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NIEMYSORE
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ABSTRACT
In this dissertation work, two space-vector-based pulse-width modulation (PWM) (SVPWM) strategies named equal- and proportional-duty SVPWMs are described, which are used to synthesize a four-level waveform from an open-end winding configuration of an induction motor.
Two isolated dc-link voltages, which are in the ratio of 2 : 1, are employed to achieve this objective. Both of these PWM strategies achieve the avoidance of overcharging of the dc-link capacitor of an inverter operating with lower dc-link voltage by its counterpart with higher dc-link voltage.
Implementation of these PWM strategies requires only the instantaneous three-phase reference voltages, eliminating the need of sector identification or lookup tables. The numbers of samples per cycle for individual inverters are selected in such a way that the quarter-wave, half-wave, and three-phase symmetries are achieved for the dual-inverter drive despite unequal dc-link voltages for the constituent inverters. It is also shown that one of the two PWM techniques, called the proportional-duty SVPWM, results in a better spectral performance and lower switching power loss in the overall dual-inverter system compared to the equal-duty SVPWM
INTRODUCTION
In this dissertation work, two decoupled synchronous space-vector-based PWM (SVPWM) switching strategies are described for the four-level inverter configuration. The SVPWM technique suggested is extended to the four-level configuration. With this PWM strategy, the dc-link capacitor of the inverter with lower voltage is not overcharged by that of the one with higher dc-link voltage. It is shown that this PWM strategy obviates the need for sector identification. The implementation of this PWM strategy requires only instantaneous phase reference voltages and does not require any lookup tables.
The principles described which render the quarter-wave symmetry (QWS), half-wave symmetry (HWS), and three-phase symmetry (TPS) for a two-level voltage source inverter (VSI) are extended to obtain these waveform symmetries for the output voltage waveform of the four level inverter. The fact that the output voltage waveform is obtained by the superposition of the output of one inverter with the output of the other inverter is exploited in this paper to judiciously choose the number of samples for the individual inverters, which constitute the dual-inverter drive system
LITERATURE SURVEY
. A Decoupled SVPWM Technique for a Dual Inverter Four-level Open-end Winding
Induction Motor Drive with Voltage Balancing of DC-link Capacitors.
B. Venugopal Reddy, Student Member, IEEE, and V. T. Somasekhar
Four-level inversion is realised by connecting two two-level inverters, fed from both ends of open-end winding induction motor with Asymmetrical DC link voltages in the ratio of 2:1. This inverter scheme produces 64 space-vector combinations distributed over 37 space-vector locations. All the switching combinations are not helpful to achieve four level
inversion, some of the combinations overcharge DC-link capacitor of the inverter operating with a lower DC-link voltage of Vdc/3 by other inverter with higher DC-link voltage of 2Vdc/3.
Decoupled space vector PWM switching strategy is described to eliminate the overcharging effect. Isolated DC power supplies are employed to avoid the zero-sequence currents.
CONCEPT OF SPACE VECTOR
The concept of space vector is derived from the rotating field of AC machine which is used for modulating the inverter output voltage. In this modulation technique the three phase quantities can be transformed to their equivalent two-phase quantity either in synchronously rotating frame (or) stationary frame. From this two-phase component the reference vector magnitude can be found and used for modulating the inverter output.
When this 3-phase voltage is applied to the AC machine it produces a rotating flux in the air gap of the AC machine. This rotating flux component can be represented as single rotating voltage vector. The magnitude and angle of the rotating vector can be found by means of Clark’s Transformation in the stationary reference frame
DECOUPLED SVPWM STRATEGY WITH EQUAL SWITCHING DUTY
The proposed decoupled SVPWM strategy is based on the fact that a space vector Vsr can be constructed by taking the difference between two individual space vectors, which are anti-phased to each other. Since the dc-link voltages of individual inverters are in the ratio of 2 : 1, it stands to reason that they should be made to output space vectors, whose magnitudes are proportional to their respective dc-link voltages. Thus, inverter-1 and inverter-2 are individually operated with reference voltage vectors 2Vsr/3 and −Vsr/3, respectively. Subtraction of the latter component from the former achieves the desired reconstruction of the reference vector (Vsr). The basic principle of the decoupled SVPWM strategy is as shown below
DECOUPLED SVPWM STRATEGY WITH PROPORTIONAL SWITCHING DUTY
The switching power loss Ploss in an inverter employing space-vector modulation is proportional to both the dc-link voltage (Vdc) and the number of samples per one revolution of the reference voltage space vector (Nsamp), which, in turn, is proportional to the switching frequency. Hence, one may express switching loss
Ploss = KVdc*Nsamp.
The constant of proportionality K depends on the operational parameters of the inverter and the switching times of the power semiconductor devices, used to constitute the inverter. Thus, it would be an advantageous proposition to switch the inverters with switching frequencies, which are proportional to their dc-link voltages. In other words, the inverter with higher dc-link voltage (i.e., with a voltage of 2/3 • Vdc) should be switched with a lesser frequency, ideally at half of the switching frequency, compared to the one operating with a lower dc-link voltage (i.e., with a voltage of 1/3 • Vdc
CONCLUSION
• Two decoupled SVPWM strategies, namely, equal- and proportional-duty SVPWMs, have been described for a four-level dual-inverter-fed open-end winding induction motor drive.
• Two unequal dc-link voltages, which are in the ratio of 2 : 1, are used to achieve this objective. The zero-sequence current is not allowed to flow in this circuit, as two isolated dc power supplies are employed to feed individual inverters.
• It is shown that, with the equal-duty SVPWM strategy, the gating waveforms of only one inverter need to be generated, from which the gating waveforms for the other inverter could be deduced.
• It is shown that both of the schemes achieve the avoidance of overcharging of the dc-link voltage of the inverter operating with lower dc-link voltage.
• Moreover, the proportional-duty SVPWM displays a lesser harmonic contamination (i.e., a lesser THD) compared to the equal-duty SVPWM, which would result in lesser iron loss in the motor and, therefore, an enhanced performance of the overall drive system