Doubly fed electric machines, Doubly fed induction generator (DFIG), or slip-ring generators, are
electric motor
An electric motor is a machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a electromagnetic coil, wire winding to gene ...
s or
electric generator
In electricity generation, a generator, also called an ''electric generator'', ''electrical generator'', and ''electromagnetic generator'' is an electromechanical device that converts mechanical energy to electrical energy for use in an externa ...
s, where both the
field magnet
Field magnet refers to a magnet used to produce a magnetic field in a device. It may be a permanent magnet or an electromagnet. When the field magnet is an electromagnet, it is referred to as a field coil.
Although the term usually refers to magne ...
windings and
armature windings are separately connected to equipment outside the machine.
By feeding adjustable frequency
AC power
In an electric circuit, instantaneous power is the time rate of flow of energy past a given point of the circuit. In alternating current circuits, energy storage elements such as inductors and capacitors may result in periodic reversals of the d ...
to the
field winding
A field coil is an electromagnet used to generate a magnetic field in an electro-magnetic machine, typically a rotating electrical machine such as a motor or generator. It consists of a coil of wire through which the field current flows.
In a ...
s, the
magnetic field
A magnetic field (sometimes called B-field) is a physical field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular ...
can be made to rotate, allowing variation in motor or generator speed. This is useful, for instance, for generators used in
wind turbine
A wind turbine is a device that wind power, converts the kinetic energy of wind into electrical energy. , hundreds of thousands of list of most powerful wind turbines, large turbines, in installations known as wind farms, were generating over ...
s. Additionally, DFIG-based wind turbines offer the ability to control
active and
reactive power
In an electric circuit, instantaneous power is the time rate of flow of energy past a given point of the circuit. In alternating current circuits, energy storage elements such as inductors and capacitors may result in periodic reversals of the ...
.
Introduction

Doubly fed electrical generators are similar to
AC electrical generator
In electricity generation, a generator, also called an ''electric generator'', ''electrical generator'', and ''electromagnetic generator'' is an electromechanical device that converts mechanical energy to electrical energy for use in an extern ...
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
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
''Power Grid'' is the English-language version of the second edition of the multiplayer German-style board game ''Funkenschlag'', designed by Friedemann Friese and first released in 2004. ''Power Grid'' was released by Rio Grande Games.
I ...
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
A power inverter, inverter, or invertor is a power electronic device or circuitry that changes direct current (DC) to alternating current (AC). The resulting AC frequency obtained depends on the particular device employed. Inverters do the op ...
. 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
A field coil is an electromagnet used to generate a magnetic field in an electro-magnetic machine, typically a rotating electrical machine such as a motor or generator. It consists of a coil of wire through which the field current flows.
In a ...
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
An insulated-gate bipolar transistor (IGBT) is a three-terminal power semiconductor device primarily forming an electronic switch. It was developed to combine high efficiency with fast switching. It consists of four alternating layers (NPNP) that ...
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
Nikola Tesla (;["Tesla"](_blank)
. ''Random House Webster's Unabridged Dictionary''. ; 10 July 1856 – 7 ...
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
A cycloconverter (CCV) or a cycloinverter converts a constant amplitude, constant frequency Alternating current, AC waveform to another AC waveform of a lower frequency by synthesizing the output waveform from segments of the AC supply without an ...
'' 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 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
An insulated-gate bipolar transistor (IGBT) is a three-terminal power semiconductor device primarily forming an electronic switch. It was developed to combine high efficiency with fast switching. It consists of four alternating layers (NPNP) that ...
'' 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 turbine
A wind turbine is a device that wind power, converts the kinetic energy of wind into electrical energy. , hundreds of thousands of list of most powerful wind turbines, large turbines, in installations known as wind farms, were generating over ...
s. It is based on an
induction generator
An induction generator (or asynchronous generator) is a type of alternating current (AC) electrical generator that uses the principles of induction motors to produce electric power. Induction generators operate by mechanically turning their roto ...
with a multiphase wound rotor and a multiphase
slip ring
A slip ring is an electromechanical device that allows the transmission of Electric power, power and electrical Signal, signals from a stationary to a rotating structure. A slip ring can be used in any Electromechanics, electromechanical system ...
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
Voltage, also known as (electrical) potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a Electrostatics, static electric field, it corresponds to the Work (electrical), ...
source converter that controls both the rotor and the grid currents. Thus
rotor
ROTOR was an elaborate air defence radar system built by the British Government in the early 1950s to counter possible attack by Soviet bombers. To get it operational as quickly as possible, it was initially made up primarily of WWII-era syst ...
frequency
Frequency is the number of occurrences of a repeating event per unit of time. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...
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
Vector control is any method to limit or eradicate the mammals, birds, insects or other arthropods (here collectively called " vectors") which transmit disease pathogens. The most frequent type of vector control is mosquito control using a varie ...
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 transistor
An insulated-gate bipolar transistor (IGBT) is a three-terminal power semiconductor device primarily forming an electronic switch. It was developed to combine high efficiency with fast switching. It consists of four alternating layers (NPNP) that ...
s and
diode
A diode is a two-Terminal (electronics), terminal electronic component that conducts electric current primarily in One-way traffic, one direction (asymmetric electrical conductance, conductance). It has low (ideally zero) Electrical resistance ...
s 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 20–60ms 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
In electrical power engineering, fault ride through (FRT), sometimes under-voltage ride through (UVRT), or low voltage ride through (LVRT), is the capability of electric generators to stay connected in short periods of lower electric network vol ...
, 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
In an electric circuit, instantaneous power is the time rate of flow of energy past a given point of the circuit. In alternating current circuits, energy storage elements such as inductors and capacitors may result in periodic reversals of the ...
. 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).[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, .] 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.
See also
*Variable-frequency transformer
A variable-frequency transformer (VFT) is used to transmit electricity between two ( asynchronous or synchronous) alternating current frequency domains. The VFT is a relatively recent development. Most asynchronous grid inter-ties use high-voltage ...
References
External links
*
*
{{DEFAULTSORT:Doubly Fed Electric Machine
Electric motors
Electrical generators