Research On The Multipulse Rectifier System Based On The Delta-polygon Connected Phase-shifting Autotransformer

Due to the non-linear and time-varying characteristics of power semiconductor devices,large amounts of current harmonics has been injected into the power grid by the rectifier system,which can result in low efficiency of the equipment and adverse influence on the users in the vicinity.Thus,eliminating harmonic components and achieving energy conservation and emissions reduction has become a very important research topic.Since multipulse rectifier(MPR)has the advantages of low cost,simple structure,and high reliability,it is widely used in high power AC-DC conversion and is found to be one of the most effective and efficient methods to suppress harmonics.Furthermore,phase-shifting transformer is the core part of MPR to realize harmonic mitigation.Compared with isolated transformer,autotransformer can drastically reduce cost,volume,weight,and loss of the MPR.On the basis of analyzing operating principle of MPR,delta-polygon connected autotranformer based 24-pulse AC-DC converter is proposed and design process of the 24-pulse delta-polygon autotransformer is obtained.Moreover,the proposed autotransformer is modified to make the rectifier system appropriate for retrofit applications.The equivalent kilo-voltampere(kVA)rating of the autotransformer is calculated to be only 17.4% of the load rating.Then,the 24-pulse rectifier system based on delta-polygon auto-wound transformer is modeled,simulated,and analyzed.Simulation results confirm that the total harmonic distortion(THD)of AC mains current remains below 6.1% and the power factor(PF)is close to unity under varying load conditions.Hence,the 24-pulse AC-DC converter can be used in applications requiring THD of input line current lower than 8.0%.To make the rectifier system suitable for more stringent applications,36-pulse AC-DC converter based on delta-polygon auto-connected transformer is presented.Design process of the 36-pulse delta-polygon autotransformer is given and structure of the autotransformer is also modified to make the scheme appropriate for retrofit applications.The equivalent kVA rating of the 36-pulse autotransformer is calculated to be only 18.5% of the load rating.Then,the delta-polygon autotransformer based 36-pulse rectifier system is modeled,simulated,and analyzed.Simulation results indicate that the THD of utility current maintains lower than 4.3% and the PF is near unity under wide operating range of the load.Therefore,the 36-pulse rectifier can be used in applications requiring THD of supply current lower than 5.0%.Twelve-pulse delta-polygon autotransformer rectifier based on active harmonic suppression method at DC-side is proposed and compensation strategy is analyzed,because the 24-pulse and 36-pulse delta-polygon autotransformers have the drawbacks of complicated structure and difficult to manufacture.Then,the 12-pulse AC-DC converter based on active harmonic suppression method at DC-side is modeled and simulated.Simulation results demonstrate that the THD of utility current maintains lower than 2.4% and the PF is near unity under wide operating range of the load.The 12-pulse rectifier also reduces the effects of the unbalanced voltage on harmonic components of the line currents.The total power rating of the system is only 23% of load power.Therefore,the combination of MPR and active harmonic suppression method at DC-side can achieve better harmonic suppression performance and reduce cost and losses.Besides,it can not only eliminate the effects of the unbalanced voltage on power quality but also improve reliability of the rectifier system.