INVESTIGATIONS ON CYCLOCONVERTER-FED INDUCTION MACHINE SYSTEM IN GENERATING MODE

Abstract

The rapidly growing global industrialization has resulted in an ever-increasing rate of energy demand. This, alongwith the perpetual depletion of conventional energy resources, pose this planet with an acute energy crises There is a need to explore means of replenishing the fossil fuel energy resource. Renewable energy resources and the possible means of tapping these resources, therefore, has become the subject of prime importance. Amongst the various renewable energy resources, wind, by virtue of its several attractive features, ranks as a promising renewable resource. Extensive research activities have been in progress on the development, manufacture and erection of cost competitive, efficient and reliable wind energy conversion schemes (WECs). Due to the highly fluctuating nature of wind, the shaft of the generator coupled to the wind turbine rotates at variable speed resulting in a variable frequency output. A constant frequency output can be obtained by maintaining a constant shaft speed by means of electrical controls, mechanical controls or both. Such wind energy conversion schemes are classified as the Constant Speed Constant Frequency (CSCF) systems. Alternatively the generator is allowed to run at variable speeds and the variable frequency output is converted to constant frequency output using frequency converters. These schemes are classified as the Variable Speed Constant Frequency (VSCF) systems. In the past the non availability of cost effective frequency converters made the less efficient CSCF systems virtually obligatory for grid connected schemes; however with the continuous decline in the cost of static frequency converters, the relatively more efficient VSCFs have emerged as the state-of-art. In a wind powered system power in wind is converted into mechanical power by the aerodynamics of the system. The efficiency of conversion is represented by the so-called "Power Coefficient" (Cp) of the wind turbine which varies with the ratio of blade tip speed to wind speed (X). Usually the rated speed of the turbine corresponds to the maximum value of Cp , for a given wind speed. If the shaft speed is held constant, as in CSCFs, the efficiency of conversion will decrease at wind speeds other than the rated value. However, if the rotor speed is allowed to follow the variations in 11 wind speed such that the optimum ratio of blade tip speed to wind speed (A.optlmum)IS maintained at all wind speeds, maximum efficiency of conversion can be ensured. It has been reported in literature that a 10 to 20 percent additional power output can be obtained with a VSCF as compared to a CSCF. Induction machines of cage type are characterised by low unit cost, less maintenance requirement, rugged brushless construction , no synchronization requirement, self-protected against short circuits and absence of hunting phenomenon and therefore qualify as the most suitable generating unit in wind energy conversion systems. Wind speeds range between 16-96 km/h and the rotor speeds of conventional wind turbine lie between 50-300 rpm depending on the shape and design. For the lower rotor speed of 50 rpm, a directly coupled machine would require a notional 120 pole construction for a 50 Hz grid for super-synchronous operation. Therefore, in conventional WECs the generator is invariably coupled with the turbine through a mechanical transmission to raise the shaft speed to super-synchronous speeds. To ensure maximum power transfer, the VSCF system requires that the generator load line be adjusted to match turbine speed torque characteristic every time the speed changes. From this standpoint, the slip-ring induction machine has been widely employed in VSCF WECs as the double output induction generator (DOIG). The apparent slip resistance added to the rotor by the rectifier-inverter cascade provides a variable load line. DOIG has the disadvantage of high initial cost on account of the relatively expensive slip-ring machine and the step-up mechanical transmission. Also the losses in the gearbox result in a reduced overall conversion efficiency. The present research envisages to introduce a gearless wind energy conversion scheme that employs the squirrel-cage machine. A cage induction machine has a fixed rotor resistance and when fed from a fixed frequency supply it has only one speed torque characteristic and consequently the machine can operate within a small range of slip (1-5 %) resulting in a CSCF system. However, if its synchronous speed could be varied in accordance with the turbine speed in such a way that the machine is forced to operate in the super-synchronous regime at any turbine speed, a VSCF system could be realized. This requires adjustable frequency operation. A cycloconverter being capable of bi-directional power flow is used as the frequency converter between the generator and the grid. By controlling the frequency and amplitude of the cycloconverter output in voltage, the machine is fed with excitation VARs at an appropriate low frequency for the super-synchronous operation and to develop the required steady state torque corresponding to the optimal turbine speed. Thus the need of a step-up gear box is obviated and power can be generated at low variable frequency, at any shaft speed and fed to the grid at constant grid frequency. With conventional grid-connected WECs, the grid suffers from switching transients resulting from the switching-in and switching-out of the WECs at the so-called, "Cut-in" and Cut-out" speeds which could be quite frequent depending on the fluctuations in wind speed. With the proposed scheme since generation is possible at all shaft speeds, the system can be left permanently connected to the grid. Afurther advantage offered by this scheme is that wind energy is harnessed even at low wind speeds which goes unutilized below the "Cut-in" speed in the conventional WECs. Behaviour of the proposed scheme under both steady-state and transient conditions has been analysed by computer simulation. Typical results have been presented to demonstrate the feasibility of the scheme. For the analysis, a laboratory size cage induction machine of 2.2 kW, 4-pole, 415 volt 50 Hz. delta connected has been used. The cycloconverter is a 3-phase, 6-pulse circulating current cycloconverter in bridge configuration . Taking currents as state variables, a mathematical model of the machine is developed in the synchronously rotating orthogonal axes reference frame. The magnetization characteristic of the machine is introduced into the simulation by the method of linear segmentation to obtain the magnetizing inductances. The model neglects core loss as the machine would be operating at low frequencies; however, saturation of the main flux path is incorporated based on the "cross-coupling" phenomenon. A detailed qualitative discussion on the magnetic coupling between windings at space quadrature and the means of accommodating its effect mathematically in the machine equations is presented. Sudden input power impulse due to gusts in wind is a common feature of WECs. These input impulses induce a momentary line to line short circuit through the frequency converter. Besides other advantages, the circulating-current mode of the cycloconverter eliminates the possibility of such short circuits making the system prone to these input impulses. Simulation of the cycloconverter has been based on the cosine IV wave crossing principle for determining the instants of the triggering pulses to the thyristors. The direct coupling of the generator with the turbine requires that the frequency and amplitude of the cycloconverter output voltage be adjusted every time the turbine speed changes, so that the machine is retained in super-synchronous speed and develops a torque to counter balance the applied torque at a given turbine speed. This is achieved by controlling the frequency and modulating index of the cycloconverter. The wind turbine used in the present simulation study is a horizontal axis 1500 Watt, fixed blade turbine with a rotor radius of 1.525 meters. To assess the steady state performance of the system has been obtained over a wide range of wind speed keeping the ratio of blade tip speed to wind speed at the optimal value C^-optimum)- Waveforms for speed, torque, machine current, supply line current, input power, power fed to grid, power factor on the low and the high frequency sides and efficiency of conversion have been obtained for each optimum discrete rotor speed. The induction generator is operated at constant flux over the entire range of operating frequencies. The transient behaviour of the system is also studied under certain realistic operating conditions. In case of rotating machinery, the transient peak in the developed torque is responsible for imposing undue strain on the shaft leading to a possible breakdown. In the present study, the transient condition that needs particular attention is that resulting from high fluctuations in the input power caused by the characteristic unpredictable gusting of the wind. Fluctuations in voltage and frequency are the common realistic grid conditions. The transient behavior of the system is evaluated from transient plotsof torque, speed, current, input power, output power corresponding to step change in wind speed and step change in grid voltage and frequency. The study also includes simulation of currents on the high frequency side of the cycloconverter to investigate the nature and harmonic content of these currents. Waveforms generated for a wide range of operating frequencies have been analyzed for harmonic content using the Fast Fourier Transforms. The study revealed an acceptable level of Total Harmonic Distortion (THD) in these currents which is attributable to the circulating current mode of the cycloconverter. However, the power factor was observed to be quite low. This is because of the extra reactive power drawn by the cycloconverter, in addition to that required by the induction generator, on account of the phase modulation of triggering pulse used. To relieve the supply mains from the reactive power burden, an Active Power Filter (APF) has been proposed in the present system, which also serves to eliminate all harmonic components from the supply currents. The APF is connected in shunt with the cycloconverter- induction generator set and consists of three single-phase voltage source inverters comprising of IGBTs as the switching devices and a 1500 uE capacitor on the dc bus. Simulation of the Active Power Filter is based on the concept of "Instantaneous Reactive Power " and the Hysteresis-Based-Carrier-less PWM Current Control for the timing of gating pulses to the IGBTs. The fundamental reactive current component, the harmonic reactive current component and the harmonic active current component are all compensated by the APF resulting in only the fundamental active current component to flow in the supply mains. The performance of the APF has been validated by obtaining waveforms of the high frequency side current at several operating frequencies and compared with those obtained without the APF. Harmonic analysis of these waveforms shows that, except for the high frequency switching harmonics, all other cycloconverter-generated harmonic components have been eliminated and unity power factor condition is achieved. To conclude, the research problem undertaken and the ensuing results of the various simulation analysis made, establish the viability of the proposed gearless VSCF wind energy conversion scheme with appreciable overall conversion efficiencies at all wind speeds. With the simple configuration, on account of the line-commutated cycloconverter used, the scheme provides several advantages over the conventional schemes as discussed earlier. The ability to operate at unity power factor and to generate the high quality of output power further enhances the attractive features of this wind energy conversion scheme.