Sustainable Energy Technologies and Assessments 8 (2014) 172–180
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Sustainable Energy Technologies and Assessments
journal homepage: www.elsevier.com/locate/seta
Original Research Article
Application of variable frequency transformer (VFT) for grid
interconnection of PMSG based wind energy generation system
Farhad Ilahi Bakhsh ⇑, Dheeraj Kumar Khatod
Alternate Hydro Energy Centre, Indian Institute of Technology Roorkee, Roorkee 247667, India
a r t i c l e
i n f o
Article history:
Received 30 April 2014
Revised 26 August 2014
Accepted 9 September 2014
Keywords:
Grid interconnection
Permanent magnet synchronous generator
(PMSG)
Power flow
Variable frequency transformer (VFT)
Wind energy conversion system (WECS)
a b s t r a c t
Conventional grid connected permanent magnet synchronous generator (PMSG) based wind turbine
generator (WTG) incorporates sophisticated power electronic control systems which produces harmonics
and deteriorates the quality of the power supply. Recently, a new technology i.e. variable frequency
transformer (VFT) has emerged as a flexible ac link to transfer power in-between asynchronous power
grids. Hence, this paper aims to explore the possibility of grid integration of PMSG based WTG using
VFT. The proposed scheme does not employ any power electronic based interface for grid integration
of PMSG based WTG. To validate the proposed scheme, a simulation model has been developed under
MATLAB/Simulink environment. A series of studies on power flow from the PMSG to the grid using
VFT at different PMSG speeds has been carried out with this model. Moreover, the response characteristics of power transfer plots under various torque conditions are obtained. Further the efficiency, THD of
output voltage and current have been compared with the conventional system. The proposed system is
having almost the same efficiency but with negligible THD. The cost analysis of both systems has also
been carried out. From the cost analysis, it is observed that the proposed system is cheaper than conventional system.
Ó 2014 Elsevier Ltd. All rights reserved.
Introduction
Nowadays, wind energy has emerged as one of the most promising renewable energy resource. With the priority status accorded
to it in many countries, the share of wind power in relation to overall installed capacity has increased significantly and in some countries, the share of wind in relation to the overall installed capacity
is already approaching the 50% mark [1]. It is predicted that by
2020 up to 12% of the world’s electricity would be supplied from
wind power [2].
In wind energy conversion systems (WECS), there are two
operating modes of wind turbine generator (WTG) i.e. fixed-speed
and variable-speed operating modes. In fixed-speed operation,
wind turbines are equipped with a generator (mostly induction
type) connected directly to the grid. Since the frequency of the grid
is fixed, the speed of the turbine is controlled by the gearbox gears
ratio and by the number of poles of the generator. In order to
increase the amount of output power, some fixed-speed turbine
designs are equipped with a two speed generator and this way they
can operate at two different speeds. Though the use of induction
generator offers several advantages, such as robust design, simple
⇑ Corresponding author.
http://dx.doi.org/10.1016/j.seta.2014.09.003
2213-1388/Ó 2014 Elsevier Ltd. All rights reserved.
construction and relatively small price, it has also some major disadvantages, such high starting current and demand for reactive
power [3]. Variable-speed wind turbines have many advantages
over fixed-speed generation such as increased energy capture,
operation at maximum power point, improved efficiency and
power quality. Hence, variable-speed wind turbines are today the
dominating type of turbines. In this scheme, the generator (either
synchronous or induction) is connected to the grid by an electronic
converter system which incorporates sophisticated power electronic control systems. Moreover, these power electronic converters cause harmonic distortion and thereby deteriorate the quality
of power supplied. Further, they require suitable compensation
in order to meet the standards for harmonic pollution which further increases the cost and complexity of the system [4,5].
On the other hand, the permanent-magnet synchronous
generator (PMSG) is one of the frequently used generators in the
wind energy generation system. It is one of the best solutions for
small-scale wind power plants. Low-speed multi-pole PMSG are
maintenance-free and may be used in different climate conditions.
A conventional megawatt-scale wind power plant consists of a lowspeed wind turbine rotor, gearbox, and high-speed electric generator [6]. The use of a gearbox causes many technological problems in
a wind power plant, as it demands regular maintenance, increases
F.I. Bakhsh, D.K. Khatod / Sustainable Energy Technologies and Assessments 8 (2014) 172–180
the weight of the wind plant, generates noise, and increases power
losses [7]. These problems may be avoided using a direct-drive lowspeed PMSG.
The interaction between the wind energy generation system and
the grid is an important aspect in the planning of WECS. It is essential to ensure that the grid is capable of staying within the operational limits of frequency and voltage for all foreseen combination
of WECS and consumer load, and to keep, at the same time the grid
transient stability [8]. The biggest problem faced during integration
of the wind energy generation system into the power system or grid
is the quality of power. The wind based power generation fluctuates
over wide range. Thus, supplying power to ac grid is often repeatedly disconnected due to reduced input power (low wind speed)
which reduces the speed below its cut-in speed. This condition is
frequent, in case of renewable energy based systems (RES), e.g.
wind turbine based power generation, where the level of wind
speed does not remain constant and therefore repeated disconnection, islanding and re-switching become common. Thus, to avoid
these conditions which ultimately cause adverse impacts on the
power quality of the ac grid; it is desirable that the generator should
be kept connected to the ac grid, in spite of the low power available
at the input or from the prime mover side to avoid re-switching of
the generator. Injecting wind power generated to the grid through
power electronic conversion system can effectively reduce power
fluctuations. However, power electronic conversion is complicated
due to the need of closely coordinate harmonic filtering, controls,
and reactive compensation. Moreover, power electronic conversion
system need conversion plants at both sides which increases cost
and undesirably require significant space due to the large number
of switches and filter banks [9].
However, inherent power fluctuations and the grid interconnection of wind energy systems are among some of the major challenges faced by wind turbine power generation companies. These
above challenges can be answered well by using a new power
transmission technology named variable frequency transformer
(VFT) [10–12]. Recent studies have indicated that the application
of VFT in wind power generation could provide effective wind
power utilization at small as well as large scale. In the paper VFT
is used for grid interconnection of PMSG based wind energy generation system. Here PMSG is connected to the gird using VFT irrespective of the level of PMSG speed. Thus, avoids the conditions
for repeated disconnection, islanding and re-switching of the
PMSG which ultimately causes stress on the grid. Further, the
requirement of costly rectifier and an inverter is omitted. No frequency control is required. Moreover, the proposed method is simple and does not produce harmonics. This paper has been
organised in VII Sections. The Section II of the paper describes
the conventional PMSG based WECS. Section III discusses the modeling and analysis of the proposed PMSG based WECS. Sections IV
and V presents the MATLAB simulation model and results of the
conventional system and the proposed system, respectively. Section VI draws the conclusion of the paper. Finally, Section VII discusses the future aspects of the proposed system.
Conventional PMSG based WECS
In a conventional PMSG based WECS, the wind turbine is connected to the rotor of the PMSG with or without gear box. The output power of the PMSG is fed to the power system or grid through a
power electronic conversion system. The power electronic conversion system consists of an ac-to-dc rectifier followed by a dc-to-ac
inverter as shown in Fig. 1. Since the output of power electronic
conversion system produces harmonics therefore, a filter is used
to reduce the total harmonic distortion (THD) and thus improves
the quality of power supply. In fact a control system is required
173
for gate pulse control of IGBT’s used in the power electronic conversion system.
Proposed system
In the proposed system, VFT is used as a flexible ac link for connecting PMSG to the power system or grid. Here, the output power
of the PMSG is fed to the power system or grid through a VFT. The
wind turbine is connected to the rotor of the PMSG without gear
box. The stator winding of the PMSG is connected to the rotor
winding of the WRIM. The stator winding of the WRIM is connected to the power system or grid as shown in Fig. 2. A dc drive
motor is used to apply torque at the rotor of WRIM in order to control the power flow from PMSG to the power system or grid.
Modeling of proposed system
In the modeling, the VFT is modeled as a doubly-fed wound
rotor induction machine (WRIM) mechanically coupled to the dc
drive motor. A dc drive motor is used to apply torque, Td to the
rotor of the WRIM which adjusts the position of the rotor relative
to the stator. The three phase windings are provided on both stator
side and rotor side of the WRIM. The grid is connected to the stator
winding of the WRIM, energized by voltage, Vg with phase angle,
hg. The stator winding of the PMSG is connected to the rotor winding of the WRIM, energized by voltage, Vp with phase angle, hp. The
rotor of the PMSG is mechanically coupled with the dc motor,
where dc motor is working as prime mover of the PMSG in order
to simulate the wind turbine as shown in Fig. 3. In order to transfer
power from PMSG to grid, the rotor of the WRIM is externally
rotated by applying dc motor torque. The power flow through
VFT is proportional to the magnitude of the applied torque
[13–15]. Here, in the power flow process, only real power transfer
has being discussed.
Analysis of proposed system
The power flow through VFT can be approximated as follow
[14]:
PVFT ¼ Pmax sin hnet
ð1Þ
where, PVFT is the power flow through VFT (from PMSG to grid), and
Pmax the maximum theoretical power flow possible through the VFT
will occurs when the net angle hnet is near 90°. The Pmax is given by:
Pmax ¼
Vp Vg
X pg
ð2Þ
where, Vp is the voltage magnitude of PMSG on rotor side, Vg the
voltage magnitude of grid on stator side and Xpg the total reactance
between PMSG and grid terminals.
Also
hnet ¼ hg ðhp þ hpg Þ
ð3Þ
where, hp is the phase-angle of PMSG voltage on rotor, with respect
to a reference phasor, hg is phase-angle of grid voltage on stator,
with respect to a reference phasor and hpg the phase-angle of the
machine rotor due to hp with respect to stator due to hg.
Thus, the power flow through the VFT is given by:
PVFT ¼
Vp Vg
sinðhg ðhp þ hpg Þ
X pg
ð4Þ
For stable operation, the angle hnet must have an absolute value
significantly less than 90°. The power transmission or power flow
will be limited to a fraction of the maximum theoretical level given
in (2). Here, the power transmission equations are analyzed based
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Fig. 1. The conventional PMSG based WECS.
where, Pg is the electrical power out of the stator windings i.e. into
the grid, Pp is electrical power into the rotor windings i.e. output of
PMSG and Pd is mechanical power from the torque-control dc drive
motor.
Since the machine behaves like a transformer, the ampere-turns
must balance between stator and rotor:
N s Ig ¼ N r I p
ð6Þ
where, Ns is the number of turns on stator winding, Nr is number of
turns on rotor winding, Ig is current out of the stator winding and Ip
the current into the rotor winding.
Both the stator and rotor windings link the same magnetic flux,
therefore
Fig. 2. Proposed system for PMSG based WECS.
on assumption that VFT is an ideal and lossless machine, with negligible leakage reactance and magnetizing current. The power balance equation requires that the electrical power flowing out of the
stator winding must flow into the combined electrical path on the
rotor winding and the mechanical path to the dc drive motor, i.e.
Pg ¼ Pp þ Pd
ð5Þ
V g ¼ Ns f g wa ;
ð7Þ
V p ¼ Nr f p wa ;
ð8Þ
and V p =Nr ¼ V g =Ns f p =f g
ð9Þ
where, fg is the frequency of voltage on stator winding (Hz), fp is frequency of voltage on rotor winding (Hz), and wa the air-gap flux.
The nature of the machine is such that in steady state, the rotor
speed is proportional to the difference in the frequency (electrical)
on the stator and rotor windings,
f rm ¼ f g f p ;
ð10Þ
and xrm ¼ f rm 120=Np
ð11Þ
Fig. 3. Model representation of the proposed system.
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Fig. 4. MATLAB Simulink model of the conventional system.
Table 1
Power fed to the grid at different input speed of PMSG.
p
PMSG speed (rpm)
PMSG output power (Pp) in W
Power fed to grid (Pg) in W
% Efficiency ¼ pg 100
3600
3000
2400
1500
2445
2135
1811
1180
2249
2010
1674
1045
91.98
94.14
92.43
88.56
p
where, frm is the rotor mechanical speed in electrical frequency
(Hz), Np is number of poles in the machine, and xrm the rotor
mechanical speed in rpm.
Combining the above relationships gives the power exchanged
with the drive system as
It shows that the drive torque ‘Td’ is proportional to the grid current and the air gap flux. Since the VFT will operate almost near
constant flux, thus the drive torque is proportional to the grid current only.
Pd ¼ Pg Pp
MATLAB analysis of conventional system
¼ V g Ig V p Ip
¼ V g Ig ðNr V g =Ns f p =f g Þ ðNs Ig =Nr Þ
MATLAB simulation model
¼ V g Ig ð1 f p =f g Þ
or; Pd ¼ Pg ð1 f p =f g Þ
ð12Þ
It shows that the electrical power flowing into the grid being
proportional to mechanical power of the dc drive motor, grid frequency and PMSG frequency. Hence, if the grid frequency and
PMSG frequency are kept constant, then the electrical power flowing into the grid is being only proportional to mechanical power of
the dc drive motor.
Also
T d ¼ Pd =f rm
¼ V g Ig ð1 f p =f g Þ=ðf g f p Þ
¼ V g Ig =f g
¼ Ns f g wa Ig =f g
¼ Ns wa Ig
ð13Þ
Fig. 4 shows the model of the conventional system in MATLAB
Simulink. For simulation of conventional system in MATLAB, PMSG
is simulated with the permanent magnet synchronous machine SI
units with rating 26.13 Nm, 560 V dc, 3000 rpm, 27.3 Nm. The
grid is simulated with three-phase source. The three-phase source
is connected to inverter side of the power electronic converter system and permanent magnet synchronous machine is connected to
rectifier side of the power electronic converter system. Here, the
universal diode bridge is used as a rectifier and universal insulated
gate bipolar junction transistor (IGBT) bridge is used as an inverter
with pulse width modulation (PWM) scheme. The grid is kept at
400 V (L–L) and 50 Hz and the PMSG is kept at different levels of
input speed. To simulate various power transfer functions, other
blocks are also used. Then this simulated model, as shown in
Fig. 4, is used to analyze the performance of the conventional
method.
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Fig. 5. Waveforms showing rectifier output voltage, inverter output voltage and current without and with filter at 3000 rpm.
Fig. 6. MATLAB Simulink model for the proposed system.
F.I. Bakhsh, D.K. Khatod / Sustainable Energy Technologies and Assessments 8 (2014) 172–180
177
Table 2
Power fed to the grid at 3000 rpm.
Dc
motor
torque
(Nm)
Dc
motor
input
power,
Pd (W)
PMSG
output
voltage
(V)
PMSG
output
power,
Pp (W)
Power
fed into
grid, Pg
(W)
% Efficiency ¼ p
0
2
4
6
8
10
12
14
0
62.35
100.9
128.8
151.7
171.7
189.6
205.9
159.4
159.4
159.6
159.3
159.7
159.3
159.7
159.3
2136
2137
2137
2137
2134
2138
2138
2133
31.57
344.7
661.2
967.1
1259
1566
1894
2191
1.48
15.67
29.54
42.68
55.08
67.80
81.37
93.67
pg
d þpp
100
MATLAB simulation results
Table 1 shows the power fed to grid by conventional PMSG
based WECS at different values of input PMSG speed. It is evident
from this table that the efficiency of the conventional system
increases with the speed of PMSG and gets a maximum value of
around 94% at rated speed. If the speed of PMSG is further
increased, the efficiency starts decreasing.
Fig. 5 shows the different waveforms of the conventional system with or without filter when PMSG is operated at rated speed.
It is clear from Fig. 5 that the rectifier output voltage becomes
Fig. 8. The power fed to grid at 3000 rpm with the applied torque.
almost constant at 0.015 s. However, the inverter output voltage
without filter is not sinusoidal. Therefore, filter is required to bring
the waveform close to sinusoidal. With filter, the waveform of the
inverter output voltage becomes almost sinusoidal. Similarly, the
waveform of inverter output current is more close to sinusoidal
with filter as compared to that without filter.
Fig. 7. Waveforms showing PMSG (speed, output power) and grid (phase ‘A’ voltage, input power) at 10 Nm.
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Table 3
Power fed to the grid at 3600 rpm.
Dc
motor
torque
(Nm)
Dc
motor
input
power,
Pd (W)
PMSG
output
voltage
(V)
PMSG
output
power,
Pp (W)
Power
fed
into
grid,
Pg (W)
% Efficiency ¼ p
0
2
4
6
8
10
12
14
16
0
62.35
100.9
128.8
151.7
171.7
189.6
205.9
221.1
191.1
191.2
191.3
191.1
191.3
191.0
191.4
191.0
191.4
2445
2446
2446
2445
2443
2447
2450
2440
2445
37
331.6
646.6
951.6
1247
1556
1880
2173
2446
1.51
13.22
25.39
36.97
48.06
59.42
71.22
82.13
91.75
pg
d þpp
100
Fig. 10. The power fed to the grid at 2400 rpm with the applied torque.
Table 5
Power fed into the grid at 1500 rpm.
Fig. 9. The power fed to the grid at 3600 rpm with the applied torque.
Table 4
Power fed into the grid at 2400 rpm.
Dc
motor
torque
(Nm)
Dc
motor
input
power,
Pd (W)
PMSG
output
voltage
(V)
PMSG
output
power,
Pp (W)
Power
fed into
grid, Pg
(W)
% Efficiency ¼ p
0
2
4
6
8
10
11.5
0
62.35
100.9
128.8
151.7
171.7
185.3
128
127.7
128.1
127.7
128.1
127.8
128
1811
1811
1812
1812
1809
1811
1814
51.43
363.5
682.3
991.4
1279
1579
1845
2.84
19.40
35.67
51.08
65.23
79.64
92.28
pg
d þpp
100
Dc
motor
torque
(Nm)
Dc
motor
input
power,
Pd (W)
PMSG
output
voltage
(V)
PMSG
output
power,
Pp (W)
Power
fed into
grid, Pg
(W)
% Efficiency ¼ p
0
2
4
6
7
0
62.35
100.9
128.8
140.7
81.41
81.18
81.10
81.44
81.08
1180
1179
1180
1180
1177
92.79
399.9
720.1
1044
1163
7.86
32.21
56.22
79.77
88.26
pg
d þpp
100
4 kW, 400 V, 50 Hz WRIM. The grid is simulated with three-phase
source. The three-phase source is connected to the stator side of
WRIM and PMSG (26.13 Nm, 560 V dc, 3000 rpm, 27.3 Nm) is
connected to rotor side of WRIM as shown in Fig. 6. The drive
motor is simulated with dc series motor of 0.5 HP and the output
torque of the dc motor is applied to the rotor of WRIM as mechanical torque Tm. The grid is kept at 400 V (L–L) and 50 Hz and the
PMSG is kept at different levels of the input speed. To simulate various power transfer functions, other blocks are also used. Then this
simulated model, as shown in Fig. 6, is used to analyze the performance of the proposed system.
MATLAB simulation results
MATLAB analysis of proposed system
MATLAB simulation model
For simulation in MATLAB, VFT is simulated with the asynchronous machine SI units which is basically a doubly-fed, 3 phase,
At rated speed
The PMSG is operated at rated speed i.e. 3000 rpm and the
speed is kept same for all operating conditions. The grid is kept
at 400 V (L–L) and 50 Hz. Then the power flow into the grid under
different torque conditions applied to the VFT are obtained and
presented in Table 2.
It is evident from this Table 2 that when the torque applied to
the VFT is zero, the power flow through the VFT is very small
(almost negligible). As the torque applied to the VFT increases,
the magnitude of power flow from PMSG side to the grid side also
increases. When a torque of 14 Nm is applied, then the magnitude
of power fed to grid is more than the output of the PMSG. This is
because some amount of power from the dc motor of the VFT is
fed to grid. Moreover, almost maximum power is fed into the grid
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In this case also, the efficiency is increasing with increase in dc
motor torque. The plot of power fed to grid at different values of dc
motor torque applied to the VFT is shown in Fig. 9.
Below rated speed
The PMSG is operated at 2400 rpm and 1500 rpm i.e. below
rated speed. Here, again the speed is kept same for all operating
conditions and the grid is kept at 400 V (L–L), 50 Hz.
At 2400 rpm
Table 4 gives the power fed to the grid at 2400 rpm under different values of VFT torque. A plot between the VFT torque and
the power fed to grid is shown in Fig. 10.
Fig. 11. The power fed to the grid at 1500 rpm with the applied torque.
at this value of torque. Thus, by controlling the applied torque of
VFT, the proposed system can operate at maximum efficiency.
The output waveforms at 10 Nm dc motor torque are shown in
Fig. 7.
It is clear from Fig. 7 that the VFT output voltage or grid voltage
is almost sinusoidal and contains negligible harmonics. The power
flow into the grid with the applied torque achieved in Table 2 is
shown in Fig. 8.
Above rated speed
The grid is kept at 400 V (L–L) and 50 Hz. The PMSG is operated
at 3600 rpm i.e. above rated speed. At this speed, the power flow
into the grid under different values of the dc motor torque applied
to the VFT are obtained and given in Table 3.
At 1500 rpm
Table 5 gives the power fed to the grid under different values of
torque applied to the VFT at 1500 rpm. A plot between the power
fed to grid and the torque applied to the VFT is shown in Fig. 11.
From Tables 2–5 it is evident that the efficiency increases with
the applied dc motor torque to the VFT. However, the output
power of the PMSG remains constant. Further, from Figs. 8–11 it
is clear that the power fed to the grid is proportional to the torque
applied to the VFT. Thus, from Tables 2–5 and Figs. 8–11 it is concluded that VFT can be used as a flexible ac link to connect the
PMSG with the grid when wind speed or speed of the PMSG is at,
above or below synchronous speed. Here, PMSG is connected to
the grid through VFT at different input speeds i.e. at different frequencies without synchronization. Hence, no frequency control is
required and thus it avoids the conditions for repeated disconnection, islanding and re-switching of the alternator which ultimately
causes stress on the grid.
The efficiency, THD of output voltage and THD of output current
of proposed system have been compared with the conventional
system in Table 6.
From Table 6, it is clear that in conventional system the THD of
inverter output voltage without filter is very high which is not
Table 6
Comparison of simulation results.
PMSG
speed
(rpm)
Conventional system
% THD of inverter
output voltage
without filter
% THD of inverter
output current
without filter
% THD of inverter
output voltage with
filter
% THD of inverter
output current with
filter
Efficiency
in %
Proposed system
% THD of VFT
output
voltage
% THD of VFT
output
current
Efficiency
in %
3600
3000
2400
1500
163.32
139.88
110.54
101.51
3.07
3.12
3.44
5.42
0.90
0.89
0.92
1.04
1.82
1.94
2.36
3.97
91.98
94.14
92.43
88.55
0.02
0.08
0.03
0.01
0.52
0.57
0.20
0.43
91.75
93.67
92.28
88.26
Table 7
Comparison of cost (kEuro).
Equipments
Iron (ton)
Copper (ton)
PM (ton)
Total (ton)
PMSG active material cost (kEuro)
PMSG construction cost (kEuro)
WRIM active material cost (kEuro)
Dc motor active material cost (kEuro)
(WRIM + Dc motor) construction cost (kEuro)
Gearbox cost (kEuro)
Converter cost (kEuro)
Total cost (kEuro)
Direct-drive PMSG
PMSG with single stage gearbox
Conventional system
Proposed system
Conventional system
Proposed system
18.1
4.3
1.7
24.1
162
150
–
–
23.47
5.91
1.7
31.08
162
150
30
10
40
–
–
392
18.1
4.3
0.41
24.1
43
50
–
–
23.47
5.91
0.41
31.08
43
50
30
10
40
120
–
293
–
120
432
120
120
333
180
F.I. Bakhsh, D.K. Khatod / Sustainable Energy Technologies and Assessments 8 (2014) 172–180
within permissible limits. Moreover, it decreases with the decrease
in PMSG speed. The THD of the inverter output current without filter
is within permissible limit, but at 1500 rpm it is not within permissible limits. Moreover, it increases with the decrease in PMSG speed.
So, as we reduce the PMSG speed the THD of inverter output voltage
reduces but the THD of inverter output current increases. Hence, filter is required in order to bring the THD within permissible limits.
After filter both THD’s are within permissible limits. Further, the efficiency of conventional system is maximum at rated speed and it
decreases by either increasing or decreasing the PMSG speed.
Whereas, in proposed system the THD of VFT output voltage and current are very less than the conventional system and is within permissible limits. Thus, no filter is required. Moreover, the efficiency
of proposed system is also maximum at rated speed and it decreases
by either increasing or decreasing the PMSG speed, but a little bit
smaller than the conventional system which is less than 0.5%
(almost equal). Hence, from Table 6 we conclude that the proposed
system is having almost same efficiency but with negligible THD.
Further, the cost analysis of both the systems have been done
for 3 MW rating in Table 7. The data for the cost analysis has been
taken from [16].
From Table 7, it is clear that the proposed system is cheaper
then the conventional system by 40 kEuro.
Conclusion
This paper presents the possibility of grid interconnection of
permanent magnet synchronous generator (PMSG) based wind
turbine generator (WTG) using variable frequency transformer
(VFT). For this purpose, a number of simulation studies have been
carried out on power flow from PMSG to the grid. From the simulated results, it is evident that VFT is viable technology to connect
the PMSG to the grid. It can transfer power at different levels of
PMSG speed without synchronization. Hence, no frequency control
is required. Further, the requirement of power electronic converter
system is omitted. Thus, it does not produce harmonics. Moreover,
the real power transfer is directly proportional to the torque
applied at the rotor of the VFT and the magnitude of the power
transfer is controllable by the applied torque. Hence, the proposed
system can be operated at maximum efficiency by controlling the
applied torque. The response characteristics of power transfer plots
under various torque conditions are obtained. The MATLAB/Simulink model developed is successfully used to demonstrate the real
power flow from the PMSG to the grid.
In conventional system, at rated speed i.e. 3000 rpm the THD of
inverter output voltage is 139.88% which is very much. Thus, filter
is required to make it within permissible limit which increases the
cost of the system. After filter it becomes 0.89% which is within
permissible limit, but with the proposed system the THD of output
voltage is 0.08% which is around 10 times lesser than the conventional method with filter. Hence, no filter is required as in case of
conventional system. At rated speed, the THD of inverter output
current without filter (3.12%) and with filter (1.94%) is within permissible limit, but with the proposed system it is very less i.e.
0.57%. At the same time the efficiency of conventional system
(94.14%) is little bit higher than the proposed system (93.67%)
i.e. 0.47% which is less than 0.5% (almost equal). Similar observations are found at other speeds also.
From the obtained results, it is concluded that the proposed system have almost same efficiency but with negligible THD. Further,
cost analysis has also been carried out for the proposed system and
it has been compared with that of conventional system. From the
cost analysis, it is observed that the proposed system is cheaper
than conventional system.
Future aspects
The proposed system can be analyzed for large power rating
(MW range). Fault and closed loop analysis of the proposed system
can be performed. Analysis of proposed system with single stage
gearbox (incorporating wind turbine model) can also be performed. Practical setup can be installed for actual realization of
the system.
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