I. INTRODUCTION
AS THE fast growing capacity and more significant
impacts to the power grid by wind turbine system
(WTS), the power electronic technologies used in wind power
application have changed dramatically during the last 30
years [1]–[18]. In the 1980s, the power electronics for wind
turbines was just a soft starter used to initially interconnect
the squirrel-cage induction generator with the power grid,
and only simple thysistors were applied and they did not
need to carry the power continuously [2]. In the 1990s the
power electronic technology was mainly used for the rotor
resistance control of wound-rotor induction generator, where
more advanced diode bridges with a chopper were used to
control the rotor resistance for generator [4], especially at
nominal power operation to reduce mechanical stress and
loading. Since 2000, even more advanced back-to-back (BTB)
power converters were introduced in large scale which started
to regulate the generated power from the wind turbines. First
mostly in the partial-scale power capacity for the doubly fed