Selection and operation of low-voltage high-power inverter

The correct selection of the frequency converter is essential to the normal operation of the electrical control system of mechanical equipment. To choose a frequency converter, first of all, we must choose the voltage level of the frequency converter reasonably according to the actual situation of the company. Try to choose a low-voltage frequency converter within a certain power range (below 1000Kw). The speed range, static speed accuracy, starting torque and the requirements of the use environment determine which control mode and protection grade inverter to choose. 
  The frequency converter is mainly used to adjust the speed of AC motors. It is an ideal speed regulation scheme. With the rapid development of the Chinese economy, the market’s requirements for transmission products are gradually increasing. , High speed control accuracy, good dynamic response, etc., play an increasingly important role in many speed control applications. In addition to excellent speed control performance, it also has significant energy-saving effects, especially for inverter applications. For fans and water pumps, this type of load has a large number of applications in various sectors of the national economy, with extremely wide distribution, huge power consumption, and limited speed regulation range (generally 50%-100%), which is a promising development. Application areas. Taking a power plant as an example, the transmission system capacity of a large number of auxiliary boilers is generally large, and there is a serious power waste problem when running at a constant speed. The use of high-power frequency converters can support or modify the boiler fans and water pump equipment of the power plant, such as boiler blowers, boiler induced draft fans, boiler water pumps, condensate pumps, slag washing pumps, mortar pumps, etc., and the power saving effect is very significant. At the same time, it can achieve high automation control, making the boiler operation safer and more reliable. For example, the frequency conversion speed control of the induced draft fan makes the negative pressure of the furnace more stable; the frequency conversion speed control of the primary fan makes the air supply more stable, and the combustion of coal is more thorough; the frequency conversion speed control of the mortar pump It can protect the mortar pump motor and prevent the pump from blocking and overloading and burning the motor. 1. The status quo of high-power motors in domestic power plants    
  The power of most of the fans and water pumps of domestic power plant boiler auxiliary machines is 200kW-2000 kW. According to domestic practice, 6kV motors are used for motors above 200kW, so that the main auxiliary machines of power plants are all 6kV motors. The power sector is from the perspective of reducing line loss. We hope to increase the power supply voltage and vigorously promote 10kV. From the point of view of simplifying the configuration, users naturally hope that the motor and frequency converter above 200Kw use 6kV or 10kV. The advantage of this configuration is that it can save the purchase cost and installation work of a motor. The inverter can be directly installed with a bypass switch. When the inverter fails, it can be directly switched to the 6kV or 10kV factory bus through the bypass switch without interrupting the operation. However, the use of 6kV or 10kV “direct” frequency conversion is unreasonable from a technical and economic point of view. At present, all “direct” high-voltage frequency conversions are not true direct frequency conversions. They have transformers on the input side or rely on the series connection of electronic devices, so the motor and frequency converter do not need to be consistent with the grid voltage. From an economic point of view, the price of a 560kW high-voltage inverter is more than double the price of a low-voltage 660V or 690V inverter, and the price is more than 1.2 million. It shows that within a certain power range, even considering the cost of replacing motors and adding rectifier transformers, it is much more economical to choose a low-voltage solution than a high-voltage solution. The later equipment maintenance costs and technical requirements for repair are all required to be low. 2. Comparison of high and low voltage inverter structure 
     7kV (690V frequency converter), composed of IGBTs, generally only one power device is used for each bridge arm. The structure of the high-voltage inverter is relatively complicated. Through years of continuous research, a high-voltage inverter made with multiple technology, represented by Beijing Lead & Huafu, is a means to achieve high-power conversion with low-power devices. The so-called multiple technology is that each phase is composed of several low-voltage PWM power units connected in series or parallel at its output end in some way (such as a transformer). Each power unit is powered by a multi-winding isolation transformer, and the low-voltage units are superimposed in series to achieve high voltage. Output or parallel to achieve the purpose of large-capacity output. The other is a two-level conversion circuit high-voltage inverter with multiple tubes directly connected in series represented by Chengdu Jialing, which mainly uses power devices in series and parallel to meet the requirements of the system’s high-voltage frequency conversion. This kind of device is used as a single device, using the more mature circuit topology control strategy and control method of low-voltage inverter to realize high-voltage inverter products. The difficulty of this is that the series switching tube needs dynamic voltage equalization and static voltage equalization, which has high requirements on the overall drive control circuit, and also needs to solve many technical problems such as du/dt, anti-common-mode voltage, sine wave filtering and so on. The other is the diode clamp type three-level conversion circuit high voltage inverter represented by the Department of Electrical Engineering of Tsinghua University. The purpose is to overcome the voltage equalization problem required when power devices are directly connected in series. The clamping diode concept avoids the dynamic voltage equalization problem caused by the direct series connection of devices. In addition to the above-mentioned three mainstream topological structure forms of high-voltage inverters, in recent years, there have been various types of high-voltage inverters such as flying capacitor clamping three-level conversion circuits, capacitor bootstrap multi-level conversion circuits, etc. The form is accompanied by its own corresponding control method and theory. Any control method is relatively low voltage. The ordinary two-level inverter is complicated, and it is accompanied by a series of problems to be solved, such as power device selection and stray parameter processing. , Insulation, electromagnetic interference, heat dissipation and cooling, field analysis of devices, energy control, du/dt and di/dt effects, transient process control and other problems are solved. These are all low-voltage inverters that have never appeared or have rarely appeared. Problems, or problems that low-voltage inverters can easily solve, so the use of low-voltage inverters is more reliable and simple for the system. Three, inverter selection   
   The correct selection of the frequency converter is essential to the normal operation of the electrical control system of mechanical equipment. To choose a frequency converter, first of all, we must choose the voltage level of the frequency converter reasonably according to the actual situation of the company. Try to choose a low-voltage frequency converter within a certain power range (below 1000Kw). The speed range, static speed accuracy, starting torque and the requirements of the use environment determine which control mode and protection grade inverter to choose.
      3.1 Reasonable selection of inverter voltage level    
  At present, the reasonable voltage rating of the inverter is determined by the withstand voltage level of the power device, and the possible power range of the inverter at a certain voltage level is determined by the current carrying capacity of the power device; it is subject to the voltage of the power electronic device and the motor. Due to du/dt limitation, 10kV inverter must be multi-level and multiple devices connected in series. The circuit is complicated, the price is expensive, the reliability is poor, and the maintenance cost is high. For a 10kV inverter, if 1700V IGBT devices are used, 10 strings and three-phase for 120 devices are required; if 3300V is used The device also needs 5 series for 60 devices, the quantity is huge, and the reliability will be affected. On the other hand, the devices used in high-voltage inverters generally have small currents, and the current capabilities of the devices cannot be fully utilized. Taking 710kW as an example, the current of a 10kV motor is only about 50A. The current withstand voltage of 1700V IGBT current has reached 2400A, 3300V device current Up to 1600A, high-current devices are not used, but a large number of low-current devices are used in series, and its structure is extremely unreasonable. Even if the motor power reaches 2000kW, the current is only about 150A, which is still very small. In practical applications, in order to isolate the level, improve the input current waveform and reduce the harmonics, most high-voltage inverters have an input transformer on the input side. Since there is a transformer, the voltage of the inverter and the motor does not need to be the same as the grid voltage. It is not possible to use 6kV or 10kV. Therefore, there is a problem of reasonable voltage levels for frequency converters and motors. In the past, the low-voltage 200kW boundary of the motor mainly considered the direct starting of the motor. The starting current is 5-7 times the rated current. If a 10kV/380V power transformer is used, its capacity is 2000kVA and the short-circuit impedance is about 6%. Considering the instantaneous trip value of the switch, the instantaneous trip value of the switch is very large. If you want to improve the stability of the power system and the quality of power supply, the impedance of the transformer is also hoped to be further reduced; but the smaller the impedance, the short-circuit current of the system will increase, which is not conducive to the selection of electrical equipment. The breaking capacity of the switch in the line Further improvement is necessary, which will inevitably lead to an increase in the selection cost and an increase in the cost. If the voltage drop of the 380V busbar is limited to about 50% when the motor is started, the transformer capacity must be increased. The short-circuit current will be too large and the low-voltage switch will be difficult to withstand. After the frequency converter is used for speed regulation, the starting current is limited to the rated current value. The capacity of low-voltage frequency converters can be made very large at present. Companies such as ABB have mastered the parallel output technology of inverter units, and the capacity of low-voltage frequency converters can reach 2900kW. It is possible for us to choose a large number of low-voltage high-power inverters. In practice, we can change the incoming line transformer 10Kv/6kV or 6Kv/6kV, which was only used for isolation, to 6Kv/690V, 6Kv/380V incoming line transformers, and then 690V and 380V power supply voltages can be obtained, which is widely used. Low-voltage high-power inverters provide the possibility. At present, the capacity of domestic 660V or 690V low-voltage motors has reached 1000-1200Kw, which makes the selection more convenient. 
     3.2 Load torque characteristics of mechanical equipment  
    In practice, people often divide production machinery into three types according to different load torque characteristics: constant torque load, constant power load and fan and water pump load.     
 3.2.1 Constant torque load   
   In this type of load, the load torque TL has nothing to do with the speed n, TL always remains constant or basically constant at any speed, and the load power increases linearly with the increase of the load speed. Frictional loads such as conveyor belts, mixers, extruders and feed mechanisms of mechanical equipment, as well as gravity loads such as cranes, hoists, and elevators, are all constant torque loads.  
    When the inverter drives a load with constant torque, the output torque at low speed must be large enough, and there must be enough overload capacity. If you need to run at a low speed for a long time at a steady speed, you should consider the heat dissipation capacity of the standard cage asynchronous motor to avoid excessive temperature rise of the motor. 3.2.2 Constant power load  
    The characteristic of this type of load is that it needs to remain unchanged. The main shaft of the metal cutting machine tool, the rolling mill, the paper machine, and the coiler and decoiler in the film production line are all constant power loads. 
     The constant power nature of the load should be in terms of a certain speed range. When the speed is very low, limited by the mechanical strength, TL cannot increase indefinitely, and it becomes a constant torque property at low speed. The constant power area and constant torque area of ​​the load have a great influence on the selection of the transmission scheme. When the motor is in constant flux speed regulation, the maximum allowable output torque does not change, which belongs to constant torque speed regulation; after exceeding the field weakening point, it enters the field weakening speed regulation zone, the output voltage of the inverter does not increase, the speed increases, and the torque Decrease, the maximum allowable output torque is inversely proportional to the speed, which belongs to constant power speed regulation.      
3.2.3 Loads
      such as fans and pumps The torque of such loads is proportional to the second power of the speed, and the power is proportional to the third power of the speed. Various fans, water pumps and oil pumps

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