Grzegorz Iwanski

Direct Power Control of Doubly-Fed-Induction-Generator-Based Wind Turbines Under Unbalanced Grid Voltage

In this paper, the behavior of a doubly fed induction generator (DFIG) is studied under unbalanced grid voltage conditions. It is shown that if no special control efforts are employed, the behavior of the generator is deteriorated, basically due to two reasons: electromagnetic torque oscillations and nonsinusoidal current exchange with the grid. These phenomena are first analyzed theoretically as a function of the stator active and reactive instantaneous power exchange by the stator of the DFIG and the grid-side converter (GSC). This analysis provides the main ideas for generation of the active and reactive power references for the rotor-side converter (RSC) and the GSC, controlled by means of direct power control techniques. Therefore, this paper proposes a new algorithm that generates the RSC power references, without the necessity of a sequence component extraction, in order to eliminate torque oscillations and achieve sinusoidal stator currents exchange. On the contrary, the GSC power references are provided by means of voltage and current sequence extraction. Finally, simulation and experimental results successfully validate the proposed power reference generation methods.

DFIG-Based Power Generation System With UPS Function for Variable-Speed Applications

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.

Sensorless Direct Voltage Control of the Stand-Alone Slip-Ring Induction Generator

In this letter, a stand-alone power system with doubly fed induction generator is described. Direct voltage control based on the stator voltage vector, represented in rotating polar frame, does not need any feedback from the rotor speed or position. Using positive- and negative-sequence components, an unbalanced load can be supplied

Sensorless direct voltage control method for stand-alone slip-ring induction generator

A control method for stand alone mode operation of doubly fed induction generator is presented. The method bases on the stator voltage vector control represented in the synchronously rotated polar coordinates. The generated stator voltage is controlled by rotor currents. Amplitude of stator voltage and its frequency are controlled independently. The output signals from the voltage and the frequency regulators are the reference signals for the rotor current amplitude and frequency. This way, in spite of the variable generator speed, the fixed stator voltage amplitude and frequency, and controlled voltage phase are achieved. Developed direct voltage control method permits the DFIG the stand alone and the grid connected operation. After the mains outage the DFIG instantly supplies loads. Methods of supply grid voltage faults detection are developed. One method bases on the generated voltage vector amplitude, whereas a second bases on the voltage frequency. Both methods allows soft transients of generator disconnection from the grid. The controlled voltage phase permits for soft synchronization and grid connection after the mains reappearing. This way the load is protected from voltage phase rapid change. Applied methods allows the load to be uninterruptible supplied. Topology of power system, controller operation and results from laboratory tests of the DFIG are presented in the paper

Power management in an autonomous adjustable speed large power diesel gensets

The autonomous power generation system with DFIG dedicated for diesel driven gensets is presented. Characteristics of torque and power versus speed of internal combustion engine ICE are analyzed at an angle of application in electrical power generation. Features and advantages of variable speed operation versus fixed speed operation of internal combustion engine are described. The vector control method for generation of fixed amplitude and frequency output voltage is presented. The PLL vectorial synchronization of the actual stator voltage with the reference vector allow the rotor speed and position sensorless operation. Variable speed operation of the ICE engine reduces fuel consumption what is important in continuously operated diesel and hybrid systems with renewable sources. Results of standalone operation of 2.2 kW doubly fed induction generator supplying nonlinear load are presented.

Rotor current PI controllers in the method of output voltage control of variable speed standalone DFIG

Doubly fed induction generator DFIG operating in standalone mode is described. Analysis of single phase model allow for the selection of the filtering capacitance connected to the stator. The standalone DFIG is compared to the classical synchronous generator with DC excitation current. Method of output voltage control using PI controllers of the rotor current is shown. Decoupling between the amplitude and frequency of the stator voltage allow for the simple prototyping and step response based tuning of the PI controllers. Rotor current PI controllers are used in the rotating frame connected with a rotor current vector. They are designed using simplified model of the rotor excited slip ring induction machine. Results of simulation with 250 kW slip ring induction generator are shown. Laboratory results of 2.2 kW DFIG supplying the different type of load is presented.

Extended Direct Voltage Control of the Stand-Alone Double Fed Induction Generator

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.

Positive and Negative Sequence based Sensorless Control for Stand-Alone Slip-Ring Generator

A stand alone power generation system with doubly fed induction generator and its control method is presented in the paper. A slip-ring induction generator is equipped with the stator connected capacitances for filtering the switching frequency harmonics produced by the rotor connected power electronics converter. The proposed direct voltage control method, based on the stator voltage vector reference, provides fixed frequency and amplitude of the generated stator voltage operation. Different types of load (linear nonlinear) can be supplied keeping high quality voltage. A star connected stator provides the four wires system, supplying a single phase or unbalanced load. However the high load asymmetry produces the unbalanced stator voltage and in this case the additional part of the voltage control method is necessary for voltage asymmetry correction. As the main part of the control method bases of the positive sequence component of the stator voltage vector, the asymmetry correction algorithm bases on the negative sequence components. To extract both positive and negative sequence components of the stator voltage vector, two independent rotating frames are used. The coordinates systems rotates in opposite directions with synchronous and counter to synchronous angular speed. A PSIM software is used for control method design and prototyping. A proposed stand alone power generation system was build and tested. The developed stator voltage control with voltage asymmetry algorithm is proved by simulations and by oscillograms from laboratory tests

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

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.