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			{{Short description\|Electric motor}}
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			'''Doubly fed electric machines''', '''Doubly fed induction generator''' (DFIG), or '''slip-ring generators''', are ]s or ]s, where both the ] windings and ] windings are separately connected to equipment outside the machine.
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			By feeding adjustable frequency ] to the ]s, the ] can be made to rotate, allowing variation in motor or generator speed. This is useful, for instance, for generators used in ]s.<ref>{{cite web \|title=Generators for wind turbines Standard slip ring generator series for doubly-fed concept from 1.5-3.5 MW \|publisher=] \|date=2014 \|url=http://new.abb.com/docs/default-source/ewea-doc/generators-for-wind-turbines-standard-slip-ring-generator-series-for-doubly-fed-concept.pdf \|access-date=April 24, 2018}}</ref> Additionally, ]-based wind turbines offer the ability to control ] and ].<ref>M. J. Harandi, S. G. Liasi and M. T. Bina, "," 2019 International Power System Conference (PSC), Tehran, Iran, 2019, pp. 181-187, {{doi\|10.1109/PSC49016.2019.9081565}}.</ref><ref>M. Niraula and L. Maharjan, “Variable stator frequency control of stand-alone DFIG with diode rectified output”, 5th International symposium on environment-friendly energies and applications (EFEA), 2018.</ref>
	'''Doubly-fed electric machines''' are ]s or ]s where both the ] windings and ] 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 ]s.

	==~~History~~==		==Introduction==
			]
	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,<ref>http://ethw.org/Power_electronics</ref> 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.<ref>Leonhard, W.: Control of Electrical Drives. 2nd Ed. Springer 1996, 420 pages. ISBN 3-540-59380-2.</ref> 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.<ref> Transactions of the American Institute of Electrical Engineers, Volume: 51 Issue: 1 Date: March 1932 , Page(s): 121 - 127.</ref><ref> Transactions of the American Institute of Electrical Engineers, Volume: 61, Issue: 5, May 1942, Page(s): 255 - 260.</ref>
			Doubly fed electrical generators are similar to ] ]s, but have additional features which allow them to run at speeds slightly above or below their natural synchronous speed. This is useful for large ]s, because wind speed can change suddenly. When a gust of wind hits a wind turbine, the blades try to speed up, but a ] is locked to the speed of the ] and cannot speed up. So large forces are developed in the hub, gearbox, and generator as the power grid pushes back. This causes wear and damage to the mechanism. If the turbine is allowed to speed up immediately when hit by a wind gust, the stresses are lower with the power from the wind gust still being converted to useful electricity.

			One approach to allowing wind turbine speed to vary is to accept whatever frequency the generator produces, convert it to DC, and then convert it to AC at the desired output frequency using an ]. This is common for small house and farm wind turbines. But the inverters required for megawatt-scale wind turbines are large and expensive.
	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.

			Doubly fed generators are another solution to this problem. Instead of the usual ] fed with DC, and an ] winding where the generated electricity comes out, there are two three-phase windings, one stationary and one rotating, both separately connected to equipment outside the generator. Thus, the term ''doubly fed'' is used for this kind of machines.
	Another concept using static frequency converter had a '']'' 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 <ref> Proceedings of the 1997 IEEE/ASME Joint Railroad Conference, March 18–20, 2007, Boston, MA, Pages: 29-33.</ref> and run the turbines in pumped storage plants.<ref> EPE Conference 2003, Toulouse.</ref>

			One winding is directly connected to the output, and produces 3-phase AC power at the desired grid frequency. The other winding (traditionally called the field, but here both windings can be outputs) is connected to 3-phase AC power at variable frequency. This input power is adjusted in frequency and phase to compensate for changes in speed of the turbine.<ref>{{cite journal \|url=http://web.mit.edu/kirtley/binlustuff/literature/wind%20turbine%20sys/DFIGinWindTurbine.pdf \|title=Doubly Fed Induction Generator Systems for Wind Turbines \|author1=S. MÜLLER \|author2=S. \|display-authors=etal \|publisher=IEEE \|journal=IEEE Industry Applications Magazine \|volume=8 \|issue=3 \|pages=26–33 \|date=2002\|doi=10.1109/2943.999610 }}</ref>
	Today the frequency changer used in applications up to few tens of megawatts consists of two back to back connected '']'' inverters.

			Adjusting the frequency and phase requires an AC to DC to AC converter. This is usually constructed from very large ] semiconductors. The converter is bidirectional, and can pass power in either direction. Power can flow from this winding as well as from the output winding.<ref>L. Wei, R. J. Kerkman, R. A. Lukaszewski, H. Lu and Z. Yuan, "Analysis of IGBT power cycling capabilities used in Doubly Fed Induction Generator wind power system," 2010 IEEE Energy Conversion Congress and Exposition, Atlanta, GA, 2010, pp. 3076-3083, {{doi\|10.1109/ECCE.2010.5618396}}.</ref>
	Several brushless concepts have also been developed in order to get rid of the slip rings that require maintenance.

	==~~Classification~~==		==History==
			With its origins in ] with multiphase winding sets on the rotor and stator, respectively, which were invented by ] in 1888,<ref>{{cite web \|url=http://ethw.org/Power_electronics \|title=Power electronics - Engineering and Technology History Wiki \|website=ethw.org}}</ref> 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.<ref>Leonhard, W.: Control of Electrical Drives. 2nd Ed. Springer 1996, 420 pages. {{ISBN\|3-540-59380-2}}.</ref> 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 ] drive where the slip power is fed back to the AC grid by motor generator sets.<ref>{{Cite journal \| doi=10.1109/T-AIEE.1932.5056029\| title=Automatic Control for Variable Ratio Frequency Converters\| year=1932\| last1=Shively\| first1=E. K.\| last2=Whitlow\| first2=Geo. S.\| journal=Transactions of the American Institute of Electrical Engineers\| volume=51\| pages=121–127\| s2cid=51636516}}</ref><ref>{{Cite journal \| doi=10.1109/T-AIEE.1942.5058524\| title=A Study of the Modified Kramer or Asynchronous-Synchronous Cascade Variable-Speed Drive\| year=1942\| last1=Liwschitz\| first1=M. M.\| last2=Kilgore\| first2=L. A.\| journal=Transactions of the American Institute of Electrical Engineers\| volume=61\| issue=5\| pages=255–260\| s2cid=51642497}}</ref>
	Electric machines are either ] with one multiphase winding set that actively participates in the energy conversion process or ''Double Fed'' with two active winding sets. An active winding set (or ] winding set has at least two AC phases with independent electrical ports, which is necessary for the production of a ] with an ] that actively participates in the energy conversion process. Since both winding sets of a doubly-fed electric machine actively participate in the energy conversion process, a doubly-fed electric machine operates to twice synchronous speed or twice the constant torque range with a given frequency of excitation and as a result, both armature winding sets contribute to electromechanical power production. Many confuse the singly-fed slip-energy recovery induction and the field-excited synchronous electric machines with two electrical ports as doubly-fed but only one port or winding set actively participates in the energy conversion process and as a result, these electric machines are not designed for operation to twice synchronous speed with a given frequency of excitation. A practical doubly-fed electric machine system must operate between sub-synchronous and super synchronous speed without control discontinuity.

			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 ]-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.
	Only practical with the evolution of control technology that avoids instability and discontinuity over the sub-synchronous to super-synchronous speed range, particularly at synchronous speed where induction ceases to exist, there are now three varieties of doubly-fed electric machine systems: 1) the Doubly-Fed Induction Electric Machine (DFIM), which is the ] with an active winding set on the rotor and stator, respectively, and flux vector controlled rotor excitation through a multiphase slip-ring assembly; 2) the Brushless Doubly-Fed Induction Electric Machine (BDFIM), which is the ] with cascaded active winding sets of unlike pole-pairs on the stator assembly of which one is flux vector controlled and a flux focusing rotor assembly; and 3) Synchro-Sym electric machine system, which is the only ]. Unlike the doubly-fed electric machine topologies that always rely on a slip in speed between the ] and ] (i.e., slip-induction) for operation with pronounced instability potential at synchronous speed where induction ceases to exist, the Synchro-Sym electric machine system has a brushless real time control method that eliminates reliance on slip-induction, multiphase slip ring assemblies, or any derivative of rotor flux vector control to provide stable synchronous operation to twice synchronous speed for a given frequency of excitation with active rotor and stator winding sets.

			Another concept using static frequency converter had a '']'' 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 {{frac\|16\|2\|3}} Hz railway grid in Europe.<ref>{{Cite book \| doi=10.1109/RRCON.1997.581349\| isbn=0-7803-3854-5\| chapter=Modern rotary converters for railway applications\| title=Proceedings of the 1997 IEEE/ASME Joint Railroad Conference\| year=1997\| last1=Pfeiffer\| first1=A.\| last2=Scheidl\| first2=W.\| last3=Eitzmann\| first3=M.\| last4=Larsen\| first4=E.\| pages=29–33\| s2cid=110505314}}</ref> Cycloconverter powered machines can also run the turbines in pumped storage plants.<ref> EPE Conference 2003, Toulouse.</ref>
	The symmetrical circuit topology and operational relationships of the wound-rotor doubly-fed synchronous electric machine cores with active winding sets on the rotor and stator, respectively, become the classic study for all other electric machines by de-optimizing their symmetry with asymmetry; for instance, by replacing the ''symmetrical'' circuit topology provided by the rotor active winding set with the ''asymmetrical'' circuit topology provided by a passive permanent magnet assembly, which has no active power port and as a result, cannot actively participate in the energy conversion process. A true doubly-fed electric machine must have two or more active winding sets (ports) excited with bi-directional power in order to produce a ] for active contribution in the energy conversion process and for providing practical operation from sub-synchronous to super-synchronous speed without regions of discontinuity, such as about synchronous speed where induction ceases to exist.

			Today the frequency changer used in applications up to few tens of megawatts consists of two back to back connected '']'' inverters.
	The DFIM and BDFIM rely on speed-based asynchronism (or slip) between the rotor and stator windings to induce speed-synchronized current onto the rotor winding set. However at the low slip experienced about synchronous speed, the time critical measurement or excitation synthesis of shallow time-differential signals makes stability increasingly elusive. The BDFIM has eliminated the multiphase slip-ring assembly and partially improved stability by sacrificing size, cost, and efficiency with incorporation of a second stator ]. In contrast, the BDFSM as provided by the Synchro-Sym electric machine system brushlessly propagates instantaneously derived speed-synchronized multiphase excitation to the rotor winding set without discontinuity and without relying on slip-induction as exhibited by ], although slip-induction may be experienced as in all singly fed and doubly-fed induction electric machines.

			Several brushless concepts have also been developed in order to get rid of the slip rings that require maintenance.
	Doubly-fed electric machines can be categorized as either wound-rotor synchronous doubly-fed electric machines or a variety of induction doubly-fed electric machines. Under the same electric machine design constraints equally available to all but without considering the excitation control means of the rotor armature, the wound-rotor doubly-fed synchronous electric machine (WRSDFM) has at least the following attributes: 1) With similarly rated armature winding sets on the rotor and stator respectively, only the WRSDFM has a rotor assembly that actively participates in the energy conversion process with the stator assembly to provide twice the power density as all other electric machine systems by assuming the rotor and stator assemblies occupy the same physical volume; 2) With twice the power density as other electric machines, only the WRSDFM is half the cost as all other electric machine systems for a given power rating, by assuming the physical size of the electric machine for a given power rating determines the amount of materials with the WRSDFM showing twice the power density; 3) With efficiency normalized to power rating at a given frequency and voltage of excitation, the WRSDFM with twice the power rating at a given frequency and voltage of excitation has higher efficiency than all other electric machine systems; 4) With a dual ported (or symmetrical) transformer topology, which avoids saturating the core with increasing torque current, only the WRSDFM has multiply higher peak torque potential compared to all other electric machine systems.

	===Features of doubly-fed machines===
	The wound rotor doubly-fed ''synchronous'' electric machine as provided by the Synchro-Sym electric machine system is the only electric machine that operates with rated torque to twice synchronous speed for a given frequency of excitation and without control discontinuity at synchronous speed where slip-induction ceases to exist (i.e., 7,200 rpm with one pole-pair doubly-fed machine when both stator and rotor are fed with 60 Hz versus 3,600 rpm for otherwise similar singly-fed electric machine). In high power applications two or three pole-pair machines are common. Higher speed with rated torque under the same pole count, voltage, and excitation frequency as any other electric machine, means that doubly-fed machines have lower cost per kW, higher efficiency, and higher power density as other electric machines, including rare-earth permanent magnet synchronous electric machines.

	In concept, any multiphase electric machine can be converted to a wound-rotor doubly-fed electric motor or generator by changing the rotor assembly to a multiphase wound rotor winding set. If the rotor winding set can transfer bi-directional active or ''working'' power to the electrical system, the conversion result is a wound-rotor doubly-fed electric motor or generator with higher speed and power rating than the original singly-fed electric machine. These advantages can be achieved without core saturation, all by electronically controlling half or less of the total motor power for full variable speed control.

	As do all electromagnetic electric machines, doubly-fed machines need torque current and magnetic flux to produce torque. Because there are no permanent magnets in the doubly-fed machine, magnetizing current is needed to produce magnetic flux. Magnetizing current and torque current are orthogonal vectors and do not add directly. Since the magnetizing current is much smaller than the torque current, it is only significant in the efficiency of the machine at very low torque. Furthermore, magnetizing current of the wound rotor doubly-fed electric machine can be shared between the stator and rotor windings. If all magnetizing current is supplied by the rotor windings, the stator will only have torque current and so unity ]. However, by optimal current sharing the total I<sup>2</sup>R loss can be minimized.

	At synchronous speed the rotor current has to be DC, as in ordinary synchronous machines. If the shaft speed is above or below synchronous speed, the rotor current must be AC at the slip frequency. Thus the rotor winding requires reactive power when it is used to magnetize the machine in non-synchronous operation.

	Torque production requires that rotor current also has a torque producing component in addition to magnetization. Thus active power is present in the rotor in addition to reactive power.

	The frequency and the magnitude of the voltage is proportional to the difference between the speed of the machine and the synchronous speed (the slip). At standstill, the frequency will be the same as the frequency in the stator; the voltage magnitude is determined by the ratio of the stator and rotor winding turns. Thus if the number of turns is equal, the rotor has the same voltage as the stator. The doubly-fed machine is a transformer at standstill. The transformer-like characteristics are also present when it is rotating, manifesting itself especially during transients in the grid.

	Due to the voltage and current behavior described above the rotor will either require or generate active power from sub-synchronous to super-synchronous speeds with the potential torque rating of an armature winding set but only with the invention of stable synchronized bi-directional excitation that does not rely on induction, which ceases to exist at synchronous speed, for true synchronous operation; otherwise, the doubly-fed electric machine is an induction (or asynchronous) electric machine producing torque and operating as a motor with the rotor generating power at speeds below synchronous speed (subsynchronous operation). At standstill all active power fed in the stator (excluding losses) is returned via the rotor. If the motor has rated torque, rated active power is circulating through the stator, rotor and frequency converter and only losses are taken from the grid. The mechanical power being the angular speed multiplied by the torque of the motor is zero at standstill. Thus like all electric machines, the efficiency of the machine is not very good at low speeds because losses depend on the current that is required to produce torque but little or no mechanical power is produced.

	If the machine is operating as a motor at speeds over the synchronous speed (supersynchronous operation), the mechanical power is fed through both the stator and rotor for higher rated power than a singly-fed electric machine; otherwise, the machine acts as a generator over the same speed range but only with stable synchronized excitation that does not rely on induction. As a consequence the efficiency is now better than with singly-fed motors, since efficiency is normalized to rated power. For example, the doubly-fed electric machine with equal stator and rotor turns produces same torque at double speed (and thus twice the power) as a singly-fed electric machine. The losses, being roughly proportional to the torque, are quite the same. Thus efficiency, which is the power taken divided by the total power produced, is better than singly-fed electric machines. Naturally one has to take into account the loss of the power electronic control equipment. However, the frequency converter of the doubly-fed machine has to control only 50% or less of the power of the machine, and thus has about half of the loss of the singly-fed machines' frequency converter that has to pass through 100% of the power.

	Since efficiency is the ratio between the output power to the input power, the magnetic core efficiency of a wound rotor doubly-fed machine, which has just two winding sets of loss but shows twice the power for a given frequency and voltage of operation, is comparable to the magnetic core efficiency of permanent magnet machines with just one winding but without magnetizing current. Coupled with the low power electronic controller, the wound-rotor doubly-fed electric machine system would be more efficient than permanent magnet machine systems without magnetizing current.

	For operation as a generator a similar situation exists. At subsynchronous speeds the stator is generating the power but part of it has to be fed back to rotor. At supersynchronous speeds both the rotor and stator are producing power to the grid.

	Thus the current rating of the rotor converter is defined by the maximum active current required by the torque production and the maximum reactive current required to magnetize the machine.

	With synchronous speed determined by the excitation frequency, only Doubly-fed electric machines can operate to supersynchronous speeds. They can operate with the same constant torque as singly-fed electric machines to twice synchronous speed if each active winding (i.e., ]) is rated at half the total power of the doubly-fed electric machine (or the same rated power of the armature of a singly-fed electric machine) and can provide continuous operation between sub-synchronous through supersynchronous speed range. As a result, for a given frequency and voltage of excitation (and pole count), the doubly-fed electric machine (with two ]) is rated at twice the rated power of singly-fed electric machine (with a single but similarly rated ]). For the reasons provided, specific power (KW/Kg) or ] (KW/L) has no comparative relevance unless excitation frequency, excitation voltage, and pole count are similar in the comparison, which determines the speed of operation, and as a result, the core of a ] should show nearly twice the power density under the same design constraints equally available to all, such as the same torque, same air-gap area, same air-gap flux density (which is determined by the saturation limit of the electrical steel core), etc.

	Doubly-fed machines do not produce more continuous rated torque per volume than singly-fed machines, since torque is a function of at least the air-gap area and air-gap flux density (which is determined by the saturation limit of the electrical steel core) equally available to all.<ref>Norbert L. Schmitz and Willis F. Long, “The Cycloconverter driven Doubly-fed Induction Motor,” IEEE Transactions on Power Apparatus And Systems, page 526, column 1, paragraph 6, Vol. PAS-90, No. 2, March/April 1971, pp. 526-531.</ref> With electric machine torque and size virtually the same with the same air-gap area and flux density, the bigger power rating is due to the higher speed attainable without weakening the magnetic flux. The short time maximum torque of a ''wound rotor doubly-fed electric machine'' is, however, much higher than all other electric machines, including induction or permanent magnet machines, because increasing torque current does not directly increase air-gap flux, which quickly leads to core saturation for all other electric machine types. In practice, torque current increase is only limited by the temperature of the windings and the maximum current capability of the rotor frequency converter.

	With one of the two armature winding sets residing on the rotor and stator body, respectively, the rotor real estate of the wound-rotor doubly-fed machine actively participates in the energy conversion process, which is different from all other electric machines, including permanent magnet synchronous machines. As a result, the magnetic core of the wound-rotor doubly-fed electric machine shows highest power density.

	Changing of the direction of the rotation requires the swap of two stator phases near zero speed if symmetrical speed range in both directions is required.

	It is common to dimension the doubly-fed machine to operate only at a narrow speed range around synchronous speed and thus further decrease the power rating (and cost) of the frequency converter in the rotor circuit.

	Typical applications of doubly-fed machines have been high power pumps and fans, hydro and wind generators, shaft generators for ships etc. where operating speed range has been quite narrow, less than ±30% of the synchronous speed and only small power is required in the subsynchronous range.

	Due to the high rotor to stator winding turns ratio that is typical in these applications and the high voltage thus induced in the rotor at standstill, the starting of this kind of restricted operating speed range motor drive is usually done with rotor resistors in induction motor mode. When speed is in the operating speed range, the resistors are disconnected and the frequency converter is connected to the rotor. If the starting torque is low enough it is also possible to short circuit the stator and use the frequency converter in the induction motor control mode to accelerate the motor to the operating speed range. Generators, naturally, don't usually need any additional starting means because wind or water is used to accelerate the machine to the operating speed range.

	===Electronic control===
	The controller, a '']'', conditions bi-directional (four quadrant), speed synchronized, and multiphase electrical power to at least one of the winding sets (generally, the rotor winding set). Due to the lack of damper windings used in synchronous machines, the wound-rotor doubly-fed electric machines are susceptible to instability without stabilizing control because torque is a function of position.<ref> Transactions of the American Institute of Electrical Engineers, Volume: 60 Issue: 10 Oct. 1941, Page(s): 923 - 924</ref><ref> Proceedings of the IEE - Part C: Monographs, Volume: 105 Issue: 7 Date: March 1958, Page(s): 319 - 329</ref><ref> Proceedings of the Institution of Electrical Engineers, Volume: 114 Issue: 6, June 1967, Page(s): 791 - 796</ref> Pioneering work of Drs. Albertson, Long, Novotny, and Schmitz.<ref> Transactions of the Power Apparatus and Systems, Volume: 83 Issue: 8 Date: Aug. 1964, Page(s): 858 - 864.</ref><ref> Transactions of the Industry and General Applications, Volume: IGA-7 Issue: 1 Date: Jan. 1971, Page(s): 95 - 100.</ref><ref>, AIEE Great Lakes District Meeting, Date: April 1962, Page(s): 652-657.</ref> from the engineering department of the University of Wisconsin realized this must be overcome with instantaneous control. Like any synchronous machine, losing synchronism will result in alternating torque pulsation and other related consequences.

	Doubly-fed electric machines require electronic control for practical operation and should be considered an electric machine ''system'' or more appropriately, a ].<ref> Vdm Verlag Dr. Müller (August 2008), ISBN 978-3-639-03244-4</ref>

	==Wound-rotor doubly-fed electric machine==

	===Construction===
	Two multiphase winding sets with similar pole-pairs are placed on the rotor and stator bodies, respectively. Since the rotor winding set actively participates in the energy conversion process with the stator winding set, utilization of the magnetic core real estate is optimized in contrast to all other electric machine types.

	The doubly-fed machine operation at unity stator power factor requires higher flux in the air-gap of the machine than when the machine is used as wound rotor induction machine. It is quite common that wound rotor machines not designed to doubly-fed operation saturate heavily if doubly-fed operation at rated stator voltage is attempted. Thus a special design for doubly-fed operation is necessary.

	A multiphase ] assembly is traditionally used to transfer power to the rotating winding set and to allow independent control of the rotor winding set. The slip ring assembly requires maintenance and compromises system reliability, cost and efficiency. Attempts to avoid the slip ring assembly are constantly being researched with limited success (see ]).

	===Control===
	Although the multiphase slip ring assembly reduces reliability and requires regular maintenance, it allows easy control of the rotor (moving) winding set so both multiphase winding sets actively participate in the energy conversion process with the electronic controller controlling half (or less) of the power capacity of the electric machine for full control of the machine.

	This is especially important when operating at synchronous speed, because then the rotor current will be DC current. Without slip rings the production of DC current in the rotor winding is only possible when the frequency converter is at least partly located in the rotor and rotating with it. This kind of rotor converter naturally requires its own winding system (preferably using high frequency in the 10 kHz range for compact size) for power transfer out of or into the rotor. Furthermore, there are thermal and mechanical constraints (for example centrifugal forces) of the power electronic assembly in the rotor. However, high speed alternators have had electronics incorporated on the rotor for many years. Furthermore, high frequency wireless power transfer is used in many applications because of improvements in efficiency and cost over low frequency alternatives.

	===Efficiency===
	Neglecting the slip ring assembly, the theoretical electrical loss of the wound-rotor doubly-fed machine core in supersynchronous operation is ] to the most efficient electric machine systems available (the synchronous electric machine with permanent magnet assembly) under similar operating metrics. The efficiency is similar because the total current is split between the rotor and stator winding sets while the electrical loss is proportional to the square of the current flowing through the winding set. Further considering that the electronic controller handles less than 50% of the power of the machine, the wound-rotor doubly-fed machine theoretically shows nearly half the electrical loss of other machines of similar rating.

	===Power density===
	Neglecting the slip ring assembly and considering similar air-gap flux density, the physical size of the magnetic core of the wound-rotor doubly-fed electric machine is smaller than other electric machines because the two active winding sets are individually placed on the rotor and stator bodies, respectively, with virtually no real-estate penalty. In all other electric machines, the rotor assembly is passive real estate that does not actively contribute to power production. The potential of higher speed for a given frequency of excitation, alone, is an indication of higher power density potential. The continuous constant-torque speed range is up to 7200 rpm @ 60 Hz with 2 poles for a given design torque compared to 3600 rpm @ 60 Hz with 2 poles for other electric machines. In theory, the core volume is nearly half the physical size ] to other machines of similar rating.

	===Cost===
	Neglecting the slip ring assembly, the theoretical system cost of a wound-rotor doubly-fed electric machine is nearly 50% less ] to other machines of similar rating because the power rating of the electronic controller, which is the significant cost of any electric machine system, is 50% (or less) than other electric motor or generator systems of similar rating.

	===Peak Torque===
	With the symmetrical or dual-ported transformer topology of two active winding sets on the rotor and stator, respectively, the wound-rotor doubly-fed electric machine core produces nearly twice the peak torque of any electric machine by similarly increasing torque current without affecting air-gap flux density or saturating the magnetic core because the torque currents (and flux production) on each side of the air-gap are neutralized. For all electric machines, peak torque current increases dissipation while reducing efficiency.

	==Double fed induction generator==

	'''DFIG''' for Double Fed Induction Generator, a generating principle widely used in ]s. It is based on an ] with a multiphase wound rotor and a multiphase ] assembly with brushes for access to the rotor windings<!-- (see ])-->. It is possible to avoid the multiphase slip ring assembly (see ]), but there are problems with efficiency, cost and size. A better alternative is a ].

	]

	The principle of the DFIG is that rotor windings are connected to the grid via slip rings and back-to-back ] source converter that controls both the rotor and the grid currents. Thus ] ] 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 ] or ].<ref>{{US patent\|6448735}}</ref> DTC has turned out to have better stability than current vector control especially when high reactive currents are required from the generator.<ref> Wind Energy 2008; 11:121-131.</ref>

	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 ]s and ]s of the converter, a protection circuit (called ]) 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 ]<ref>an ]: for example {{US patent\|7164562}}</ref> 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.<ref> 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.</ref>

	As a summary, a doubly-fed induction machine is a wound-rotor doubly-fed electric machine <!--(see 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 ]. This has important consequences for ] and allows the machine to support the grid during severe voltage disturbances (]). Second, the control of the rotor voltages and currents enables the induction machine to remain ] 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.

	==Brushless doubly-fed versions==

	==~~=Brushless~~ ~~doubly-~~fed induction ~~electric machine=~~==		==Doubly fed induction generator==
			Doubly fed induction generator (DFIG), a generating principle widely used in ]s. It is based on an ] with a multiphase wound rotor and a multiphase ] assembly with brushes for access to the rotor windings. It is possible to avoid the multiphase slip ring assembly, but there are problems with efficiency, cost and size. A better alternative is a brushless wound-rotor doubly fed electric machine.<ref>{{cite journal \|title=Overview of research and development status of brushless doubly-fed machine system \|publisher=] \|journal=Chinese Journal of Electrical Engineering \|volume=2 \|issue=2 \|date=December 2016}}</ref>
	'''Brushless doubly-fed ''induction'' electric machine''' is constructed by adjacently placing two multiphase ] sets with unlike pole-pairs on the ] body. With unlike pole-pairs between the two winding sets, low frequency magnetic induction is assured over the speed range. One of the stator winding sets (power winding) is connected to the grid and the other winding set (control winding) is supplied from a frequency converter. The shaft speed is adjusted by varying the frequency of the control winding. As a doubly-fed electric machine, the rating of the frequency converter need only be fraction of the machine rating.

			]
	The brushless doubly-fed electric machine does not utilize core real-estate efficiently and the dual winding set stator assembly is physically larger than other electric machines of comparable power rating. In addition, a specially designed rotor assembly tries to focus most of the mutual ] to follow an indirect path across the air-gap and through the rotor assembly for inductive coupling (i.e., brushless) between the two adjacent winding sets. As a result, the adjacent winding sets are excited independently and actively participate in the electro-mechanical energy conversion process, which is a criterion of doubly-fed electric machines.

			The principle of the DFIG is that stator windings are connected to the grid and rotor winding are connected to the converter via slip rings and back-to-back ] source converter that controls both the rotor and the grid currents. Thus ] ] 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 ] or ] (DTC).<ref>{{US patent\|6448735}}</ref> DTC has turned out to have better stability than current vector control especially when high reactive currents are required from the generator.<ref>{{cite journal \|doi=10.1002/we.254 \|volume=11 \|issue=1 \|title=About the active and reactive power measurements in unsymmetrical voltage dip ride-through testing \|year=2008 \|journal=Wind Energy \|pages=121–131 \|last1=Niiranen \|first1=Jouko\|bibcode=2008WiEn...11..121N }}</ref>
	The type of rotor assembly determines if the machine is a ] or ] doubly-fed electric machine. The constant ] speed range is always less than 1800 ] @ 60 ] because the effective pole count is the average of the unlike pole-pairs of the two active winding sets. Brushless doubly-fed electric machines incorporate a poor electromagnetic design that compromises physical size, cost, and electrical efficiency, to chiefly avoid a multiphase ] assembly.

			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 ]s and ]s of the converter, a protection circuit (called ]) is used.<ref name= Crowbar_LVRT/>
	===Brushless wound-rotor doubly-fed electric machine===
	The '''brushless ''wound-rotor'' doubly-fed electric machine''' ('''BWRSDF''') incorporates the electromagnetic structure of the wound-rotor ] doubly-fed electric machine but replaces the traditional multiphase slip ring assembly with a brushless real-time emulation control means that independently powers the ] multiphase winding set with speed and phase synchronized AC excitation automatically and without delay as hypothesized by electric machine experts since the 1960s to avoid torque angle instability and slip induction discontinuity.<ref name="autogenerated640"> Electric Machines & Drives Conference (IEMDC, 2013 IEEE International), pages 640-647.</ref><ref> Frederick W. Klatt, Electric Machines & Drives Conference (IEMDC, 2013 IEEE International), pages 640-647.</ref> Without experiencing control discontinuity between sub-synchronous and super-synchronous speeds while under stable instantaneously controlled excitation, the ] multiphase winding set of only the BWRSDF becomes an equal power contributing (or active) participant in the electrical to mechanical power conversion process in conjunction with the ] multiphase winding set (or ]) and as a result, the rotor winding set provides the second ] for a dual-armature (or doubly-fed) electric machine system. In accordance with ] operating principles, the rotor armature cannot rely on ] as a result of a slip between the speeds of ] and ] winding sets because '''slip-induction''' ceases to exist about either side of synchronous speed, which always leads to discontinuous operation. Without the reliance on slip-induction for practical operation, the '''brushless wound-rotor ] doubly-fed electric machine''' operationally differs significantly from ], which traditionally include a multiphase slip-ring assembly, and the so-called ] that rely on very different principles of unlike pole-pair slip-induction for operation. The doubly-fed induction electric machines have been relegated to high power generator applications, such as wind turbines, where their higher efficiency and lower cost as a result of doubly-fed prevail over instability, which is externally mitigated by the inertia of the prime mover.

			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 ]<ref>an ]: for example {{US patent\|7164562}}</ref> 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{{nbsp}}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 ], it is common to wait until the dip ends because it is otherwise not possible to know the phase angle where the reactive current should be injected.<ref>{{Cite book \| doi=10.1109/PES.2008.4596687 \| isbn=978-1-4244-1905-0\| chapter=Low voltage ride-through analysis of 2 MW DFIG wind turbine - grid code compliance validations\| title=2008 IEEE Power and Energy Society General Meeting - Conversion and Delivery of Electrical Energy in the 21st Century\| year=2008\| last1=Seman\| first1=Slavomir\| last2=Niiranen\| first2=Jouko\| last3=Virtanen\| first3=Reijo\| last4=Matsinen\| first4=Jari-Pekka\| pages=1–6\| s2cid=41973249}}</ref>
	Considering the ] (and voltage) of all electric machines is determined by the number of pole pairs and the frequency of excitation, electric machines with a 2 pole (i.e., one pole-pair) ] operate at 3600 ] under 60 hertz of excitation but only a true BWRSDF can achieve stable and contiguous operation to the super-synchronous speed of 7200 RPM with the same torque performance, which is tantamount to twice the power density of all other electric machines, including permanent magnet and reluctance electric machines, by reasonably assuming the rotor and stator armature assemblies are equally power rated and occupy virtually the same physical volume as was qualified by at least pioneering electric machine experts from the 1960s, such as Professors D.W. Novotny, Norbert L. Schmitz, and Willis F. Long from the electrical engineering department of the University of Wisconsin.<ref name="autogenerated640"/> Since efficiency and cost are normalized to power rating, other ramifications are half the cost (e.g., half the material for a given power rating) and half the electrical loss (e.g., torque current is divided equally between the two armatures for lower electrical loss, which is a function of the square of the current (I<sup>2</sup>R), and the electronic controller need only control the power of one armature (or half the power) of the electric machine).<ref>"A Simple Position-Sensorless Algorithm for Rotor-Side Field-Oriented Control of Wound-Rotor Induction Machines," Rajib Datta and V.T. Ranganathan, IEEE Transactions On Industrial Electronics, Vol. 48, No. 4, August, 2001, pp. 786-793</ref> Operationally comparable to a dual ported (i.e., symmetrical) ], the air-gap flux (i.e., flux linkage between the ] and ]) remains virtually constant regardless of the level of flux producing torque current and as a result, the '''brushless wound-rotor doubly-fed electric machine''' ('''BWRSDF''') can achieve multiply higher peak torque density than all other electric machines without saturating the magnetic core, which is an attractive attribute for '''transmission-less''' electric propulsion systems, such as electric vehicles.<ref name="cycloconverter526">“The Cycloconverter driven Doubly-fed Induction Motor,” Norbert L. Schmitz and Willis F. Long, IEEE Transactions on Power Apparatus And Systems, Vol. PAS-90, No. 2, March/April 1971, pp. 526-531</ref> In addition, this same attribute allows the air-gap flux density of only the BWRSDF to be designed closer to the saturation limit of the magnetic steel core without operational concern and effectively, allowing a higher air-gap flux density than other electric machines. Like any ] with excess ] capacity in conjunction with field control (i.e., field weakening) and ] control, the BWRSDF can be a ] with the ability to balance the electrical ] system, such as the ], with leading to lagging or unity ] adjustment.

			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 ]. This has important consequences for ] and allows the machine to support the grid during severe voltage disturbances (]; LVRT).<ref name= Crowbar_LVRT>M. J. Harandi, S. Ghaseminejad Liasi, E. Nikravesh and M. T. Bina, "An Improved Control Strategy for DFIG Low Voltage Ride-Through Using Optimal Demagnetizing method," 2019 10th International Power Electronics, Drive Systems and Technologies Conference (PEDSTC), Shiraz, Iran, 2019, pp. 464-469, {{doi\|10.1109/PEDSTC.2019.8697267}}.</ref> Second, the control of the rotor voltages and currents enables the induction machine to remain ] 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.
	Without practical invention, pioneering experts have only theorized that stable excitation control of the rotor armature for a true BWRSDF must be '''a real-time''' (i.e., instantaneous) '''emulation control''' ('''RTEC''') method that without delay, automatically and sensorlessly compensates for the destabilizing ] effects from at least external shaft perturbations further complicated by the effects of slip-induction.<ref>“Direct Torque and Frequency Control of Double-Inverter-Fed Slip-Ring Induction Motor Drive,” Gautam Poddar and V. T. Ranganathan, IEEE Transactions On Industrial Electronics, VOL. 51, NO. 6, December 2004, pp 1329-1337. http://ieeexplore.ieee.org/xpl/periodicals.jsp</ref><ref name="cycloconverter526"/><ref>“Parametric Pump-Down of Synchronous Machine Oscillations,” D. W. Novotny and N. L. Schmitz, AIEE Great Lakes District Meeting, Fort Wayne, Ind., April 25–27, 1962. Page 652-657.</ref> Very different from RTEC, today's state-of-art ] rely on the imprecision and costly time delays of '''simulation''' control methods that sequentially measure speed and position, then estimate the rotor time constant, and finally, synthesize the excitation frequency and waveform, all by an offline digital electronic processor, which further aggravate the instability and slip-induction issues as a result of control delays and imprecisions that have prevented the realization of a true BWRSDF electric machine in the past. For brushless operation, rotor multiphase power cannot propagate through the traditional multiphase slip-ring assembly with its issues of additional cost, efficiency, size, maintenance, suitability, and reliability.

			==See also==
	Without including the multiphase slip-ring assembly or RTEC, the basic symmetrical topology and relationships of the BWRSDF are the classic introductory textbook study of electric machines and by simply de-optimizing their high performance with asymmetry, the relationships of the BWRSDF become the study for all other electric machines, including permanent magnet, induction, and reluctance electric machines. So although the attractive performance attributes of the BWRSDF have been studied and understood for decades, the formidable invention of a practical brushless RTEC ('''BRTEC''') has not been forthcoming and as a result, the BWRSDF has been kept from practical application until recently.
			*]

	==References==		==References==
Line 133:		Line 47:

	==External links==		==External links==
	* {{cite journal\|last=Dufour\|first=Christian\|author2=Bélanger, Jean\|year=2004\|url=http://www.opal-rt.com/sites/default/files/technical_papers/dufour_windturbinepaper2004.pdf\|title= Real-Time Simulation of Doubly Fed Induction Generator for Wind Turbine Applications}}		* {{cite journal \|last=Dufour \|first=Christian \|author2=Bélanger, Jean \|year=2004 \|url=http://www.opal-rt.com/sites/default/files/technical_papers/dufour_windturbinepaper2004.pdf \|title=Real-Time Simulation of Doubly Fed Induction Generator for Wind Turbine Applications \|access-date=2011-02-17 \|archive-url=https://web.archive.org/web/20110105201507/http://www.opal-rt.com/sites/default/files/technical_papers/dufour_windturbinepaper2004.pdf \|archive-date=2011-01-05 \|url-status=dead }}
	* {{cite journal\|url=http://www-control.eng.cam.ac.uk/~pcr20/papers/roberts_BDFM_dissertation_2005.pdf\|title=Study of Brushless Doubly-Fed (Induction) Machines; Contributions in Machine Analysis, Design and Control\|first=Paul C. \|last=Roberts\|year=2004\|publisher=Emmanuel College, University of Cambridge}}		* {{cite journal \|url=http://www-control.eng.cam.ac.uk/~pcr20/papers/roberts_BDFM_dissertation_2005.pdf \|title=Study of Brushless Doubly-Fed (Induction) Machines; Contributions in Machine Analysis, Design and Control \|first=Paul C. \|last=Roberts \|year=2004 \|publisher=Emmanuel College, University of Cambridge \|url-status=dead \|archive-url=https://web.archive.org/web/20130319104947/http://www-control.eng.cam.ac.uk/~pcr20/papers/roberts_BDFM_dissertation_2005.pdf \|archive-date=2013-03-19 }}
	{{electric motor}}		{{electric motor}}

Latest revision as of 19:36, 14 December 2024

Electric motor

Doubly fed electric machines, Doubly fed induction generator (DFIG), or slip-ring generators, 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. Additionally, DFIG-based wind turbines offer the ability to control active and reactive power.

Introduction

Doubly fed electrical generators are similar to AC electrical generators, but have additional features which allow them to run at speeds slightly above or below their natural synchronous speed. This is useful for large variable speed wind turbines, because wind speed can change suddenly. When a gust of wind hits a wind turbine, the blades try to speed up, but a synchronous generator is locked to the speed of the power grid and cannot speed up. So large forces are developed in the hub, gearbox, and generator as the power grid pushes back. This causes wear and damage to the mechanism. If the turbine is allowed to speed up immediately when hit by a wind gust, the stresses are lower with the power from the wind gust still being converted to useful electricity.

One approach to allowing wind turbine speed to vary is to accept whatever frequency the generator produces, convert it to DC, and then convert it to AC at the desired output frequency using an inverter. This is common for small house and farm wind turbines. But the inverters required for megawatt-scale wind turbines are large and expensive.

Doubly fed generators are another solution to this problem. Instead of the usual field winding fed with DC, and an armature winding where the generated electricity comes out, there are two three-phase windings, one stationary and one rotating, both separately connected to equipment outside the generator. Thus, the term doubly fed is used for this kind of machines.

One winding is directly connected to the output, and produces 3-phase AC power at the desired grid frequency. The other winding (traditionally called the field, but here both windings can be outputs) is connected to 3-phase AC power at variable frequency. This input power is adjusted in frequency and phase to compensate for changes in speed of the turbine.

Adjusting the frequency and phase requires an AC to DC to AC converter. This is usually constructed from very large IGBT semiconductors. The converter is bidirectional, and can pass power in either direction. Power can flow from this winding as well as from the output winding.

History

With its origins in wound rotor induction motors with multiphase winding sets on the rotor and stator, respectively, which were 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. Cycloconverter powered machines can also 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.

Doubly fed induction generator

Doubly fed induction generator (DFIG), 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, 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 stator windings are connected to the grid and rotor winding are connected to the converter 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 insulated-gate bipolar transistors 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 it is otherwise 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

"Generators for wind turbines Standard slip ring generator series for doubly-fed concept from 1.5-3.5 MW" (PDF). ABB. 2014. Retrieved April 24, 2018.
M. J. Harandi, S. G. Liasi and M. T. Bina, "Compensating Stator Transient Flux during Symmetric and Asymmetric Faults using Virtual Flux based on Demagnetizing Current in DFIG Wind Turbines," 2019 International Power System Conference (PSC), Tehran, Iran, 2019, pp. 181-187, doi:10.1109/PSC49016.2019.9081565.
M. Niraula and L. Maharjan, “Variable stator frequency control of stand-alone DFIG with diode rectified output”, 5th International symposium on environment-friendly energies and applications (EFEA), 2018.
S. MÜLLER; S.; et al. (2002). "Doubly Fed Induction Generator Systems for Wind Turbines" (PDF). IEEE Industry Applications Magazine. 8 (3). IEEE: 26–33. doi:10.1109/2943.999610.
L. Wei, R. J. Kerkman, R. A. Lukaszewski, H. Lu and Z. Yuan, "Analysis of IGBT power cycling capabilities used in Doubly Fed Induction Generator wind power system," 2010 IEEE Energy Conversion Congress and Exposition, Atlanta, GA, 2010, pp. 3076-3083, doi:10.1109/ECCE.2010.5618396.
"Power electronics - Engineering and Technology History Wiki". ethw.org.
Leonhard, W.: Control of Electrical Drives. 2nd Ed. Springer 1996, 420 pages. ISBN 3-540-59380-2.
Shively, E. K.; Whitlow, Geo. S. (1932). "Automatic Control for Variable Ratio Frequency Converters". Transactions of the American Institute of Electrical Engineers. 51: 121–127. doi:10.1109/T-AIEE.1932.5056029. S2CID 51636516.
Liwschitz, M. M.; Kilgore, L. A. (1942). "A Study of the Modified Kramer or Asynchronous-Synchronous Cascade Variable-Speed Drive". Transactions of the American Institute of Electrical Engineers. 61 (5): 255–260. doi:10.1109/T-AIEE.1942.5058524. S2CID 51642497.
Pfeiffer, A.; Scheidl, W.; Eitzmann, M.; Larsen, E. (1997). "Modern rotary converters for railway applications". Proceedings of the 1997 IEEE/ASME Joint Railroad Conference. pp. 29–33. doi:10.1109/RRCON.1997.581349. ISBN 0-7803-3854-5. S2CID 110505314.
A. Bocquel, J. Janning: 4*300 MW variable speed drive for pump-storage plant application. EPE Conference 2003, Toulouse.
"Overview of research and development status of brushless doubly-fed machine system". Chinese Journal of Electrical Engineering. 2 (2). Chinese Society for Electrical Engineering. December 2016.
U.S. patent 6,448,735
Niiranen, Jouko (2008). "About the active and reactive power measurements in unsymmetrical voltage dip ride-through testing". Wind Energy. 11 (1): 121–131. Bibcode:2008WiEn...11..121N. doi:10.1002/we.254.
^ M. J. Harandi, S. Ghaseminejad Liasi, E. Nikravesh and M. T. Bina, "An Improved Control Strategy for DFIG Low Voltage Ride-Through Using Optimal Demagnetizing method," 2019 10th International Power Electronics, Drive Systems and Technologies Conference (PEDSTC), Shiraz, Iran, 2019, pp. 464-469, doi:10.1109/PEDSTC.2019.8697267.
an active crowbar: for example U.S. patent 7,164,562
Seman, Slavomir; Niiranen, Jouko; Virtanen, Reijo; Matsinen, Jari-Pekka (2008). "Low voltage ride-through analysis of 2 MW DFIG wind turbine - grid code compliance validations". 2008 IEEE Power and Energy Society General Meeting - Conversion and Delivery of Electrical Energy in the 21st Century. pp. 1–6. doi:10.1109/PES.2008.4596687. ISBN 978-1-4244-1905-0. S2CID 41973249.

External links

Dufour, Christian; Bélanger, Jean (2004). "Real-Time Simulation of Doubly Fed Induction Generator for Wind Turbine Applications" (PDF). Archived from the original (PDF) on 2011-01-05. Retrieved 2011-02-17. {{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. Archived from the original (PDF) on 2013-03-19. {{cite journal}}: Cite journal requires |journal= (help)

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