Analysis and performance of a switched reluctance generator for wind energy conversion

The concept of operation for the switched reluctance generator is divided into two sections, first is the excitation stage and second is the generating stage. This paper discuss the determination of the coil excitation in the SRG, the switching phenomena, requirement to protect the machine from electrical damage as well as the electric losses limitations. I. INCTRODUCTION Climate change is one of the focus issues that leaders world wide are discussing and trying to find an acceptable solution before it is too late. The technology is one of the major areas that have been focus on to deal with these issues. The scientist world wide are trying to find a better and more effective way to use the nature energy or to minimise the carbon dioxide from feeding into the environment, as well as increasing the efficiency of the electric machine. Renewable energy became an important topic when it comes to deal with climate changes, renewable energy in all faces, solar system, hydro system, wind energy, co-generation are the aim for improvements in many country due to its impact on the climate changes. Wind energy is one of the nature ongoing sources that help the life cycle of the planet to continue, wind energy is not a simple breeze that can push a tree it could be strong to take buildings off, it is one of the modern energy sources to generate electricity. Using an ongoing source to produce electric power for the human daily need, will give a more stability and cheap way to maintain the human demand under control, no need to worry about the coal or fuel reserve, as soon the wind stop from the planet the life will end. The IPCC (the intergovernmental panel on climate change) sent an important warning to the word nations for the need of use a low-carbon to avoid the dangerous complications to the climate changes and the danger on the society. Due to many factors wind energy became a main renewable source to convert nature energy into electric power for daily use. The consumption of the electric energy increases on a high rate in the past decade, this increase forces the international society to find more efficient way to overcome the climate change that caused by this increase. The majority of the high capacity wind generator used are induction generator, this machine have few disadvantages such as winding losses and low efficiency, a new machined called switched reluctance generator started to develop to be used as wind generator, this machine has few advantages over the AC conventional machine [1]. The boom in the technology market helped in securing a high efficiency output for the switched reluctance generator and made it easier to build more effective controller. The main advantages that the switched reluctance generator has over the conventional AC machine that will be highlighted in this paper are: 1. higher efficiency 2. eliminate to electric losses in the rotor 3. fast response 4. cheap manufacturing cost These four points made the international research take a deeper look in using the SRG as possible sources for the wind energy conversion. SRG is a simple machine that consisted of a winding around the stator poles and of a silent rotor poles, this gives the SRG the fast response under the change of load condition due to lighter rotor. The elimination of the rotor winding in the induction generator made the SRG cheaper to manufacturing also did eliminate the electric losses in the winding. The higher the efficiency the lower the cost per KWH for the wind energy In this paper the following has been used during the experiment: • 1 hp switched reluctance machine 8/6 rated current at 5 amps • BUZ71 MOSFET power transistors • IGBT • 55100 Mini Flange Mount Hall Effect Sensor • Permanent magnet • Bipolar BC107 • 74HC/HCT74 Dual D-type flip-flop • AD8564 Quad 7 ns single supply comparator • DC power supply for the excitation • AC 50 Hz power supply for the inductance test of the winding II. PRINCIPAL OF OPERATION The basic operation of the machine can be divided into tow sections, first the operation as a motor (SR motor) and the second is to operate as generator (SRG), in this paper the concern is in the machine when operation under as a generator, the operation cycle will be divided into two stages, first is called excitations and the second is generation stage, the on and off mode between these two stages called the firing angles for the machine. During one cycle, the machine will be excite, using external sources which could be a capacitor or a battery, the magnetic field that created by the excitation will be converted into electric power during the second stage of the cycle using the wind energy as a mechanical source. The firing angel will depend on the characteristics of the machine (inductance, flux density, rated values), the inductance of the machine will determine the output of the machine, when operation the machine as generator, the flux density for the motoring stage must be minimum to avoid losses and when operation as motor the generating stage should be at its lowest value, this will be explained in more details in this paper. Inductance As mentioned earlier, depend on the inductance characteristics will determine the type of the machine and the characteristics of the output power. The analysis of the inductance will be on the non-linear inductance model which is based on the Fourier series of inductance; the inductance is a function of the excitation current and the rotor angle ) , ( i L θ . The value of the inductance is constant and periodic with period equal to 2π/Pr where, Pr is the number of rotor poles. By applying Fourier series and taken into consideration the symmetrical of the inductance about the y-axis in the section between [ ] r r P P / , / π π − , the following equations represent the relation between the inductance, the current and the rotor angle: ∑ ∞ = + = 1 0 Pr cos ) ( ) ( ) , ( n n n i L i L i L θ θ (1) The result of the first 4 harmonics will be acceptable as a final result of the inductance calculation; therefore: θ θ θ θ Pr 3 cos ) ( Pr 2 cos ) ( Pr cos ) ( ) ( ) , ( 3 2 1 0 i L i L i L i L i L + + + = (2) it is clear that the inductance of the SRG depend on the rotor position as well as the excitation current. Recall the requirement of the coil excitation and the current behaviour in the coil as stated in the equation below. ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ − = − L R t increase R v i exp 1 (3) ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ = − L R t decrease R v i exp (4)