PLL Grid Synchronization of the Standalone DFIG based Wind Turbine or Rotary UPS

A phase locked loop PLL is applied in the three phase power electronics converters to synchronize with grid voltage vector to obtain unity power factor. There are many concepts of synchronization which allow obtain a sinusoidal reference current in spite of the strongly distorted grid voltage. This paper presents another application of the phase synchronization. The variable speed induction generator is synchronized with the grid. Before connection to the grid it operates autonomously with output voltage vector controlled in synchronously rotated polar frame. Connection of unsynchronized rotating DFIG has an impact on the stator current. Even if the generator is unexcited, the peak of magnetizing current produces disturbances in the grid voltage. In case of the variable speed rotary UPS operating in standalone mode, the output voltage must be synchronized with the grid after the grid voltage recovery. It is necessary due to the fact, that the load supplied from the UPS must be protected from the rapid change of the voltage phase during connection of the generator to the grid. The PLL synchronization is also used to achieve a fixed amplitude and frequency of the generated voltage during standalone operation of DFIG. The use of synchronization between actual and reference voltage vector eliminates information about rotor speed or. Proposed methods were verified in 2.2 kW laboratory system.