Research Of 18-pulse Delta-polygon Autotransformer Rectifier System With Current Injection

Rectifiers are widely used in power converters,but rectifiers contain nonlinear semiconductor elements,which make the rectifiers usually inject large amounts of current harmonics into the power grid.Harmonics bring an additional burden to the grid,not only reduce the operating efficiency of electrical equipments,but also have a negative impact to the users.Therefore,the suppression of harmonic components in power grid is a very meaningful research content.Multipulse rectifier(MPR)has the advantages of simple structure and low cost,and high reliability.MPR is more suitable for high power applications,it is effective to suppress the harmonic currents.Phase-shifting transformer is the core component of MPR system.Compared to traditional isolated transformer,autotransformer has lower cost,volume,capacity and loss of MPR.Compared to other structure types of autotransformers,using delta-polygon autotransformers in MPR system makes the phase-shifting transformer has lower equivalent capacity,the input currents have lower harmonic contents,and the input power factor(PF)is higher.In this paper,the MPR with 18-pulse delta-polygon autotransformer was selected as the object of study.Then analyzed the rectifier circuit in the cases with and without interphase transformer(IPT),symmetric and asymmetric autotransformer.In the rectifier circuit without IPT,the asymmetric autotransformer has smaller capacity.In MPR systems,increasing the pulse number can reduce the harmonic contents of the input currents more effectively,but it makes the phase-shifting transformer has a complex structure and difficult to manufacture,the effect of high order harmonic suppression is not obvious.Therefore,based on the study of 18-pulse 20 degrees phase shift angle delta-polygon autotransformer rectifier with IPTs,two kinds of DC side active harmonic suppression methods were proposed,the compensation currents were injected to the DC side of the two and three sets of rectifier bridges.The compensation principle of injection currents was analyzed,and the two rectifier circuits were modeled and simulated.The results show that when the load changes or the three-phase voltage is unbalanced,the two methods can reduce the THD of the gird side currents from about 10% before compensation to 6% and 3% after compensation,the input PFs are close to unity.The injection currents reduce the harmful effects of unbalanced three-phase voltage.The capacity of electromagnetic components in rectifier systems are respectively 32.8% and 30% of the load power.Hence,combining the MPR with the active harmonic suppression technique at DC side can achieve better suppression effect,reduce the cost and loss and the influence of unbalanced source voltage,also improve the reliability of rectifier systems.