Application of the hottest high power frequency co

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Application of high-power frequency converter in cement plant with the development of high-power frequency conversion technology, high-voltage frequency conversion devices have been more and more used in working condition enterprises. Cement plant is a large power consumer. In recent years, due to the need of energy conservation and consumption reduction, some high-power equipment in cement enterprises have also begun to adopt frequency conversion speed regulation. The equipment using frequency converter is increasing, such as high-temperature fan, centrifugal fan, water pump, disc feeder, screw conveyor, crusher, belt conveyor and exhaust fan. High-power frequency converter is used to replace the traditional hydraulic coupling for high-temperature fan The DC device regulates the speed of rotary kiln and the valve regulates the air volume of large fan. After application, the utility model not only saves the electricity expense, but also improves the product quality, increases the flexibility in use, and is more adaptable to different process requirements. In particular, after the frequency transformation of the fan variable all aluminum car began to appear, the traditional production mode of adjusting the air pressure or air volume by changing the opening mode in the past has been changed, the labor intensity has been reduced, the accuracy of adjustment and control is good, the product qualification rate has been improved, and the power saving effect can reach 30%-60%. However, high-power frequency conversion technology (especially high-voltage frequency conversion) is under development. So far, there is no nearly unified topology like low-voltage frequency converter, and engineers' understanding of it is also gradually maturing. In order to use the high-power frequency converter technically, economically and reasonably, the following will systematically analyze its control mode, technical performance and applicable working conditions, and describe its use, selection and problems needing attention in the cement plant

1 structural form of high-power inverter

high power inverter can be divided into "high low high" inverter, "high low" inverter and "high high" inverter according to the main circuit topology. "High high" type is divided into current type, neutral point clamped three-level voltage type and unit series multi-level voltage type. Among them, the "high low high" frequency converter requires large transformers on both sides, with high losses. When the frequency conversion system is started, the load capacity will be reduced, which greatly improves the stability and reliability of the system, and has large harmonics, which limits its development. "High high" current type frequency converter is generally of two-level structure. The motor bears a large du/dt and needs a voltage equalizing and buffer circuit. The technology is complex, there are many devices, the device volume is large, it is difficult to adjust and maintain, the power factor is low, and it changes with the load change. It is not easy to compensate, and the output harmonic and common mode voltage have an impact on the motor. The motor needs to be derated and insulated. The utility model has the advantage that the regenerative energy of the load can be fed back to electricity without an external circuit. At present, it is mainly used in super power occasions

due to the above reasons, at present, cement plants seldom use "high-low-high" and high-voltage current type frequency converters in the selection of high-power frequency converters, and generally use "high-low", three-level voltage type and multi-level voltage type frequency converters

(1) in the "high low" structure, the frequency converter is low-voltage, and the transformer is used at the power input side to change the high voltage into low voltage. Due to the use of low-voltage frequency converter, the capacity of the frequency converter is limited, and the tension and pressure are separately calibrated). Therefore, the motor needs to use a special variable-frequency low-voltage motor, whose voltage level is generally 690V. This structure is widely used for equipment below 800KW in the cement plant. However, once this type of frequency converter fails, the motor cannot be put into power frequency operation, and the generated harmonic transfer switch should be turned to the "load" gear with high wave

(2) neutral point clamped three-level voltage source inverter. The rectifier circuit usually adopts 12 pulse or 24 pulse diode rectifier structure, the inverter part adopts IGBT or IGCT, and the inverter part adopts clamp circuit, which solves the problem of series connection of power devices and makes the phase voltage output have three levels. Compared with the two-level structure, the voltage borne by the inverter is reduced and the output voltage waveform is greatly improved. The three-level voltage source inverter has high efficiency, good dynamic performance and strong overload capacity. The disadvantage is that although the side configuration can realize 12 or 24 pulse rectification and reduce side harmonics, the du/dt is still large, and the total harmonic of motor current can still reach more than 17%. Therefore, special motors are generally required. If ordinary motors are to be used, output filters must be added. In addition, its maximum output voltage can only reach 4.16kV. With this frequency converter, we must adopt flexible methods to change the voltage of the motor or add a step-up transformer at the output side, which is the biggest problem restricting its use

(3) unit series multilevel voltage type inverter. Multiple low-voltage PWM variable-frequency units are connected in series to realize direct high-voltage. The electric voltage is reduced by the isolation transformer to supply power to the power unit. The single-phase variable-frequency power unit is connected in series at the output end to realize the high-voltage output of variable-voltage and variable-frequency, which is directly supplied to the high-voltage motor

the transformer at the input side of this frequency converter adopts multi-phase shift technology, and the output side adopts multi-level sinusoidal PWM technology. The harmonic is low. Without output filter, thd can be

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