Research Of Three-phase AC-DC Converter Based On Modified SVPWM Strategy

Three-phase bidirectional AC-DC converters are now widely used in electrical vehicle charging stations,battery formation systems,micro grids,etc.Voltage of energy storage devices such as batteries and super capacitors vary in a wide range with the state of charge(SOC).The key problem here is how to realize high efficiency energy conversion while ensuring wide voltage range regulation,which is one of the principle problems of these three-phase AC-DC conversion systems.Due to multiple power losses during energy transformation,efficiency of conventional two-stage AC-DC converters is low in general.A quasi-single-stage AC-DC structure based on three-phase three-port bidirectional AC-DC converter is investigated.The three-port bidirectional AC-DC converter can manage an adjustable DC load port,a high-voltage DC bus port and an AC grid port simultaneously.The DC load port can be directly connected to batteries.Single-stage conversion of part of the energy can be achieved with the three-port AC-DC converter and energy conversion stages are reduced accordingly.The structure,implementation methods and control schemes of quasi-single stage AC-DC converters are analyzed in detail.The modulation strategy of three-phase three-port bidirectional AC-DC converters is pointed out to be the key problem during implementation.To find appropriate modulation strategies for three-phase three-port AC-DC converters,symmetrical three-level SVPWM strategies are analyzed.The law of relationship between voltage vector and direction of port current is summarized.From this,a modified SVPWM strategy of three-phase three-port AC-DC converter is obtained.Except for accomplishing regulation of AC current,the modified SVPWM strategy can transfer part of grid-side energy to low-voltage DC port within single stage conversion.Detailed analysis of basic principle and characteristics of the modified SVPWM strategy is given.Characteristics of quasi-single-stage structure and traditional two-stage structure are compared with each other.Results of power loss analysis verify effectiveness of quasi-single-stage structure primitively.A 3kW prototype of quasi-single-stage AC-DC converter is built to verify feasibility of the modified SVPWM strategy.Steady state and dynamic waveforms of both rectifying and inverting direction are given.Efficiency of the investigated quasi-single-stage structure is higher than two-stage structure within the whole operating range.