Grid Connection to Stand Alone Transitions of Slip Ring Induction Generator During Grid Faults

A grid connected power generation systems based on the superior controllers of an active and reactive power are useless during a grid failures like grid short-circuit or line braking. Therefore the change of operation mode from grid connection to stand alone allows for uninterruptible supply of a selected part of grid connected load. However, in the stand alone operation mode the superior controllers should provide fixed amplitude and frequency of the generated voltage in spite of the load nature. Moreover, a soft transition from grid connection mode to stand alone operation requires that, the mains outage detection method must be applied. A grid voltage recovery requires change of the generator operational mode from stand alone to grid connection. However, the protection of a load from rapid change of the supply voltage phase is necessary. This may be achieved by synchronization of the generated and grid voltages and controllable soft connection of the generator to the grid. The paper presents the transients of controllable soft connection and disconnection to the grid of the variable speed doubly fed induction generator (DFIG) power system. A description of the mains outage detection methods for the DFIG is based on the grid voltage amplitude and frequency measurement and comparison with a standard values. Also an angle controller, between generated and grid voltages, for synchronization process is described. The short description of the sensorless direct voltage control of the autonomous doubly fed induction generator (ADFIG) is presented. All the presented methods are proved based on PSIM simulation software and in a laboratorial conditions and the oscillograms with a test results are presented in the paper. A 2.2 kW slip-ring induction machine was applied as a generator and 3.5 kW DC motor was used as a primary mover to speed adjusting. A switching and sampling frequencies are equal to 8 kHz. For filtering the switching frequency distortions in the output voltage external capacitances equal to 21 muF per phase are connected to the stator. The control algorithm is implemented in a DSP controller build on a floating point ADSP-21061 with an Altera/FPGA support