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Doubly-fed electric machines are electric motors or electric generators where both the field magnet windings and armature windings are separately connected to equipment outside the machine. By feeding adjustable frequency AC power to the field windings, the magnetic field can be made to rotate, allowing variation in motor or generator speed. This is useful, for instance, for generators used in wind turbines.

History

With its origins in wound rotor induction motors with multiphase winding sets on the rotor and stator, respectively, that was invented by Nikola Tesla in 1888, the rotor winding set of the doubly-fed electric machine is connected to a selection of resistors via multiphase slip rings for starting. However, the slip power was lost in the resistors. Thus means to increase the efficiency in variable speed operation by recovering the slip power were developed. In Krämer (or Kraemer) drives the rotor was connected to an AC and DC machine set that fed a DC machine connected to the shaft of the slip ring machine. Thus the slip power was returned as mechanical power and the drive could be controlled by the excitation currents of the DC machines. The drawback of the Krämer drive is that the machines need to be overdimensioned in order to cope with the extra circulating power. This drawback was corrected in the Scherbius drive where the slip power is fed back to the AC grid by motor generator sets.

The rotating machinery used for the rotor supply was heavy and expensive. Improvement in this respect was the static Scherbius drive where the rotor was connected to a rectifier-inverter set constructed first by mercury arc-based devices and later on with semiconductor diodes and thyristors. In the schemes using a rectifier the power flow was possible only out of the rotor because of the uncontrolled rectifier. Moreover, only sub-synchronous operation as a motor was possible.

Another concept using static frequency converter had a cycloconverter connected between the rotor and the AC grid. The cycloconverter can feed power in both directions and thus the machine can be run both sub- and oversynchronous speeds. Large cycloconverter controlled doubly-fed machines have been used to run single phase generators feeding 16 2/3 Hz railway grid in Europe and run the turbines in pumped storage plants.

Today the frequency changer used in applications up to few tens of megawatts consists of two back to back connected IGBT inverters.

Several brushless concepts have also been developed in order to get rid of the slip rings that require maintenance.

Double fed induction generator

DFIG for Double Fed Induction Generator, a generating principle widely used in wind turbines. It is based on an induction generator with a multiphase wound rotor and a multiphase slip ring assembly with brushes for access to the rotor windings. It is possible to avoid the multiphase slip ring assembly (see brushless doubly-fed electric machines), but there are problems with efficiency, cost and size. A better alternative is a brushless wound-rotor doubly-fed electric machine.

The principle of the DFIG is that rotor windings are connected to the grid via slip rings and back-to-back voltage source converter that controls both the rotor and the grid currents. Thus rotor frequency can freely differ from the grid frequency (50 or 60 Hz). By using the converter to control the rotor currents, it is possible to adjust the active and reactive power fed to the grid from the stator independently of the generator's turning speed. The control principle used is either the two-axis current vector control or direct torque control (DTC). DTC has turned out to have better stability than current vector control especially when high reactive currents are required from the generator.

The doubly-fed generator rotors are typically wound with 2 to 3 times the number of turns of the stator. This means that the rotor voltages will be higher and currents respectively lower. Thus in the typical ± 30% operational speed range around the synchronous speed, the rated current of the converter is accordingly lower which leads to a lower cost of the converter. The drawback is that controlled operation outside the operational speed range is impossible because of the higher than rated rotor voltage. Further, the voltage transients due to the grid disturbances (three- and two-phase voltage dips, especially) will also be magnified. In order to prevent high rotor voltages - and high currents resulting from these voltages - from destroying the IGBTs and diodes of the converter, a protection circuit (called crowbar) is used.

The crowbar will short-circuit the rotor windings through a small resistance when excessive currents or voltages are detected. In order to be able to continue the operation as quickly as possible an active crowbar has to be used. The active crowbar can remove the rotor short in a controlled way and thus the rotor side converter can be started only after 20-60 ms from the start of the grid disturbance when the remaining voltage stays above 15% of the nominal voltage. Thus it is possible to generate reactive current to the grid during the rest of the voltage dip and in this way help the grid to recover from the fault. For zero voltage ride through it is common to wait until the dip ends because with zero voltage it is not possible to know the phase angle where the reactive current should be injected.

As a summary, a doubly-fed induction machine is a wound-rotor doubly-fed electric machine and has several advantages over a conventional induction machine in wind power applications. First, as the rotor circuit is controlled by a power electronics converter, the induction generator is able to both import and export reactive power. This has important consequences for power system stability and allows the machine to support the grid during severe voltage disturbances (low voltage ride through, LVRT). Second, the control of the rotor voltages and currents enables the induction machine to remain synchronized with the grid while the wind turbine speed varies. A variable speed wind turbine utilizes the available wind resource more efficiently than a fixed speed wind turbine, especially during light wind conditions. Third, the cost of the converter is low when compared with other variable speed solutions because only a fraction of the mechanical power, typically 25-30%, is fed to the grid through the converter, the rest being fed to grid directly from the stator. The efficiency of the DFIG is very good for the same reason.

References

1. http://ethw.org/Power_electronics
2. Leonhard, W.: Control of Electrical Drives. 2nd Ed. Springer 1996, 420 pages. ISBN 3-540-59380-2.
3. Shively, E. K.; Whitlow, Geo. S.: Automatic Control for Variable Ratio Frequency Converters. Transactions of the American Institute of Electrical Engineers, Volume: 51 Issue: 1 Date: March 1932 , Page(s): 121 - 127.
4. Liwschitz, M. M., Kilgore, L. A.: A Study of the Modified Kramer or Asynchronous-Synchronous Cascade Variable-Speed Drive. Transactions of the American Institute of Electrical Engineers, Volume: 61, Issue: 5, May 1942, Page(s): 255 - 260.
5. Pfeiffer, A.; Scheidl, W.; Eitzmann, M.; Larsen, E.: Modern rotary converters for railway applications. Proceedings of the 1997 IEEE/ASME Joint Railroad Conference, March 18–20, 2007, Boston, MA, Pages: 29-33.
6. A. Bocquel, J. Janning: 4*300 MW variable speed drive for pump-storage plant application. EPE Conference 2003, Toulouse.
7. U.S. patent 6,448,735
8. J. Niiranen: About the active and reactive power measurements in unsymmetrical voltage dip ride-through testing Wind Energy 2008; 11:121-131.
9. an active crowbar: for example U.S. patent 7,164,562
10. Seman, S.; Niiranen, J.; Virtanen, R.; Matsinen, J.-P.: Low voltage ride-through analysis of 2 MW DFIG wind turbine - grid code compliance validations. Proceedings of the 2008 IEEE Power and Energy Society General Meeting - Conversion and Delivery of Electrical Energy in the 21st Century, 20–24 July 2008, Pittsburgh, PA, 6 pages.

External links

• Dufour, Christian; Bélanger, Jean (2004). "Real-Time Simulation of Doubly Fed Induction Generator for Wind Turbine Applications" (PDF). {{cite journal}}: Cite journal requires |journal= (help)
• Roberts, Paul C. (2004). "Study of Brushless Doubly-Fed (Induction) Machines; Contributions in Machine Analysis, Design and Control" (PDF). Emmanuel College, University of Cambridge. {{cite journal}}: Cite journal requires |journal= (help)

• Electric motors
• Electrical generators