Unidirectional High-Frequency-Link DC to Three-Phase AC Conversion: Topology, Modulation and Converter Design

Abstract

In recent years, stringent restrictions on greenhouse gas emission due to the present global warming scenario is driving governments and power utilities worldwide behind electricity generation using renewable energy sources. Conventionally, for grid integration of a large scale photovoltaic (PV) system, a three-phase voltage source inverter followed by a line frequency transformer (LFT) is used. The inverter generates line frequency (50/60 Hz) AC from the DC output of PV. The LFT provides galvanic isolation and thus reduces the circulation of leakage current, and ensures safety. Few limitations with the conventional system are a) huge volume as the LFT is bulky, (b) quite expensive due to large amount of iron and copper used in LFT and (c) the inverter is hard switched. The converter topologies with high-frequency galvanic isolation have attractive features like high power density and are less expensive. Hence these converters are promising alternatives to the conventional solution. The three-phase inverter topologies with high-frequency transformer are generally of two types- a) multi-stage and b) single-stage. In multi-stage, interstage DC link is voltage sti as lter capacitor is used. In a single-stage solution, the intermediate DC link is pulsating as lter capacitor is avoided to improve reliability. Though these converters have high power density, they employ large number of active switches on both the sides of the transformer to process power and hence have relatively lower e ciency compared to the conventional solution. The active switch count can be reduced in case of unidirectional applications like grid integration of PV, fuel-cell where the active power ows from DC source to AC grid. The converter e ciency can be further improved by reducing the switching loss. In this work, we have investigated four new unidirectional single-stage three-phase inverter topologies with low or negligible switching loss. To reduce the switching loss, the active switches of the introduced topologies are either line frequency switched or high-frequency soft-switched. The soft-switching is achieved without additional snubber circuit. The pulse width modulation is implemented on the input DC side converters which are soft-switched. The active switches of the grid interfaced converter are low frequency switched and thus enabling the use of high voltage blocking inherently slow semiconductor devices for direct medium voltage grid integration. The topologies are gradually improved to achieve soft-switching of the DC side converters throughout the line cycle. The conditions on dead time to ensure soft-switching are derived through detailed circuit analysis. The operations of these topologies are experimentally veri ed on hardware prototypes with power range 2-6kW. Out of four introduced topologies, three topologies can support only unity power factor operation. An additional shunt compensator is needed for any reactive power support. The fourth topology can support up to 0.866 power factor operation though it has relatively higher conduction loss. The performances of the introduced topologies are compared with multi-stage and conventional solutions. Though the new topologies have relatively higher switch counts, the converter power losses, lter requirements are comparable with the conventional solution with line frequency transformer, and have high power density.