09-11-2012, 04:11 PM
ADVANCED MODULATING TECHNIQUES FOR DIODE CLAMPED MULTILEVEL INVERTER FED INDUCTION MOTOR
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ABSTRACT
The general function of the multilevel inverter is to synthesize a desired AC voltage from several levels of DC voltages. As the number of voltage levels increases the harmonic content decreases significantly. These multilevel inverters are used to increase inverter operating voltage, to minimize THD with low switching frequency, to reduce EMI due to lower voltage steps. The advantages of this multilevel approach include good power quality, good electromagnetic compatibility, low switching losses and high capability. This project proposes to study various multilevel inverter topologies, to simulate various modulating techniques for diode clamped multi level inverter fed induction motor. These modulating techniques include sinusoidal pulse width modulation, modified reference modulating techniques. i.e., trapezoidal reference, staircase reference, stepped reference, third harmonic injected reference and offset line voltage injected reference with triangular carrier waves. The main objective of this study is to reduce total harmonic distortion, comparison of THD and fundamental component for different modulation techniques. The Simulated Induction motor model is connected at the end to observe the Stator current harmonics and speed-torque characteristics.
INTRODUCTION
Traditional two level and three level high frequency pulse width modulation (PWM) inverters for motor drives have several problems associated with high frequency switching, which produce common-mode voltages and high voltage change (dv/dt) rates to the motor windings. The concept of utilizing multiple small voltage levels to perform power conversion was introduced. These converters recently have found many applications in the medium and high power applications. Recent advances in power electronics made the multilevel concept practical [8, 10]. In fact, the multilevel is so advantageous that several major drives manufacturers have obtained recent patens on multilevel power converters and associates switching techniques. Furthermore, several IEEE conferences now hold entire session on multilevel conversion. It is evident that the multilevel concept will be a prominent choice for power electronic systems in future years, especially for medium-voltage operations [8].
Diode clamped multi level inverter (DCMLI)
The most commonly used multilevel topology is the diode clamped inverter [4], in which the diode is used as the clamping device to clamp the dc bus voltage so as to achieve steps in the output voltage. Figures 2 and 3 show the circuit for a diode clamped inverter for a three-level and a five-level inverter. The key difference between the two-level inverter and the three-level inverter are the diodes D1a and D2a. These two devices clamp the switch voltage to half the level of the DC bus voltage. In general the voltage across each capacitor for an m-level diode clamped inverter at steady state is Vdc/m-1. Although each active switching device is only required to block Vdc/m-1, the clamping devices have different ratings. The diode-clamped inverter provides multiple voltage levels through connection of the phases to a series of capacitors. According to the original invention, the concept can be extended to any number of levels by increasing the number of capacitors. Early descriptions of this topology were limited to three-levels where two capacitors are connected across the dc bus resulting in one additional level.
MODULATION TECHNIQUES
Pulse width modulation (PWM) control strategies development concerns the development of techniques to reduce the total harmonic distortion (THD) of the current. It is generally recognized that increasing the switching frequency of the PWM pattern reduces the lower-frequency harmonics by moving the switching frequency carrier harmonic and associated sideband harmonics further away from the fundamental frequency component. While this increased switching frequency reduces harmonics, resulting in a lower THD by which high quality output voltage waveforms of desired fundamental r.m.s value and frequency which are as close as possible to sinusoidal wave shape can be obtained. Any deviation from the sinusoidal wave shape will result in harmonic currents in the load which result in electromagnetic interference (EMI), harmonic losses and torque pulsation in the case of motor drives. The quality of the output waveform will improve with increase in switching frequency. Higher switching frequency can be employed for low and medium power inverters, whereas, for high power and medium voltage applications the switching frequency is of the order of 1 kHz.