Voltage-induced stresses during Low Voltage Ride Through (LVRT) in the drive train of wind turbines with DFIG

Abstract

Nowadays   stochastic   occurrence   of   main   grid   disturbances (LVRT,  FRT)  leads  to  high  level  vibrational  excitation  of  the drive train during operation of wind turbines. Regardless  of  the  drivetrain  design  as a  direct,  hybrid  or  multi-stage  geared  one  in  this  case  high  mechanical  and  electrical stresses should be expected. This voltage-induced stress resulting from  the  generator-load  input  function,  in  which  the  air  gap torque  of  the  generator  changes  with  high  dynamic  rates.  The momentum   is   sufficient   to   excite   any   significant   natural frequencies  of  the  above-mentioned  power  train  variants  and  of all mechanically coupled systems, such as the rotor blades, main frames, nacelle and tower structures.  The  effects  within  the  drivetrain  are  shown  by  simulation  by means  of a  physically  discretized  torsional  oscillator  model  of a typical  multi-megawatt  wind  turbine  with  a  conventional  multi-stage gearbox and fast running DFIG drivetrain design. A general method  for  a  controller  setting  will  be  discussed  to  handle  these abormal grid side conditions. Using FOC for the DFIG-converter system  electrical  stress  will  be  limited  by  the  rotor  current control.