FIELD OF THE INVENTION
The present invention is related to a method and an apparatus for operation of
Doubly Fed Induction Generator ( DFIG). More particularly, the invention relates
to a low cost operational method of doubly fed induction generators( DFIG)
without use of power electronic devices to achieve high power density, high
torque potential and high efficiency.
BACKGROUND OF THE INVENTION
Doubly fed electric machines are electric motors or electric generators that have
windings on both stationary and rotating parts, wherein both windings transfer
significant power between generator shaft and the electrical system. Usually the
stator winding is directly connected to the three-phase grid and the rotor winding
is fed from the grid through a rotating or static frequency converter.
Doubly Fed Induction Generator ( DFIG) 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. Brushless doubly fed induction machines
incorporate a poor electromagnetic design that compromises physical size, cost,
and electrical efficiency, to chiefly avoid a multiphase slip ring assembly.

A 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 means to
independently power the rotor winding set with a multiphase AC power. For
stable operation, the frequency and phase of the multiphase AC power must be
synchronized and instantaneously adjusted to the stator excitation frequency
including the speed and position of the shaft, which is particularly difficult at
synchronous speed where induction no longer exists.
It is to be noted that 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.
In all the above configurations, energy conversion systems generally having
rectifier and inverter devices, to overcome the problems associated with these
systems. Moreover, such energy conversion systems for example, the variable
speed wind turbine generators providing the source of energy to utility power
grids, require a matched constant output frequency at preferably optimum power
output. Thus, the variable frequency AC from such turbine generators must be
converted to a constant frequency AC for use by the utility power grid. Generally,
this conversion is accomplished through an intermediate conversion from
variable frequency AC to DC by a rectifier, and subsequent inversion to a fixed-
frequency AC by means of an inverter. Unfortunately, such prior art systems are

insufficient, relatively expensive, difficult to maintain, and relatively unreliable as
an electricity source.
During expensive research and experimentation, the present inventors
recognized that it can be overcome with low cost solutions, then high power
density, low cost, high efficiency, and high torque can be realized.
US 7579702 entitled 'Electric power converting device and power converting
method for controlling doubly-fed induction generator' teaches a synchronous
generator for generating auxiliary electric power independently of a doubly-fed
induction generator so as to generate electricity even in a system power-free
environment, wherein a grid-side converter is composed of a three-phase four-
wire converter so as to generate a balanced voltage even in an unbalanced load
condition and automatically synchronize a stator voltage of a doubly-fed
induction generator and a system voltage with each other.
US 7629705 entitled ' Method and apparatus for operating electrical machines',
teaches a wound rotor, synchronous, 60 Hz, three phase, doubly-fed induction
generator (DFIG), comprising a generator stator magnetically coupled to a
generator rotor. The control system consists of Converters on stator as well as
rotor side connected to the grid and related electronic hardware. The control
system facilitates the electrical machine remaining electrically connected to the

grid during and subsequent to voltage disturbances on the grid beyond a
predetermined range for an undetermined period of time.
US 7411309 entitled 'Control system for doubly fed induction generator;,
discloses a controller for a doubly fed induction generator which adjusts control
signals to a rotor side converter and line side converter to adjust rotor current
when a voltage transient on a utility grid occurs, so that the doubly fed induction
generator can ride through the transient. The controller can also turn off the
transistors of the rotor side converter to reduce rotor current and/or activate a
crowbar to reduce the voltage of the DC link connecting the converters when
significant voltage transients occur on the grid. This permits continued operation
of the DFIG system without disconnecting from the grid.
US 6278211 entitled 'Brushless doubly-fed induction machines employing dual
cage rotors;, describes a brushless twin stator squirrel cage induction generators
(BTSIG) wherein first and second generating machines are connected at their
rotors by connecting the conductors in the first cage rotor to the conductors in
the second cage rotor in a reverse phase sequence. This system introduces
assembly and mechanical difficulties.
US 8193654 entitled 'Variable speed power generator having two induction
generators on a common shaft', teaches a variable speed power generator
having two induction motor rotors placed in a single common housing on the

same shaft namely the primary and the secondary, while the stator of the
primary machine is connected to the mains; the stator of secondary rotor is fed
from a power electronic converter to control the speed and power factor of the
primary machine. The machine, hereinafter termed a brushless doubly-fed twin
stator induction generator (BDFTSIG) can operate in a synchronous mode
wherein a single frequency current passes through both sections of the common
rotor.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a low cost operational
method of doubly fed induction generators( DFIG) without use of power
electronic devices to achieve high power density, high torque and high efficiency.
Another object of the invention is to propose a low cost operational method of
doubly fed induction generators( DFIG) without use of power electronic devices
to achieve high density, high torque and high efficiency, which uses a phase-
shifting transformer to introduce a change in the supply voltage and phase.

A still another object of the invention is to propose a low cost operational
method of doubly fed induction generators( DFIG) without use of power
electronic devices to achieve high power density, high torque and high
efficiency, which uses a step-up transformer to obtain a change in voltage
magnitude.
A further object of the invention is to propose a low cost operational method of
doubly fed induction generators( DFIG) without use of power electronic devices
to achieve high power density, high torque and high efficiency in which an on-
load tap changer in combination with a programmable logic controller (PLC) is
used to change the tapings of the transformer and obtain desired magnitude and
phase shift.
SUMMARY OF THE INVENTION
Accordingly, there is provided a low cost operational method of doubly fed
induction generators( DFIG) without use of power electronic devices to achieve
high power density, high torque and high efficiency, comprising the steps of
providing a DFIG machine having three phase windings in the stator and rotor,
the stator winding and the rotor windings being respectively fed from mains and
a low-frequency source via a set of slip rings matching the slip-frequency of the
machine, providing an auxiliary slip-ring type induction machine coupled to the
DFIG machine; a phase shifting transformer (PST) connected to the stator of the

auxiliary machine and receiving power from the mains at a voltage and
frequency identical to that supplied to the DFIG, wherein the PST comprises two
windings in each phase which are connected in a zig-zag configuration to effect a
phase change, and a plurality of tapings on the winding to control the phase
angle, wherein the magnitude of the voltage fed to the stator of the auxiliary
induction motor is controlled by a step-up transformer having tapings on its
winding, wherein said controlling of phase angle and magnitude of the voltage
fed to the auxiliary induction motor is effected by an on board tap changer in
combination with a programmable logic controller (PLC), and wherein a slip
frequency voltage depending on said phase angle, voltage magnitude, and speed
is induced in the rotor windings of the auxiliary induction motor which in turn is
fed to the rotor windings of the DFIG machine to allow corresponding change in
the speed and power factor of the DFIG machine.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - shows a schematic test device illustrating the connection of a doubly
fed induction generator (DFIG).
Figure 2 - shows a phasor diagram of a phase shifting transformer (PST) of the
(DFIG).
Figure 3 - shows an electrical connection network of the PST, on board tap
changer and a step up transformer of the (DFIG).
Figure 4 - shows a resultant phasor diagram of the PST.

DETAIL DESCRIPTION OF THE INVENTION
CONTROL OF DFIG USING A PHASE SHIFTING TRANSFORMER
A DFIG machine consists of three phase windings in the stator and rotor. While
the stator winding is fed from the mains, the rotor windings are accessed
through set of slip rings and are fed from a low frequency source which is
matched to the slip-frequency of the DFIG machine. In this scheme the three-
phase rotor windings are connected, by means of a set of slip rings, to an
auxiliary slip-ring type induction machine which is coupled to the main machine
with the help of suitable couplings. The control of the auxiliary induction machine
is effected by feeding its stator winding using a phase shifting transformer (PST).
The phase shifting transformer is fed from the normal mains supply of the same
frequency and voltage that is fed to the main DFIG machine. The PST introduces
a phase shift in the supply voltage and alters the voltage magnitudes as
required. Depending upon the voltage magnitude, the phase-angle and speed,
the slip-frequency voltage is induced in the rotor winding of the auxiliary
machine, which is fed to the rotor windings of the main DFIG machine with the
help of slip-rings. A pre-determined shift in phase and magnitude of the voltage
changes the speed of DFIG and its power-factor. The connection scheme used in
test bed is shown in figure 1.

PHASE SHIFTING TRANSFORMER (PST):
The purpose of using a phase shifting transformer (PST) is to introduce a change
in the supply voltage and phase. The working of the PST is explained here. In
order to get a phase shift in the R-phase voltage, a negative B-phase voltage is
introduced in the R-phase voltage. The sum of the two voltages results in a shift
in phase and magnitude of the R-phase voltage as explained in the phasor
diagram (Figure 2).
Utilizing the same principle, the phase shift of all the phases are effected. In
order to obtain a change in voltage magnitude and isolation, a step-up
transformer is used, The schematic diagram of the complete PST along with the
step-up transformer is shown in (Figure 2).
The voltage and phase shift depends on the tapings used in the transformer. An
on-load tap changer in combination with a known programmable logic controller
(PLC) is used to change the transformer tapings and obtain the required
magnitude and phase shift. The theoretical calculations are shown in the test
result in the vector diagram is shown in Figure 4.

In the present invention, the main machine's rotor windings are accessed
through slip-rings. The main machine is a known DFIG machine. Another
auxiliary machine of slip-ring induction motor type, which is approximately, 20%
of the rating of the main machine, is coupled to the main machine and the slip-
rings of the two machines are inter-connected as shown in Figure 1. Further, the
stator of the auxiliary machine is fed from a phase shifting transformer, and the
speed and power-factor of the main machine is suitably controlled by controlling
the tapings of the phase shifting transformer. In this manner the use of power-
electronic devices are avoided. The three major items namely the main DFIG
machine, the auxiliary slip-ring induction machine, the phase shifting transformer
and the step-up transformer are electro-mechanical components which are
reliable and are commonly manufactured machines.

WE CLAIM
1. A low cost operational method of doubly fed induction generators( DFIG)
without use of power electronic devices to achieve high power density,
high torque and high efficiency, comprising the steps of:
providing a DFIG machine having three phase windings in the stator and
rotor, the stator winding and the rotor windings being respectively fed
from mains and a low-frequency source via a set of slip rings matching
the slip-frequency of the machine;
providing an auxiliary slip-ring type induction machine coupled to the
DFIG machine;
a phase shifting transformer (PST) connected to the stator of the
auxiliary machine and receiving power from the mains at a voltage and
frequency identical to that supplied to the DFIG,
wherein the PST comprises two windings in each phase which are
connected in a zig-zag configuration to effect a phase change, and a
plurality of tapings on the winding to control the phase angle, wherein the
magnitude of the voltage fed to the stator of the auxiliary induction motor

is controlled by a step-up transformer having tapings on its winding,
wherein said controlling of phase angle and magnitude of the voltage fed
to the auxiliary induction motor is effected by an on board tap changer in
combination with a programmable logic controller (PLC), and wherein a
slip frequency voltage depending on said phase angle, voltage magnitude,
and speed is induced in the rotor windings of the auxiliary induction motor
which in turn is fed to the rotor windings of the DFIG machine to allow
corresponding change in the speed and power factor of the DFIG
machine.
2. The method as claimed in claim 1, wherein the PST introduces a
negative B-phase voltage to achieve a phase shift in the R-phase voltage,
and wherein the sum of the two voltages results in a shift in phase and
magnitude of the R-phase voltage.
3. The method as claimed in claim 1, wherein the rating of the
auxiliary Induction motor is approximately 20% of that of the DFIG
machine.

ABSTRACT

The invention relates to a low cost operational method of doubly fed induction
generators( DFIG) without use of power electronic devices to achieve high power
density, high torque and high efficiency, comprising the steps of providing a
DFIG machine having three phase windings in the stator and rotor, the stator
winding and the rotor windings being respectively fed from mains and a low
frequency source via a set of slip rings matching the slip-frequency of the
machine; providing an auxiliary slip-ring type induction machine coupled to the
DFIG machine; a phase shifting transformer (PST) connected to the stator of the
auxiliary machine and receiving power from the mains at a voltage and
frequency identical to that supplied to the DFIG, wherein the PST comprises two
windings in each phase which are connected in a zig-zag configuration to effect a
phase change, and a plurality of tapings on the winding to control the phase
angle, wherein the magnitude of the voltage fed to the stator of the auxiliary
induction motor is controlled by a step-up transformer having tapings on its
winding, wherein said controlling of phase angle and magnitude of the voltage
fed to the auxiliary induction motor is effected by an on board tap changer in
combination with a programmable logic controller (PLC), and wherein a slip
frequency voltage depending on said phase angle, voltage magnitude, and speed
is induced in the rotor windings of the auxiliary induction motor which in turn is
fed to the rotor windings of the DFIG machine to allow corresponding change in
the speed and power factor of the DFIG machine.