Advantages of using a Switched Reluctance Generator (SRG) for

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

                           Eleonora Darie, Costin Cepisca, Emanuel Darie


Abstract

Wind energy found to be one of the most useful solutions to help in overcoming the air pollution and
global. There is no agreed solution to conversion of wind energy to electrical energy. Climate change is a
contemporary issue, and the international community has accepted the dangers of green house gas
emissions. Renewable energy is one of the hot topics when it comes to dealing with green house gas
emissions treatments. Wind generation is one of the renewable energy power sources that help in reducing
the carbon dioxide from our atmosphere. Using of the wind energy has become increasingly important as a
renewable energy source and therefore is an increasing interest in exploiting it using a Switched
Reluctance Machine as a generator and optimizes its characteristics in this domain. This work analyzes the
generator mode of the Switched Reluctance Machine in the direct coupling to the turbine shaft and
coupled to the shaft through a gearbox. The principles of operation of this machine are simple, well
known and based on reluctance torque. The machine has a stator of wound-up salient poles that after
energizing synchronized with the position of the rotor develops a torque that tends to align the poles in a
way that diminishes the reluctance in the magnetic circuit. Currently the synchronous and induction
machines dominate the market of wind energy applications, although, the SRM has been the subject
of current investigation and it shows to be a valid alternative for this field.

Keywords: emissions, energy source, switching reluctance motor.


1       Introduction
Climate change is a contemporary issue, and the international community has accepted the dangers of
green house gas emissions. Renewable energy is one of the hot topics when it comes to dealing with green
house gas emissions treatments. Wind generation is one of the renewable energy power sources that help
in reducing the carbon dioxide from our atmosphere.
In the last decades the Switched Reluctance Machine (SRM) has become an important alternative in
various applications in the industrial and domestic markets, namely as a motor showing good mechanical
reliability, high torque-volume ratio and high efficiency, plus low cost. Although less evangelized as a
generator, there are a few studies of its application in the aeronautical industry and in integrated
applications in wind based energy generators.
The principles of operation of this machine are simple, well known and based on reluctance torque. The
machine has a stator of wound-up salient poles that after energizing synchronized with the position of the
rotor develops a torque that tends to align the poles in a way that diminishes the reluctance in the magnetic
circuit [2].
Currently the synchronous and induction machines dominate the market of wind energy applications,
although, the SRM has been the subject of current investigation and it shows to be a valid alternative for
this field [2], [3] and [4].
Comparing with the classical solutions of machines integrated in wind applications, a Switched
Reluctance Generator (SRG) shows a simplified construction associated with the inexistence of permanent
magnets or conductors in the rotor, which results in lower manufacturing costs; in addition both the
machine and the power converter are robust. The low inertia of the rotor allows the machine to respond to
rapid variations in the load.
Associated with these characteristics, these machines have a control system that allows rapid changes in
the control strategy such that the performance of the machine is optimized.
The structure of the SRM is not as stiff as the synchronous machines and due to its flexible control system;
it is capable of absorbing transient conditions, thus supplying more resilience to the mechanical system [7].
The machine has an inherent fault tolerance, especially when under an open-coil fault (in the windings) and
in the power converter (external faults) [4]. Under normal operation, each phase of SRG is electrically and
magnetically independent from others.
The SRM is generally felt to be louder than conventional machines. However an adequate mechanical
design can do a lot to improve these figures and new control techniques (current control strategy with a
torque reference) permits further improvements.

2       Mode of operation by SRG
2.1     SRG Characteristics
In electrical drives with variable reluctance (Figure 1), the torque is function of the regular position of the
rotor due the double salient poles. The operation of the machine as a generator is obtained by energizing
the windings of the stator when the salient poles of the rotor are away from their aligned position due to
the rotating motion of the prime mover.




Figure 1: The Switched Reluctance Generator in the wind turbine.

The SRM is characterized by the mode of controlling its phase current. For this problem the power
electronic converter is used, which functions in a way that the phase currents of the machine are imposed
for certain positions of the rotor. In this work is used the standard topology of the converter usually
applied in SRM drives, given that it provides a greater flexibility regarding its control and better fault
tolerance.
The control system of this converter must regulate the magnitude and even the wave shapes of the phase
currents to fulfill the requirements of torque and output power available and to ensure safe operation of the
generator. This implies that the electronic switches associated with the controller are fully controlled
devices.
The topology (Figure 2) used power transistors (IGBT or MOSFET) that work as electronic switches. The
capacitor shown in this topology prevents fluctuations in the voltage Vs.
If losses are neglected the output energy over each stroke exceeds the excitation by the mechanical energy
supplied [6]. On considers that there is no magnetic saturation and each phase is magnetically independent
from others.
Figure 2: Circuit diagram of the four phase converter for SRG.

The SRM is characterized by the mode of controlling its phase current. For this problem the power
electronic converter is used, which functions in a way that the phase currents of the machine are imposed
for certain positions of the rotor. In this work is used the standard topology of the converter usually
applied in SRM drives, given that it provides a greater flexibility regarding its control and better fault
tolerance.
If losses are neglected the output energy over each stroke exceeds the excitation by the mechanical energy
supplied [6]. On considers that there is no magnetic saturation and each phase is magnetically independent
from others.
In these terms, the expression of the instantaneous power, p, available in the SRG is expressed as follow:


                                                        1 ⎡ n d L j (θ ) 2 ⎤
                             p (θ , i1 , i2 ,L in ) =     ⎢∑            ij ⎥ω ,                          (1)
                                                        2 ⎣ j =1 d θ       ⎦

where: n - the number of phases; j – the phase number; θ – rotor position; ω – rotor speed; ij – the current
phase, Lj(θ) – the inductance of phase j as the function of θ.
The average of power available P, resulting from the operation of the machine as a generator, is (with
excluding the losses) equal to the mechanical power. The values can be obtained from the expression of
the average value of the torque Tm using (2) and (3):

                                                        P = Tm ⋅ ω ,                                     (2)


                                                    2π / N r
                                                               ⎛ n 1 d Lj 2
                                           N
                                       Tm = r
                                           2π           ∫      ⎜∑          )
                                                               ⎜ 2 dθ i j dθ ,                           (3)
                                                        0      ⎝ j =1

where: Nr is the number or rotor poles.
The above equations enable us to infer that the obtained power is approximately constant and it reaches a
maximum when the dwell angle is located, in the descending section of the phase inductance profile,
which corresponds to the highest average torque [5], [7].
For this type of machines the torque ripple appears mainly in the commutation zones related with the
sequential process of establishing and removing the phase currents.
The imposition of phase current waveform using the current control with an adjusted hysteresis band and a
sufficient input voltage, allow the torque ripple reduction.
In this way the ripple can be minimized, thus controlling the phase’s currents commutation precisely
phased relative to the rotor position. For that effect, the current control is done is done using the
trapezoidal phase reference torque model [8], two adjacent phases can be supplied at the same time to
ensure the continuity in the generated torque.
The SRM is capable of operating continuously as a generator by keeping the dwell angle so that the bulk
                                                                           d Lj
of the winding conduction period comes after the aligned position, when           <0.
                                                                           dθ
The waveforms of the phases reference current i * , results from the desired torque T * and is calculated by
                                                j

the following equation:


                                                           2 T j*
                                                i* =
                                                 j
                                                         d L j (θ ) ,                                     (4)
                                                            dθ

and are themselves the reference signals to be treated using the feedback pulse with modulation (PWM)
with adjusted hysteresis band.
2.2     The current control of SRG
The block diagram from the Figure 3, indicates the current control with the torque reference applied to the
                                                         * *
8/6 SRG. The waveforms of the reference currents, i1* , i2 , i3 and i 4 , on calculated using the trapezoidal
                                                                      *



model torque associated to each phase,   T1* ,T2* ,T3* and T4* .




Figure 3: The current control with the torque reference applied to the 8/6 SRG.
2.3     SRG Simulations
On used for simulations an 8/6 SRG, with Pn=2.4 kW, 4 phase.
In these simulation examples of the SRG operation, the converter voltage used was Vs=800 V, which
allow reduced torque ripple and the rotor speed is 1000 rpm.
The Figure 4 shows the phase current resulting from the trapezoidal phase torque.




Figure 4: The phase current.

In Figure 5, is indicated the total instantaneous torque for the 8/6 SRG.




Figure 5: Total Torque.

In order to achieve higher performance in SRG operation and higher efficiency in the conversion on
includes optimal dwell angle control to further reduce the torque ripple.
3       About conversion Methods of Wind Energy
The capture 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 power at wind speeds above 10 m/s.
However, they can be adjusted to the local wind profile.
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 (Cp), which is expressed by the
following, where Pmec is the mechanical power of the turbine and Pw is the available wind power.

                                                            Pmec
                                                    Cp =
                                                             Pw .                                        (5)

The power Pw is related with the wind speed Vw calculated by (6),

                                                      1
                                              Pw =      ⋅ ρ ⋅ A ⋅V w3 ,                                  (6)
                                                      2

where ρ is the air density (ρ = 1.225 kg/m3) and A is the cross-sectional area of the turbine rotor.
When considering the generator efficiency (η), the output power is given by (7).


                                                     ⋅ ρ ⋅ A ⋅V w3 ( ⋅C p ),
                                                   1
                                          Pout =                   η                                     (7)
                                                   2

Cp (8) varies with the Speed Ratio (λ), given in (9):


                                          C p = 0 , 22 ⋅ ρ ⋅ V w3 ⋅(η ⋅ C p ) ,                          (8)


                                                             r ⋅ω
                                                       λ=
                                                              Vw ,                                       (9)

where: r is the rotor radius, ω is the rotor speed.
The low rotor speeds of the turbine bring about small turbulences in the air flow. With high speeds the
turbine behaves as a wall for the wind. Therefore the priority is to adapt the wind speed to the rotor speed
with the purpose of obtaining a greater conversion efficiency, which results in a maximum Cp [1].

4       Wind System Simulation
This work presents two modes of mechanical coupling of the turbine to the generator: the direct coupling
to the turbine shaft, direct - drive wind turbine (Figure 6) and the SRG coupling to the turbine shaft
through a gearbox (Figure 6) [1].
4.1     Turbine Generator direct coupling
The rotor speed ω of approximately 100 rad/s is too high and not compatible for this type of wind
turbines, in normal wind conditions.
Figure 6: Direct drive wind turbine with SRG.

The Figure 7 shows the electric power generated by the machine coupled with this turbine, where its
average power value corresponds to the power of the system excluding losses in the generator.




Fig. 7. The 4 phase SRG instantaneous power versus rotor position.

Associated with the required high rotor speed for the good performance of the SRG, the fact that the rotor
diameter is small brings about the problem that the wind speed is not sufficient to overcome the combined
turbine-generator inertia, namely at the starting stage.
4.2     Indirect coupling with gearbox
The Figure 8 indicates the SRG coupling to the turbine shaft through a gearbox.




Fig. 8. The indirect coupling with gearbox.
Assuming that the losses in the gearbox are negligible, and given that the input and output power
( ω1T1 = ω 2T2 ), the transmission ratio rt, varies in the inverse of the torque’s ratios:

                                                        ω1 T2
                                                 rt =      = .                                                 (10)
                                                        ω 2 T1

Figure 8 shows the behavior of the electric power generated by the machine, when coupled with a turbine
having a rotor diameter of 5m for a constant wind speed of 8m/s.
With the gearbox the rotor speed 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 applications. Therefore it is reasonable to foresee that in the
medium power wind systems, the SRG allow good performance in extracting the energy carried by the
wind. On the downside we can point out the fact that the SRG is noisier than the other conventional
systems.
Nevertheless the current control based on torque reference covered in this paper attenuates this problem;
especially via a reduction of the torque ripple.

References
[1] G. Gail, A. D. Hansen, Controller design and analysis of a variable speed wind turbine with doubly fed induction
generator, European Wind Energy Conference, pp. 500-508, 2006.
[2] P. Lobato, A. J. Pires, Methodology based on energy-conversion diagrams to optimize switched reluctance
generators control, ICEM 2004 Proceedings, no. 158, pp. 700-705, 2004.
[3] P. Chancharoensook, M. F. Rahman, Control of a Four-Phase Switched Reluctance Generator: Experimental
Investigations, IEMDC 03 Proceedings, vol. 2, pp. 842-848, 2003.
[4] P. Lobato, A. J. Pires, A New Control Strategy Based on Optimized Smooth-Torque Current Waveforms for
Switched reluctance Motors, Electromotion'03 Proceedings, vol. 2, 610-615, 2003.
[5] V. Akhmatov, A. H. Nielsen, Variable speed wind turbines with multipole synchronous permanent magnet
generators, Proceedings Wind Energineering, vol. 27, no. 6, pp. 531-548, 2003.
[6] A. D. Hansen, Wind models for predictions of power fluctuations from wind farms, Proceedings APCWEV 2001,
no.89, pp. 9-18, Kyoto, Japan, 2001
[7] H. Henao, E. Bassily, A new control angle strategy for switched reluctance motor, Proceedings EPE '97, vol.3,
pp. 613-618, Trondheim, Norway,1997,
[8] A. Grauers, Efficiency of three wind energy generator systems, IEEE Transactions on Energy Conversion, vol.
11, no. 3, pp. 650-657, 1997.


Author
Eleonora Darie, Assoc. Prof. PhD, Technical University of Civil Engineering, Electrotechnical Department, B-dul
Pache Protopopescu Nr. 66, sector 2, Bucharest, Romania, 0742139208, eleonora_darie@yahoo.com.
Costin Cepisca, Prof. PhD, University Politechnica of Bucharest, Electrotechnical Department, Splaiul Independentei
313, sector 6, Bucharest, Romania, 021/402.95.57, costin@wing.ro.

Emanuel Darie, Assoc. Prof. PhD, Police Academy of Bucharest, Engineering Department, Str. Aleea Privighetorilor
Nr. 1, sector 1, Bucharest, Romania, 021/317.55.23, edarie_darie@yahoo.com.