| Since there are nonlinear power electronic devices in the rectifying system,a large number of harmonics will be injected into the power grid,which will affect the normal operation of related facilities in the power system.Therefore,how to eliminate grid-side harmonic has been one of the research hotspots in recent years.Multi-pulse rectifier is widely used in high-power rectifiers because of its simple structure,low failure rate and remarkable harmonic suppression ability.To reduce the harmonic produced in the multi-pulse rectifier further,the DC-side harmonic injection technology is combined with the multi-pulse rectifier.In 12/18-pulse rectifiers based on zigzag autotransformer,several topologies of zigzag phase-shifting autotransformer are designed and their equivalent capacities are calculated.The design process and simulation model of the zigzag 12/18-pulse rectifier are given,and the simulation results are compared with the theoretical ones.Within the range of the phase-shifting angle of the phase-shifting transformer,the influence of the phase-shifting angle on the gridside harmonic distortion rate,ripple coefficient of DC voltage and equivalent capacity of the12/18-pulse rectifier is analyzed.The grid-side harmonic distortion rate of the 12/18-pulse zigzag rectifier is calculated theoretically.The simulation results show that when the phaseshifting angle is π/9,2π/9,4π/9 or 5π/9,the harmonic distortion rate of the grid-side current in the rectifying system can be minimized.To improve the harmonic suppression ability of zigzag 12-pulse rectifier,a DC-side active injection strategy based on zigzag autotransformer is adopted.In the strategy,compensating currents are generated by controlling the auxiliary circuit composed of a low capacity inverter,and are directly injected into the DC-side of the rectifier,so that the current waveform on the grid-side of rectifier is approximately a sine wave.The phase-shifting transformer is zigzag,which can block the zero-sequence current components and reduce the equivalent capacity of the rectifier.After active injection,the harmonic content of grid-side current in the rectifier is only 1.17%.The results show that the designed multi-pulse rectifier can not only suppress the current harmonic,but also has a lower equivalent capacity.Based on the zigzag 18-pulse rectifier,a grid-side harmonic suppression strategy is designed by adding an auxiliary circuit to the DC-side inter-phase reactor.Under the assumption that the current on the gird-side of the rectifier is the standard sinusoidal waveform,the current that needs to be compensated on the DC-side of the rectifier can be calculated from the current and voltage relationship on the AC-DC side.Six compensating currents generated by the active auxiliary circuit are injected into the DC-side of the rectifier to eliminate the gridside harmonics.The theoretical and emulational results show that the designed rectifier can effectively suppress the harmonics,and the harmonic distortion of grid-side current is about1.45%.Based on the zigzag 12-pulse rectifier,the passive suppression method of the DC-side is studied.By adding a resistor or adding a diode auxiliary circuit on the inter-phase reactor to consume or offset the excess harmonic energy,the harmonic suppression ability of the zigzag rectifier can be improved significantly.The simulation results show that the designed circuit is correct and efficient. |
