Adil Sarwar

Multilevel converter topology for solar PV based grid-tie inverters

A solar PV based grid tie inverters are used for dc-ac conversion. The conventional line commutated ac-to-dc inverters have square-shaped line current which contains higher-order harmonics. The line current with the high harmonic contents generates EMI and moreover it causes more heating of the core of distribution/ power transformers. Alternatively, PWM based inverters using MOSFET/IGBT switches are also used for the same purpose. However, apart from higher switching losses, the power handling capability and reliability of these devices are quite low in comparison to thyristors/ SCR. Nevertheless, the conventional thyristor based forced commutated inverters are not suitable for PWM applications due to the problems of commutation circuits. A pure sinusoidal line current or waveform with low harmonic contents is the most desirable for grid tie inverters. In the present work, various multilevel line commutated inverter topologies has been proposed and analyzed which improves the wave shape and hence reduces the total harmonic distortion (THD) of the line current in a grid tie line commutated inverter. The scheme has successfully been implemented and tested. Moreover, the performance of the proposed topology is far better than the conventional line-commutated inverter. It reduces THD, losses, switching stress and EMI.

Measurement of Speed and Calibration of Tachometers Using Rotating Magnetic Field

Fast speed measurement and control signals are required for feedback control applications. In the proposed technique, a rotating magnetic field (RMF) is used to measure the speed and the variation in speed with high accuracy. In addition to a 400 Hz, three-phase input voltage circuit and an output zero-crossing detector circuit, a contact-type synchro is used as a primary transducer. A balanced three-phase ac voltage is applied to the stator windings of synchro, which produces an RMF. The frequency of electromotive force induced in the rotor circuit of synchro depends upon the speed of rotor or rotating member. The RMF revolves at a speed of 24 000 r/min, which is purposely kept very high in comparison with the speed of the rotor or rotating member. Thus, the measurement becomes very fast and the change in speed is sensed within 2.5 ms. The time response of the proposed scheme is also compared with the time response of a dc tachogenerator. A digital storage oscilloscope and a microcontroller are used for measurement of the output. The 16-b digital output of the microcontroller is made available as a feedback signal for control applications. The high accuracy of measurement of the proposed technique is also successfully used for calibration of different contact- and noncontact-type tachometers.

Performance analysis of carrier based PWM technique for three level diode clamped multilevel inverter with different reference signals

Multilevel Inverters (MLI) are used in electrical drives in industries especially for medium/high power applications due to the low total harmonic distortion (THD) contents in voltage waveform and high utilization factor. For control of output voltage several pulse width modulation techniques are used. Carrier based PWM (CBPWM) techniques are quite simple compared to space vector modulation. In this paper diode clamped MLI has been selected and its performance with several conventional CBPWM techniques has been analyzed using MATLAB simulation. The analysis is based on four different reference signals taken with carrier wave. The variation of THD with amplitude modulation index is shown for all CBPWM technique. Gate signal generation for all the CBPWM techniques using TMS320F28335 has also been included in this paper.