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Application of variable frequency transformer (VFT) for grid interconnection of PMSG based wind energy generation system
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
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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
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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.
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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
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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. References [1] Engelhardt S, Erlich I, Feltes C, Kretschmann J, Shewarega F. Reactive power capability of wind turbines based on doubly fed induction generators. IEEE Trans Energy Convers 2011;26(1):364–72. [2] DeMeo EA. 20% electricity from wind power: an overview. In: IEEE power and energy society general meeting-conversion and delivery of electrical energy in the 21st century, Pittsburgh, USA, 20–24 July, 2008. p. 1–3. [3] Bhadra SN, Kastha D, Banerjee S. Wind electrical systems. Oxford University Press; 2006. [4] Bhende CN, Mishra S, Malla SG. Permanent magnet synchronous generatorbased standalone wind energy supply system. IEEE Trans Sust Energy 2011;2(4):361–73. [5] Mi C, Filippa M, Shen J, Natarajan N. Modeling and control of a variable-speed constant-frequency synchronous generator with brushless exciter. IEEE Trans Ind Appl 2004;40(2):565–73. [6] Kilk A. Design and experimental verification of a multipole directly driven interior PM synchronous generator for wind power applications. In: 4th international electric power quality and supply reliability workshop, Pedase, Estonia, 2004. p. 87–9. [7] Westlake AJG, Bumby JR, Spooner E. Damping the power-angle oscillations of a permanent-magnet synchronous generator with particular reference to wind turbine applications. IEE Proc Electr Power Appl 1996;143(3):269–80. [8] Gaillard A, Poure P, Saadate S, Machmoum M. Variable speed DFIG wind energy system for power generation and harmonic current mitigation. Renewable Energy 2009;34(6):1545–53. [9] Chinchilla M, Arnaltes S, Burgos JC. Control of permanent-magnet generators applied to variable-speed wind-energy systems connected to the Grid. IEEE Trans Energy Convers 2006;21(1):130–5. [10] Larsen E, Piwko R, McLaren D, McNabb D, Granger M, Dusseault M, Rollin L-P, Primeau J. Variable frequency transformer – a new alternative for asynchronous power transfer. Toronto, Ontario, Canada: Canada Power; 2004. September 28–30. [11] Doyon P, McLaren D, White M, Li Y, Truman P, Larsen E, Wegner C, Pratico E, Piwko R. Development of a 100 MW variable frequency transformer. Toronto, Ontario, Canada: Canada Power; 2004. September 28–30. [12] Dusseault M, Gagnon JM, Galibois D, Granger M, McNabb D, Nadeau D, Primeau J, Fiset S, Larsen E, Drobniak G, McIntyre I, Pratico E, Wegner C. First VFT application and commissioning. Toronto, Ontario, Canada: Canada Power; 2004. September 28–30. [13] Bakhsh FI, Irshad M, Jamil Asghar MS. Modeling and simulation of variable frequency transformer for power transfer in-between power system networks. In: Indian International conference on power electronics (IICPE), Delhi, India, 28–30 January 2011. [14] Merkhouf A, Doyon P, Upadhyay S. Variable frequency transformer – concept and electromagnetic design evaluation. IEEE Trans Energy Convers 2008;23(4):989–96. [15] Bakhsh FI, Khatod DK. Digital simulation of VFT applications between power system networks. In: Fourth international joint conference on advances in engineering and technology, NCR, India, 13–14 December, 2013. p. 60–74. [16] Polinder H, van der Pijl FFA, de Vilder G-J, Tavner P. Comparison of direct-drive and geared generator concepts for wind turbines. IEEE Trans Energy Convers 2006;21(3):725–33.
Contents lists available at ScienceDirect
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
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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
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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.
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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. References [1] Engelhardt S, Erlich I, Feltes C, Kretschmann J, Shewarega F. Reactive power capability of wind turbines based on doubly fed induction generators. IEEE Trans Energy Convers 2011;26(1):364–72. [2] DeMeo EA. 20% electricity from wind power: an overview. In: IEEE power and energy society general meeting-conversion and delivery of electrical energy in the 21st century, Pittsburgh, USA, 20–24 July, 2008. p. 1–3. [3] Bhadra SN, Kastha D, Banerjee S. Wind electrical systems. Oxford University Press; 2006. [4] Bhende CN, Mishra S, Malla SG. Permanent magnet synchronous generatorbased standalone wind energy supply system. IEEE Trans Sust Energy 2011;2(4):361–73. [5] Mi C, Filippa M, Shen J, Natarajan N. Modeling and control of a variable-speed constant-frequency synchronous generator with brushless exciter. IEEE Trans Ind Appl 2004;40(2):565–73. [6] Kilk A. Design and experimental verification of a multipole directly driven interior PM synchronous generator for wind power applications. In: 4th international electric power quality and supply reliability workshop, Pedase, Estonia, 2004. p. 87–9. [7] Westlake AJG, Bumby JR, Spooner E. Damping the power-angle oscillations of a permanent-magnet synchronous generator with particular reference to wind turbine applications. IEE Proc Electr Power Appl 1996;143(3):269–80. [8] Gaillard A, Poure P, Saadate S, Machmoum M. Variable speed DFIG wind energy system for power generation and harmonic current mitigation. Renewable Energy 2009;34(6):1545–53. [9] Chinchilla M, Arnaltes S, Burgos JC. Control of permanent-magnet generators applied to variable-speed wind-energy systems connected to the Grid. IEEE Trans Energy Convers 2006;21(1):130–5. [10] Larsen E, Piwko R, McLaren D, McNabb D, Granger M, Dusseault M, Rollin L-P, Primeau J. Variable frequency transformer – a new alternative for asynchronous power transfer. Toronto, Ontario, Canada: Canada Power; 2004. September 28–30. [11] Doyon P, McLaren D, White M, Li Y, Truman P, Larsen E, Wegner C, Pratico E, Piwko R. Development of a 100 MW variable frequency transformer. Toronto, Ontario, Canada: Canada Power; 2004. September 28–30. [12] Dusseault M, Gagnon JM, Galibois D, Granger M, McNabb D, Nadeau D, Primeau J, Fiset S, Larsen E, Drobniak G, McIntyre I, Pratico E, Wegner C. First VFT application and commissioning. Toronto, Ontario, Canada: Canada Power; 2004. September 28–30. [13] Bakhsh FI, Irshad M, Jamil Asghar MS. Modeling and simulation of variable frequency transformer for power transfer in-between power system networks. In: Indian International conference on power electronics (IICPE), Delhi, India, 28–30 January 2011. [14] Merkhouf A, Doyon P, Upadhyay S. Variable frequency transformer – concept and electromagnetic design evaluation. IEEE Trans Energy Convers 2008;23(4):989–96. [15] Bakhsh FI, Khatod DK. Digital simulation of VFT applications between power system networks. In: Fourth international joint conference on advances in engineering and technology, NCR, India, 13–14 December, 2013. p. 60–74. [16] Polinder H, van der Pijl FFA, de Vilder G-J, Tavner P. Comparison of direct-drive and geared generator concepts for wind turbines. IEEE Trans Energy Convers 2006;21(3):725–33.