Because of the high-voltage equipment power. Its power may account for a considerable part of the grid capacity, so its start-up and shutdown and operation may have an impact on the power supply voltage. In addition, if the output high harmonic component of the converter is too high, it will cause motor overheating, common-mode voltage and drV mountain. In order to realize the speed regulation and energy saving operation of fans, and take into account the technical requirements of high-voltage and large-capacity equipment, this paper designs a unit series multi-level side channel blower frequency conversion speed regulation system.
3. The structure and function unit of the speed regulating system are connected in series with multi-level voltage source frequency converter. Several low-voltage power units are connected in series to realize direct high-voltage output. As shown in the structure, the transformer used is a multiplex isolation transformer, the primary side is fed with high voltage, and the secondary side is fed with low voltage isolated from each other to supply each power. The three-phase structure of each H-bridge in the figure is similar.
Isolation transformer LJ diagram I unit series converter structure, each power unit is a three-phase input, single-phase output of the cross-continuous one-cycle converter, with a unified structure, the power unit structure as shown.
Each power unit is supplied by a set of secondary windings of the transformer, and the power units and the secondary windings of the transformer are insulated from each other. If six units are connected in series, each power unit will bear all the output current, but only 1/6 of the output phase voltage and 1/18 of the output power. For 6 kV motor system, the output voltage of each unit is 590 V adjustable, and the frequency is 050 Hz adjustable, thus achieving constant voltage-frequency ratio control.
Multi-level phase-shifted pulse width modulation (PSPWM) technology is used in the inverter part. The same phase power unit outputs the same fundamental voltage, but the carriers of each unit in series stagger a certain phase in turn, so that the superimposed sinusoidal modulation wave is 13 steps.
The output voltage is very close to the sinusoidal wave and contains almost no low-order harmonics. The output terminals of adjacent power units are connected in series to realize high voltage direct output of variable voltage and frequency conversion for high voltage motor. The output phase voltage waveform of 6 kV voltage level converter is very close to sinusoidal waveform.
Each platform step has only the unit DC bus voltage, so it is very small. The insulation of the motor will not be affected. The power unit uses lower switching frequency. In order to reduce switching losses, and can not use surge absorption circuit, improve the efficiency of the converter. Due to the adoption of multi-level phase-shifting PWM technology, the equivalent output switching frequency is very high, and the number of output levels increases, which can greatly improve the output waveform, reduce the output harmonics, and the motor heating caused by harmonics. Noise and torque ripple are greatly reduced. Therefore, this kind of converter has no special requirements for motors, and can be used for ordinary high voltage motors without reducing the quota. Because the output is very low, the surge voltage caused by the long traveling wave reflection of the output cable will not increase, which will cause the insulation damage of the motor. Therefore, there is no special restriction on the length of the cable between the converter output and the motor.
Compared with direct series converters with high voltage devices, the number of devices will be increased by using this main circuit topology. For 6kV converters, 72 low voltage IGBTs are used in six-unit series structure, but the gate driving power of low voltage IGBT is lower, its peak driving power is less than 5W, the average driving power is less than 1W, and the driving power is less than 1W. The road is very simple. Because the switching frequency is very low and the voltage equalizing circuit and surge absorption circuit are not needed, the system still has a great advantage in efficiency. At full load, the efficiency of the frequency converter can reach 98.5%, including the overall efficiency of the input transformer and the frequency converter can generally reach 97*. Because the power unit adopts the voltage source type of capacitive filter. Structure, the converter can withstand 30% of the power supply voltage drop and continue to operate (quota reduction operation), and in the power grid instantaneous power failure within five cycles can be fully loaded operation. The low voltage IGBT power module widely used in the low voltage frequency converter is used in the power unit, which is mature and reliable in technology.
Because of the structure of diode uncontrollable rectifier circuit, the converter has a strong ability to withstand surge voltage. The surge voltage caused by lightning stroke or switching operation can generate surge current through transformer (the impedance of transformer is generally about 8%), and the filter capacitor is charged by rectifier diode of power unit. The filter capacitor is sufficient to absorb surge energy into the cell. In addition, a surge absorber with varistor is installed on the primary side of the transformer. Play a further protective role. The World of Power Supply Jan I has a high input impedance, and the effect of absorbing surge voltage is far inferior to that of voltage source converter.
The power unit communicates with the main control system through optical fiber to solve the problem of isolation and interference between strong and weak currents. The power unit adopts modular structure, all power units can be exchanged, and maintenance is more convenient. Each unit has only three feeders, two output electrical connectors and a fiber optic plug to connect with the system, so the replacement of power unit is very convenient. The power unit automatic bypass technology can make the frequency converter continue to operate in the case of power unit damage, greatly improving the reliability of the system.
If the redundant power unit design scheme is adopted, even if a power unit is damaged, it can run at full load. Because of the use of diode rectifier circuit, energy can not be fed back to the grid, not four quadrant operation.