3rd International Conference on Electrical, Electronics, Engineering Trends, Communication, Optimization and Sciences (EEECOS)-2016
A Converter-Based Starting Method of a Direct power control for a Doubly
Fed Induction Machine with Centrifugal Loads
J. Suresh1
V. L. N. Sastry2
P.G scholar
Assistant Professor
Department of EEE
Sasi Institute of Technology & Engineering
Tadepalligudem, A.P, India
Jupalli.suresh292@gmail.com
Department of EEE
Sasi Institute of Technology & Engineering
Tadepalligudem, A.P, India
sastry@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 high-speed
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—Direct Power Control (DPC), V/F control, Grid
synchronization, DFIM, Centrifugal loads.
1.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 cycloconverter 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 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 vector-controlled 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.
2. 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 diode-bridge
converter are used. Therefore, the DFIM 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
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3rd International Conference on Electrical, Electronics, Engineering Trends, Communication, Optimization and Sciences (EEECOS)-2016
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
3. Control sequences and DFIM modelling
Low speed region
Setting speed region
Synchronous speed
Load Power
Power
(p.u)
Converter Power
3.1Control Sequences
The control for the whole speed range operation of the
machine can be divided into three parts: low-speed region
0
Grid
Speed (p.u)
W1
W2
W3
Fig. 2 Typical power–speed curve for centrifugal loads
KM 2
KM 1
is achieved. The stator and rotor fluxes can be given by
LOAD
Rotor
side
conver
ter
G
Grid
side
convert
er
(1)
(2)
DFIM
DC Generator
for load purpose
(3)
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 setting-speed 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 setting-speed
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 αand β-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
(4)
Where
and are the stator and rotor
along the d- and q-axes, respectively, and
and are the stator and rotor currents along the dand q-axes, respectively. and are the stator and rotor selfinductances, and is the mutual inductance;
is the
magnetizing current. is the leakage factor, and
fluxes
The stator- and rotor-side voltage equations in the d−q
frame can be expressed
(5)
(6)
(7)
(8)
Where
and
are the d- and q-axis rotor voltages,
respectively, is the grid angular frequency, and
is the slip
angular frequency.
and are the stator and rotor resistances,
respectively. Substituting the rotor flux with yield
(9)
(10)
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PI
dq/
/
abc
PI
=0
P
W
M
Rotor
side
converter
/ dq
Encoder
D
F
I
M
Voltage
calc.
KM 1
Decoupling
term calc.
Stator flux
calc.
GRID
Fig. 3 Grid Synchronization Control of DFIM
during synchronization. The time needed for the grid
synchronization process depends on the current control loop
bandwidth, which is usually in the range of several
milliseconds with the converter switching frequency of several
kilohertz. During grid synchronization, due to the large system
The grid synchronization is used for the transition
inertia, the DFIM speed will fall down slowly (level of several
from the constant volts/hertz control in the low-speed region
seconds) but will not fall below the synchronous speed.
to the vector control in the high-speed region. When the DFIM
Therefore, there is enough time for the grid synchronization
is accelerated over the synchronous speed and reaches ω2 (a process, which can be finished within several milliseconds.
speed higher than the synchronous speed), the grid Hence, the grid synchronization process is fast enough for the
synchronization process starts. Here, the rotor-side PWM will control purpose in the synchronization process, the DFIM
first be disabled to let the stator current reduce to a certain stator voltage amplitude and angle are compared with the grid
level, allowing switch KM2 to open. Then, the rotor-side voltage amplitude and angle. The error between the two will be
PWM will be re-enabled to start the grid synchronization. recorded and used to determine whether the condition for the
During this period, the DFIM is freely rotating, and the DFIM grid connection is met.
stator voltage attempts to track the grid voltage. In order to
In practice, the voltage amplitude and angle error
achieve “soft connection” to the grid, avoiding the inrush between the DFIM stator and grid will be sampled multiple
current, the DFIM stator voltage amplitude, angle, and times, e.g.,50 times, and when 40 of those errors are within a
frequency should match the grid voltage during the certain range, then the synchronization is successful, and the
synchronization process. The stator voltage amplitude is DFIM stator-side switch KM1 in Fig. 1 is closed. Otherwise,
determined by the magnetizing current. Assuming that all the another sample cycle is needed. The number of sample points
magnetizing current isprovided from the rotor side, and the threshold value will be determined by the field noise
then
, and the reference value of is given by
level and Sensitivity. It should be noted that, during grid
synchronization, the rotor-side voltage equation turns to be
(11)
which is a little different from therefore, the proportional and
integral gains for the current loop controller should be
modified accordingly
The voltage angle and frequency matching between
the grid and DFIM stator is guaranteed by accurately
(12)
measuring the grid angle and the rotor position. The rotor
(13)
position is usually obtained through the shaft encoder. The
4. Grid Synchronization Control and Direct
Power Control
4.1 Grid Synchronization Control
machine speed will not fall below the synchronous speed
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3rd International Conference on Electrical, Electronics, Engineering Trends, Communication, Optimization and Sciences (EEECOS)-2016
4.2 Direct power control
dq/
abc
PI
PI
PI
Rotor
side
converter
PWM
PI
abc/
dq
D
F
I
M
Encoder
Power calculation
PLL
abc/dq
Grid
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
(
)
(
)
(14)
(
)
(
)
(15)
As the svof is used for the controllers’ synchronization vqs
vanishes and the stator active and reactive power equations are
simplified to
(16)
(17)
According to the stator flux equations in the synchronous
frame in this condition, the stator currents can be written as
(18)
Solving a above equations
(
)
(19)
(20)
(
(21)
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.
5. MATLAB/Simulink models and Simulation
Results
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
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3rd International Conference on Electrical, Electronics, Engineering Trends, Communication, Optimization and Sciences (EEECOS)-2016
speed is regulated by the direct power control scheme. Fig. 7
shows the grid voltage, stator voltage, and rotor current during
the grid synchronization
TABLE I
DFIM simulation Parameters
Phase to phase
voltage
Stator resistance
Mutual
inductance
Proportional gain
0.178
Ω
0.071
H
0.069
H
2.944
Inertia constant
0.685
Stator inductance
Fig.5 DFIM simulink model
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
660 v
Rated stator
Current
Rotor resistance
Rotor inductance
Synchronous
speed
Integral gain
Friction factor
54 A
0.176
Ω
0.070
H
3000 H
299
0.01
5.2 Simulation Results
Fig.7 (a).stator voltage
Fig.7 (b).stator current
Fig.6 Direct power Controller simulink model
Fig.7 (c).Grid voltage
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3rd International Conference on Electrical, Electronics, Engineering Trends, Communication, Optimization and Sciences (EEECOS)-2016
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
Fig.7 (d).rotor current of direct power control of DFIM
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by the converter without the help of starting resistors or an
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