The future development direction of frequency converter

There are many new inverter brands on the market and each has its own merits. The rapid development of frequency conversion technology is based on the innovation of power electronics technology, the development of power electronic devices and materials, and the improvement of device manufacturing process levels, especially high-voltage large-capacity insulated gate bipolar transistors IGBTs, integrated gate commutated thyristors The successful development of industrial IGCT devices has enabled the rapid development of high-power frequency conversion technologies and improved performance. Through the analysis found that this type of inverter has a lot in common. The status quo of the frequency converter and its future technological development direction are analyzed and introduced below.

1. The market situation of the frequency converter:

China's 50% to 60% of the power generation capacity is used for AC motors, and the capacity is above 3kw. The rated voltage is generally 3~10kV and the motor accounts for 40%~50% of the total installed capacity of the motor. Because China's medium-voltage frequency conversion technology has not yet become industrialized and lags behind foreign developed countries, this part of the motor lacks economical and reliable speed-regulating means when the load conditions change, and a large amount of electric energy is wasted every day. Therefore, there are potential domestic Huge medium voltage high power inverter market. The State Development Planning Commission predicts that the total investment in China's converters will exceed 50 billion yuan in the next 15 years, of which 60% to 70% will be medium-voltage high-power inverters. China's high-voltage inverter market has its own particularities, including: (1) The industry is very strong, mainly concentrated in the metallurgy, electric power, water supply, petroleum, chemical, coal and other industries. In the industrial power industry, the power consumption of oil, coal and other energy industries accounted for 22.34%; chemical industry accounted for 14.73%; metallurgy accounted for 14.18%; machinery and building materials accounted for 10.96%; water supply accounted for 10.53%. (2) At present, among the industries in China, only a small number of enterprises use high-speed motors for speed control, and there are many market gaps. (3) The high voltage frequency converter is a kind of investment equipment, mainly used for energy saving and improving the production process. Whether users purchase such equipment is very much related to the government’s policy orientation. If the government promotes more efforts, the market will start faster, and vice versa. On the other hand, the market is also affected by the international and domestic economic environment and the overall economic benefits of certain domestic industries. Therefore, there are still some uncertain factors in the future market development process. (4) It is highly probable that the products of well-known brands of overseas companies will enter the Chinese market in large quantities, and all parties should be prepared.

2. Current status of frequency converter technology

Transmitting and alternating frequency conversion is the main form of early frequency conversion, and it is suitable for low-speed and large-capacity motor loads. Its main circuit switching device is in the natural off state, there is no problem of forced commutation, so the first generation of power electronic devices - thyristors can fully meet its requirements. Due to its mature technology, it is also the most developed and developed in the country. At present, there is still a certain market in China. Transmitting a frequency conversion requires a large number of thyristors in its main wiring, which has a complicated structure and a large amount of maintenance work. Due to the use of a phase shift control method, the power factor is relatively low, generally only 0.6 to 0.7, and harmonic components are large. Reactive power compensation and filtering devices increase the overall cost.

Interchange has been using a variety of topologies for frequency conversion, such as the low-in-one mode, which is essentially low-voltage frequency conversion, but it is high voltage from both the grid and the motor. Because of the existence of intermediate low pressure links, it has the disadvantages of large current, complicated structure, low efficiency, and poor reliability. Due to its early development and mature technology, it is still widely used. With the development of medium-voltage frequency conversion technology, especially the successful development of new high-power switchable devices, the Sino-Ice low one has a tendency to be phased out. The direct medium-voltage frequency conversion method has broad prospects for development due to the absence of intermediate low voltage links.

Inverters of inverters generally use self-power-off devices such as high-power MOSFETs, high-power transistors GTRs, and turn-off thyristors GTOs, among which GTR applications are the most common. However, in the case of modulation strategy development and the requirement that the output harmonic content of the inverter is smaller, the switching frequency must be increased. For this reason, GTR cannot meet this requirement, and a new component IGBT has been developed. The full name of IGBT is insulated gate bipolar transistor, which is a voltage type bipolar/M05 composite device that combines MOSET and GTR ingeniously. IGBT has high input impedance, fast switching speed, small component loss, simple driving circuit, and driving. Low power, high temperature limit, low thermal resistance, low saturation voltage, low resistance, large current capacity, strong surge immunity, wide safety zone, easy parallel connection, stable reliability, modularization, etc. Switching elements. Currently, IGBTs with a current of 2400 A, a voltage of 3300 V, and a switching frequency of 40 kHz have been used in the small, medium, and high power ranges. IGBT is not only used for low voltage inverter below 500V, but also can be used for high voltage inverter above 1000V to drive high voltage motor. This kind of medium voltage, high voltage frequency converter uses the multi-level inverter output high pressure, also may use the transformer to step down ~ the low voltage frequency changer a transformer boosts the way. Because IGBTs have exceptional advantages in performance, it is expected that they will not be replaced by newly developed components within the next decade.

The technical development trend of the inverter

1. Cell serial multilevel technology

The unit series multi-level form has advantages in terms of harmonics, efficiency, and power factor, etc., and has a wider application prospect when four-quadrant operation is not required. Among them, the three-level control has many advantages, including: (1) The use of a three-level topology can effectively solve the problem of low withstand voltage of power electronic devices, and is suitable for high voltage and high power. (2) Three-level topology A single bridge can output three levels (+ud/2, -ud/2, 0). The line (phase) voltage has more steps to simulate a sine wave, reducing the distortion of the output waveform. The harmonics are greatly reduced. (3) The multi-level voltage stepped wave reduces du/dt, making the insulation winding impact on the motor winding less. (4) The three-level PWM method shifts the first harmonic distribution band to the frequency band of 2 times the switching frequency, and uses the motor winding inductance to better suppress the influence of higher harmonics on the motor. With the three-level PWM method, the IGBT switching frequency of each power unit is 600 Hz. If 5 power units per phase are connected in series, the equivalent output phase voltage switching frequency is 6 kHz, which can reduce the switching losses and improve the efficiency of the inverter. The frequency converter can be applied to any ordinary high-voltage motor, and does not have to be derated. Although the use of such a main circuit topology will increase the number of devices, due to the reduction of driving power, the lower switching frequency and the need to use a voltage equalization circuit, the system still has a large advantage in terms of efficiency, generally up to 97%. And, thanks to the modular structure, all power units can be interchanged and maintenance is convenient. (5) The three-level topology can generate 3*3*3=27 kinds of space voltage vectors, which can bring about the freedom of the harmonic elimination algorithm and can get a good output waveform.

2. Power bus technology &n

In today's development of high-frequency power electronics and applications, parasitic electrical parameters in the system, especially the connection lines, generate huge electrical stress, which has become an important factor that threatens the reliability of power electronic devices. Parasitic inductance from the DC storage capacitors to the DC bus between the inverter's components In a typical hard-switched inverter, the over-voltage at the momentary switching can cause the device to overheat and sometimes even cause the inverter to lose control. And it exceeds the rated safety working area of ​​the device and is damaged, limiting the improvement of the switching frequency. The power bus consists of:

(1) Cable stranding is the most commonly used conventional power bus, which is simple and inexpensive, but in IGBT inverters, since the self-inductance of the cable is large, the self-inductance of the flat bus is only that of the round conductor compared to the round cross-section conductors. One-third to one-half, and it occupies only 1/10 to 1/2 of its volume.

(2) The printed circuit board bus is mainly used for small current inverters, but when the bus DC current reaches 150A, the copper layer of the circuit board is required to be very thick and the cost is too high. In addition, the perforation used to connect the multilayer wiring board is not only Take up more space, and it will affect the reliability of the machine.

(3) Bare copper busbar (planar parallel busbar) is a traditional busbar form of an IGBT module feed system widely used in industry. The disadvantage of this method is the large mutual inductance of parallel busbars.

(4) Bracket type busbars If the positive DC bus copper board is placed above the negative DC bus board and the busbars are separated by a layer of thin insulating material, the mutual inductance can be minimized due to the mutual cancellation of the magnetic field, but its The process is complicated and it is not suitable for large-scale production.

There are different disadvantages based on the above-mentioned several kinds of power buses, and a laminated power bus has been developed for this purpose. Laminated power bus is based on the theory of electromagnetic field, the connection is made into a flat section, in the same section, the thinner and wider, the smaller its parasitic inductance, the opposite current flows in the adjacent wire, and its magnetic field cancels Can also reduce the parasitic inductance. The stacked power busbars are stacked together in a thin, wide copper row. The layers are hot pressed together with a very thin, high dielectric strength material. The distance between the bus bars is uniform and uniform to reduce mutual inductance. Each copper row is reliably insulated from the other layers at the desired terminal position so that the terminals having different potentials are exposed on the same plane so as to connect all the devices in the main circuit. This integrated laminated power bus structure can withstand hundreds of kilograms of shear stress and can withstand thousands of volts across its lead electrodes. Using laminated power bus bars to combine IGBTs, rectifiers, and other modules, heat sinks, capacitors, and gate drive circuits, the connection between the laminated power bus and the device is accomplished using different terminals and connectors. The contact resistance between the connected contact surface and the busbar is very small, and the parasitic inductance is also reduced by an order of magnitude, so that the overvoltage stress of the Ldi/dt is minimized, and the power electronic device works in an optimal state.

3. Computer Control and Artificial Intelligence Technology

The use of microcomputer control technology can control and protect the inverter. In terms of control:

(1) The calculation determines the turn-on and turn-off moments of the switching elements so that the inverter outputs the required voltage according to the modulation strategy.

(2) A variety of drive speed control functions are achieved through different codes. Such as various frequency settings and execution, start, operation mode selection, torque control setting and operation, acceleration and deceleration design and operation, braking mode setting and execution.

(3) The variable speed transmission system is constituted by an interface circuit, an external sensor, and a microcomputer. In terms of protection, with the combination of external sensors and I/O circuits, it forms a complete detection and protection system and can complete a variety of self-diagnosis protection schemes. Protection functions include: (1) Undervoltage and overvoltage protection of main circuit and control circuit; (2) Undercurrent and overcurrent protection of output current; (3) Overload protection of motor or inverter; (4) Brake Overheat protection of the resistor; (5) Stall protection.

The use of artificial intelligence technology to fault diagnosis of the frequency converter constitutes a fault diagnosis system. The system is composed of monitoring, detection, knowledge base (failure model knowledge base or fault diagnosis expert system knowledge base), reasoning mechanism, human-machine dialogue interface and database. Not only can faults accurately indicate the nature and location of the fault after a fault occurs, but also the possibility of a fault can be predicted before the fault occurs. Before the start of the inverter, the diagnostic system itself, the main circuit of the inverter (including the power supply), and the control system are subject to a diagnostic inventory hazard. If you find a fault phenomenon, call the knowledge base to reason, determine the cause of the fault, and show that the fault cannot be started. If no fault is found, the display can be turned on. After starting up, diagnose diagnosis in real time. During operation, each inspection point is queried cyclically to store data and keep refreshing. If the data is found to be out of limits, it is considered that a failure may occur and the directional tracking is immediately performed. If the results of several inspections are the same, it means that there is a fault, and then the knowledge base is invoked for analysis and reasoning to determine what kind of fault and its location is. If it is displayed, if it is severe, a shutdown instruction is issued.

4. Other various technologies

In recent years, some companies at home and abroad are developing new types of high-voltage inverters that have no power network pollution. According to reports, this kind of frequency conversion device has the advantages of high power factor, high efficiency, no harmonic pollution, no special motor, etc., and adopts a number of advanced technologies: (1) Phase shifting through a zigzag transformer on the DC side of the inverter of the inverter. Achieve 30 "pulse wave rectification, which greatly enhances the harmonic suppression ability of the device, so that there is almost no phase shift between the voltage and current on the grid side, so the power factor can be close to 1. (2) The application of all-digital fiber control technology The frequency converter makes its control flexibility and reliability greatly improved.(3) Power unit standard modularization, IGBT drive circuit intelligence.