Research On Autotransformer Step-up Models Based On 12-pulse Rectifier System

In recent years,various active,passive and hybrid power filters can be installed in the power system in order to suppress harmonics.There are two ways to effectively reduce harmonics,PWM rectifier and multi-pulse rectifier,by reforming power electronic devices to produce no or as little harmonics as possible.Among them,PWM rectifier has prominent problems such as switching loss and failure rate with the improvement of power level.It is generally used in small and medium power occasions.Because of its high reliability,low cost,high efficiency and simple control,multi-pulse rectifier has become a research hotspot in recent years.Multi-pulse rectifier supplies power to the same DC-side load through phase-shifted multi-connection of multiple full-bridge or half-bridge rectifier circuits,so that the harmonics generated by one rectifier circuit can be offset by the harmonics generated by other rectifier circuits,including isolation transformer and self-transformer.There is not only magnetic coupling but also circuit coupling between the primary winding and the secondary winding of the autotransformer.Therefore,the volume,weight,cost and loss of the system can be significantly reduced compared with the isolation transformer.Therefore,the autotransformer becomes the first choice when there is no need for electromagnetic isolation.In multi-pulse rectifier,the autotransformer converts the output voltage of the network side into the input voltage of the DC side.The 12-pulse rectifier system is simpler than other rectifier systems and can meet the harmonic requirement of most electrical equipment.Therefore,12-pulse rectifier is widely used in multi-pulse rectifier system.In order to meet the voltage requirements of grid-connected or power supply inverters and some rectifier loads,based on 12-pulse rectifier system,the relationship between phase-shifting angle of polygon autotransformer and zigzag autotransformer and current harmonic coefficient,DC load voltage ripple coefficient and equivalent capacity of system magnetic devices is derived.The equivalent capacities of the magnetic devices at different phase shifting angles are compared while ensuring the minimum harmonic content of the AC side current and the minimum ripple coefficient of the DC side load voltage.When both polygon and zigzag transformers adopt new phase-shifting angles,the harmonic content and load voltage ripple coefficient at the grid side are the smallest,and the equivalent capacity of DC side magnetic devices is the same as that of traditional phase-shifting angles.Compared with the traditional phase-shifting angle,the capacity of transformer increases slightly,but it can be applied to boost voltage situation,which enhances the application scope of the system.The correctness of the theoretical analysis is verified by simulation.In order to solve the problem of small boost range,it is necessary to design a new boost topology based on the original autotransformer topology.For zigzag autotransformer and star autotransformer,the boost topology of each transformer is analyzed and designed.In order to minimize the harmonic content of input current and the ripple coefficient of load voltage measurement,the phase shift angle of 30 degrees is adopted in the design of the topology.On this basis,the relationship between the voltage ratio and the equivalent capacities of transformers and other magnetic devices is theoretically derived.After considering the winding turns and transformer capacity under different conditions,the optimal boost topologies of star-shaped autotransformer and zigzag autotransformer are obtained at the same boost ratio.Compared with the traditional variable voltage model,the proposed boost topology does not produce non-characteristic harmonics.The correctness of the theoretical analysis is verified by simulation.