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New non-PWM power unit in perfect harmonic-free high-voltage inverter
1 Introduction With the development of variable frequency speed control technology, high voltage variable frequency speed control technology as a large capacity transmission has been widely used. The high-voltage motor can realize stepless speed regulation by using high-voltage frequency converter, which can meet the requirements of motor speed control in the production process, save energy and reduce production cost [1]. Since the introduction of the first generation of perfect harmonic high-voltage inverters in 1994, Robincon has been leading the market in Europe, America, Japan and China due to its excellent performance, high reliability and simple maintenance. Compared with ordinary high-voltage inverters, harmonic high-voltage inverters have improved a large grade from the aspects of inverter control performance, reliability assurance, and manufacturing process [1]. However, up to now, the rectification part of the power unit of this perfect high-voltage inverter adopts unidirectional diode series connection, and the inverter part output adopts multi-electric translation phase-phase pwm technology, and each power unit pulse control is controlled by pwm. The control pulse waveform of the inverter is generated by comparing the reference sine wave and the triangular wave. In order to further improve the energy saving of high-voltage inverters and reduce the phenomenon of grid pollution and electromagnetic interference, this paper describes a new type of power unit, namely non-pwm power unit. Non-pwm is not really a pwm principle, but the triangular carrier used to generate the pulse waveform is replaced by a rectangular wave whose frequency is constant and whose amplitude changes. In order to introduce the new power unit, we will start with a brief introduction to the ordinary perfect harmonic high-voltage inverter. 2 Perfect Harmony High Voltage Inverter Principle Perfect Harmony High Voltage Inverter uses several frequency conversion power units in series to achieve direct high voltage output. The frequency converter has low harmonic pollution to the power grid, high input power factor, good output waveform quality, and no characteristics of motor additional heating, torque ripple, noise, dv/dt and common mode voltage caused by harmonics. With the output filter, you can use ordinary asynchronous motors, including domestic motors. 2.1 Power unit series multi-level structure As shown in Figure 1, the inverter has 15 power units, from a1 to a5, b1 to b5, and c1 to c5. The output voltage of each power unit is 690v, and the power unit itself has the same structure [2]. The 6kv or 10kv grid voltage is supplied to the power unit through the secondary side multiplexed isolation transformer. The power unit is a three-phase input, single-phase output AC-DC-AC pwm voltage source inverter structure, which realizes high voltage direct conversion of variable voltage frequency conversion. Output, supply high voltage motor. Taking the 6kv output voltage level as an example, each phase consists of five power units with a rated voltage of 690v connected in series. The output phase voltage is up to 3450v and the line voltage is about 6kv. Each power unit is powered by a set of secondary sides of the input transformer. The power units are insulated from each other between the secondary windings of the transformer. The secondary winding adopts the extended-edge delta connection method to realize multiplexing, so as to achieve the purpose of reducing the input harmonic current [2]. For the 6kv voltage class inverter, it is a 36-pulse rectifier circuit structure, and the input current waveform is close to a sine wave. Due to the low input current harmonic distortion, the input power factor of the inverter can reach 0.95 or higher. 2.2 Traditional pwm power unit The traditional pwm power unit circuit structure is shown in Figure 2. The power unit is a three-phase input single-phase output AC-DC-AC pwm voltage source type inverter, and the secondary side of the phase shifting transformer outputs three-phase AC power and is rectified by a three-phase diode of the power unit, and a straight DC current is formed by the filter capacitor. Then, after four h-type single-phase inverter bridges composed of four igbts, pwm control is implemented. The output of the inverter adopts multi-electrical phase-shifting pwm technology, and the power unit of the same phase outputs the fundamental voltage of the same amplitude and phase, but the carriers of the series connected units are offset from each other by a certain electrical angle to realize multi-level pwm. After the superposition, the equivalent switching frequency and level of the output voltage are greatly increased, and the output voltage is very close to a sine wave. Each power unit pulse control is controlled by spwm, and the control pulse waveform of the inverter is generated by comparing the reference sine wave and the triangular wave. 3 Non-pwm (npwm) power unit 3.1 Circuit and characteristics of non-pwm power unit As mentioned above, the ordinary pwm power unit has already completed the output close to sine wave well, but this pwm method cannot avoid grid pollution and electromagnetic interference. phenomenon. Based on the past knowledge precipitation, a unique power unit implementation is proposed based on the structure of the common power unit. The structure is shown in Figure 3. The non-pwm power unit is composed of two parts: the input unit is composed of a thyristor three-phase controllable rectifier bridge, and the output unit is composed of an inverter bridge composed of igbt, and its output voltage state is 1, 0, -1. If each phase is superimposed of five cells, then 11 different voltage levels can be generated, so that a perfect harmonic-free system can synthesize a more perfect sinusoidal output voltage waveform. The characteristics of the power unit structure are as follows: (1) The adjustment of the amplitude of the output voltage is realized by an anti-parallel logic reversible logic loop-free controllable rectifier circuit. (ud=2.34u2cos) (2) The change of the frequency of the output voltage is realized by the inverter unit module. The whole system separates the voltage regulation and the frequency modulation, but the principle of frequency conversion and voltage transformation remains unchanged. (3) Anti-parallel reversible logic The loop-free rectification side can automatically realize energy feedback to save energy. (4) Since the switching frequency of the output inverter module is not higher than the output frequency, the phenomenon of grid pollution and electromagnetic interference caused by the traditional pwm mode is avoided, which will indicate the birth of a new type of energy-saving green high-voltage inverter. - Non-pwm (npwm) power unit high voltage inverter. The use of anti-parallel reversible logic loopless rectification not only can complete the rectification function well, but also realize energy feedback, and return excess energy to the grid, saving energy consumption. Since the loop-free reversible system adopts the control principle, only one group of the two sets of bridges is put into operation at any time, and the other group is turned off, so there is no circulation between the two sets of bridges. The switching process between the converters is controlled by the logic unit and is therefore referred to as a logic loopless system. The rectangular wave modulation principle is similar to the original triangular wave modulation. In the case of a sinusoidal wave as a modulated wave, the original triangular wave is replaced by a rectangular wave as a carrier. Taking a power unit as an example, a rectangular wave is used to cut a sine wave of the same frequency. When the amplitude of the sine wave is greater than the amplitude of the rectangular wave, the voltage is output, and the output is zero for the rest of the time. The output of the unit. By superimposing the outputs of the nine cells, you can get a voltage output that is perfectly close to the sine wave. The secondary winding of the input side isolation transformer is stepped down by phase shifting to provide an independent power supply for each power unit, which is equivalent to a 30-pulse uncontrollable rectification input for 6kv, eliminating most of the harmonic current caused by a single power unit. The generation of harmonics on the grid side is greatly suppressed; the harmonic content of the grid side caused by the inverter can meet the most stringent requirements for the harmonic content of the "Power Quality Harmonics of the Utility Network", without installing an input filter and protecting the peripheral equipment. Harmonic interference. In the normal speed range, the power factor is greater than 0.95, no power factor compensation capacitor is needed; the rectangular wave is used as the carrier, which greatly reduces the output harmonic content, and the output waveform is close to the perfect sine wave. The total harmonic content can be obtained without the output filter device. (thd) is reduced to below 2. The use of anti-parallel circuits increases the number of thyristors. It seems to increase the cost, but because the inverter part uses a rectangular wave with the same frequency as the sine wave as the carrier, the frequency of the tube is greatly reduced. In the case of mass production, the low-frequency switch tube can be used instead of the high-priced igbt. The cost is reduced; since there is no high frequency, the high frequency protection circuit is omitted, and in addition to reducing the grid pollution, the cost is also reduced. Therefore, this design can not only make up for the increase in cost caused by the increase of thyristors, but also further reduce the total cost. 3.2 Computer simulation of non-pwm power unit The modulation mechanism of the pwm wave outputted by each unit in the ordinary high-voltage inverter is generated by a sinusoidal wave as a signal wave and a triangular wave as a carrier modulation. The idea of ​​implementing non-pwm power is quite different. In each unit, a rectangular wave whose frequency is constant and whose amplitude changes is used as a carrier to modulate the non-pwm wave required for output. We carried out computer simulation of the inverter with rated output voltage of 10kv. Each phase consists of nine power units with rated voltage of 650v connected in series. The output phase voltage can reach up to 5850v and the line voltage can reach about 10kv. The simulation circuit of the non-pwm power unit is shown in Figure 4, and the simulation results are shown in Figure 5. It can be seen from the simulation results that the new non-pwm power unit can output a more perfect sinusoidal waveform than the ordinary pwm power unit. 3.3 Implementation method of non-pwm power unit The digital signal processor dsp is used in the control system. Dsp is a microprocessor with special structure. The inside of dsp chip adopts Harvard structure with program area and data division. It has a special hardware multiplier. It uses extensive pipeline operation and provides special dsp instructions, which can be used to quickly Implement a variety of digital signal processing algorithms. The main control component in the system control circuit uses tms320lf2407dsp chip, multi-chip dsp collaborative operation, through the controller local area network (can) to communicate with each other, complete the transfer of control parameters, thereby realizing the phase-shifting npwm pulse triggering, and Handle various fault interruptions in a timely manner. Taking the high-voltage inverter with rated output voltage of 10kv as an example, the implementation method of the whole control system is as follows: The system adopts the master-slave multi-cpu control system. The control circuit is composed as shown in Figure 6. Assuming that each phase of the actual main circuit is a 5-unit series structure, the entire circuit has a total of 15 power units. For three power units in the same position in each phase, one dsp is used for control, so that 15 power units can form five symmetric subsystems from five dsps. Then use one dsp as the main control chip, sample and calculate the control signal and necessary information processing. For the three power units in each subsystem using the same carrier signal, the sinusoidal modulated wave signals differ by 120 electrical degrees; the five power units of each phase share a single sinusoidal modulated wave signal. The subsystem clock is given by the main control unit and transmitted through the optical fiber, so that the clock of the whole system is consistent and there is no drift. The sub-control chip can determine the frequency of the sinusoidal modulated wave according to a given step size parameter, so that the frequency of the output voltage can be determined. The sub-control system also requires the necessary protection of the power unit. Since the protection signal of the inverter system is relatively large and the protection methods are also different, the protection signals mainly considered in this design are: overcurrent, overvoltage, undervoltage and overheating. In these four typical fault situations, the sub-cpu will block all the npwm trigger signals output by it, and at the same time send the necessary information to the main cpu, so that it can respond and process the necessary situation and pass the human-machine interface. A fault condition is displayed. The main control dsp is mainly responsible for sampling, real-time calculation, v/f look-up table evaluation, pi algorithm control, etc. for a given signal and a feedback signal, and is connected to the human-machine interface system through a data and address bus and a serial communication interface. Thereby completing the reception and display of information. At the same time, it also analyzes and processes the information sent by the subsystem to monitor the operation of the system. At the same time, the main control chip will perform the necessary protection and processing on the motor circuit. This can make full use of the powerful computing and real-time processing capabilities of the dsp processor. The master chip and the slave chip are connected to each other through a controller area network (can). Thereby completing some necessary information and data transfer between each other. The information and data that need to be transmitted in this system mainly include: modulation frequency signal, modulation depth coefficient, comparison output control word, protection interrupt information, and initialization setting value information. The use of such a control circuit not only ensures the function realization and stable operation of the system, but also effectively saves equipment costs, and has strong function expansion and upgrade capabilities. 4 Conclusion Compared with the traditional high-voltage inverter pwm power unit, the non-pwm power unit has further improved in terms of input and output waveforms and control performance, and is more resistant to grid pollution and electromagnetic interference than pwm power units. It is superior and has high market application value. (This article Source: Alibaba)