic filter and a power factor compensation device. The output waveform is good, there is no problem of additional heating and torque ripple, noise, output dv/dt, common mode voltage, etc. caused by harmonics, and an ordinary asynchronous motor can be used. The output voltage of the unit series multi-level inverter can reach 10kV or even higher.
Compared with the above three types of high-voltage inverters, the input and output waveforms of the unit series multi-level inverter are good, 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 gradually becomes a high-voltage variable frequency speed regulation. Mainstream program. China's high-voltage motors are mostly 6kV and 10kV grades. At present, three-level inverters are limited by the withstand voltage of the device. It is still difficult to achieve this level of direct high-voltage output, and the output voltage of the unit-series multi-level inverter can reach 10kV or even more. High, so it is widely used in China, especially in the energy-saving field such as fan and water pump, almost forming a monopoly situation. The high-voltage motors used in Jinan Iron and Steel are common cage asynchronous motors with voltage ratings 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 technical characteristics of the manufacturer
1 unit series multi-level inverter principle
(1) The unit series multi-level inverter adopts several independent low-voltage power units in series to realize high-voltage output. The principle is shown in Figure 1.
Figure 1 Principle of voltage superposition
(2) Taking the 6kV output voltage level as an example, the main circuit topology of the inverter is shown in Figure 2.
Figure 2 main circuit topology
(3) The grid voltage is supplied to the power unit after the voltage is reduced by the secondary transformer of the secondary side. The power unit is a three-phase input, single-phase output AC-DC-AC PWM voltage source inverter, see Figure 3.
Figure 3 Power unit structure
The principle is summarized, the output terminals of adjacent power units are connected in series to form a Y-connected structure, and the high-voltage direct output of the variable voltage frequency conversion is realized, and the high-voltage motor is supplied. Each power unit is powered by a set of secondary windings of the input transformer, and the power units are insulated from each other between the secondary windings of the transformer. For inverters with a rated output voltage of 6kV, each phase consists of five power units with a rated voltage of 690V connected in series. The output phase voltage can reach up to 3450V and the line voltage can reach about 6kV. Each power unit is subjected to the full output current, but only provides 1/5 phase voltage and 1/l5 output power. Therefore, the voltage level and series number of the unit determine the inverter output voltage, and the rated current of the unit determines the inverter output. Current. Since the entire power unit is connected in series, there is no voltage equalization problem caused by the series connection of the devices.
2 unit series multi-level inverter technology advantages
Since Siemens Robincom introduced the first inverter in 1994, after more than ten years of continuous development, the unit series multi-level inverter has gradually formed the following relatively complete technologies.
(1) Input transformer multiplex design
The input transformer is multiplexed to achieve the goal of reducing harmonic current. The input power factor is high and it is not necessary to use an input harmonic filter and a power factor compensation device. Taking the 6kV inverter as an example, the 15 secondary windings of the transformer are divided into five different phase groups by using the extended triangle connection. With a mutual difference of 12°, a 30-pulse diode rectifier circuit structure is formed, so theoretically less than 29 harmonics can be eliminated, and the input current waveform is close to a sine wave. The total harmonic current distortion can be less than 1%.
(2) Inverter output multi-electric translation phase-phase PWM technology
In the PWM modulation, the phase shifting PWM is adopted, that is, the modulation signals of each unit of the same phase are the same, and the carrier signals are mutually different by one electrical angle and are paired with each other. Thus the output of each unit is a PWM wave of the same form, but differs from each other by an angle. The equivalent switching frequency of the output voltage after the superposition is greatly increased. By changing the amplitude and frequency of the reference wave, the high voltage output of the variable voltage variable frequency can be realized. In fact, in order to improve the power utilization rate, the reference wave is not a strict sine wave, but a certain third harmonic is injected to form a "saddle type" waveform.
(3) Power unit bypass technology
A bidirectional thyristor (or two anti-parallel SCRs) is connected in parallel at each power unit output T1, T2. When the power unit fails, the unit is blocked and the SCR is turned on to form a bypass. After bypassing, the circuit can continue to operate, except that the output voltage drops slightly. If the load is important, a redundant design can be made to install the spare power unit. The power unit bypass technology greatly improves the reliability of the unit series multi-level inverter, and largely compensates for the problem of reduced reliability due to the large number of components.
3 unit series multi-level inverter manufacturer technical performance characteristics
The unit series multi-level voltage source type inverter was invented and patented by Robin Kang Company of the United States, and was named as the perfect harmonic-free frequency converter. Since Robincon's patent application is only in the United States, the technology belongs to well-known technology in China, and the high-voltage inverter produced by Beijing Li Dehuafu and other companies also adopts this structure. In addition to the above two, there are many manufacturers at home and abroad such as TMEIC (Toshiba, Mitsubishi), Fuji, and Oriental Hitachi to produce this type of inverter.
The following is a comparison of the performance indexes of three high-voltage inverters, Robin Kang, Lead Huafu, and TMEIC, and select the products suitable for the actual situation of the current power grid and equipment in China. The performance of the three inverters is shown in Table 2.
Table 2 Comparison of inverter performance
Comparing the performance of three inverters, the high-voltage inverter has the following characteristics:
(1) The control mode can basically achieve the encoderless vector control mode;
(2) The unit bypass function is continuously improved, especially the Robincon's center point offset scheme is safer;
(3) The price of high-voltage inverters is declining, which is not an unattainable thing for domestic enterprises.
If considering factors such as cost, the inverter of domestic companies is the first choice, and the main domestic high-voltage inverter supporting industries, such as multi-pulse rectifier transformers, large-capacity electrolytic capacitors, radiators, high-voltage flexible cables, etc., are basically mature, and the maintenance cost will be very high. low. If the cost factor is not taken into account, since the introduction of the first unit series multi-level inverter, Robincon has been in the leading position in this technical field, and the sales performance of 500 units per year also shows that the company's products are excellent. Technical performance.
Conclusion
The high-voltage motor can realize stepless speed regulation by using high-voltage frequency converter, meet the requirements of motor speed control in the production process, to improve the output and quality of the product, and can save energy and reduce production cost. Although the one-time investment is high, the investment can generally be recovered within 2 to 3 years. The application of high-voltage (medium-voltage) frequency converter in Jinan Steel's high-pressure fan and water pump motor system will not only save energy, but also have a significant impact on the development of Jinan Steel's circular economy.
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