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The proposed controller can balance individual dc capacitor voltages when H-bridges run with different switching patterns and have parameter variations. It has two advantages: 1) the controller can work well in all operation modes (the capacitive mode, the inductive mode, and the standby mode) and 2) the impact of the individual dc voltage controller on the voltage quality is small. This topology suggests further improvement Here we have to using fuzzy logic controllers in the control scheme to reduce total harmonic distortion. A three phase cascaded H-bridge inverter this control method uses a discrete-time model of the system to predict the future value of the current for all voltage vectors, and selects the vector which minimizes a cost function. Analytical solutions of pulse width-modulation (PWM) strategies for multilevel inverters are used to identify that alternative phase opposition disposition PWM for diode clamped inverters produces the same harmonic performance as phase-shifted carrier PWM for cascaded inverters, and hybrid PWM for hybrid inverters, when the carrier frequencies are set to achieve the same number of inverter switch transitions over each fundamental cycle. In order to improve the performance, a phase-shifted carrier based pulse width modulation technique is used. A mathematical model of the system is derived, based on which a controller for the scheme is designed. The effectiveness of the scheme is verified through detailed simulation study. Index Terms: DC voltage regulation, fuzzy logic controllers, voltage source converter (VSC), Pulse width modulation (PWM), static compensator (STATCOM) I. Introduction The static synchronous compensator (STATCOM) is a flexible ac transmission system device, which is connected as a shunt to the power system, for generating or absorbing reactive power (5). A STATCOM works in the capacitive mode if it injects reactive power to the power system and in the inductive mode it absorbs reactive power from the power system. A multi pulse converter uses more than one voltage source converter (VSC), with common dc link, operating with nearly fundamental switching frequency, and the output of each module is connected in series through the multi pulse transformer. By adjusting the triggering pulses of different VSCs, specified total harmonic distortion (THD) of the injected current is achieved with reduced switching losses as compared to that of single-VSC-based solution. The major drawback of this scheme is the high cost and complex structure of the bulky multi pulse transformer. Multilevel converter technology is a very efficient alternative for medium voltage and high-power applications and also for other applications where high-quality voltages and currents are required (1), (2). The other commonly used multilevel topology, i.e., cascaded converter topology comprises several single-phase H-bridge/full-bridge converters, with separate dc links. The following are the two associated problems of this topology: 1) The size of the dc-link capacitor required is high because the instantaneous power involved with each module varies at twice the fundamental frequency. 2) Regulating voltage across a large number of self supported dc-link capacitors makes the controller design complex.

Controlled Strategies Using DC Voltage for Asymmetric Twin Converter for a High Power STATCOM