Advantages of using a Switched Reluctance Generator (SRG) for wind energy applications

Transcription

1 Advantages of using a Switched Reluctance Generator (SRG) for wind energy alications Eleonora Darie, Costin Ceisca, Emanuel Darie Abstract Wind energy found to be one of the most useful solutions to hel in overcoming the air ollution and global. There is no agreed solution to conversion of wind energy to electrical energy. Climate change is a contemorary issue, and the international community has acceted the dangers of green house gas emissions. Renewable energy is one of the hot toics when it comes to dealing with green house gas emissions treatments. Wind generation is one of the renewable energy ower sources that hel in reducing the carbon dioxide from our atmoshere. Using of the wind energy has become increasingly imortant as a renewable energy source and therefore is an increasing interest in exloiting it using a Switched Reluctance Machine as a generator and otimizes its characteristics in this domain. This work analyzes the generator mode of the Switched Reluctance Machine in the direct couling to the turbine shaft and couled to the shaft through a gearbox. The rinciles of oeration of this machine are simle, well known and based on reluctance torque. The machine has a stator of wound-u salient oles that after energizing synchronized with the osition of the rotor develos a torque that tends to align the oles in a way that diminishes the reluctance in the magnetic circuit. Currently the synchronous and induction machines dominate the market of wind energy alications, although, the SRM has been the subect of current investigation and it shows to be a valid alternative for this field. Keywords: emissions, energy source, switching reluctance motor. Introduction Climate change is a contemorary issue, and the international community has acceted the dangers of green house gas emissions. Renewable energy is one of the hot toics when it comes to dealing with green house gas emissions treatments. Wind generation is one of the renewable energy ower sources that hel in reducing the carbon dioxide from our atmoshere. In the last decades the Switched Reluctance Machine (SRM) has become an imortant alternative in various alications in the industrial and domestic markets, namely as a motor showing good mechanical reliability, high torque-volume ratio and high efficiency, lus low cost. Although less evangelized as a generator, there are a few studies of its alication in the aeronautical industry and in integrated alications in wind based energy generators. The rinciles of oeration of this machine are simle, well known and based on reluctance torque. The machine has a stator of wound-u salient oles that after energizing synchronized with the osition of the rotor develos a torque that tends to align the oles in a way that diminishes the reluctance in the magnetic circuit []. Currently the synchronous and induction machines dominate the market of wind energy alications, although, the SRM has been the subect of current investigation and it shows to be a valid alternative for this field [], [3] and [4]. Comaring with the classical solutions of machines integrated in wind alications, a Switched Reluctance Generator (SRG) shows a simlified construction associated with the inexistence of ermanent magnets or conductors in the rotor, which results in lower manufacturing costs; in addition both the

2 machine and the ower converter are robust. The low inertia of the rotor allows the machine to resond to raid variations in the load. Associated with these characteristics, these machines have a control system that allows raid changes in the control strategy such that the erformance of the machine is otimized. The structure of the SRM is not as stiff as the synchronous machines and due to its flexible control system; it is caable of absorbing transient conditions, thus sulying more resilience to the mechanical system [7]. The machine has an inherent fault tolerance, esecially when under an oen-coil fault (in the windings) and in the ower converter (external faults) [4]. Under normal oeration, each hase of SRG is electrically and magnetically indeendent from others. The SRM is generally felt to be louder than conventional machines. However an adequate mechanical design can do a lot to imrove these figures and new control techniques (current control strategy with a torque reference) ermits further imrovements. Mode of oeration by SRG. SRG Characteristics In electrical drives with variable reluctance (Figure ), the torque is function of the regular osition of the rotor due the double salient oles. The oeration of the machine as a generator is obtained by energizing the windings of the stator when the salient oles of the rotor are away from their aligned osition due to the rotating motion of the rime mover. Figure : The Switched Reluctance Generator in the wind turbine. The SRM is characterized by the mode of controlling its hase current. For this roblem the ower electronic converter is used, which functions in a way that the hase currents of the machine are imosed for certain ositions of the rotor. In this work is used the standard toology of the converter usually alied in SRM drives, given that it rovides a greater flexibility regarding its control and better fault tolerance. The control system of this converter must regulate the magnitude and even the wave shaes of the hase currents to fulfill the requirements of torque and outut ower available and to ensure safe oeration of the generator. This imlies that the electronic switches associated with the controller are fully controlled devices. The toology (Figure ) used ower transistors (IGBT or MOSFET) that work as electronic switches. The caacitor shown in this toology revents fluctuations in the voltage Vs. If losses are neglected the outut energy over each stroke exceeds the excitation by the mechanical energy sulied [6]. On considers that there is no magnetic saturation and each hase is magnetically indeendent from others.

3 Figure : Circuit diagram of the four hase converter for SRG. The SRM is characterized by the mode of controlling its hase current. For this roblem the ower electronic converter is used, which functions in a way that the hase currents of the machine are imosed for certain ositions of the rotor. In this work is used the standard toology of the converter usually alied in SRM drives, given that it rovides a greater flexibility regarding its control and better fault tolerance. If losses are neglected the outut energy over each stroke exceeds the excitation by the mechanical energy sulied [6]. On considers that there is no magnetic saturation and each hase is magnetically indeendent from others. In these terms, the exression of the instantaneous ower,, available in the SRG is exressed as follow: n d L ( θ ) = = dθ ( θ i, i, Li ) i ω, n, () where: n - the number of hases; the hase number; θ rotor osition; ω rotor seed; i the current hase, L(θ) the inductance of hase as the function of θ. The average of ower available P, resulting from the oeration of the machine as a generator, is (with excluding the losses) equal to the mechanical ower. The values can be obtained from the exression of the average value of the torque T m using () and (3): P = T m ω, () T m = N r π π / N n r 0 = d L i dθ ) dθ, (3) where: N r is the number or rotor oles. The above equations enable us to infer that the obtained ower is aroximately constant and it reaches a maximum when the dwell angle is located, in the descending section of the hase inductance rofile, which corresonds to the highest average torque [5], [7]. For this tye of machines the torque rile aears mainly in the commutation zones related with the sequential rocess of establishing and removing the hase currents. The imosition of hase current waveform using the current control with an adusted hysteresis band and a sufficient inut voltage, allow the torque rile reduction.

4 In this way the rile can be minimized, thus controlling the hase s currents commutation recisely hased relative to the rotor osition. For that effect, the current control is done is done using the traezoidal hase reference torque model [8], two adacent hases can be sulied at the same time to ensure the continuity in the generated torque. The SRM is caable of oerating continuously as a generator by keeing the dwell angle so that the bulk d L of the winding conduction eriod comes after the aligned osition, when < 0. dθ The waveforms of the hases reference current * * i, results from the desired torque T and is calculated by the following equation: * T i = d L ( θ ), (4) dθ and are themselves the reference signals to be treated using the feedback ulse with modulation (PWM) with adusted hysteresis band.. The current control of SRG The block diagram from the Figure 3, indicates the current control with the torque reference alied to the * * * 8/6 SRG. The waveforms of the reference currents, i, i, i3 and i *, on calculated using the traezoidal 4 * * * model torque associated to each hase, T, T T and T., 3 * 4 Figure 3: The current control with the torque reference alied to the 8/6 SRG.

5 .3 SRG Simulations On used for simulations an 8/6 SRG, with P n =.4 kw, 4 hase. In these simulation examles of the SRG oeration, the converter voltage used was V s =800 V, which allow reduced torque rile and the rotor seed is 000 rm. The Figure 4 shows the hase current resulting from the traezoidal hase torque. Figure 4: The hase current. In Figure 5, is indicated the total instantaneous torque for the 8/6 SRG. Figure 5: Total Torque. In order to achieve higher erformance in SRG oeration and higher efficiency in the conversion on includes otimal dwell angle control to further reduce the torque rile.

6 3 About conversion Methods of Wind Energy The cature of the wind energy, in an efficient way, requires the existence of a constant wind flow sufficiently strong [7]. Currently wind turbines are designed to achieve a maximum ower at wind seeds above 0 m/s. However, they can be adusted to the local wind rofile. The maximum theoretical efficiency for the wind to energy conversion is 59.3% (Betz's Limit). The effective efficiency conversion is given by the Power Coefficient (C ), which is exressed by the following, where P mec is the mechanical ower of the turbine and P w is the available wind ower. P C = P The ower P w is related with the wind seed V w calculated by (6), mec w. (5) P = ρ A 3 w V w, (6) where ρ is the air density (ρ =.5 kg/m3) and A is the cross-sectional area of the turbine rotor. When considering the generator efficiency (η), the outut ower is given by (7). P out 3 = ρ A V w η ( C ), (7) C (8) varies with the Seed Ratio (λ), given in (9): C = 0, ρ V η 3 w ( C ), (8) λ = r ω, (9) where: r is the rotor radius, ω is the rotor seed. The low rotor seeds of the turbine bring about small turbulences in the air flow. With high seeds the turbine behaves as a wall for the wind. Therefore the riority is to adat the wind seed to the rotor seed with the urose of obtaining a greater conversion efficiency, which results in a maximum C []. 4 Wind System Simulation This work resents two modes of mechanical couling of the turbine to the generator: the direct couling to the turbine shaft, direct - drive wind turbine (Figure 6) and the SRG couling to the turbine shaft through a gearbox (Figure 6) []. 4. Turbine Generator direct couling The rotor seed ω of aroximately 00 rad/s is too high and not comatible for this tye of wind turbines, in normal wind conditions. V w

7 Figure 6: Direct drive wind turbine with SRG. The Figure 7 shows the electric ower generated by the machine couled with this turbine, where its average ower value corresonds to the ower of the system excluding losses in the generator. Fig. 7. The 4 hase SRG instantaneous ower versus rotor osition. Associated with the required high rotor seed for the good erformance of the SRG, the fact that the rotor diameter is small brings about the roblem that the wind seed is not sufficient to overcome the combined turbine-generator inertia, namely at the starting stage. 4. Indirect couling with gearbox The Figure 8 indicates the SRG couling to the turbine shaft through a gearbox. Fig. 8. The indirect couling with gearbox.

8 Assuming that the losses in the gearbox are negligible, and given that the inut and outut ower ω T = ω ), the transmission ratio r t, varies in the inverse of the torque s ratios: ( T r t = = ω ω T T. (0) Figure 8 shows the behavior of the electric ower generated by the machine, when couled with a turbine having a rotor diameter of 5m for a constant wind seed of 8m/s. With the gearbox the rotor seed of the turbine was reduced to less than half of the value obtained in the first. 5 Conclusions The SRG is a valid alternative in wind energy alications. Therefore it is reasonable to foresee that in the medium ower wind systems, the SRG allow good erformance in extracting the energy carried by the wind. On the downside we can oint out the fact that the SRG is noisier than the other conventional systems. Nevertheless the current control based on torque reference covered in this aer attenuates this roblem; esecially via a reduction of the torque rile. References [] G. Gail, A. D. Hansen, Controller design and analysis of a variable seed wind turbine with doubly fed induction generator, Euroean Wind Energy Conference, , 006. [] P. Lobato, A. J. Pires, Methodology based on energy-conversion diagrams to otimize switched reluctance generators control, ICEM 004 Proceedings, no. 58, , 004. [3] P. Chancharoensook, M. F. Rahman, Control of a Four-Phase Switched Reluctance Generator: Exerimental Investigations, IEMDC 03 Proceedings, vol., , 003. [4] P. Lobato, A. J. Pires, A New Control Strategy Based on Otimized Smooth-Torque Current Waveforms for Switched reluctance Motors, Electromotion'03 Proceedings, vol., 60-65, 003. [5] V. Akhmatov, A. H. Nielsen, Variable seed wind turbines with multiole synchronous ermanent magnet generators, Proceedings Wind Energineering, vol. 7, no. 6, , 003. [6] A. D. Hansen, Wind models for redictions of ower fluctuations from wind farms, Proceedings APCWEV 00, no.89,. 9-8, Kyoto, Jaan, 00 [7] H. Henao, E. Bassily, A new control angle strategy for switched reluctance motor, Proceedings EPE '97, vol.3, , Trondheim, Norway,997, [8] A. Grauers, Efficiency of three wind energy generator systems, IEEE Transactions on Energy Conversion, vol., no. 3, , 997. Author Eleonora Darie, Assoc. Prof. PhD, Technical University of Civil Engineering, Electrotechnical Deartment, B-dul Pache Protooescu Nr. 66, sector, Bucharest, Romania, , eleonora_darie@yahoo.com. Costin Ceisca, Prof. PhD, University Politechnica of Bucharest, Electrotechnical Deartment, Slaiul Indeendentei 33, sector 6, Bucharest, Romania, 0/ , costin@wing.ro. Emanuel Darie, Assoc. Prof. PhD, Police Academy of Bucharest, Engineering Deartment, Str. Aleea Privighetorilor Nr., sector, Bucharest, Romania, 0/ , edarie_darie@yahoo.com.