Doubly fed electric machines
are electric
motors or electric generators that have windings on both
stationary and rotating parts, where both windings transfer significant power
between shaft and electrical system. Usually the stator winding is directly
connected to the three-phase grid and the three-phase rotor winding is fed from
the grid through a rotating or static frequency converter.
Doubly fed machines are typically
used in applications that require varying speed of the machine's shaft in a
limited range around the synchronous speed, for example ± 30%, because the
power rating of the frequency converter is reduced similarly. Today doubly fed
drives are the most common variable speed wind turbine concept.
History
The doubly fed machine has its
origins in wound rotor induction motors with multiphase winding sets on the
rotor and stator, respectively, with the rotor winding set connected to
resistors via 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.
Classification
Electric machines are either Singly
Fed with one winding set that actively participates in the energy
conversion process or Doubly Fed with two active winding sets. Having
two electrical ports, many confuse the singly-fed slip-energy recovery
induction and the field-excited synchronous electric machines as doubly-fed;
however, the port of only one winding set is actively excited while the port of
the other winding set passes dissipative power for passive participation in the
energy conversion process.
Only practical with the
evolution of control technology, there are now three varieties of doubly fed
electric machine systems: 1) the Doubly Fed Induction Machine (DFIM), which is
the conventional
wound-rotor doubly fed electric machine 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 Machine (BDFIM), which is the brushless
doubly fed induction (or reluctance) electric machine 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) the Brushless
Doubly-Fed Synchronous Machine (BDFSM), which has the traditional
DFIM circuit topology with a rotor and stator active winding set but with a
brushless real time control method replacing the slip ring assembly and rotor
flux vector controller.
The symmetrical circuit
topology and operational relationships of the wound-rotor doubly-fed electric
machine core 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 active
winding sets (ports) excited with bi-directional power for practical operation
from sub-synchronous to super-synchronous speed without regions of
discontinuity, such as about synchronous speed.
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. In contrast, the BDFSM brushlessly propagates instantaneously
derived speed-synchronized multiphase excitation to the rotor winding set
without discontinuity and without relying on slip induction, although
slip-induction is experience beyond synchronous speed as in all doubly-fed
electric machines.
Features of doubly fed machines
The wound rotor doubly fed
electric machine is the only electric machine that can be operated with
rated torque to twice synchronous speed for a given frequency of excitation
(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 means that doubly fed
machines have lower cost per kW, higher efficiency, and higher power density.
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 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 power factor. However, by optimal current
sharing the total I2R 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 rotor 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 depending on the speed and torque. If the machine is producing torque and
operating as a motor, the rotor will generate power if the speed is 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 in both through the stator and rotor. As a consequence
the efficiency is now better than with singly fed motors. 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.
Doubly fed electric machines
outperform the others in supersynchronous speeds. They can operate at constant
torque to twice synchronous speed if each active winding is rated at half the
total power of the machine and can provide continuous operation between
sub-synchronous through supersynchronous speed range.
It is important to note,
however, that doubly fed machines do not produce more continuous rated torque
per volume than singly fed machines. 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 leads to core saturation. 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 frequency
converter, 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. Pioneering work of Drs. Albertson, Long, Novotny, and
Schmitz. 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 variable-frequency drive.
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 slip ring
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 Brushless
doubly fed induction electric machines).
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 comparable
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 compared to 3600 rpm @
60 Hz with 2 poles for other electric machines. In theory, the core volume
is nearly half the physical size compared
to other machines of similar rating.
Cost
Neglecting the slip ring
assembly, the theoretical system cost is nearly 50% less compared
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 is an abbreviation 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.
Brushless doubly fed versions
Brushless doubly fed induction
electric machine
Brushless doubly fed induction
electric machine is constructed by adjacently placing two multiphase winding
sets with unlike pole-pairs on the stator
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 magnetic
field 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 type of rotor assembly
determines if the machine is a reluctance
or induction doubly fed electric
machine. The constant torque speed range is always less than 1800 rpm @ 60 Hz 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 slip ring
assembly. Although brushless doubly fed electric machines have not seen commercial
success since their conception in the early 1970s, the promise of a low cost,
highly efficient electronic controller keeps the concept under perpetual study,
research, and development.
The brushless wound-rotor
doubly fed electric machine 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 control means to
independently power the rotor winding set with multiphase AC power. Since it
does not rely on slip for operation, it differs from brushless
doubly fed induction electric machines, which rely on very different
principles of unlike pole-pair induction for operation.
Without an independent means of
exciting the rotor winding set, the torque of the wound-rotor doubly fed
electric machine is dependent on both slip and position, which is a classic
condition for instability, and torque cannot be produced at synchronous
speed where slip ceases to exist. For stable operation, the
frequency and phase of the multiphase AC power must be instantaneously
synchronized and fixed to the stator excitation frequency and the speed and
position of the shaft, which is not trivial at any speed and particularly
difficult about synchronous speed where induction (or slip) no longer exists.
If these conditions are met without relying on induction, all the attractive
attributes of the synchronous wound-rotor doubly fed electric machine,
such as high power density, low cost, ultra-high efficiency, and ultra-high
torque potential are realized without the traditional slip-ring assembly and
instability problems.
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