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The aim of this paper is to analyze the potential of switched reluctance generator (SRG) in wind energy application. The machine comprises of switched reluctance generator, power converter and controller. In this paper the main ele-ments that form the generator system is discussed. It also highlights the common type of converter and structure used for SRG in wind energy application and types of control strategy available. Using power converter for switching the generator can operate over a wide speed range. Its applications in high speed area such as starter/generator for air-craft and gas turbine has been established, however the low/medium speed operation is still at an early stage of re-search. In order to subject the machine to various parameters, offline modeling is being investigated to produce the best optimum design.

Wind is a natural source of energy related to the low and medium speed range. In order to extract energy from the wind, machine that can be controlled to generate at variable speed are preferred/of advantage to ensure optimum performance. Among the common types of machines used for wind energy are: double fed induction generator (DFIG), induction generator and also synchronous generator. The potential candidate to the group is the Switched Reluctance Generator (SRG) which comprises of SR generator, power converter and controller as shown in

SRG has attracted researches to investigate the potential of the machine mainly due to its attractive characteristics over conventional machine as stated below:

• Simplified construction with rotor only consists of laminated steel

• Concentrated phase winding is only on the stator poles

• Absence of permanent magnet which gives low manufacturing cost.

• Has higher reliability since each phase is electrically and magnetically independent

• Low inertia since it does not have windings or magnets on the rotor hence machine can operate at low wind speed [

• Requires power converter circuit which controls the instant of energizing and de-energizing to match its speed and load.

Extensive ongoing research has been done on the low and medium speed range which studies the suitability of using SRG in wind energy applications. The objective of generator control in wind energy application is to optimize the energy captured to produce maximum output power. Speed control strategy has been used in [3,4] which requires a good design of the controller to control the output power. The authors in [

From literature review above the research on SRG focus on the control strategy. Few papers has discussed on its structure [9-11] and power converter [12-16].

The research on SRG in wind energy application is still ongoing and further research on the machine is required before it can be commercially available.

The operation of the machine as a generator is by controlling the switching sequence of the power converter during the decreasing inductance profile. It is referred to as a doubly salient pole due to the salient pole of its stator and rotor structure. Salient pole refers to the structure of the element protruding from the yoke into the air gap. Rotor and stator are made of steel laminations and only the stator poles has windings concentrated around it [

The operation of switched reluctance machine as motor has made a good progress however its operation in generating mode is slowly evolving.

The operation of SRG depends entirely on synchronized excitation of the set of stator windings to create continuous rotation of rotor. The movement of rotor with respect to excited stator phase varies the inductance of the machine periodically from maximum to minimum hence

torque and power is produced. During the aligned position of rotor and stator, inductance is maximum and minimum during the non-aligned position [

The operation of Switched Reluctance Generator (SRG) is similar to Switched Reluctance Motor (SRM). However, for SRG the excitation of stator phase must be made when rotor is moving pass the stator when inductance is decreasing as shown in

inductance region. Movement of rotor in an out of alignment with the stator poles creates variation of reluctance flux path. It can be seen that this variation creates conversion of energy. Hence in every cycle the flux must be established and returned to zero before excitation of the next phase.

The torque (T) is produced by the tendency of the rotor moving to the excited stator phase winding where minimum reluctance occurs independent of direction of current (i) flow as shown by equation below:

Where (L) is the phase inductance and (θ) is the rotor position.

The phase of the stator must be excited in synchronism with rotor position to produce continuous torque. If the phase is excited when the poles are unaligned as in

The voltage equation for one phase winding takes into account the following assumptions; mutual coupling between phases is almost zero, no fringing effect of flux around pole corners and all flux between poles crosses the air gap in radial direction is as follows:

Flux, is a function of phase current and rotor position, hence can further be extended as follows

Also

Differentiate Equation (4) as a function of current

Since Equation (3) can be written as

where is the terminal voltage, is the phase current, is the flux linkage (volt.sec), is the phase resistance, phase inductance, indicates the rotor position and is the angular velocity of the rotor (rad/s).

The last parameter on the right hand side is the back EMF of the machine and it appears during separation of poles and with increase in speed. From the back EMF equation it can be seen that represents the slope of the inductance profile as shown in _{phase}) depends on back

EMF which varies with speed (). V_{r} is the voltage across the phase resistance (R_{ph}) and V_{L} is the voltage across the phase inductance (L_{ph}).

Furthermore it can be seen that as rotor pole moves out of alignment with the stator pole, it gains speed during decreasing inductance slope. Hence, after both switches are turned off, current will be large with the aid of back EMF. Phase current will eventually subside as back EMF opposes the negative voltage and becomes zero.

During motoring mode, when the rotor moves into alignment with the stator pole, speed increases and the inductance increases as well. The back EMF that results from the change of inductance together with saturation effects aids the voltage in reducing the phase current. As compared to motoring mode the control of firing angles during generating mode is more complicated due to this back EMF effect [

The most common converter circuit for SRG is the Asymmetric Half Bridge Converter (AHBC). It consists of two switching devices and two power diodes per phase.

• C-dump power converter

• R-dump converter

• Bifilar type converter

• Split DC converter

Some converter circuits are only suitable for even number of phases due to arrangements of capacitor and switches that require balance of power [

Due to the advantages that AHBC offers in line with the requirement makes it a popular choice. It has the ability to shape the phase current using 3 switching sequence such as positive voltage, zero voltage and negative voltage. This form of switching is suitable during low and medium speed where soft switching is required under the current chopping mode. It is also able to operate at higher speed during single pulse mode where the switches are closed for excitation and open for de-fluxing namely known as hard switching [

When switches are closed as in

The heart of the SRG depends on its control strategy which is through proper switching of power electronic devices as shown in

As observed, in generating mode single pulse operation is common for both low and high speed. The difference lies in the control strategies applied. Unlike in motoring single pulse mode is used for high speed whereas pulse width modulation for low speed.

SRG control strategy based on the determination of the firing angles could be classified into three groups. These groups are:

• fixing both the turn on and turn off angles

• one of the angles may be fixed whilst varying the other

• vary both turn on and turn off angles

As there will be variation in wind velocity, the best strategy is to vary both the turn on and turn off angles in order to achieve optimum performance. There are few studies which highlight the strategy for varying both the firing angles such as: flux linkage [

also curve fitting [

Apart from the firing angles, the SRG structure also plays a role in terms of efficiency. Higher number of poles is suggested in [

Various methods has been used to maximize the energy captured during low/medium speed such as: speed feedback control, using area of magnetization curve by controlling reference current, closed loop output power control.

Output power of SRG depend on various variables such as: excitation current, excitation voltage, rotor position and also rotor speed. The parameters above can be controlled to achieve high efficiency of the SRG. However the challenge lies on how to determine the best parameters that will produce the optimum performance. Based on that, the modeling of the SRG is proposed using MATLAB/SIMULINK. The software has all the tools to model the dynamics of electrical machines. The simulations are performed using block diagrams and special MATLAB functions.

At this stage of research a simulation model of the SRG comprising of the converter, controller and generator operating at steady state is proposed. Assuming that there is no mutual interaction between phases and neglecting flux linkage the representation of SRG is performed for one phase. Electromagnetic characteristics and torque can be calculated individually for each phase hence additional phases can be added. At a later stage the model will then be subject to testing with a prototype for practical implementation.

The SRG is modeled as an inductor and a resistor in series as shown in

The details of the subsystems are as follows:

The position sensor as shown in

The converter used is the standard Asymmetric Half Bridge Converter (AHBC) as shown in

The controller function which ensures commutation of power switches at turn on and turn off angles is performed by the switch function seen in

The phase winding constructed using mathematical functions as in Equation (2) is shown in

The SRG used for simulation is based on parameters from a 3 phase 6/4 machine. The maximum and minimum phase inductances are 60mH and 8mH. Currently the simulation is tested by fixing both the firing angles with constant speed.

It can be seen in

Overall from the simulations performed, it can be seen that the amount of power generation depends on placement of firing angles. A good control strategy which synchronizes the phase current with rotor position would provide more efficient SRG system. If we advanced turn on angle it gives ample time for current to build up. However the disadvantage of advancing too far is that it will be in the positive torque region which reduces the efficiency. The firing angles must vary to accommodate changes in speed to maximize the output power. The simulation model needs to be enhanced to cater for variation in the above parameters.

The paper made an attempt to view the potential of the SRG in wind energy application. Its operation, converter and also its control strategies have been discussed. Based on literature review SRG has the potential to be used in wind energy application due to its simple structure, ability to operate at low speed due to low inertia of the machine, ease of maintenance and advantage in control strategy. The use of power converter for switching allows it to operate at variable speed operation. By proper placement/excitation of firing angles the output power can be maximized. Future work will look into enhancing the model to cater for various parameters such as the control methods in order to vary the firing angles, varying speed and also include load. The final model will allow user to study the characteristic of the machine and choose the optimum design based on its parameters.