Investigation of Line-Commutated Three-Phase Converters Under Voltage Unbalance

Abstract

Power  converters  operated  in  unsymmetrical  three-phase  elec-trical  systems  will  generally  work  asymmetrically.  This  intro-duces  disadvantages  for  the  operation  of  the  electrical  power  system,  like  non-characteristic  harmonics.  In  the  case  of  con-trollable  power  converters,  such  as  thyristor  converters,  it  is  possible  to  operate  the  converter  with  asymmetrical  firing  an-gles. This offers the opportunity to react on the system’s unbal-ance  optimizing  the  asymmetrical  operation  of  the  converter  to  lower the disadvantages. In  this  paper,  a  method  is  presented  to  calculate  the  behaviour  of  electrical  equipment  based  on  line-commutated  current  con-verters.  Thereby,  the  operation  of  the  essentially  non-linear  system is described as sectional continuous linear problem. For its solution an approach with differential equations in the range of space-phasors is used. It is shown to be possible to optimize the  unsymmetrical  operation  with  respect  to  certain  quality  criteria by using unsymmetrical firing angles.