The power generation system with a doubly fed induction generator (DFIG), which can be used as an autonomous power system after the loss of mains in a distributed generation network, is described. After the mains outage, a fixed frequency and an amplitude of the output voltage are obtained, despite the variable rotor speed. For this reason, it can be successfully applied in the variable-speed wind turbines, adjustable speed water plants, or diesel engines. Moreover, the stand-alone operation of DFIG is useful in a flywheel-based high-energy rotary uninterruptible power supply system. An output voltage is controlled directly by the synchronization of an actual voltage vector with the reference vector represented in a synchronously rotating polar frame. The rotor current angular speed is obtained as a result of vectorial phase-locked loop operation. Any sensors or estimators of the rotor speed or position are unnecessary. Both amplitude and angle control loops are linear. The use of stand-alone operation in grid-connected systems requires mains outage detection. Also, the grid voltage recovery requires a method of synchronization and soft connection of a generator to the grid. The proposed methods of output voltage control, synchronization, and detection of mains loss were tested in a laboratory system.

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Absfrocf -The following paper presents a variable speed stand alone power generating system. The system consists of a double fed induction machine and power electronic converter The simple generation system can be applied to wind and water turbines as well as to other prime-movers especially where is not stable or adiusted. The system is not connected to the rotor. A topology and a control system allow for stand alone operation. An external power source was used for preliminary excitation. After that, the system can operate autonomously and take excitation power from the stator. The equipped with any mechanical sensor, and in spite of the variable speed the stand alone operation generator, its stator voltage frequency is fixed and equals 50Hz.control system is based on stator voltage space \'ertor referred to rotating frame. The speed of the rotating frame responds to II. Slnhu A L W L OPeKr\llON OF DFIG S v S l L M stator frequency. The system is controlled by rotor current which amplitude is adjusted by controller of the stator voltage component related to x axes, whereas the rotor current frequency is controlled by stator voltage component related to y axes. In this way the stator output voltage frequency is speed independent and the control system operates without any mechanical sensor. For design a simulation program PSIM with DLL files is applied. The results of controllers calculation in a wide speed range are presented. A laboratory system was built and tested. As a controller a DSP SHARC7M system with support ALTERATM/ FPGA is used. The results of laboratory tests present the generating system in transients when a step load is applied.

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A stand-alone power system with a doubly fed induction generator DFIG is described. A Direct Voltage Control DVC for the stand alone operation is based on the stator voltage vector represented in rotating polar frame. A synchronization of the produced stator voltage vector with a reference voltage vector produces an output signal, used as a reference rotor current angular speed. This way the control method does not need any information of the rotor speed or position. An amplitude and frequency control uses the positively rotated polar frame and positive sequence components, whereas the negative sequence components for voltage asymmetry correction are used. Using positive and negative sequence components an unbalanced load can be supplied.

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 2.2kW laboratory system.

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