A CONVERTER-BASED STARTING METHOD OF A DIRECT POWER
CONTROL FOR A DOUBLY FED INDUCTION MACHINE WITH
CENTRIFUGAL LOADS
1
J. SURESH, 2V. L. N. SASTRY
1
P.G scholar, Department of EEE, 2Assistant Professor, Department of EEE
Sasi Institute of Technology &Engineering, Tadepalligudem, A.P, India
E-mail: 1Jupalli.suresh292@gmail.com, 2sastry@sasi.ac.in
Abstract— Now a day’s adjustable speed drives has grasped more attractive usage for applications like pump, compressor,
and other centrifugal load applications rather than conventional constant speed drives due to their high efficiency and
flexibility of operation. In general to start up a Doubly Fed Induction Machine (DFIM) an additional starting resistors or an
auto transformer is needed. In this paper a converter-based starting method for a DFIM with centrifugal loads by constant
volts/hertz control is proposed, instead of using the additional starting resistors or autotransformer. It is also discussed a grid
synchronization control scheme for motor starting and a Direct power control scheme for the operation of machine in highspeed region. Hence it is stated that the above said machine is operated in the whole speed region effectively by combining
constant volts/hertz control and direct power control. Finally the proposed control technique is simulated in MATLAB/
Simulink environment and the results are produced.
Keywords— Diect Power Control (DPC), V/F control, Grid synchronization, DFIM, Centrifugal loads.
different control strategies can be applied and the
motor can be started successfully by a power
converter. Second part is setting speed region which
is above the synchronous speed where the control
scheme is implemented easily with a standard one
quadrant motor drive converter. The control strategies
for motor starting, grid synchronization and vectorcontrolled speed regulation are developed based on
the machine mathematical model. For the machine
with different turn’s ratios between the stator and
rotor windings, it is preferable to start the machine
from the lower voltage side, thus reducing the
converter voltage rating. The starting methods from
the rotor side and the stator side are both developed in
the paper The rest of the paper is organized as
follows. Section 2 introduces the Doubly Fed
Induction Machine (DFIM). Section 3 presents the
control sequences and modeling of the Doubly Fed
Induction Machine. Section 4 presents the grid
synchronization control and Direct Power Control of
the Doubly Fed Ease of Use Induction Machine. In
Section 5 Direct Power controller simulink models
and simulation results are presented, finally in section
6 conclusion presented.
I. INTODUCTION
The main feature of the DFIM is to operate the
converter at a slip power range, which is a fraction of
the total power. The DFIM operated at low speed
region can facilitate the converter to process at slip
power which enables the converter to drive
centrifugal loads like pumps, compressors and fans.
In literature most of the authors proposes the control
of slip power recovery circuit topologies which uses
either current fed dc link converter or a cyclo
converter in the rotor circuit. And the authors also
named them as Scherbius and Static Kramer circuit.
Voltage source pulse width modulated converters are
used to overcome the disadvantages of the above said
two topologies. In recent years research include
sensor less drive, brushless doubly fed machine
design, and the use of doubly fed induction
generators for wind power applications. To handle the
slip power within the small setting speed range
around synchronous speed slip power recovery circuit
is widely used. For the speed outside the preset speed
range, the converter loses its control capability.
Therefore, the machine is started by using either
additional starting resistors or autotransformer to
accelerate to the setting speed range; then, the power
converter takes over the speed control of the machine.
Obviously, the use of starting resistors and the
autotransformer increases the system size and cost.
This paper, presents an alternative method to start the
machine without the help of starting resistors and the
autotransformer by constant volts/hertz control. Since
the power converter is rated to operate the machine
with its slip power and the starting method with
constant volt/hertz will provide limited torque at a
low speed range. The total speed range is divided into
two parts, first part is the synchronous speed where
II. DOUBLY FED INDUCTION MACHINE
(DFIM)
The basic system configuration is shown in
Fig.1.TheDFIM stator winding is connected to the
grid through a switchKM1, while the rotor winding is
connected to the grid via two three-phase voltage-fed
converters. In this paper, in order to adopt a standard
one-quadrant motor drive converter topology and
reduce the system cost and control complexity, a
rotor side PWM converter and a grid-side diodebridge converter are used. Therefore, the DFIM
Proceedings of 11th IRF International Conference, 8th May 2016, Hyderabad, India, ISBN: 978-93-86083-09-8
36
A Converter-Based Starting Method of a Direct Power Control For a Doubly Fed Induction Machine With Centrifugal Loads
setting-speed region is designed above the machine
synchronous speed as shown in Fig. 2, and the slip
power only flows from the rotor to the grid. A dc
generator with resistive load is used here to emulate
the centrifugal load with shaft connection to the
DFIM. Note that, in Fig. 1, the grid-side diode-bridge
converter may reduce the system power factor,
although the converter power is only a small portion
of the whole system power (slip power). Meanwhile,
the rotor side power electronic converter can
introduce the switching harmonics to the DFIM and
grid. Certain filter inductance may be added at the
DFIM rotor side to meet the harmonic requirements
of the DFIM and grid
III. CONTROL
MODELLING
SEQUENCES
AND
axes of the stator and rotor stationary coordinates.
Respectively. and
are the stator flux angle and
the rotor position angle. In the following derivation,
all the variables are transformed to the stator side.
With a decoupled controlled between the
electromagnetic torque and the excitation current
DFIM
3.1. Control Sequences
The control for the whole speed range operation of
the machine can be divided into three parts: lowspeed region
Fig. 2 Typical power–speed curve for centrifugal
loads is achieved. The stator and rotor fluxes can be
Fig.1 Block diagram of DFIM
operation, grid synchronization, and setting-speed
region operation. Initially, in Fig. 1, switch KM1 is
open, and KM2 is closed. The machine is started and
accelerated from the rotor side like an induction
machine using constant volts/hertz control. Here,
low-speed region means the speed range below the
synchronous speed (ω1) as shown in Fig. 2, and the
constant volts/hertz control is applied. The settingspeed region is above the synchronous speed
(between ω1 and ω3 in Fig. 2), and ω3 is set based on
the load required operating range. After the machine
speed enters the setting-speed region and reaches ω2
(a speed higher than the synchronous speed), the grid
synchronization process starts, and KM2 is open; the
machine stator voltage attempts to track the grid
voltage. Then, KM1 closes, and the Direct Power
Control scheme takes over, regulating the DFIM
speed in the setting speed region.
3.2 DFIM Modeling
The speed regulation of the DFIM in the settingspeed region adopts the stator-field-oriented vector
control scheme, where the reference frame rotates
synchronously with respect to the stator flux, with the
d-axis aligned to the stator flux position as shown in
Fig. 5. In Fig. 5, ,
, and are the - and βProceedings of 11th IRF International Conference, 8th May 2016, Hyderabad, India, ISBN: 978-93-86083-09-8
37
A Converter-Based Starting Method of a Direct Power Control For a Doubly Fed Induction Machine With Centrifugal Loads
Fig. 3 Grid Synchronization Control of DFIM
of several milliseconds with the converter switching
frequency of several kilohertz. During grid
synchronization, due to the large system inertia, the
DFIM speed will fall down slowly (level of several
seconds) but will not fall below the synchronous
speed. Therefore, there is enough time for the grid
synchronization process, which can be finished
within several milliseconds. Hence, the grid
synchronization process is fast enough for the control
purpose in the synchronization process, the DFIM
stator voltage amplitude and angle are compared with
the grid voltage amplitude and angle. The error
between the two will be recorded and used to
determine whether the condition for the grid
connection is met.
In practice, the voltage amplitude and angle error
between the DFIM stator and grid will be sampled
multiple times, e.g.,50 times, and when 40 of those
errors are within a certain range, then the
synchronization is successful, and the DFIM statorside switch KM1 in Fig. 1 is closed. Otherwise,
another sample cycle is needed. The number of
sample points and the threshold value will be
determined by the field noise level and Sensitivity. It
should be noted that, during grid synchronization, the
rotor-side voltage equation turns to be which is a little
different from therefore, the proportional and integral
gains for the current loop controller should be
modified accordingly
IV. GRID SYNCHRONIZATION CONTROL
AND DIRECT POWER CONTROL
4.1 Grid Synchronization Control
The grid synchronization is used for the transition
from the constant volts/hertz control in the low-speed
region to the vector control in the high-speed region.
When the DFIM is accelerated over the synchronous
speed and reaches ω2 (a speed higher than the
synchronous speed), the grid synchronization process
starts. Here, the rotor-side PWM will first be disabled
to let the stator current reduce to a certain level,
allowing switch KM2 to open. Then, the rotor-side
PWM will be re-enabled to start the grid
synchronization. During this period, the DFIM is
freely rotating, and the DFIM stator voltage attempts
to track the grid voltage. In order to achieve “soft
connection” to the grid, avoiding the inrush current,
the DFIM stator voltage amplitude, angle, and
frequency should match the grid voltage during the
synchronization process. The stator voltage amplitude
is determined by the magnetizing current. Assuming
that all the magnetizing current
is provided from
the rotor side, then
= , and the reference value
of
is given by
The voltage angle and. The rotor position is usually
obtained through the shaft encoder. The machine
speed will not fall below the synchronous speed
during synchronization. The time needed for the grid
synchronization process depends on the current
control loop bandwidth, which is usually in the range
Proceedings of 11th IRF International Conference, 8th May 2016, Hyderabad, India, ISBN: 978-93-86083-09-8
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A Converter-Based Starting Method of a Direct Power Control For a Doubly Fed Induction Machine With Centrifugal Loads
4.2 Direct power control
Fig.4 direct power control of DFIM
The current vector is decomposed into the
components of the stator active and reactive power in
synchronous reference frame. This decouples the
active power control from the reactive power control.
The stator active and reactive power references are
determined by the maximum power point tracking
strategy and the grid requirements, respectively. The
phase angle of the stator flux space vector is usually
used for the controller synchronization. Therefore in
this paper, the stator voltage oriented frame is used
for the controller synchronization. In order to extract
the synchronization signal from the stator voltage
signal, a simple phase locked loop system used. The
stator active power and reactive power are expressed
as
So the stator active and reactive power controlled
through ids and iqs respectively. Block diagram of
the direct power control of DFIM as shown in fig 4.
V. MATLAB/SIMULINK
SIMULATION RESULTS
MODELS
AND
5.1 MATLAB/Simulink models
The proposed starting method and speed regulation of
the DFIM system are tested through simulation. The
simulation is carried out in MATLAB/Simulink with
the diagram shown in Fig. 1, and the centrifugal load
characteristics are considered in the model. The
system parameters are shown in Table I.
The simulation results are shown in Fig7.Fig8(c)
Shows the starting speed curve of the DFIM system
with centrifugal loads. The system can be
successfully started by the constant volts/hertz control
and accelerated above the synchronous speed. After
grid synchronization, the DFIM speed is regulated by
the direct power control scheme. Fig. 7 shows the
grid voltage, stator voltage, and rotor current during
the grid synchronization
As the svof is used for the controllers’
synchronization vqs vanishes and the stator active
and reactive power equations are simplified to
According to the stator flux equations in the
synchronous frame in this condition, the stator
currents can be written as
Fig.5 DFIM simulink model
Proceedings of 11th IRF International Conference, 8th May 2016, Hyderabad, India, ISBN: 978-93-86083-09-8
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A Converter-Based Starting Method of a Direct Power Control For a Doubly Fed Induction Machine With Centrifugal Loads
As shown, the DFIM stator voltage can track the grid
voltage in terms of voltage amplitude and angle. At t
= 1 s, the condition for grid connection is met, and
the grid-side switch KM1 is closed. The DFIM enters
the direct power control mode. Fig. 8 shows the direct
power controlled speed regulation (above the
synchronous speed) with consideration of the
centrifugal load characteristics. The DFIM speed is
well controlled, and the currents are sinusoidal. The
aforementioned simulation results validate the
proposed starting method and speed regulation
method
Fig.7 (d).rotor current of direct power control of DFIM
Fig.6 Direct power Controller simulink model
TABLE I: DFIM simulation Parameters
Fig.8 (a) active power
5.2 Simulation Results
Fig.8 (b) reactive power
Fig. 8 (d) speed control of direct power controlled DFIM,
CONCLUSION
This paper has presented a converter-based adjustable
speed drive to allow the DFIM to operate in the full
speed range the machine can be successfully started
and accelerated by the converter without the help of
Proceedings of 11th IRF International Conference, 8th May 2016, Hyderabad, India, ISBN: 978-93-86083-09-8
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A Converter-Based Starting Method of a Direct Power Control For a Doubly Fed Induction Machine With Centrifugal Loads
starting resistors or an autotransformer. Based on a
centrifugal load characteristic, it is possible to utilize
a partially rated power converter to start the motor by
constant volts/hertz control in the low-speed region
and regulate the speed with the direct power control
scheme in the setting-speed region. With the grid
synchronization method in this paper, the machine
can switch smoothly between different modes without
inrush current. It is possible to start the machine from
the low-voltage side, thus reducing the converter
voltage rating by controlling the switches in a certain
sequence The starting scheme can also be used for
other DFIM-driven applications which have small
load or no load in the low speed range
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