DESIGN AND ANALYSIS OF MULTILEVEL CONVERTER FOR GRID APPLICATIONS

Abstract

A lot of research is being carried out on power converters employed for grid interfacing applications and multilevel converter is perceived as gen-next technique for grid integration. Multilevel converters are advantageous over the traditional two-level converter due to increased scalability, lower harmonics content, reduced filter components, low switching frequency, less voltage stress across switching device and an increased efficiency. Multilevel converters are widely used in recent years for high voltage and high power grid applications such as integration of renewable energy sources, FACT devices, HVDC, STATCOM and BESS. In the proposed work, the grid integration of photovoltaic source is considered. Solar energy provides the clean, ever available, reliable and environment-friendly electricity generation near the load centre. Many multilevel converters (MLC) topologies like, diode clamped, flying capacitor and cascaded H-bridge converter are reported in the literature. In the proposed work, cascaded H-bridge multilevel converter is employed as it is the best suitable configuration for photovoltaic (PV) power generation because multistring PV plant naturally provides the isolated DC source for each bridge of the CHB converter. A set of 7-level, 9-level, 19-level and 43-level multilevel converters for integration of large photovoltaic system are investigated in the proposed work. Multilevel converter has the advantage of direct interfacing with high and medium voltage grid. Hence transformerless integration with the grid is employed in the proposed work to avoid a bulky and expensive step-up transformer. This system is a single-stage converter to eliminate DC-DC stage, for reducing the cost, losses and also control complexity in the system. The selection of optimum number of levels for given voltage rating is investigated in the proposed work to obtain the proper ratio of performance versus cost and complexity. The vi number of levels in the multilevel converter is selected in proposed work, considering all design considerations like the cost of IGBTs, arithmetic and logical operations (ALOs), THDs, and device voltage utilization factor (DVUF). As well-known algorithms converge fast, so these algorithms like perturb and observe (P&O), and incremental conductance (INC) methods are employed in proposed work. Each PV array is provided with a separate maximum power point tracking (MPPT) algorithm to overcome the unpredictable irradiance level variations, ambient temperature, shading effect, and other relevant factors in proposed work. Decoupled current control with SRF-PLL is used for VSC control in the proposed work. This work focuses on investigation of a suitable modulation technique with improved THD of MLC output voltage and grid current, and low switching frequency for the symmetrical CHB multilevel converter. The phase shifted PWM, SHE-PWM and NLM modulation techniques are investigated in this work. The major focus of this research work is on the investigations of high power converter based PV grid tied system for photovoltaic application, while satisfying the IEEE standards. The lower switching frequency employed for switching the converter, reduces the switching losses and reduces the acoustic noise. It ultimately reduces the size, hence the cost of the filter. The reduction of switching losses at higher power rating system is significant.