Research On The Multipulse Rectifier System Based On The Zigzag Connected Phase-Shifting Autotransformer

Due to the non-linear and time-varying characteristics of power semiconductor devices, rectifier system injects large amounts of current harmonics into the power grid, resulting 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 is 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 by many researchers. 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 MPR, zigzag connected autotransformer based 24-pulse AC-DC converter is proposed and design process of the 24-pulse zigzag autotransformer is obtained. Moreover, the proposed autotransformer is modified to make the rectifier system appropriate for retrofit applications. The equivalent kilo-volt-ampere(k VA) rating of the autotransformer is calculated to be only 18.0% of the load rating. Then, the 24-pulse rectifier system based on zigzag auto-wound transformer is modeled, simulated, and analyzed. Simulation results confirm that the total harmonic distortion(THD) of AC mains current remains below 6.0% 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%. In addition, compared with traditional ?/Y/? isolation transformer and other types of autotransformer, the use of zigzag auto-connected transformer can significantly reduce the power capacity of magnetics.To make the rectifier system suitable for more stringent applications, 36-pulse AC-DC converter based on zigzag auto-connected transformer is presented. Design process of the 36-pulse zigzag autotransformer is given and construction of the autotransformer is also modified to make the scheme appropriate for retrofit applications. The equivalent k VA rating of the 36-pulse autotransformer is calculated to be only 19.1% of the load rating. Then, the zigzag autotransformer based 36-pulse rectifier system is modeled, simulated, and analyzed. Simulation results indicate that the THD of utility current maintains lower than 4.0% 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%. The 36-pulse rectifier can mitigate harmonics more efficiently than the 24-pulse rectifier, at the expense of higher kilo-volt-ampere rating of the autotransformer and more cumbersome system structure.Twelve-pulse rectifier based on zigzag autotransformer and tap-changer is proposed, because the 24- and 36-pulse zigzag autotransformers have the drawbacks of complicated structure and difficult to manufacture. Working principle of the double-tap changer and triple-tap changer is analyzed. The zigzag autotransformer and tap-changer based 12-pulse AC-DC converter is modeled and simulated. Simulation results demonstrate that the proposed rectifier system can not only achieve similar harmonic mitigating performance to the aforementioned 24- and 36-pulse rectifiers, but also significantly simplify the structure of the phase-shifting autotransformer and reduce the amount of three-phase rectifier bridges. Therefore, employing tap-changer to replace traditional interphase reactor has many advantages, such as achieving satisfactory harmonic suppression performance, reducing losses, and improving reliability of the rectifier system.